the impact of subsoil compaction on soil functionality

The Impact of Subsoil Compaction on Soil Functionality & Landscape

DEFRA

March 2006

QM

Issue/revision Issue 1 Revision 1 Revision 2 Revision 3

Remarks Draft Interim

Report

2nd Draft Interim

report

Third Draft Final

November 2005 February 2006 March 1st 2006 March 26th 2006 Date

Prepared by Zoe Buckley Chris McDermott Chris McDermott Chris McDermott

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Checked by Chris McDermott V Piggot V Piggot V Piggot

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Authorised by Chris McDermott Chris McDermott Chris McDermott Chris McDermott

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Project number 12262058/001 12262058/001 12262058/001 12262058/001

Bristol/EIA/

Projects/1226205

8/DEFRASubsoil

CompactionRepo

rtCMMar2006

File reference Bristol/EA

Projects:/226205

8

Bristol/EIA/

Projects/1226205

8/DEFRASubsoil

CompactionRepo

rtCMFeb2006

Bristol/EIA/

Projects/1226205

8/DEFRASubsoil

CompactionRepo

rtCMFINAL2006

WSP Environmental UK Colston 33 Colston Avenue Bristol BS1 4TT Tel: +44 (0)117 930 2000 Fax: +44 (0)117 929 4624

Contents

1 Introduction 5

1.1 Background 5 1.2 Aim 5 1.3 Report Structure 6

2 Definitions 7

2.1 Soil 7 2.2 Topsoil 7 2.3 Subsoil 7 2.4 Soil Compaction 7

3 Background Review 8

3.1 Compaction Processes in soils 8 3.2 Subsoil Compaction on Construction Sites 9 3.3 Effects of Compaction on Landscape Schemes 17 3.4 Existing knowledge on Methods to Reduce and Alleviate Compaction 19

4 Case Studies 29

4.1 Methodology 29 4.2 Case Study 1: Arborfield Cross 29 4.3 Case Study 2: Northstowe New Town 31 4.4 Case Study 3: Batheaston Reservoir 34 4.5 Case Study 4: Trees within paved areas 37 4.6 Case Study 5: New Towns 38 4.7 Case Study 6: Artificial Compaction of Soils 45

5 Consultation with Landscape Contractors 46

5.1 Questionnaire 46 5.2 Discussion on the results 49

6 Recommendations for good practice 52

6.1 Introduction 52 6.3 Summary 53

7 Conclusion 54

Appendix A - Questionnaire 57

12262058 Subsoil Compaction 4

Summary The over-compaction of subsoil is almost an inevitable by product of the construction process and alleviation is necessary where trees, shrubs and grass are to be established.

The alleviation of compaction is not the responsibility of one sector of the industry. Policy makers, clients, designers, specifiers and contractors all have a role to play in minimising the occurrence of compaction.

Methods for relieving compaction are well known and the failure to realise best practice is primarily due to the lack of communication and understanding of the roles of each sector. Financial losses arising from compaction do not seem to be sufficient to be a driver for improvement. The greatest improvements in the alleviation of compaction are likely to be achieved through the production of best practice guides, particularly through such organisations as CIRIA.

Issues regarding the over-compaction of soils are gaining a higher profile in the construction industry as the requirement for sustainable drainage increases and higher development densities create new landscape challenges. This is leading to innovation in both landscape design and associated products, such as structural soils; but there is an opportunity to review standard specification clauses to ensure that they reflect current construction and landscaping trends.

Although compaction can adversely affect the growth of grass, shrubs and trees, the most significant impact is on tree growth. There is substantial evidence that a high proportion of trees within amenity landscape schemes are failing to thrive due to the over compaction of soil and other factors. This has long term implications in terms of realising the design objective, sustainability, cost and the creation of attractive living environments. Thus improvements in the alleviation of compaction should be sought, particularly in relation to tree growth.


1 Introduction

1.1 BACKGROUND

1.1.1 WSP Environmental Ltd was commissioned by the Department for Environment, Food and Rural Affairs (Defra) to determine the impact of subsoil compaction, arising out of construction, on soil functionality and landscaping products and to examine cost-effective and practical mitigation options suitable for the construction sector. Like air and water, soil is a precious resource that is essential to life. Covering most of the Earth’s land surface soil is a vital foundation to both agricultural systems and natural environments. Defra is working with partners to achieve the sustainable use and protection of soils in the built environment. The built environment can be defined as anywhere where development and construction has occurred (or is planned) and applies to both urban and rural areas.

1.1.2 The First Soil Action Plan for England was produced by Defra in 2004. The vision of the plan is:

‘to ensure that England’s soils will be protected and managed to optimise the varied functions that soils perform for society…in keeping with the principles of sustainable development and on the basis of sound evidence’.

1.1.3 Following on from the Soil Action Plan Defra and WSP Environmental completed a review of the handling and management of soils throughout the construction process in April 2005 which included issues of soil compaction.

1.1.4 Landscapes that fail due to compaction have a cost to society. Landscaping is usually required to meet social, cultural and visual objectives that evolve from the design and planning process. For example to provide attractive open space, reduce visual intrusion or create mitigating ecological habitat. These objectives will not be fully achieved if compaction results in scheme failures or if the planting fails to develop to its full potential. If plants need to be replaced the additional environmental cost of producing those plants in terms of compost, watering, labour, machinery and waste has also to be considered.

1.1.5 Landscape schemes must also respond to changes in climate and more extreme weather (such as prolonged periods of drought, heat, high wind and rain fall). Such conditions are placing greater stresses on plants and to thrive they need a growing medium that is optimally compacted for plant growth. Thus the need to understand and tackle the problems of compaction is becoming more relevant in today’s society.

1.2 AIM AND SCOPE

1.2.1 This research forms part of Defra’s commitment to encourage sustainable use and protection of soils in the built environment. The overriding aim is to clearly identify the current issues in relation to the over compaction of subsoil in relation to construction and the commercial landscape sector and provide sustainable, cost-effective and practical solutions to the problems of subsoil compaction for the construction and landscape industries.

1.2.2 The study does not cover compaction issues in relation to agriculture because the nature, scale and objectives are entirely different and substantial existing knowledge


on the subject exists. Nor does it cover compaction in relation to the maintenance of amenity grassland where it can arise due to recreational activity since techniques to alleviate sports turf compaction are well documented.

1.3 REPORT STRUCTURE

1.3.1 This research report provides background information and relevant case studies on soil functionality and subsoil compaction from existing literature, in-house professional knowledge and practical experience. The manner in which compaction becomes manifest within the landscape industry and the cost implications are assessed. Interviews and discussions with a cross-section of professionals and contractors involved in the construction and landscape industries were also undertaken as part of the research programme. The various options for avoiding and alleviating soil compaction have been assessed and, in the light of practical professional knowledge, realistic guidelines have been developed.


2 Definitions

2.1 SOIL

2.1.1 The First Soil Action Plan for England: 2004-2006 (Defra, 2004), defines soil as ‘the upper layer of the earth’s crust composed of mineral particles, organic matter, water, air and organisms in which soil forming processes have transformed the parent material’. Conventionally soil is sub-divided into topsoil and subsoil.

2.2 TOPSOIL

2.2.1 Topsoil is the layer of material on the land surface. It contains most of the organic matter and therefore supports most of the vegetation growth.

2.2.2 Naturally occurring topsoil is the dynamic product of chemical, physical and biological processes. It is the result of interaction between the inherent nature of the parent material and the prevailing environment (including human activity and vegetation). The properties of topsoil will therefore change if one or more of these factors change (BS 3882: 1994).

2.2.3 The British Standard for Topsoil (BS 3882) provides the following basic definitions for natural topsoil:

“upper layer of an in situ soil profile, usually darker in colour and more fertile than that below (subsoil) and which is a product of natural biological and environmental processes.”

2.3 SUBSOIL

2.3.1 Subsoil, is defined in the British Standard 3882 as the:

“soil layer extending between the natural topsoil and the parent material”

2.3.2 Whilst the parent material / bedrock is defined as:

“broken weathered rock or other underlying geological material from which the natural soil has developed”.

2.3.3 In essence the subsoil can be defined as the soil between the topsoil and the bedrock.

2.4 SOIL COMPACTION

2.4.1 The process of compaction comprises the loss of soil volume due to forces applied to the soil surface, large air-filled pore spaces are effectively crushed leading to smaller, water-filled pores and fewer connections between pores, which are essential for the transference of water, air, minerals and living material. The soil particles pack more closely together. Compression is more prevalent in soils under wet conditions.

2.4.2 Further compaction comprises the destruction of the soil aggregates and total collapse of aeration pores leading to deformation of the soil destroying any pore space and structure, and water is squeezed from the soil matrix. This process leads to increased internal bonding and soil strength as more particle to particle contacts are made and pore space is eliminated.

2.4.3 A heavily compacted soil would essentially show a loss of soil aggregates; destroyed aeration pores; crushed or collapsed pore spaces; and, underground extensive packing of soil particles resulting in increased soil strength.


3 Background Review

3.1 COMPACTION PROCESSES IN SOILS

3.1.1 Since bedrock varies in chemical composition, it follows that subsoil derived from it also varies. Soil forming processes also vary and intermixing can occur, further increasing subsoil diversity. Worldwide the depth of subsoil ranges from a few centimetres to hundreds of metres depending upon the local geology, but in the UK it ranges from a few centimetres in shallow soils like chalk, to up to several metres in the clay basins. Subsoil in a particular location can be of a consistent character throughout, but more typically the character changes with depth due to geological and biological processes. This can result in recognisable strata forming known as soil horizons.

3.1.2 Undisturbed subsoils exhibit a natural level of compaction and this varies between subsoil types and between soil horizons. Loose workable soils commonly have bulk densities of 0.8 to 1.3 grams per cubic centimetre ( or have 70 to 50 percent pore space), while more compacted soils have bulk densities in the range 1.6 to 1.8 (or 40 to 30 percent pore space). Typically root penetration is greatly retarded when bulk density exceeds 1.4 in dry conditions (Relf, 1997).

3.1.3 Non-cohesive soils, such as those with a high sand content can have a higher density before root growth is significantly impeded, than cohesive soils, such as those with high clay content. This is illustrated in Table 3.1. Density varies with moisture content; maximum density occurs at an optimum water content, which varies with the type of soil. At optimum water content the lubrication effect of the water allows soil particles to become easily aligned, but excessive water can result in dilution and a lower density. A good example of this is when clays absorb water, swell and become less dense.

Table 3.1: Approximate bulk densities that restrict root penetration (Handbook of Soil Science, 1999)

Texture Bulk density range for soil resistance (g/cm2)

High Low

Sandy 1.85 1.6

Coarse-loamy 1.8 1.4

Fine-loamy 1.7 1.4

Coarse-fine silty 1.8 1.4

Clayey Varies depending upon clay percent and structure

3.1.4 Soil compaction reduces the diameter and the continuity of the pores and thereby reduces the permeability of liquids and the diffusion of gases in the soil. Trouse (1996) found that infiltration was reduced from 323 to 2 mm/h after eight passes of a light vehicle (Rolf, 1994).

3.1.5 Some of the most sever compaction occurs when wet soils are trafficked, with the greatest variation in the degree of compaction occurring in clay soils and silty soils. Moisture content is one of the most important parameters to take into consideration to avoid heavy soil compaction.

3.1.6 Smearing can also occur on clay soils, this is the process by which fine-textured clay soils tend to gel when wet and often occurs as a result of site vehicular movements working on wet soils.


3.1.7 Compacted layers or zones in soils are called pans. Pans are horizons or layers in soils that are strongly compacted, indurated, or very high in clay content.

3.1.8 Compacted subsoil layers limit total soil volume available for rooting and restrict total water and nutrient availability. Artificially compacted zones (traffic pans) often are much denser than natural clay pans and have no breaks or channels, such as are found in naturally occurring pans. Traffic pans must be physically shattered to allow significant rooting. Natural subsoil clay pans, on the other hand, often have structural planes of weakness that allow roots to penetrate to some extent, even though the layer overall is very dense (Relf, 1997).

3.1.9 In the South-East of England, where development activity has been historically prevalent, there is a significant proportion of slowly permeable seasonally wet acid loamy and clayey soils, which naturally have impeded drainage with low natural fertility These clay soils are particularly prone to compaction when wet and present a significant issue for the landscape and construction industries.

3.2 SUBSOIL COMPACTION ON CONSTRUCTION SITES

Background

3.2.1 Soil compaction on construction sites occurs either deliberately when foundations and sub grades are prepared for construction or as an unintended result of vehicular traffic and excavation (cut & fill) works.

3.2.2 Although the most sustainable and potentially cost-effective long term solution to subsoil compaction is to prevent it, in practice a certain level of compaction is unavoidable. Constraints on space resulting from increasing demand, together with timing and programme constraints and economic drivers often result in development during wet weather which further increases the impact of compaction.

3.2.3 The problem of excessive soil compaction is becoming more relevant within the current development climate. Central government policy has promoted an increase in development densities and this trend is set to continue. The implication is that space on sites is limited; the majority of the site is either being built upon or used for storage, access, services and site accommodation. To promote sustainability, planning policies discourage the importation and exportation of soils from sites, requiring a cut and fill balance to be achieved. This usually requires disturbance to soils right across the site and the mechanical compaction of fill areas to meet engineering requirements. Thus compaction issues occur throughout most sites.

3.2.4 As stated by Kendle & Schofield (1992) ‘the best solution to soil compaction is to avoid the circumstances which produce it’. Avoidance/ soil preservation is obviously the best solution where practicable however on construction sites this is not generally a realistic solution. Main contractors are nearly always under pressure to meet tight deadlines and avoid penalties for claims for defects, however, compaction, in relation to plant establishment and survival is rarely an issue that causes claims for damage. This is in part due to the relatively low cost and economic value of planting, compared to buildings and infrastructure and the fact that the impact of compaction can materialise months or even years after completion of the scheme when the main equity release from the scheme has occurred. Thus compaction is usually viewed as far less of a problem than the many other constraints and issues that a main contractor has to deal with.

3.2.5 Frequently landscaping is one of the last operations on a site, but has to be undertaken within a tight timescale to prepare the site for ‘opening’. This is particularly true where the landscape will assist sales, such as housing schemes. This means that


landscaping is often undertaken in inappropriate weather conditions where the relief of compaction cannot be carried out adequately.

3.2.6 Policies also promote the development of previously developed land, which may require remediation and appropriate methods of soil handling to minimise compaction, or contain areas of uncontaminated, but compacted soils.

3.2.7 There are typically two main forms of compaction on a construction site, compaction of in situ topsoil when the unexcavated parent material is tracked over and secondly where subsoil is excavated and recompacted as fill.

Compaction of in situ subsoil

3.2.8 It is standard procedure on most construction sites to remove the topsoil and/or existing buildings or structures and commence construction within the subsoil layer. The subsoil layer forms the working surface of the majority of construction activities. Central government policy has promoted an increase in development densities and this trend is set to continue. Usually the only areas of a site that are protected from the construction process are existing landscape features to be retained such as trees, hedgerows and water bodies and these usually require minimum landscaping. Virtually all excavation work is carried out by heavy plant, often continually crossing the same area over several weeks in a variety of weather conditions. Thus the in situ subsoil, which may already be close to or exceeding the density that inhibits plant growth, is compacted further. Typically the compaction caused by machinery does not extend below 500mm, depending upon soil type, but it is the high level of compaction within this upper layer that creates a high density, poor draining strata that can severely inhibit plant growth. Clay soils are particularly prone to compaction in moist conditions when the water content acts as a lubricant to the movement of clay particles.

3.2.9 Where in situ subsoil has been compacted by machinery and lies within areas to be landscaped, it is important that this compaction is alleviated.

Compaction of subsoil used as fill

3.2.10 The excavation and redistribution of subsoil to re-contour a site is a commonplace activity on most development sites. This is not necessarily damaging, particularly if the native subsoil is naturally too compact to promote good growth. Excavation will break up the subsoil to create voids and fissures, improving drainage and offering the potential for colonisation by micro-organisms. In certain soils a small amount of compaction may actually increase plant growth by producing a better balance between air space and water retention. The correct degree of compaction is also desirable since under compaction can lead to settlement. This can lead to landscape areas settling lower than adjacent hard paving, causing maintenance difficulties and the exposure of foundations. Lawns can become uneven and low spots prone to water logging.

3.2.11 Methods to compact subsoil on site vary depending upon the nature of the subsoil, the layout of the site and the degree of compaction required to meet engineering requirements. Re-profiling of construction sites is common place in order to create building platforms, the correct levels for roads or as part of the general remediation of brownfield sites. Typically this is achieved through excavating subsoil and other materials on site or importing fill. This fill has to be compacted to achieve the structural strength for the level of engineering support required. Frequently these artificially compacted subsoils are also required to support plant growth. This section examines the various methods of compaction and accesses the implications for plant growth. These issues are also illustrated by Case Study 6 which shows that soils can be routinely artificially compacted to densities that inhibit root growth. The most commonly used methods are:


Dynamic compaction 3.2.12 Dynamic compaction involves lifting and dropping a heavy weight several times in one place. The process is repeated on a grid pattern across the site. Trials in the UK indicate that masses in the range 5 to 10 tonnes and drops in the range 5 to 10m are effective for compacting loose sand and granular fill but not clay. Dynamic compaction is not often used in the UK due to the noise and general disturbance it can cause to nearby residents.

Vibro compaction and vibro replacement 3.2.13 This is a compaction process for purely granular soils. It relies on the fact that particles of non-cohesive soil can be re-arranged into a denser condition under the influence of vibrations from specially designed poker vibrators. The action of the vibrator, usually accompanied by water jetting, reduces the inter-granular forces between the soil particles allowing them to move into a more compact configuration. After a certain compaction time, the particles are arranged in such a way that the optimum density has been achieved. In coarse-grained soils the poker may be removed slowly while still vibrating. This causes the sides of the hole to collapse and results in a depression in the ground surface. In fine-grained soils it is usual to fill the hole with coarse aggregate (up to 50mm), this is known as vibro replacement. The poker may be used to compact the stone column in layers. A typical column might be 5m deep and 500mm diameter. A line of columns at say 3m centres can be used to support a reinforced concrete ground beam effectively producing a piled foundation.

Pre-consolidation

3.2.14 Pre-consolidation involves loading the soil to apply a pressure that results in gradual settlement over a period of time. The larger the pre-load, the less time it will take to achieve the final settlement. Pre-consolidating the ground in this way tends to be an expensive solution compared with the use of piles to support localised loads such as columns, but pre-consolidation may be a cost-effective way of reducing the settlement due to lightly distributed loads such as roads or warehouses or supermarket floors, provided that material is readily available to provide the pre-loading. Pre-consolidation is normally designed to take 6 - 9 months.

Compaction in layers 3.2.15 Fills are normally compacted in layers between 100mm and 600mm thick depending upon the substrate, usually using a smooth wheeled roller, a sheeps foot roller or a pneumatic tyred roller. For granular soils, a motor on the back of a smooth wheeled roller is used to rotate an eccentric mass causing the roller to vibrate. Sheep's foot rollers produce a kneeding action which changes the shape of clods of soil and displaces air from the spaces between the clods and is best for fine soils including clays. Compaction in layers cab cause substantial smearing of the clays creating a series of pan like layers which can impede drainage and root penetration.


The effects of compaction on the functioning of sustainable drainage systems

Introduction

3.2.16 The disposal of surface water on development sites in the most sustainable manner is encouraged through government policy, primarily to reduce flood impacts further down the drainage system. The policy is being successfully implemented through planning condition and regulation by the Environment Agency which is at the forefront of this initiative. Sustainable drainage can be achieved through a variety of techniques, not all of which may be desirable or practical for a particular site. All systems aim to reduce the amount of water flowing from a site into the wider drainage network, particularly at times when drainage capacity is limited, such as during storm conditions or when the water holding capacity of soil has reached a maximum, typically during winter months. Sustainable drainage is relatively new and as discussed below, more complex than it might first appear. CIRIA and the DTI have been undertaking research since 2003 to produce Best Practice Guidance, but this is not due to be published until June 2006. CIRIA produced an interim paper in 2004, Sustainable Drainage Systems (CIRIA C609). This section considers the effect of soil compaction on the effectiveness of particular sustainable drainage systems.

Surface water storage basins / infiltration basins 3.2.17 The commonest sustainable drainage technique is to create a storage area, usually on site, that can accommodate all the surface water run off from a site during a 100 year storm period (or greater). Typically the storage area is created by lowering ground levels or bunding to create a floodable volume, which pre-flood needs to be dry, i.e. a basin. This retained storm water can then be released slowly into the wider drainage system, when the wider system has capacity, via weirs, throttles or pumping depending upon the hydrological requirements of a site. Holding water on site can also allow water to be lost through evaporation, transpiration through plants and permeation into substrata.

3.2.18 In terms of engineering function the storage area is only likely to become filled to capacity every 100 years or so, because the systems are designed for extreme events; for most of the time the storage area is likely to contain no standing water. In such circumstances it may be desirable that the soils in the base of the storage area are as permeable as possible to enable water to soak away, although in certain instances the EA may require the structure to be fairly impermeable to ensure that engineering integrity is retained and that no polluted water seeps into the ground. Typically, however, the base of the floodable area is over dug to create a permanent water body for aesthetic, recreational and biodiversity benefits. Often it is desirable to create an impermeable base to the water body to enable it to retain water, particularly if the only source of water is from run off within the development which can be unreliable in the summer months when evapo-transpiration is at its greatest.

3.2.19 It is therefore usually desirable to compact soils in the base of a permanent water body storage system but create more permeable margins that flood only periodically. Evidence suggests that infiltration rates can quickly decline as the bases become covered with sediment and organic matter. Consequently engineers typically calculate flood capacity assuming the system is largely impermeable and so over compaction of the floodable margins usually has no adverse hydrological implications, but may have biological implications.

3.2.20 Infiltration basins are specifically designed to allow water to infiltrate down to lower strata but work best in areas of free draining strata such as limestone or chalk;


geology which is often designated a Groundwater Protection Area because it feeds aquifers and springs. Often this can mean restrictions on the use of infiltration basins if there is any possibility that contaminated water can enter the groundwater system.

Maximising Permeable Surface Area 3.2.21 This technique aims to reduce the amount of hard run off areas within a development and maximise the amount of permeable surfaces, thereby reducing the amount of water that flows to a storage area. The trend for developments of higher density is potentially increasing the extent of impermeable surfaces and typically permeable ground is confined to landscape areas and permeable car parking and footways, as illustrated by photograph 1. Ideally the surface water would drain freely into these areas and disperse, but this is often impractical because either the subsoil is not free draining (such as clay) or the drainage of large amounts of water into a sub base can result in the failure of the pavement structure or a shortening of its design life. Tests have also shown that paving designed to be permeable, such as porous tarmacadam or paviors with wider joints, often becomes impermeable after a short time due to sedimentation.

3.2.22 Sites built on permeable soils or free draining fill are less common than sites on impermeable soils and so the sustainable dispersal of water through the ground is frequently not practicable. A more common technique is to pave areas in a material that can soak up water which is then dispersed through evaporation. A good example of this are car parks surfaced in a mixture of aggregates bound with clay and these are effective at reducing run off. In such instances the surface is being continually compacted by vehicular traffic, which can reduce absorption. More sophisticated techniques have been developed to overcome this problem; these usually rely on a plastic mesh to provide structural support for the vehicle which is in-filled with aggregate which allows free drainage to an underlying layer that can temporarily soak up the water. The mesh can also be in-filled with grass, but the sward can be less free draining and can die out if vehicle traffic/parking restricts sunlight and water reaching the surface.

3.2.23 The trend for increased development densities means that the extent of impermeable surface is increasing, although government guidelines setting out the minimum ratio of greenspace to a developments predicted population combined with the need for sustainable drainage means that most developments will have a degree of potentially permeable greenspace. It is important that the permeability of this green space is maximised and this relates to soil compaction.

3.2.24 The impact of compaction on the permeability of grassland and amenity landscaping can be assessed by examining research on agricultural field drainage. The outflow from a field drainage system is commonly calculated by taking into account the

Area to be drained

Maximum Length of the Catchment

Crop Type

Average Annual Rainfall

Soil Type factor (the degree of permeability of the soil)

Angle of slope

If rapid draining subsoils under arable cultivation are given a co-efficient of 1.0 for the amount of permeating water that can be expected to be picked up by the field drainage system, calculation tables typically give slow draining subsoils a co-efficient of 0.7 (30% less flow). For slow draining subsoils covered in grass the co-efficient is only 0.6, but


rises to 0.8 if the subsoil is ripped prior to installation of pipes (The Design of Field Drainage Pipe Systems ADAS 1983). The difference between runoff from sandy soils and clay soils is considered to be much greater in some guides with a 60% difference (Drainage of Development Sites: A guide. HR Wallingford/CIRIA). For sites where a field drainage system is installed sandy soils are seen as more problematic, since the speed of filtration and collection during a storm is fast, causing high peak flows that have to be attenuated; whereas clay soils slow down the passage of water. If no field drainage is in place the converse can be true in that the sandy soils can absorb water, but the clay soils can be more prone to surface water runoff.

3.2.25 Such tables have largely evolved from empirical knowledge and in reality drainage engineers often have little knowledge of the soil type and its permeability on a site at the time of calculation and so assume maximum permeability to ensure the pipes, ditches and storage features in the system have suitable capacity. Under certain site conditions they also assume minimum permeability when calculating surface water run off. This is also partly necessary because in wet winters soils can be at or close to field capacity in terms of their water holding and permeable properties and so any additional rainfall may be unable to soak into the soil and instead stands on or runs across the surface. Thus factoring in soil permeability in the design of drainage systems is an imprecise art and engineers tend to err on the side of caution to ensure the designed system copes under worst case conditions.

3.2.26 In terms of sustainable drainage soil compaction is probably most significant in relation to a soils ability to soak up water, rather than transmit water to lower strata. Lightly compacted soils are more able to soak up rain water and then allow its return to the atmosphere via evaporation and transpiration. In terms of transpiration it is equally important that the soil has an appropriate level of compaction to promote strong plant growth.

3.2.27 Studies indicate that if green permeable areas are not positively drained into an engineered drainage system they can absorb rainfall under most conditions without significant adverse impacts but site specific issues must be considered; such as whether temporary ponding during storm conditions restricts amenity use of an area, causes plant deaths or causes flooding to habitations. Rarely can soils absorb additional run off from adjacent paved areas without some form of positive drainage system being put in place, such as field drainage. Just as structural soils have been created to provide a rooting medium for trees and support pavements, there may be opportunities for designing soils with properties that maximise absorption and evaporation.

Underground storage 3.2.28 Surface water run off can be held in underground reservoirs on development sites where space is too tight to accommodate surface storage. Storage is typically in tanks or oversized drainage pipes. In such cases soil permeability is not an issue.

Soakaways, Infiltration Trenches, Filter Drains

3.2.29 Soakaways have been used for many years as an effective and sustainable way of draining water, primarily from roofs and this technique has been expanded to include infiltration trenches and filter drains. All require a degree of permeability in the underlying substrate and all can suffer from sedimentation, which can substantially reduce their effectiveness after 20 years. Excessive employment of such systems can compromise the structural integrity of surrounding sub-bases and foundations; a typical recommendation is that none are located closer than 5m to buildings, which can restrict their use on densely developed sites. Figure 1 illustrates a typical road side infiltration trench detail which is designed to prevent excess water soaking into the road sub-base

and adjacent tree pits and Photograph 2 illustrates how a sustainable drainage system can be used to create wetland habitats.

Photograph 1 An example of sustainable drainage in a car park. The parking bays comprise gravel held within a plastic grid, but the system relies primarily on evaporation to disperse water with a drainage system in the granular sub-base to remove excess water through normal methods, rather than allowing water to soak into subsoil which could cause failure of the structure, particularly the road ways which are surfaced in tarmacadam.

Figure 1: Shallow grass covered ditches are a technique to reduce water flows from a site by slowing flows and allowing loss primarily through evapo-transpiration rather than soaking into the subsoil layers. In this example the base of the ditch is given permeability by installing a gravel drain but water is prevented from soaking away to the sides by a compacted clay surround. This protects the road sub-base and prevents too much water flooding into the tree pit, causing water logging and possible death of the tree.


Photograph 2: A nature reserve created from road construction fill which has been designed to shed water to a low spot, avoiding water logging in tree planting areas while creating an aquatic habitat. Drainage water is retained on site and does not feed into the wider drainage network. Summary of compaction in relation to sustainable drainage system 3.2.30 Only sites with very permeable soils are suitable for sustainable drainage techniques where water is transmitted to lower strata, but such sites are less common and may be protected by Groundwater Protection Area designation. Excessive permeation of water can damage structural sub bases and so filtration often has to be controlled with permeable barriers For heavy clay sites this technique is not practical and water loss is usually best achieved by slowing flows and creating soil conditions that maximising absorption, with water being lost through evapo-transpiration before it reaches a catchment outflow.

3.2.31 Clearly it is important that the soil handling and cultivation of these areas follows best practice to reduce compaction and achieve maximum absorption and/or permeability. For surface water storage systems compaction is often desirable in the base of the excavated area to create a permanent water body, but permeability can be desirable within the flood storage area (free board).



3.3 EFFECTS OF COMPACTION ON LANDSCAPE SCHEMES

The relationship of compaction on plant growth

3.3.1 The ability of a growing root tip to physically penetrate soil is directly dependant on soil structure and density (which can vary with moisture content) and on the species, since different species can exert different root pressures. The genetic makeup of plant species controls the maximum depth of root penetration under optimum conditions. Actual root depth is generally less than the maximum, particularly on disturbed sites as it is controlled by soil water supply and by soil properties including fertility, aeration, hardness, soil strength and particle size.

3.3.2 Roots also require air to respire and a sufficient supply of water. This relates directly to the void structure of the soil and moisture content. Clays contain more pore space than sandy soils but have a much smaller pore size. The pore size distribution controls water transmission, not total porosity. Sandy soils have large pores and clays have small pores, which transmit water slowly. Soils with small pores retain and hold water more effectively. Optimum conditions for plant growth occurs when there are sufficient large pores to transmit water readily and sufficient small pores to store water. Thus plants grow better in well compacted, uniform, sandy soils with relative low porosity (high density) or in well graded sands where sufficient silts and clays are present to provide moisture retention. The converse is true of clays. High porosity (low relative density) clay soils allow better infiltration and water transmission to plants than do highly compacted clay soils and provide good water retention and storage.

3.3.3 Poor water transmission can result in water logging, which can rapidly cause the death of plants and this is often a problem with pans, which can result in a perched water table and high winter groundwater levels.

3.3.4 Roots utilise soil spaces for access to water, essential elements and to provide structural support. Roots survive and grow where adequate water is available, temperatures are warm and oxygen is present. Roots are generally shallow as they are limited by oxygen content and water saturation at deeper levels. In response to compaction roots thicken in diameter. Thicker roots exert more force behind the root tip and penetrate farther into compacted soil areas. For effective root growth, pore sizes must be larger than the root tip. With compaction pore space diameters become smaller. Therefore as roots thicken growth slows and more laterals are generated, of various diameters, to compensate. If lateral are small enough to fit into pore sizes of the compacted soil, then lateral growth will continue while the main axis of the root is constrained. If the pore sizes are too small for even the lateral roots, root growth will cease entirely

3.3.5 It should be noted that chronic soil compaction will prevent the growth of any plant as the soil will be too compact to allow root penetration. However, there are significant differences between tree species for tolerating various levels of soil compaction. The following is a list of species which are more tolerant of compaction and which are suitable for the UK:

Alnus spp. Alder

Crataegus spp. Hawthorn

Fraxinus spp. Ash

Ilex spp. Holly

Platanus spp. Sycamore/ Plane

Populus spp. Aspen


Salix spp. Willow

Ulmus spp. Elm

Quercus rubra Red Oak

3.3.6 Many of these species are regularly used on landscape schemes including ash and red oak as specimen trees together with hawthorn and holly particular for new hedgerow planting. Elm species are restricted due to the recurring problem of Dutch elm disease.

3.3.7 Soil compaction has been reported to be a limiting factor for the growing trees in many urban environments (Craul, 1992; Day and Bassuk). Soil compaction affects drainage (infiltration and percolation rates), aeration (oxygen diffusion rate) and root penetration (Craul, 1985; 1999). This ultimately increases the likelihood of secondary pests and diseases and decreases growth rate of trees (Harris et al, 1999). Lack of aeration and poor drainage is perhaps the most serious in terms of planting success (Kendle & Schofield, 1992b).

Differentiating compaction as a cause of death from other factors

3.3.8 The failure of a landscape scheme to attain its full potential can be due to a combination of factors such as:

Insufficient watering during establishment;

Water logging;

Compaction

Poor growing medium in terms of structure (such as free draining granular material, lack of essential minerals and poor organic matter content);

Inappropriate maintenance

Use of plants which are inappropriate to the climate or soil type;

Disease; and

Theft, vandalism, physical damage.

3.3.9 Factors can be inter-related and have a cumulative effect and so it is often difficult on first inspection to ascertain the extent to which compaction is contributing to the failure of a scheme. In many cases, however, compaction is a significant factor because it can cause water logging or drought due to poor water absorption, which can lead to poor root development, plant stress and disease.

3.3.10 Death of plants due to water logging and compaction are often manifesting soon after planting, because most species cannot tolerate prolonged periods of anaerobic conditions. In other instances the effects of compaction may not be manifest for years and may be subtle in the form of poor growth. It can also be dramatic, for example when a tree topples over due failure of its roots to penetrate beyond the topsoil layer.

3.3.11 Compaction of the subsoil often results in increased shallow rooting in the upper pervious soil levels. The consequence of having smaller volumes of colonisable space at the surface of the soil means roots and their resources are subject to much greater fluctuations in water, heat loading and mechanical damage. Drought and heat stress can quickly damage roots in this small layer of oxygenated soil.


3.4 EXISTING KNOWLEDGE ON METHODS TO REDUCE AND ALLEVIATE COMPACTION

3.4.1 The compaction of subsoils during the construction process is an inevitable and recognised problem and various techniques have been devised to avoid, minimise or alleviate compaction. As well as the physical methods there are issues that must also be considered such as site management and specification.

Methods to Reduce the Effects of Compaction in a construction environment

Management of Machinery/ Vehicular Traffic

3.4.2 The scale and severity of the damage cause by heavy machinery means that the effects are nearly always detrimental (Kendle and Schofield, 1992a). Hakansson et al (1987) reviewed 26 field experiments and reported that an axle load greater than 10 tonnes should never be used. The maximum axle load should be lower when dealing with sensitive types of soil or locations with intensive traffic under wet conditions.

3.4.3 Suggested mechanisms to minimise compaction occurring on construction sites include the following

Proper equipment is of importance in avoiding damage to the soil. The equipment should not be heavier than necessary.

Reducing the axle loads – trucks, dump trucks and excavators that cause compaction damage. This type of traffic should be limited to special driving lanes which are reserved as future roads or where haul routes have been constructed, usually with aggregate on geotextile.

Avoid Repeated Driving – the first passage causes the most damage, but increases with successive passages. Unnecessary journeys should be avoided.

Use Large Tyres Minimally Inflated – The larger the wheel diameter the less the pressure on the ground since the contact area is large and sinking is reduced. Using a wider tyre also increases the contact area.

Avoid driving when the soil is wet – Avoid driving when the soil is wet. Wet soil is very susceptible to compaction, for instance after it has rained

Clear designation of haul routes and making other sections of the site ‘no go areas’ could assist in reducing the extent of compaction. In addition suitable construction of the haul routes utilising appropriate materials (such as woven fabric under a stone surface) could assist in reducing the extent of compaction on the main vehicular routes through the site.

Overall Site Management and Responsibility 3.4.4 Site management and responsibility is crucial in order to minimise soil compaction. At present the standard procedure is for the main contractor to spread the topsoil and thereby hide any visual evidence of subsoil compaction. The landscape contractor rarely spreads the top soil (refer to Section 5). This is an area where working practices could change to achieve substantial benefits and this is discussed in more detail later.


Specification

3.4.5 The standard specification documents in the industry cover compaction issues as follows:

National Building Specification and General Construction Works Specification

250 SUBSOIL SURFACE PREPARATION

General: Excavate and/ or place fill to required profiles and levels, as section D20.

Loosening:

Light and non-cohesive subsoils: When ground conditions are reasonably dry, loosen thoroughly to a depth of 300 mm

Stiff clay and cohesive subsoils: When ground conditions are reasonably dry, loosen thoroughly to a depth of 450 mm.

Rock and chalk subgrades: Lightly scarify to promote free drainage.

Stones: Immediately before spreading topsoil, remove stones larger than ______ mm.

Other items: Remove

260 INSPECTING FORMATIONS

Give notice: Before spreading topsoil for ______ .

Notice period: ______ days.

Highway Agency Specification Series 3000

FINAL PREPARATION OF SOILS

7 Where stated in Appendix 30/4, the requirements of sub-Clauses 3004.8 to 3004.11 shall apply to subsoil to be seeded or topsoil spread under the Contract.

8 Any consolidated material is to be broken up to 300 mm depth and the top 50 mm of all soil reduced to a tilth suitable for final shaping with a grading blade(particle size 10 mm and below). All undesirable material brought to the surface including stones larger than 50 mm in any dimension, roots, tufts of grass and foreign matter is to be removed off Site, unless otherwise stated in Appendix 30/4.

3.4.6 It is notable that the clauses perhaps do not allow for the alleviation of compaction for sufficient depth, but do allow for the inspection of formations prior to topsoiling.

Direct Physical Methods to Alleviate Subsoil Compaction

Ripping

3.4.7 Subsoil compaction has been an acknowledge constraint to maximising crop yields in agriculture and so techniques and guidance to the alleviation of compaction have been available for many years; the most common and cost effective method is subsoil ripping. Ripping shatters the subsoil and introduces fissures allowing deep root, water and air penetration, all assisting the natural soil structure regeneration processes. Best practice for ripping is set out in the MAFF publication Good Practice Guide for Handling Soils: Sheet 19 Soil De-compaction by Bulldozer Drawn Tines (MAFF April 2000), but this deals primarily with agricultural practices and while much of the information is applicable to construction sites, research indicates that compaction due to construction or on brownfield sites, can be more substantial and extend throughout the profile to a metre deep.


To maximise de-compaction:

The moisture content of the soils should be at least 5% below their plastic limit.

The ripping pattern must be overlapping passes.

The tines should be sufficiently closely spaced to ensure full lateral de-compaction which also needs overlapping passes.

The use of winged tines is recommended.

Tine length and width must be compatible with the proposed depth of de-compaction.

Tine and wings must have wear plates and be in good condition.

The towing unit must be capable of working efficiently without undue weaving and track slippage.

3.4.8 The degree of improvement and the longevity of the improved soil condition after loosening is dependent upon soil type, drainage capability, the incidence of wet weather and subsequent field loadings which often caused re-compaction in the upper subsoil. Contrasting soil types have different abilities to self-regenerate and showed different responses to site conditions. Granular or sandy in nature with less than 10% clay content and little internal cohesion – deep loosening improved drainage but had the shortest life, as the upper subsoil re-compacted readily if tracked over by contracting or maintenance vehicles. Soils with above 35% clay content and some shrink-swell capability, produced the most stable structural conditions with medium sized structures and sustained the best improvement in rooting for the fewest mechanical inputs.

3.4.9 Rooting improvements due to deep loosening were substantial and rooting extended beyond 1 metre depth after 5 years. More moderate improvements (reaching at least 0.7 metres) can be achieved in the other soil types. Soil structure development can accelerate after Year 3 or 4 once root penetration reached 0.75 metres and began to aid soil drying and water movement through the profile. To ensure long-term sustainable plant growth, between 0.8 to 1.0 metres depth of uniformly loosened, well-drained and well-rooted soil is required.

3.4.10 Very significant loosening at depth is being achieved with the wedge type soil failure and hardly anything at all with the slot type. In this case the difference in disturbance was caused by using different tines, a winged tine producing the wedge type and a ripper tine with wings part-way up the leg, the slot type. The same difference can occur, however, with the same winged tine when working under different conditions or at different depths. At shallower working depths failure is usually wedge type and produces loosening. As working depth increases this changes to the slot type, where the tine cuts a slot and compacts the soil either side rather than loosening it. . For the first 0.8 - 1 metre deep disturbance operation, high lift wing types (with a wing lift height of between 100-120 mm and combined wing width of 350-450 mm) are recommended. The transition depth has been called the critical depth and it defines the maximum useful working depth for that tines under those soil conditions.

3.4.11 Figure 2 illustrates the optimal design of the winged tine foot for deep and shallow subsoil loosening operations. Table 1 indicates suitable lengths for tine legs, to ensure sufficient ground clearance by the tool frame at different working depths. The leg dimensions are not critical, but tines should be strong and as narrow as possible to minimise draught.

Figure 2: Typical dimensions of a winged tine (Defra 2000). 3.4.12 Tines which are spaced too widely will leave undisturbed and unloosened soil in between the tine slots, so it is important to generate a good overlap between the disturbance effects produced by each tine. A tine should be located directly behind each track or wheel and the remaining tines should be spaced evenly in between, at the required tine spacing. The number of tines required will vary and indicative numbers are given in Table 3.2 for different working depths and track widths.

Table 3.2: Optimum tine arrangements for different depths of operation (Defra 2000).

3.4.13 Stagger between the tines in the direction of travel is necessary to allow free soil flow; tines can be mounted on the tool frame at the required spacings in a V-shape or across two toolbars. A tracked prime mover is essential for the deepest operation, to provide maximum traction and to ensure control over the working depth. In an ideal world, deep loosening to 0.8 to 1 metre depth would be conducted using a D9 or D10 crawler to pull five tines at the optimum tine spacing of 0.75 m. Crawlers of this size, however, are not always available when needed and industrial rippers rarely meet this spacing specification.


Figure 3: Illustrating the importance of good spacing and overlapping passes (Defra 2000). 3.4.14 Attempts, under extremely compact ground conditions, to use a D8 to pull 3 winged tines to a full metre depth were unsuccessful; the maximum working depth attained (with some difficulty!) was 0.8 metres. Better use of lower available crawler or tractor power can, however, be made by conducting a split parallel pass operation as shown in Figure 6. By removing alternate tines from the toolbar and conducting a second parallel pass, working mid-way between the tine slots created in the first pass, a useful deep loosening job can be achieved. The first pass should be conducted at two-thirds or more, if possible, of the depth of the second (final) pass. Parallel passes give the best loosening - cross-ripping should not be attempted. It is recommend that 25 tonnes and 260 kW (350 hp - or D8 capacity) should be the minimum size of crawler used for the initial deep loosening pass under very compact conditions. The tractor power required for a good shallow (at 0.5 m) one pass loosening operation is in the region of 150 to 190 kW (200-250 hp), alternatively the split parallel approach can be adopted with smaller power units.

Checks to ensure effective loosening The following checks are recommended to ensure effective de-compaction:

The supervisor / operator should keep to the following five step plan to ensure good loosening:

Open up a pit to look at the soil condition and rooting. Use this information to select an appropriate tine working depth.

Ensure that the soil is dry and friable at the time of the loosening operation, particularly the soil above the compacted zone.

Use a prime mover of appropriate weight and power to perform the operation at the required depth, using the split parallel pass approach in situations where power is limited.

Use winged tines, spaced correctly on the frame with sufficient ground clearance.



Check the tines are working above critical depth (i.e. wedge failure is occurring), if not, raise working depth slightly or adopt the split parallel pass approach to achieve the desired depth.

Ensure the required numbers of passes are being performed. Keep the implement level during the operation. Check the depth is correct, uniform and well-controlled. The working depth

should always be measured from the original undisturbed soil surface and not from the top of the loosened soil heave.

The forward speed should be slow and well-controlled, at about 2-3 km/hr. The spacing between successive passes of the implement across the field

should be equal to half of the tine spacing. 3.4.15 Although ripping is the best tried and tested technique, it is unfortunately difficult to use on most construction sites because the landscape areas are too small, complex in shape and topography, difficult to access and usually contain underground services or structures that could be damaged by the process. Buried builders debris can also damage ripping machinery. Research suggests that contractors tend to relieve compaction by breaking up soil with toothed buckets mounted on small plant, including mini excavators. This technique can be equally effective if the operator is thorough and the operation is carried out in appropriate soil and weather conditions.

Deep-Jetting (High Pressure Soil Injection)

3.4.16 As described by Craul (1994) this technique involves injecting air or water at high pressure using a hydroject or ‘Gro-gun’ to fracture the soil. Compost, fertilizer or a soil lightener (Styrofoam or Perlite) is then injected into the fractures. Some of the larger landscaping contracting firms or arborists routinely use this approach. Researchers report mixed results with this approach and the sustainability of using Styrofoam and Perlite must be questioned.

Aeration and Mychorrizal Inoculation

3.4.17 The word Mycorrhizae defines the mutually beneficial relationship between plants and a specialised group of root colonising soil fungi. Mycorrhizal fungi function through a network of threads. At one end the threads attach to and enter the root tissue. It is here that the plant and fungus exchange essential materials. The plants receive mineral nutrients, water and a variety of other growth promoting substances. In exchange, the fungus receives essential sugars and compounds to fuel its own growth. Several miles of filaments can be present in less than a thimbleful of soil. Mycorrhizal fungi filaments in the soil are truly extensions of the root systems and more effective in nutrient and water absorption than the roots themselves.

3.4.18 Terravention is the trade name given by the Terravent group for the complete care process, which involves aeration of the soil in the root zone with the aim of decompacting the soil followed by an infusion of beneficial mychorhizal fungi and bacteria into the soil and finally adequately covering the surface area of the treated root zone/ soil with a good mulch (where possible).

3.4.19 Mycorrhizal fungi can help plants to tolerate and recover from soil water deficits and soil compaction by enhancing roots ability to up take nutrients and improve soil structure.

3.4.20 Smiley (2001) states that ‘while soil fracturing with compressed gases, including TerraventTM, appears to be a good theory, results to date have not proved that it is a cost-effective means of treating soil compaction’. More effective treatments for soil compaction, according to Smiley (2001) include roto-tilling or mechanically breaking up of soil in areas in advance of root growth (Rolf, 1992; Craul, 1994; Day and Bassuk,


1994; Smiley 1997a, 1997b), radial trenching (Watson et.al. 1996; Smiley 1997a, 1997b) and air excavation (Smiley 1999).

3.4.21 On the other hand The Royal Botanic Gardens of Kew have officially endorsed and advocated the use of ‘Terravent’ soil decompactor for their veteran/ heritage trees (Kirkham, 1999; www.rbgkew.org.uk/plants/trees/decompaction.html). In the great hurricane in 1987 the whole root plate of the Turner’s Oak, one of the oldest trees at Kew, lifted and settled back in the ground which appeared to assist in rejuvenating the tree. This was one of the factors that initiated the present proactive decompaction programme for mature trees in the arboretum, at Kew, which began in 1998. This programme involves relieving compaction around the root crown, mulching over the turf and injecting a mixture of beneficial mycorrhizal fungi and bacteria. Loosening of the soil and injection of the mycorrhizal fungi is being performed using a ‘Terravent TM’ soil compactor.

3.4.22 Although certainly an area for continued research and investigation and often debated within the industry (http://lists.tree-care.info/uktc/archive/2001/msg01575.php) at present its main use is for targeting individual trees as does not appear to offer a cost effective solution, at present, for large development sites/ new landscape schemes. In addition all research work in this area has found it hard to distinguish the multi-faceted elements (aeration/ decompaction, inoculation and mulching).

In-Direct Biological Methods to Alleviate Compaction

Biological Processes - Earthworms

3.4.23 As described by Kendle & Schofield (1992b) improvement to soil structure will occur due to the action of plants, animals and the weather. The formation of a good soil structure is encouraged by worms, by roots and by soil drying and cracking (which in turn is encouraged by roots). Frost cracking is also helpful although it does not happen on all soils as it needs the correct moisture conditions together with a slow freezing process. Frost only affects the surface of a soil and does not help subsoil compaction and associated deep drainage problems. Unfortunately these natural remedies are unlikely to assist on most construction sites which have suffered from deep/ sub-soil compaction resulting in poorly drained soils and a high bulk density that nothing can easily penetrate (plant roots being no exception).

3.4.24 Subsoils tend to have either no or very low earthworm populations and so sites stripped to subsoil layers or filled with subsoil have low earthworm populations. This may be compounded by the fact that re-spread topsoil may have low numbers due to storage and handling methods, or because the topsoil has been artificially mixed or screened. Adding earthworms to re-spread soils on a construction site can have substantial benefits in terms of rapidly establishing a good physical and biological soil structure, but the technique is rarely used (Butt, KR et al (1995). The use of such a technique is illustrated in Case Study 3.

Replacing Soil

3.4.25 On the majority of construction sites the topsoil will have been removed and the subsoil used as a platform base for development, storage and vehicles. It is common to see topsoil brought onto site (or used from stockpiles) and spread over an area so that it is two to four inches deep. This often insufficient as stated by Bassuk (2005) two to four inches may be useful for grass re-establishment however such an amount is of no benefit to trees and shrubs. Bassuk considers trees and shrubs need at least eighteen inches to three feet of new soil for good growth. Generally the larger the plant and/ or the more water it requires, the greater the replaced soil depth should be.

Topsoil/ Compost


3.4.26 The development of previously used land (brownfield sites) will be a continuing feature of future development. Most of these sites have a surplus of topsoil from cut and fill exercises although this is often removed from site. Large savings can be made in the development of brownfield sites if topsoil can be retained or if topsoil is manufactured on site using site subsoil and imported compost for the organic matter.

3.4.27 Compost is the product resulting from the controlled biological decomposition of organic material that has been sanitized through the generation of heat and stabilized to the point that it is beneficial to plant growth (Landscape Institute, 2004). True compost is made from biodegradable (organic) materials that are biologically decomposed over a relatively short period of time. Unfortunately in the UK, many types of organically based products are called compost. Common commercial products such as potting composts and many soil improvers are typically peat based, and not true composts.

3.4.28 The Compost Specification for the Landscape Industry sets out guidance on how soil preparation in conjunction with the correct application of compost can be used to alleviate the poor soil conditions, including subsoil compaction, resulting in benefits in terms of plant establishment, growth and cost savings (LI, 2004).

3.4.29 The organic matter in many landscape soils may often be less than 2%, for example on brownfield sites. However, for good plant growth, rooting conditions in the soil need to be favourable. Plant roots require air, water and nutrients, plus a firm anchor to support the top growth. Soil organic matter is essential for the provision of these elements and should be raised to at least 4-5%, and higher from some soils, depending on soil texture. Compost is able to provide organic matter in a relatively stable form that can raise the soil organic matter levels and provide benefits described, resulting in improved plant survival, growth, quality of plants and cost savings.”

3.4.30 Compost is an excellent source of organic matter for landscape soils. Its major benefits relate to its content of organic matter, which can improve soil structure and supply nutrients. It increases the productivity of soils, while reducing plant losses through its many benefits including:

Nutrient supply

Reduced nutrient losses and improved cat ion exchange capacity

Better plant survival and growth

Reduction in soil compaction

Improvement in soil water handling capacity

Control erosion

Micro organisms increase soil aggregation, recycle nutrients and suppress soil borne diseases.

Cost benefits (LI, 2004)

3.4.31 Compost is an extremely versatile product that can improve the physical, chemical and biological characteristics of soil. Also, the use of compost in landscape and topsoil treatment practices, as a soil ameliorant, has been more thoroughly researched than any of its competing products (LI, 2004).

3.4.32 As ‘true’ composts are typically produced from recycled biodegradable (organic) materials greater volumes of compost are becoming accessible to the landscape industry. As such Landscape Architects are being and will be approached more frequently to specify it, instead of virgin products (e.g. peat and topsoil) and existing soil ameliorants (e.g. mushroom compost). The composting industry provides


the landscape industry with products that are not only effective and economical, but also annually renewable (LI, 2004).

3.4.33 All compost products should be produced to the British Standards Institution Publicly Available Specification PAS 100 (October 2002). This specification covers the range of biodegradable materials used to make compost, their quality and traceability, the minimum requirements for the process of composting and the quality of the end product.

Structural Soils

3.4.34 Structural soils have been designed to form a structurally sound sub base for pavements, whist providing a rooting medium for street trees. Typically they have a high sand content which provides good structural support with an appropriate void structure for aeration, which are filled with silts, clays and organic matter.

3.4.35 CU-Structural SoilTM is one such proprietary product, a two-part system comprised of a rigid stone ‘lattice’ to meet load-bearing requirements and a quantity of soil to meet tree requirements for root growth. The lattice of load bearing stones provides stability as well as interconnected voids for root penetration, air and water movement (Bassuk et al, 2005).

3.4.36 Since among soil textures, clay has the most water and nutrient-holding capacity, a heavy clay loam or loam with a minimum of 20% clay is selected for the CU-Structural SoilTM system. According to Bassuk, with carefully chosen uniformly-graded stone and the proper stone to soil ratio, a medium for healthy root growth is created that also can be compacted to meet engineers’ load-bearing specifications. The intention is to ‘suspend’ the clay soil between the stones without over-filling the voids, which would compromise aeration and bearing capacity. CU-Structural SoilTM utilises Gelscape® hydrogel as a non-toxic, non-phytotoxic tackifier, in addition to stone and soil components.

3.4.37 Structural soils provide a specific construction solution for the issue of compaction within new paved areas requiring landscaping. Whilst structural soils have a limited application they are an important illustration of the conflicting demand of engineers’ load-bearing specifications against the need for a suitable growing medium for planting.


Summary of Potential Methods

3.4.38 The following table provides a summary of methods which can be used to alleviate/ reduce the influence of compaction on new planting. An overview of potential advantages and disadvantages for the various methods have been considered, however it should be stressed that the best method will be site specific and it is essential that appropriate site and soil analysis work is undertaken in order that the best method for the specific site conditions is undertaken. Methods for reducing and/or alleviating compaction are considered further in the Research Section of this report:

Table 3.3: Summary of Methods to Alleviate Compaction

Method Description Advantages Disadvantages

Methods to Alleviate Compaction on New Planting

Mechanical Ripping Effective method.

Cost effective for large, accessible areas.

Difficult to avoid services

Not practical/ cost effective for smaller less accessible areas such as raised beds.

Manual Ripping Suitable for small sites.

Effective method.

Protects Services

Time consuming.

May not be practical for larger/ extensive sites due to time/ man hour costs required.

Small Excavation machinery (mini-diggers)

Effective method.

Limited size of machinery reduces issue with service runs.

Relatively expensive.

Time and site management issues.

Methods to Reduce Effects of Compaction on New Planting

Increased Topsoil depth Effective Method, providing sufficient depth for purpose.

Generally additional topsoil available on site following construction works.

Issue of storing topsoil appropriately during the construction works.

Incorporation of compost Effective, providing sufficient depth for purpose.

Sustainable/ recycled material, readily available.

Additional benefit of increasing number of soil organisms.

Transportation issues Cost

Improve drainage Effective method to alleviate one of the key effects of compaction.

Cost

Not always practical

Use plant species more tolerant to compaction

Cost effective and easy solution

Limits species available to achieve desired effect.


4 Case Studies

4.1 METHODOLOGY

4.1.1 A series of key issues regarding soil compaction and the landscape industry has been identified following discussions with developers, designers and contractors and these are illustrated by case studies. These discussions have also been used to produce a suitably tailored questionnaire included in Appendix A which was sent to 125 landscape contractors, all of which were members of the British Association of Landscape Industries. 38 replies were received, of which 30 were fully answered; these 30 were used for analysis.

4.2 CASE STUDY 1: ARBOURFIELD CROSS

4.2.1 This case illustrates the difficulties of avoiding and alleviating compaction within a construction environment and illustrates the manifestation of compaction on the success of a scheme.

4.2.2 At Arborfield Cross near Wokingham, Wokingham District Council converted a series of T junctions at the entrance into the village into a roundabout. This required the relocation of the war memorial to the side of the roundabout and the implementation of a highly ornamental landscape scheme to compensate for the visual impact of the junction and provide a new setting for the memorial.

4.2.3 The subsoil is heavy clay, but the good quality topsoil was stripped from the site and re-spread. It was possible to protect the setting of the monument from construction traffic since it was located to one side of the roundabout within land that had previously been pasture. In contrast all the ground within and around the roundabout was severely compacted by construction machinery in all weather conditions. Due to space constrictions on the site and the completion deadline this was unavoidable.

4.2.4 On completion of the construction works the site was handed to the landscape contractor, which operated under a direct contract with Wokingham District Council. The contractor was handed a site sub-soiled and top-soiled by the main contractor and with approximately two thirds of it badly compacted. The compaction was anticipated by the landscape architect and allowances had been made in the bill for deep cultivation and the importation of additional topsoil and compost, however, two factors prevented the full alleviation of compaction within the roundabout island. The landscape works started on the 1st October 2004, but had to be completed by Armistice Day, November 11th 2004. This coincided with a period of heavy rainfall that resulted in the site being waterlogged. Conditions were inappropriate to carry out soil improvement works, and yet the deadline meant that work had to proceed. The planting works actually exacerbated the compaction as the ground became puddled by the feet of the workmen. In common with most junctions, a web of underground services crossed through the roundabout, which contained many manholes and service covers. It was therefore not possible to rip or deeply cultivate the roundabout, or mound it to establish drier ground conditions above the in situ subsoil.

4.2.5 While the newly finished scheme looked immaculate for Armistice Day, it was not until late spring that the problems of compaction became manifest. Of the 18 trees in the roundabout, all leafed out six but subsequently died due to water logging. Shrub under planting in these areas also died, representing 36% loss of plants on the roundabout, compared with less than 2% loss within un-compacted areas. Significantly shrubs specified for their drought tolerance suffered the most, with complete loss of some Mexican Orange Blossom (Choisya ternata). Photograph 3 illustrates plant loss due to compaction and Photograph 4 illustrates the typical signs of water logging. In

contrast the shrubs planted around the memorial in un-compacted ground thrived, (see photograph 5). Deaths were clearly the result of water logging and ironically exacerbated by the compost and bark mulch which prevented the soil drying out.

4.2.6 To resolve the problem additional free draining topsoil was brought in to elevate the worst affected areas by 300mm. Services and the surrounding plants that had established preclude higher mounding. The mulch was removed and although some drought resistant species were replanted, other species more tolerant of water logging and compaction were also planted, such as Variegated Dogwood. While the contractor and landscape architect have a high degree of confidence that the adverse affect of compaction on shrub planting can be overcome, the fate of the trees is less certain and harder to resolve.

Photograph 3: Plant death due to subsoil compaction and subsequent water logging at Arborfield Cross.

Photograph 4: Standing water due to compaction, moss was also starting to grow, which is another indication of poor soil conditions.


Photograph 5: Planting on land that was previously pasture and which was undamaged by contractors machinery showed excellent growth.

4.3 CASE STUDY 2: NORTHSTOWE NEW TOWN

4.3.1 This case study illustrates how the over-compaction of soils is becoming increasingly difficult to avoid on most sites, irrespective of size, particularly to soil handling, drainage and most of all build density.

4.3.2 Northstowe is a proposed new town of 9,000 dwellings being jointly promoted by South Cambridge District Council and the developer JJ Gallagher. The site covers 594 hectares north of Cambridge and comprises a disused airfield, farmland and a golf course. Currently a planning application has been made and determination is awaited. After five years of work and consultation a strategic master plan has been produced, this identifies zones for residential, mixed use, employment, retail and open space. The site lies within the Cambridgeshire clay lands and so over-compaction of clay soils is anticipated to be a problem. For such a large site with extensive preplanning it might be expected that extensive areas of public open space comprising parks and sports fields could be isolated from the construction process, thereby avoiding compaction. In reality only a few areas of existing woodland and hedgerow to be retained can be protected from the construction process, equivalent to only 3% of the site. The reasons are as follows:

As is common with most developments, Northstowe is required to have no adverse impact on downstream hydrology; drainage water generated by the town must be stored on site and then released slowly when the surrounding system is below capacity. The attenuation area has been designed as a Water



Park on the eastern side of the site which is multi-functional and meets landscape, recreational and ecological requirements, however, its primary function is to hold storm drainage water. Thus this entire area has to be excavated to create a series of lakes and bunds, with the subsoil compacted to meet engineering requirements, particularly as the water park is so large it falls under the stringent regulations of the Reservoirs Act.

As is common with most authorities, South Cambridge District Council stipulate that as much soil as possible is retained on site to promote sustainability and avoid unnecessary lorry journeys. It has been estimated that the town will generated 1.3 million cubic metres of soil and builders rubble, many from building foundation and service trenches. A small proportion of this soil is required to raise some low spots that might otherwise be prone to flooding, and for fill against road embankments. The remainder must be deposited over the site in a manner that does not create a landform that is alien to the Cambridgeshire Countryside, i.e. it should look flat. The solution is to spread spoil over large areas of the site, but to a shallow depth. The subsoil will have to be layer compacted by rollers to avoid subsidence, The existing land is predominately grassland with a good soil structure and while it might seem sensible to protect this grassland from construction where it resides within an area for open space or sports fields, this too has proved to be impractical. Initially there will be insufficient fill to build the infrastructure works, such as the highways, requiring borrow pits to be dug within future sport field areas. Towards the end of the development there will be a surplus of fill from the house building process, which will be used to back fill the borrow pits. Thus even the sports fields will be on compacted reworked ground. The extent of soil disturbance is illustrated by Figure 4 on the next page.

Development densities are between 45 and 70 dwellings per hectare, leaving little room for landscaping within built areas and inevitably resulting in the compaction or even complete loss of surface subsoils. It is likely that internal landscape areas will have to be entirely re-soiled.

4.3.3 Northstowe illustrates that subsoil compaction is very difficult to avoid on most modern construction sites and methods to alleviate it must be built into the construction process.

Figure 4: Illustrating how the over-compaction of soils is becoming increasingly difficult to avoid on most sites, irrespective of size, particularly to soil handling, drainage and most of all build density


4.4 CASE STUDY 3: BATHEASTON RESERVOIR

4.4.1 This case illustrates how compaction can be ameliorated through biological techniques even in the absence of topsoil. Batheaston Reservoir lies within the Cotswold Area of Outstanding Beauty on the edge of Bath. In 1990 the reservoir became surplus to the requirements of Wessex Water and since it was concrete lined and unattractive it was decided to restore the valley back to its original landscape; originally a chalk stream flowing through fields and hedgerows. The reservoir was filled using a mixture of clay subsoil and fractured limestone rock; material excavated during the construction of the Batheaston Bypass. Two pools were created connected by a chalk stream fed from piped springs that originally flowed into the reservoir. Hedgerows were planted to recreate the field pattern and the subsoil sown with a native wildflora mix. Photograph 6 illustrates the site on completion of the subsoil filling and 7 and 8 show its subsequent development.

Photograph 6: Batheaston Reservoir on completion of filling with subsoil. No topsoil was used in the restoration.

4.4.2 Although seed germination was good, the growth of plants was slow, consistent with the low nutrient status of the subsoil. After two years the sward still only covered less than 50% of the surface, comprising predominately wild flowers, not grass, which failed to establish in the poor soil. While the landscape architects and ecologists were content to let nature take its course, there was local pressure to ‘green up the valley’,


particularly as part of it was to become public open space. It was also noted that the subsoil, which had been ripped and was not initially over compacted, had become naturally compacted within the surface layer. The mix of clay and stone developed almost concrete-like qualities during the summer months.

4.4.3 Re-cultivation of the surface would destroy the existing sward and re-compaction was likely to occur. It was decided to ameliorate the subsoil with organic matter and a low dose of fertilizer. Earthworms were also introduced which would start to aerate the soil and draw down organic matter. Pelleted treated sewage sludge was spread at a low density over the subsoil to add organic mater and fertilizer. 10,000 earthworms were supplied by Ecological Earthworms and introduced to the site by placing them under small piles of compost scattered throughout the site.

4.4.4 These techniques were very successful and almost 100% sward cover was achieved within a year, with up to 70% of this being wildflower and not grass. Within 5 years an embryonic top soil structure had established in the surface of the subsoil, Photograph 9.

Photograph 7: Batheaston Reservoir three years later following amelioration with pelleted sewage sludge and earthworms.


Photograph 8: The restoration winter 2006 showing abundant growth.

Photograph 9: This excavation through the soil profile shows how within less than a decade biological action has created a dark embryonic topsoil horizon which has better drainage characteristics. The original yellow subsoil has been excavated from below 150mm and placed next to it for comparison.


4.5 CASE STUDY 4: TREES WITHIN PAVED AREAS

4.5.1 Research suggests that the most significant adverse affect of compaction is the limitations it can place on tree growth. Trees are usually planted to provide a variety of benefits, including aesthetics, softening/screening of built form, establishment of a green structure and provision of shade. There are also ecological and environmental benefits. Typically a trees ability to confer these benefits is dependant on it achieving good growth. Unfortunately many trees planted in the urban environment, particularly in paved areas, fail to thrive. In most cases this can be entirely attributable to a lack of sufficient appropriate rooting environment.

4.5.2 Typically pavement construction, services and compacted subsoil limit the volume of un-compacted rooting medium that can be provided within a tree pit. This constraint is a well known problem to landscape architects and landscape contractors, but in many cases cannot be adequately overcome purely due to the physical lack of space within a site. The problem is becoming exacerbated as ever increasing build densities further restrict room for tree planting. The failure to provide an adequate rooting zone is most evident in modern car park design, where standard procedure is to plant trees in small island beds or at the inter-section of four parking bays. Trees remain stunted, and show poor growth, largely due to insufficient suitable rooting medium which can cause problems of drought, nutrient uptake, disease and stability. Photographs 10, 11 and 12 illustrate the problem which is widespread across the UK.

Photograph 10: Differential growth of plane trees within a supermarket car park. The trees within the slightly larger island beds are thriving, whilst those planted in constrained tree pits at the intersection of four bays are not. Although taken in October 2005, only the intersection planes are showing autumn colour, an indication of the greater stress that they are under.


Photograph 11: Differential growth of alders within a Park & Ride car park. Despite being planted in island beds, the alders in the foreground are very stunted and failing to realise their design objective. Compaction is likely to be one of several inhibiting factors. The alders in the middle distance are growing within a broad grass verge and are thriving. Taken in October 2005, the stressed trees are showing early autumn colour.

4.5.3 One option is to design schemes with fewer trees, but ensure that the trees have sufficient room to grow to their full potential in terms of the rooting zone and canopy space. A single large mature tree can have a far greater beneficial impact than several stunted trees. Conversely it could be argued that if trees planted in paved areas, such as car park island beds, reached their full size, they would be too large for their location, and consequently a certain inhibition of tree growth is desirable. Plane trees, for example, are commonly planted within car park island beds, and most have been planted in the last 50 years. Should these trees reach their full potential over the next 200 years, it is likely that in most cases, they would be too large for their location and cause structural damage to paved surfaces.

4.5.4 In reality most commercial developments, such as supermarkets and business parks, are likely to be redeveloped within a far shorter period than the life of the trees, which may result in the felling of trees long before they reach maturity. Such an example is the Western Riverside Development in Bath, where trees planted in association with a J Sainsbury and Homebase store development built in the 1980’s are to be felled to make way for the redevelopment of the site. The trees could not be retained within the new scheme because the current planning guidance requires a far higher density of development on the site.

4.5.5 Thus a balanced approach needs to be taken. In some developments designers should strive to provide conditions for adequate growth to meet the design objectives and not necessarily maximum growth, while in other locations, such as public open spaces, designers should try and ensure that trees are provide with an adequate uncompacted rooting zone.

4.6 CASE STUDY 5: NEW TOWNS

4.6.1 New Towns are good laboratories for empirical observations on the effects of compaction because they represent significant areas of amenity landscaping carried out over a relatively short period in association with major construction activity. Three new


towns are briefly assessed to illustrate the increase in build densities that has occurred over the century, they are:

Hampstead Garden Suburb, London early 1900’s

Milton Keynes, Buckinghamshire, from 1960’s – still expanding

Port Marine, Portishead near Bristol 1995- still under construction

Hampstead Garden Suburbs

4.6.2 Hampstead Garden Suburb is, regarded by many as the finest example of the garden suburb movement. Construction started in 1907 and the suburb is characterised by its layout, the unified design of the buildings, the quality of the architecture, the harmony of the materials used and the strong green infrastructure of mature trees, hedges and gardens (Photographs 12 and 13). It can be assumed that the suburb was built without substantial mechanisation in the form of excavators and large trucks and so soil compaction is unlikely to have been a substantial problem. The area survives almost entirely in its original form and Barnet Council together with a variety of organisations, (including the Hampstead Garden Suburb Trust) and the local community actively work to preserve and maintain the character and fabric of the Conservation Area.

Photographs 12 &13: In Hampstead Garden Suburb low build densities and substantial areas of green space in the form of gardens and verges have allowed trees to develop considerable stature.


4.6.3 The suburb is now one of the leafiest in London and typically the largest trees are found growing in broad grass verges or in gardens where compaction was unlikely to be prevalent at the time of planting. These trees have reached maturity and are of a comparable size to trees of similar age and species in rural areas. These trees have also benefited from being correctly spaced to allow them to reach maturity. The trees in paved areas are often less successful but this may be due to other factors other than compaction. The garden suburb is chosen as an example of the maximum that can be achieved in terms of amenity planting within a built development, unconstrained by the over-compaction of soils.

Milton Keynes

4.6.4 Construction of Milton Keynes started in the late 1960’s and it is now the largest new town in the UK with a population of over 250,000. It continues to grow. A high provision of connecting green space throughout the town in combination with quality landscaping throughout urban areas was one of the main design objectives and this has been fully realised.

4.6.5 Whilst one would not expect trees in Milton Keynes to match the stature of those in Hampstead Garden Suburb, because they have had only 25-30 years to mature, it is noticeable that many of the trees in the denser town centre are surprisingly small for their age. This is partly because the trees were planted at close centres and competition for water, light and nutrients must be limiting growth (Photographs 14 and 15). Although many trees were planted in broad verges it can be surmised that the poor growth might also be attributed to compaction that occurred during the construction period and was not adequately ameliorated. The effect is even more marked in trees within tree pits in paved areas, many of which lack vigour.

Photograph 14: Despite being planted in a broad verge these plane trees in the centre of Milton Keynes have shown little growth over 25 years. Other factors are likely to be limiting growth, aside from compaction, such as the tight spacing between trees. In the medium term the town may benefit from the restriction in growth since it reduces maintenance problems, but in the long term this may be disadvantageous if the life span of the trees is shortened.


4.6.6 The density of planting means that the trees contribute an appropriate amount of greenness to the town, which is probably one of the greenest towns in England and it could be argued that the trees are easy to maintain because they remain small in stature. The issue in this instance is one of life span. Will these trees live to a ripe old age or will a systematic program of replacement have to be carried out. If it is the latter and largely due to the over-compaction of soils, it illustrates that compaction is an important issue in relation to the success of tree growth within amenity landscape schemes and has cost and sustainability implications. In contrast there are virtually no areas of failed or poorly growing shrubs within Milton Keynes, the majority of the shrubs appear vigorous and require annual trimming, suggesting that compaction has a less noticeable effect on shrub growth.

Photograph 15: These street trees were planted in the centre of Milton Keynes over 25 years ago, but have shown little growth over that time and their long term viability is questionable. Compaction is likely to be only one of several factors limiting growth.


Photographs 16 and 17: The landscapes in these photographs are typical of those found throughout the UK, where shrub growth is not limited by compaction or other factors, but requires regular pruning to keep it in check. In contrast, tree growth is restricted by several factors including compaction. Significantly in these cases the close planting of trees and the unlikely event that they will be thinned is limiting their growth.


Port Marine, Portishead nr Bristol

4.6.7 Port Marine is a 6000 dwelling community currently under construction and is typical of the high density developments now commonplace across the United Kingdom designed to maximise housing provision and minimise land take. Detached houses with gardens are rare and confined to the outer edges, the majority of properties are apartments or terraced houses. The suburbs of the last century have been replaced by a return to traditional streets where you step out of your front door onto the pavement. Front gardens are either absent or comprise a narrow strip of defensible space landscaping. Trees are confined mainly to primary distributor roads or within public open space due to the paucity of front gardens and diminutive nature of back gardens (Photographs 18, 19 and 20).

Photograph 18: As development density increases, soft landscape areas have essentially become planters within a hard landscape and should be treated as such by deep filling with a high quality growing medium.

4.6.8 In Port Marine apartment gardens are often roof gardens above underground car parking where the growing medium is a highly specified man made product. While this increased urbanity may appear retrograde it has resulted in a high quality, award winning development with a traditional urban character which is a refreshing change to suburbia. In such developments it is very important that the limited amount of planting grows and thrives to soften and provide counterpoint to the dense built form. This can only be achieved if adequate rooting areas are provided and soil conditions are optimised. Many of the landscape areas are in effect ‘planters’ within a hard landscape and have no connectivity with the original landscape. The high build density means that these landscape areas are likely to have been compacted by the build process and it is imperative that they are treated as planters where the mixture of compacted subsoil and


building debris is removed to an appropriate depth and back filled with a high quality growing medium.

4.6.9 Landscape costs are now a much smaller factor in terms of the profitability of development schemes since the value of the built form per hectare has increased substantially over the years due to the rise in property prices and development densities, and yet the area for soft landscape has decreased dramatically. Clients also recognise that high quality landscaping confers quality to the overall development and adds value in a very cost effective manner. There is empirical evidence, that developers are willing to spend more money per square metre on soft landscape as a result. There should be no reason why the problems of over-compaction cannot be fully dealt with if designers adequately specify for its alleviation by the landscape contractor, even if this means re-excavating beds topsoiled by main contractors to alleviate over-compacted subsoil underneath.

Photograph 19: A street in Port Marine illustrating the increasingly urban townscape of modern developments. Landscaping is confined to key areas, such as the tree planting at the end of the street. In such instances it should be possible to provide the correct soil conditions, which combined with the use of semi-mature stock and the provision of space for the tree to mature, should result in a successful scheme.


Photograph 20: At Port Marine the designers made space for the trees, provided a decent rooting area and made sure that the verges were excavated to remove any compaction, which were then back filled with an appropriate growing medium. These semi-mature trees were planted in 2004 and already compare favourably with some of the 25 year old street trees in Milton Keynes.

4.7 CASE STUDY 6: ARTIFICIAL COMPACTION OF SOILS

4.7.1 In order to minimise costs and promote sustainability development sites are often re-contoured to create building platforms using the substrates on site. This case study is an example of the increasing practice of compacting soils on a site for engineering processes, which can lead to an over-compaction of substrate for plant growth.

4.7.2 Following closure, the 125 hectare Massey Ferguson factory site at Coventry was remediated for a mixed use housing development. Following demolition of the buildings this required the excavation of the natural soils to a depth of 2.3m and deposition as engineered fill to a maximum depth of 2.7m to create a building platform. The substrate was natural clay and this was placed in approximately 0.5m layers by tracking with a vibrating Cotswold Roller towed by a tracked caterpillar unit. The Performance Criteria required a maximum 7% air voids and a settlement of no more than 6mm at 120kN/m2 of pressure. This represents a bulk density of between 1.869-1.950 g/cm3 in moist conditions and 1.6-1.7g/cm3 when dry. Loose workable soils commonly have bulk densities of 0.8 to 1.3 grams per cubic centimetre ( or have 70 to 50 percent pore space), while more compacted soils have bulk densities in the range 1.6 to 1.8 (or 40 to 30 percent pore space). Typically root penetration is greatly retarded



when bulk density exceeds 1.4 in dry conditions (ref, 1997). Clearly the engineering operation at the Massey Ferguson site resulted in compaction that far exceeds the optimum for plant growth. Such operations typically do not draw a distinction between construction areas and soft landscape areas, partly because the incorporation of any ‘soft areas’ can result in the wider failure of the platform, and because remediation is often carried out in advance of any detailed plans for a site. Consequently the soft landscape areas within the final scheme are more than likely to be sited on artificially compacted ground.

4.7.3 The compaction specification also stipulated that a 200mm thick strata of free draining material be placed after every 1000mm depth of clay fill to ensure the ground did not become excessively waterlogged which might result in the clay expanding. Thus sites artificially filled and compacted in this way might offer better drainage characteristic for plant growth than natural subsoils. It may be possible to specify such a free draining strata closer to the final surface to provide a suitable rooting layer for plants.

5 Consultation with Landscape Contractors

5.1 QUESTIONNAIRE

5.1.1 Discussions with landscape professionals have also been used to produce a suitably tailored questionnaire included in Appendix A which was sent to 125 landscape contractors, all of which were members of the British Association of Landscape Industries. 38 replies were received, of which 30 were answered in full; these 30 were used for analysis.

5.1.2 The full results of the survey are presented in detail in Appendix A. The findings can be summarised as follows:

5.1.3 Landscape contracting companies are only occasionally involved in the sub soiling and topsoiling of construction sites. The work is usually carried out by the main contractors earthworks subcontractor (such subcontractors skills are primarily engineering rather than horticultural, i.e. excavating to set dimensions and profiles, installing foundations, services and below ground building work).

5.1.4 40% of the landscape contractors are never entirely satisfied with the conditions of the soils on site, 17 % are occasionally satisfied, 37 % only frequently. Only 6% are satisfied most of the time. The primary cause of dissatisfaction is insufficient depth of topsoil, while poor topsoil, compacted soils and soils with a high content of building debris were rated second equally in terms dissatisfaction. Other issues cited include contamination, weed and weed seed bank content and poor drainage.

5.1.5 When pricing schemes landscape contractors typically allow for less than 10% of plant deaths which can be attributable to a wide range of factors not just soil compaction. Losses appear to be greater for trees and transplants than ornamental shrubs where less than 5% loss is typical. Factoring out other costs such as preliminaries, cultivation, mulch etc. the percentage as a total of the contract sum is likely to be less than 10% and results indicate that only between 5% and 30% of these costs are attributable to compaction.

5.1.6 Landscape contractors cited poor drainage due to compaction as the main reason for plant failure with poor topsoil coming second and compaction inhibiting root growth, drought and poor specification third. Other reasons cited for deaths included no provision for maintenance, vandalism, third party damage, having to plant in inappropriate weather conditions and poor design.


5.1.7 Landscape contractors considered that compaction was only a significant problem on less than 30% of sites but 70% of contractors considered that it was only a problem on less than 10% of sites. However, when asked whether compaction was a significant problem on the long term amenity function of landscapes they took a contrary view and over 80% considered that it was.

5.1.8 63% of landscape contractors estimated that they carried out operations to relieve compaction on less than 10% of sites; although 23% of contractors considered it necessary on more than 30% of sites. 70% never returned to a site to alleviate compaction problems and 36% only occasionally, again indicating that it is not considered to be a significant (or financially imperative) problem.

5.1.9 Techniques used to alleviate compaction included machine ripping (employed by 90% of contractors), hand digging, soil amelioration and improving drainage. Less than 20% used soil injection or biological methods. The favoured method of machine ripping was with a 360 excavator with either tines or a toothed bucket. For small areas hand digging, pedestrian rotovators or mini diggers were favoured.

5.1.10 84% of contractors did not consider that adequate provision was made in bills of quantities to alleviate compaction. One contractor thought that this was because the alleviation of compaction was usually present as a clause in the specification, but this never translated into an item in the bill that a contractor could price. Consequently it was neglected.

5.1.11 Almost 100% of landscape contracts had a defects liability period, although 60% were only for one year and only 10% were for over 2 years. Thus there is an incentive for contractors to avoid any deaths through compaction, although as indicated earlier the financial loss is low.

5.1.12 97% of contractors would prefer to have greater control over the quality and manner in which topsoil and subsoils are spread over sites and the number one reason for dissatisfaction was contractors covering up compacted subsoil with a veneer of topsoil. Other factors cited, and given a fairly even weighting were the presence or underground services, inappropriate weather conditions, no allowance in the bills and time constraints.

5.1.13 The following points were made in the additional comments section:

‘Better supervision of main contractors and the ‘subbies’ preparation and spreading of topsoil would improve the situation’

‘No supervision prior to topsoiling although the soil spec is in the bill of quantities’

‘The success of any landscape depends on careful soil handling and soil quality selection, plus good use of the appropriate plant and machinery suitable for the works’

‘Construction companies are not interested’

‘Compaction and drainage is the most common problem – ie trying to solve problems on site which have resulted from compaction but out of sight. It would be so easy to resolve at the consultation phase rather than a year later (and cheaper)!’

‘There would be no need for any problems if correct horticultural practices and working partnerships are used’

Willerby landscapes work with Client-Contractor and Landscape Architect to ensure the preparation etc. are carried out to a good horticultural standard, thus ensuring a quality scheme’

‘Generally too much compost/mushroom compost specified, which compacts and sours the soil, especially on sites over time’


‘Contractors have a duty of care to address the issue and deal with it. Our experience is always to raise risks such as compaction or drainage, specifications etc and provide the main contractor/project manager or his representatives with solutions. Better awareness of the risks is required by all parties’


5.2 DISCUSSION ON THE RESULTS

5.2.1 The findings of the survey and back ground research were discussed with landscape professionals to quantify the extent of the problem of compaction and suggest methods of working that could improve the situation.

Estimate of Direct Financial Loss due to Compaction

5.2.2 On average landscape contractors typically factor in a need to replace around 10% of stock due to failure, which may be due to a combination of factors. This is an average and losses can be much higher on certain schemes, particularly in drought years.

5.2.3 Research suggests that approximately 10% of plants die and of these 5-30% of can be attributed to compaction. If plants typically represent 70% of the capital works cost of a soft scheme (if all costs are taken into account including preliminaries, traffic management, mulch, tree stakes etc.) it can be estimated that for every £100,000 of capital soft landscape works the cost of replacing plants due to losses arising out of compaction is less than £1000 or <1%. It can be surmised that the cost to alleviate compaction in terms of additional soil cultivation or even additional visits to site to supervise topsoil spreading by the main contractor may be in the region of £1000, which would further reduce the financial incentive to reduce compaction (assuming no allowance in the bill for these works).

5.2.4 While it is accepted that landscape contractors do not necessarily avoid alleviating compaction it is not always perceived as a substantial problem and there is no real financial or legal incentive to bring about a change in practices. If plants have died due to compaction the solution is to alleviate the localised compaction before replacing the plants, often cheaper than alleviating compaction over a wider area at the start of the contract. The only occasion were compaction becomes a more significant problem is if it results in poor drainage and water logging. This can only be rectified by improving the drainage or by substituting more water tolerant plants. Poor drainage can be attributed to a design failing and not necessarily the fault or financial liability of the landscape contractor, who may offer a solution to the problem at additional cost to the client.

Landscape Contractors Dissatisfaction with Soils Conditions on arriving at a site

5.2.5 The main factor at the heart of many issues to do with soil handling is the fact that most earthworks are undertaken by the main contractor’s ground workers and not the landscape contractor. Soil handling on a site is not a one off operation but is often a series of small sequential, but often ad hoc operations driven by the building process such as backfilling over service trenches or around foundations. On many schemes it would be impractical for landscape contractors to be involved in this process and there appears to be no financial advantage to promote this. Supervision of earthworks by landscape contractors would only be realistic for large areas destined to be high quality green space.

5.2.6 A far more realistic and useful practice would be for main contractors to delay topsoiling until subsoil areas have been inspected by the landscape contractor. Areas of compaction, excessive content of builder’s material, contamination or inappropriate subsoil can be identified and site specific actions determined to resolve the problem. In theory this should be stipulated by a clause in the specification, (the NBS specification includes such a clause) but in reality it is difficult to arrange and police due to the phasing of ground works. Most landscape contractors regard having to deal with poor soils on a construction site as part of their role which is why they consider the practice of


covering up over-compacted sub soils with topsoil, either intentionally or otherwise, so irksome since it prevents them doing their job. Discussions with landscape architects indicate that they also regard it as the role of landscape contractors to cope with the aftermath of the building process and expect the landscape contractor to make allowances for this when they price. There is evidence that some landscape contractors see greater topsoil depth as a solution in some circumstances and there may be some benefit in considering this. Most plants root in the upper horizon, including trees and so as long as compaction does not cause water logging, sufficient depth of topsoil would be beneficial.

5.2.7 Landscape contractors have also commented that landscape architects rarely specify or design any drainage in schemes and consultation with landscape architects confirms this as a common failing. Many larger landscape areas would benefit from a system of land drains being installed. The move towards sustainable drainage systems means that designers are no much more aware of drainage within soft landscape areas and attitudes are now changing for the better.

5.2.8 In areas of dense built form the areas for planting can be limited which makes it even more important that what little planting there is, is successful. The smaller volumes of soils in these areas can reduce the chances of success due to limitations in rooting area, nutrients and water. In such instances designers should take a ‘belt and braces’ approach to specifying to ensure that an appropriate growing environment is created. Plant beds and tree pits should be excavated to an appropriate depth and backfilled with good quality soils, appropriately cultivated and ameliorated. The Overseeing Organisation should check on site that this has been done.

Estimate of social and cultural costs due to compaction

5.2.9 While nearly all landscape professionals agree that the over-compaction of soils affects the growth of plants, does this ultimately affect the quality of our lives? Shrubs and grass areas appear to grow adequately despite over-compaction of soils. In most schemes too much growth, requiring continual trimming, appears to be the major problem, not inhibition of growth or plant failure. If amenity grass areas are well used they are likely to become compacted from use and there are tried and tested techniques for alleviating compaction in amenity grass areas. It is accepted as an ongoing maintenance requirement.

5.2.10 The effect of over-compacted soils on tree growth probably does affect the quality of our lives and significantly the lives of future generations. There is strong evidence that many trees planted within UK urban areas in the last half of the twentieth century as part of new developments or streetscape improvements suffer from poor growth, greater instance of disease and have shorter lives than similar species within rural areas. Over-compaction of their rooting environment is just one of many contributing factors such as drought, pollution, heat, vandalism, planting density and limited rooting volume. Designers and specifiers need to create the right conditions to allow urban trees reach maturity for future generations to enjoy, but there are many constraints within our urban environment that makes this hard to achieve. Trees need room below ground and above ground to reach their full potential and so the provision of majestic trees within our cities is primarily a space planning/design issue rather than one of soil over-compaction and so cannot be resolved through soils policy alone.

5.2.11 Compaction primarily affects trees in urban areas. A substantial amount of tree planting is carried out in association with large infrastructure projects in rural areas, particularly highways. Planting is usually carried out on ground that has been subject to re-engineering, such as cuttings and embankments. Such planting is usually undertaken for a specific environmental purpose such as integrating the road into the landscape or screening traffic. Such planting rarely fails to achieve 100% coverage and usually grows


to fulfil its design objective. Typically, however, the intention is to produce a swath of woody growth and not large timber. Trees are often planted too close together as transplants and are rarely thinned later on, which prevents the development of large timber. Indeed the Highway Agency favours this approach since very mature trees adjacent to highways can become liabilities and increase management costs. Thus if the over-compaction of soils is limiting this type of amenity planting, it is not noticeable and may even be advantageous.

5.2.12 It can be concluded that soil compaction only has a significant adverse effect socially and culturally on the establishment of mature trees, particularly in urban areas, but compaction is only one of several plant growth issues that have to be tackled if the situation is to be improved.


6 Recommendations for good practice

6.1 INTRODUCTION

6.1.1 Current best practice policy and guidance on soil handling, treatment and site preparation for landscape works has been reviewed in section 3.4. This section will review existing best practice guidance, its relevance and whether further clarification or a new approach is required in relation to subsoil compaction and landscape planting.

6.2 PROPOSED NEW GUIDANCE & RECOMMENDATIONS

6.2.1 It must be accepted that over-compaction of soils is an inevitable by product of the construction industry and because it occurs it does not mean that any one party is at fault. A desirable end product is that landscape areas are free from compaction. The research suggests that the responsibility of achieving this is not down to one party and best practice is best achieved by identifying the roles and responsibilities of the various parties, as set out below:

Clients

Should recognise that high quality landscapes can add value to a development, but to maximise success landscape designers should be involved at the start of the development design process.

Should recognise how the over compaction of soils can adversely affect the quality of landscape schemes

Should make sufficient monies available within development budgets for high quality landscaping

Designers

Should ensure that landscape areas are sited in appropriate locations.

Should consider drainage issues and design drainage systems for soft landscape areas if appropriate.

Should ensure that soft landscape areas are as free as possible from services.

Identify the areas of native soil that lie within future landscape areas. If possible these should be fenced off to isolate them from the construction area, in a similar way that trees are protected under British Standard.

Specify plants tolerant of compaction or water logging, where compaction in areas where compaction will be difficult to alleviate.

Specifiers

Should ensure that the specification has adequate provision for alleviating compaction and that key operations in the specification are translated through to items in the bill of quantities that a landscape contractor can price.

Should ensure that specifications for imported soils are adequate.

Should ensure that soil depths are adequate.

Should ensure appropriate maintenance periods are in place.

Haul routes across future landscape areas should be appropriately constructed and fenced to limit the area of potential damage.


Encourage the use of Tool Box Talks between designers, landscape contractors and ground workers.

Main Contractors and Ground Workers

To work to best practice in all aspects of earthworks including handling, quality of material imported, avoiding contamination and most importantly making provision for landscape contractors and/or designers and specifiers to inspect subsoil areas prior to topsoiling.

Develop a good working relationship with the landscape contractor.

Landscape Contractors

Should expect compaction as a problem on most sites and ensure that adequate monies are included within bills for relieving compaction.

Identify problems of compaction early on and to build up a good working relationship with the main contractor and ground worker.

Inspection of subsoil before topsoiling and agreeing procedures for alleviating compaction.

Policy makers

• Should raise awareness of compaction issues

• Should encourage all of the above to work together

• Promote alleviation through best practice guidance

6.3 SUMMARY

6.3.1 There is increasing awareness of compaction problems in the landscape industry, which combined with new techniques and products, more sophisticated urban design and increasing budgets for landscape means that the opportunities for reducing compaction are greater than ever. The way forward is for clients, designers, specifiers, main contractors, ground workers and landscape contractors to each fulfil their role in helping to resolve the issue.


7 Conclusion

7.1.1 The over-compaction of soils is an inevitable by product of the construction industry, particularly as build densities increase in line with central government policies. Landscape contractors accept that over-compaction of soils is a problem and generally try to alleviate it where it occurs and indeed consider it part of their role in the construction industry. However practical working arrangements on site usually make its alleviation difficult. Currently there is little incentive to improve working practices, legal claims related to over compacted soils are extremely rare and the financial loss associated with replacing plants that die due to over compacted soils is small. If compaction is so bad that plants die in localised areas, the usual practice is to relieve compaction or poor drainage just within these small areas during the defects period. This approach is reactive rather than preventative.

7.1.2 While nearly all landscape professionals agree that the over-compaction of soils affects the growth of plants, does this ultimately affect the quality of our lives? Shrubs and grass areas appear to grow adequately despite over-compaction of soils. The need to continually cut grass and trim shrubs within amenity landscapes seems to be a more significant problem and cost.

7.1.3 The effect of over-compacted soils on tree growth probably does affect the quality of our lives and significantly the lives of future generations, but compaction is only one of several plant growth issues that have to be tackled if the situation is to be improved. Trees also need room below ground and above ground to reach their full potential and so the provision of majestic trees within our urban areas is primarily a space planning/design issue rather than one of soil over-compaction and so cannot be resolved through soils policy alone.

7.1.4 Current policies to maximise sustainable drainage within developments is leading to greater awareness amongst designers and contractors regarding soil permeability, soil type, soil handling and soil condition, which bodes well for the future. However the incorporation of sustainable drainage in modern high density developments can be technically difficult and good practice guidance is still evolving.

7.1.5 The opportunities for reducing compaction are greater than ever with new techniques and products coming to the market, a more sophisticated attitude to urban design, and increasing Client awareness of the importance of landscape, which in relation to the overall value of a development is now subject to far fewer cost constraints.

7.1.6 The problem of over-compaction of soils within the amenity landscape industry does not appear to be so significant that legislation is warranted and it would be difficult to create any effective legislative policy and enforce it. There is, however, substantial room for improvement, particularly because the trend for higher density development creates a new set of issues, not least that the remaining soft landscape areas must perform well. In light of new development trends current specifications such as the NBS should be reviewed in relation to subsoil and topsoil supply, handling and cultivation. Clients, designers, specifiers, main contractors, ground workers and landscape contractors must also rise to these new challenges and work together to resolve compaction issues. This co-operation can be promoted through specification and guidance and with organisations such as CIRIA.


REFERENCES

ADAS 1983 The Design of Field Drainage Pipe Systems

Bassuk, N et al (2005) Using CU-Structural SoilTM in the Urban Environment. Cornell University.

Bending, N A D & Mc Rae, S G (1999) Soil Forming Materials and Their Use in Land Reclamation. DETR. October 1999.

Bradshow, A D (1983) TopSoil Quality – Proposals for a New System. Landscape Design, Vol 141.

BS 3882 (1994) British Standard 3882:1994. Specification for Topsoil. 1994.

BS 3998 (1989) British Standard 3998. Recommendations for Tree Works.

Butt, KR et al (1995) An Earthworm Cultivation and Soil Inoculation Technique for Land Restoration. Ecological Engineering. January 1995.

Bullock, P (1991) Soils in the Urban Environment

CIRIA C609 2005 Sustainable Drainage Systems, Hydraulic, Structural and Water Quality advice Swilson, R Bray, P Cooper.

Costello L R et. al. (2002) Measuring the Impact of Site Development on the Physical and Chemical Properties of Landscape Soils. Elvenia J Slosson Endowment Progress Report 2002.

Craul, P (1994) Reducing Soils Compaction. Landscape Architecture. Vol. 84, Pt 12, pp 34-36.

Craul, P (1992) Urban Soil in Landscape Design. 1992

DEFRA (2004) The First Soil Action Plan for England: 2004-2006.

DEFRA & WSP Environmental (2005) Soils in the Built Environment: A Strategy for the Construction Sector.

Grabosky, J; Bassuk, N & Trowbridge, P (2002) Structural Soils: A New Medium to Allow Urban Trees to Grow in Pavement. Landscape Architecture Technical Information Series (LATIS). American Society of Landscape Architects (ASLA).

Grabosky, J; Bassuk, N; Trowbridge, P & Urban, J (1998) Structural Soils: An Innovative Medium Under Pavement that Improves Street Tree Vigour. Urban Horticultural Institute. http://www.hort.cornell.edu/uhi/outreach/csc/article.html

Handbook of Soil Science (1999) M.E. Sumner Ed. CRC Press. Boca Raton, FL.

Hakansson et al. 1987 Soil Compaction: Effects on Forest Growth and seedling Development.

HR Wallingford/CIRIA Drainage of Development Sites: A Guide

JCLI (2002) Handling and Establishing Plants

Kendle, T (1990) Soil Ameliorants for Landscape Planting. Plant User Specification No. 3, Part 1 & 2.

Kendle, T & Schofield, J (1992) Saving Our Soil, Countering Compaction. Landscape Design, May pp 36-29.

Kirkham, T (1999) Tree Restoration. Kew Scientist. October 1999 Issue 16.

Landscape Institute (2004) Compost Specifications for the Landscape Industry. Landscape Institute in association with BALI, WRAP and NBS.

Lindsey, P et. al. (1992) Redesigning the Urban Forest from the Ground Below. Arboricultural Journal, Vol 16, pp 26-39.


MAFF (2005) Good Practice Guide for Handling Soils. Sheet 19: Soil Decompaction by Bulldozer Drawn Tines. April 2005.

Rolf, K (1994) A Review of Preventative and Loosening Measures to Alleviate Soil Compaction in Tree Planting Areas . Arboricultural Journal, Vol 18, pp 431–448.

Relf, D. (1997) Managing compacted and Heavy Soils. Contact: Diane Relf, Extension Specialist, Environmental Horticulture. Posted November 1997 http://www.ext.vt.edu/departments/envirohort/factsheets3/soils/JUL93PR1.HTML

Smith, K & May, P (1998) The Challenge of Urban Soils. Landscape Australia. Feb-April, pp 38-42.

Smiley 2001 Journal of Arboriculture 27 (6) Nov 2001

Sorvig, K (2001) Soil Under Pressure. Landscape Architecture. June, pp 36-43.

Trouse 1996. Soil Compaction and Growth of Woody Plants SCi. Plant. Anal. 23 1321-1331

Appendix A - Questionnaire

APPENDIX A

Survey Results

1. When carrying out landscape planting contracts on construction sites how often is your company responsible for the main land forming earthworks?

23

67

10

00

10

20

30

40

50

60

70

Never Occasionally Frequently Most of thetime

Question 1

Percentage

2. How often is your company responsible for topsoiling a site?


23

50

27

00

5

10

15

20

25

30

35

40

45

50


Question 2

Percentage

3. On sites where the earthworks have been implemented by the main contractor, how often are you entirely satisfied with the condition of the soils?

40

17

37

6

0

5

10

15

20

25

30

35

40


Question 3

Percentage


4. What are the main physical problems that you encounter in preparing a site for planting? (please number in order of severity with 1 being the most severe)

Poor Quality Topsoil

Insufficient Depth of Topsoil

Compacted soils

Topsoil/Subsoil with a high content of builder debris/ stone

(Other please specify)

Replies under ‘Other’ included:

Poor Drainage

No topsoil

Excessive weed content or weed seed bank in soils

Contaminated soils

Soils purporting to be topsoils

Spillage of oils and contamination by the building process

0

0.5

1

1.5

2

2.5

3

3.5

4

Poortopsoil

Insufficienttopsoil

Compactedsoils

Buildersdebris

Question 4

Response weighting


0

1

2

3

4

5

6

7

8

9

10

Poor topsoil Insufficienttopsoil

Compaction Buildersdebris

Question 4

Number of times cited as theprimary problem


5. What percentage of plant loss (for any reason, not just compaction) do you typically encounter for the following, one year after planting?

a) Standard and Extra Heavy Standards:

50

40

10

00

5

10

15

20

25

30

35

40

45

50

< 5% 5-10% 10-30% >30%

Question 5a

Percentage

b) Feathered Trees and Whips:

40 40

20

00

5

10

15

20

25

30

35

40

< 5% 5-10% 10-30% >30%

Question 5b

Percentage


c) Ornamental Shrub Planting

80

20

0 00

10

20

30

40

50

60

70

80

< 5% 5-10% 10-30% >30%

Question 5c

Percentage

6. Typically what percentage of the capital cost of a scheme do you allow for plant replacements?

40

57

30

0

10

20

30

40

50

60

< 5% 5-10% 10-30% >30%

Question 6

Percentage


7. In your opinion what is the main reason for plant failure on development sites? (please number in order of importance, with 1 being the most important)

Poor Quality Topsoil

Poor drainage resulting from soil compaction

Poor root growth resulting from soil compaction

Drought

Poor Plant Stock

Pests/ Diseases

Inappropriate Specification

Other (please specify)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Poor Topsoil CompactionPoor

Drainage

CompactionRoot Grow th

Drought Poor PlantStock

Pest/Disease PoorSpecif ication

Question 7

Response Weighting

Responses under ‘Other’ include:

No provision for maintenance

Having to plant bare root stock too late in spring

Having to plant container grown stock in the hot dry weather conditions

Poor design

Inappropriate choice of plants

Vandalism

Main contractor damage when rectifying construction defects


0

2

4

6

8

10

12

Poor Topsoil CompactionPoor

Drainage

CompactionRoot Grow th

Drought Poor PlantStock

Pest/Disease PoorSpecif ication

Question 7

Number of times cited as primary problem

8. Of the plants that die, what percentage of deaths would you typically attribute to problems of soil compaction?

54

13

26

7

0

10

20

30

40

50

60

< 5% 5-10% 10-30% >30%

Question 8

Percentage


9. On how many of your sites do you perceive soil compaction as a significant problem?

27

43

23

7

0

5

10

15

20

25

30

35

40

45

< 5% 5-10% 10-30% >30%

Question 9

Percentage

10. In your opinion is compaction a significant problem in the long term with regard to the amenity function of the following:

a) Grassland (not sports turf) 76% Yes 24% No

b) Shrubs: 90% Yes 10% No

c) Trees: 80% Yes 20% No


11. On what percentage of sites do you carry out de-compaction as part of the soil preparation prior to planting?

40

23

14

23

0

5

10

15

20

25

30

35

40

< 5% 5-10% 10-30% >30%

Question 11

Percentage

12. Once a scheme is complete, how often do you have to return to a site to alleviate compaction problems that have subsequently materialised?

57

36

7

00

10

20

30

40

50

60


Question 12

Percentage


13. What techniques have you used to overcome compaction?

0

10

20

30

40

50

60

70

80

90

100

MachineRipping

HandDigging

SoilAmelioration

ImprovingDrainage

Soil Injection BiologicalMethods

Question 13

Percentage of contractors employingtechnique

14. Of the above techniques which do you use most frequently?

Machine ripping with a 360 excavator

Toothed bucket

Rotovator

Tractor mounted tines at 1m spacings 400-600mm centres

360 excavator with boom mounted tines and =low pressure tyres

Vibrating Ripper

Tractor drawn ripper

15. If you use machinery to alleviate compaction what is your preferred choice for a medium size landscape/ development scheme?

Hand digging

Pedestrian rotovator

Small 360 excavator

3 or 1.5 tonne rubber tracked 360

Mini excavator with ripper toothed bucket


16. In your opinion do specifiers make adequate provision in bills/ schedules for the alleviation of compaction by physical methods?

Yes 16% No 84%

17. In your opinion do specifiers make sufficient provision in the bills/ schedules for the amelioration of compaction with compost or additional topsoil?

Yes 26% No 74%

18. For newly planted schemes undertaken by your company, what percentage are subject to a maintenance/defect period?

18 replied 100% 12 replied between 70-95%

19. For schemes with a maintenance/defects period, what is the commonest length?

60

30

10

00

10

20

30

40

50

60

0-1 Year 1-2 Years 2-5 Years Over 5 Years

Question 19

Percentage

20. Would you prefer to have greater control over the quality and manner in which topsoil and subsoil is spread on construction sites?

Yes 97% No 3%


21. Which of the following factors hinder your company’s ability to relieve compaction? (please number in order of severity with 1 being the most severe)

Underground services restricting the ability to de compact

Having to work in inappropriate weather conditions to meet completion deadlines

Compacted subsoil being hidden by main contractor topsoiling

No allowance in the bills of quantities

Lack of time between completion of the construction works and the need to complete the landscaping

Other (please specify)

0

2

4

6

8

10

12

14

UndergroundServices

Weatherconditions

Compactionhidden bytopsoiling

No allow ancein bills

Lack of time

Question 21

Number of times cited as theprimary problem


0

0.5

1

1.5

2

2.5

3

3.5

4

UndergroundServices

Weatherconditions

Compactionhidden bytopsoiling

Noallow ance in

bills

Lack of time

Question 21

Weighting of Response

Replies under the additional Comments’ section ‘Better supervision of main contractors and the ‘subbies’ preparation and spreading of topsoil would improve the situation’

‘No supervision prior to topsoiling although the soil spec is in the bill of quantities’

‘The success of any landscape depends on careful soil handling and soil quality selection, plus good use of the appropriate plant and machinery suitable for the works’

‘Construction companies are not interested’

‘Compaction and drainage is the most common problem – ie trying to solve problems on site which have resulted from compaction but out of sight. It would be so easy to resolve at the consultation phase rather than a year later (and cheaper)!’

‘There would be no need for any problems if correct horticultural practices and working partnerships are used’

Willerby landscapes work with Client-Contractor and Landscape Architect to ensure the preparation etc. are carried out to a good horticultural standard, thus ensuring a quality scheme’



‘Generally too much compost/mushroom compost specified, which compacts and sours the soil, especially on sites over time’

‘Contractors have a duty of care to address the issue and deal with it. Our experience is always to raise risks such as compaction or drainage, specifications etc and provide the main contractor/project manager or his representatives with solutions. Better awareness of the risks is required by all parties’

the impact of subsoil compaction on soil functionality

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