IntroductionThere are a number of fairly common techniques available that allow grass to be incorporated into a pavement to provide 'the best of both worlds', ie, the appearance of grass but the load bearing capability of a well-constructed pavement or driveway. They can be utilised in those areas where the hard permanence of a typical pavement might be undesirable, such as in conservation areas, roadside verges, emergency services access, canal towpaths, farm tracks or rural settings, and they are also used for erosion control in some situations, although the specification and type of system(s) used can be completely different to that outlined below.

Different proprietary systems provide varying ratios of hard-pavement to grass, and the best system for any given project will need to take into account these differences. Some systems can only be installed by specialist contractors, while others can be installed by competent workmen or even diy-ers. It should also be noted that special grasses ought to be selected for planting the various systems, as most ordinary seed mixtures are unlikely to be sufficiently hard-wearing to withstand being trafficked. A few suggested seed mixtures are given further down the page.

The 5 techniques considered on this page are:-

o Hopsack Paving

o Spaced Paving

o Cellular Paving

o Reinforced Turf

o Mesh Protected Turf

All these techniques rely on a sub-base to give the paving its strength, although the types of sub-base recommended will vary with type of system chosen, existing ground conditions, and anticipated usage. A typical construction detail for each technique is given below.

Hopsack PavingThis is the simplest of the 4 techniques, and utilises readily-available, plain rectangular concrete paviors, laid to a pattern that leaves significant 'holes' or 'pockets' that are filled with soil and seeded.

This technique can be used for residential driveways, or in the garden. For path use in a garden with reasonably firm and stable soils, the sub-base can be omitted, but be prepared for the bricks to move.

A Hopsack pattern created from standard 100x200mm

block pavers, giving 50x50mm pockets for soil.

Any rectangular blocks can be used to create a hop-sack pattern resulting in varying sizes of 'pockets' in the finished pavement. The edges of a hopsack pavement sill need to be solid, either against an existing sound structure, such as a wall, or with an edging unit or soldier course laid on concrete, as described on the Block Paving Page.

For residential driveways, patios and paths, a 100mm sub-base is usually sufficient (see Sub-base Page), but heavier loads, such as large vans, may need 150mm thickness. For trackways, car-parking areas etc., consult a paving contractor or civil engineer for specific advice.

Once the blocks have been laid, the pockets should be filled with a well-draining, friable soil, or soil/sand loam, and compacted as for normal block paving. It's worth spending a few quid, if necessary, to get a decent planting medium, as the grasses will have only this small pocket of soil to survive upon for the next few years. The soil in the pockets can be seeded immediately with the selected grass mixture, although the soil will settle over the ensuing weeks and may need 'topping-up', or it can be left to settle for 4-6 weeks and topped-up prior to seeding.

The relatively small proportion of grass to each square metre of this type of grass paving make it less reliable than other techniques, and there is a tendency for the grass to die off within a couple of seasons, but it does have its uses, and, if fed with a liquid fertiliser and watered in dry weather, it can look quite attractive in the right situation.

Typical Hopsack Paving construction detail

Spaced PavingThis technique again relies on commonly available block paviors, that are laid on a prepared sub-base and bedding layer, with 'spacers' between adjacent blocks giving a wide, but consistent, joint, that is then filled with the selected soil prior to seeding. Different manufacturers have different proprietary systems, but they all utilise a square or rectangular 'standard' block with some form of spacer, often plastic, approximately 30-45mm wide. The number of spacers per block is determined by intended usage (more spacers used on trafficked areas) and type of block. Full instructions will be supplied by the spacer/block manufacturers.

Aquada Paving System by Marshalls

Again, this technique can be used for driveways, paths, patios, and, with a more substantial sub-base, for trackways, car parks and other large-scale, low traffic-speed sites.

Typical plan layout of spaced paving showing spacers being used to create 30-

45mm joint between 240x160mm blocks

The choice of block, and the width of the joints, determine the ratio of grass:hard-paving, and this allows this ratio to be manipulated, by choosing larger or smaller blocks, to suit individual site requirements.

For example, using 240x160mm blocks with 35mm joints results in approx 29% grass by area, whereas using 160x160mm blocks, this ratio becomes 33% grass joint by area.

Again, soil and seed need to be carefully selected, before filling the joints, and the grass should be allowed to establish itself before allowing vehicular traffic to use the pavement.

Grass does seem to survive better in the joints of spaced paving than it does in the small 'pockets' of hopsack paving, and, along with feeding and watering in dry spells, it may even need the occasional attention of a mower.

Cellular PavingSee also Cellular Systems Page

This is the most widely used type of grass-paving, and is claimed to be structurally sounder and stronger than other forms. Basically, specially-shaped, interlocking 'cell' pavers are laid on a prepared bedding layer over a sub-base, and the 'cells' filled with the chosen soil and seed. There are a good number of proprietary systems available, in 3 formats; pre-cast concrete cells, plastic cells, and cast in-situ concrete cells.

Different proprietary systems have differing bedding layer requirements, and you will need to check with manufacturers whether their system requires a granular, a grit sand, or a sand/soil bedding layer.

Sigma block from Brett

Turfstone by Tobermore

Generally speaking, these pre-cast concrete units are laid using the same methods as outlined on the block paving page.

This technique is suitable for all but the very heaviest applications. The scale of the project, and its intended use, will determine the best system to be used. Some manufacturers are not equipped to cope with orders of less than 300m², although it may be possible to arrange for a local builders' merchant to obtain a smaller quantity. These systems are quite popular with local authorities looking to provide hard-standing or emergency access to otherwise 'green areas', such as pumping stations on residential estates, and access trackways for service vehicles, fire engines, etc..

12 month old GrassGuard system

Established GrassGuard system

Grasscrete® is a proprietary system that relies on placing disposable plastic 'pots' at equal spaces over a prepared base and then pouring concrete to fill the spaces in between the 'pots', which act as temporary formers for the eventual pavement. Once the concrete has hardened, the flimsy plastic pots can be removed, and a selected soil mix used to fill the resulting voids. The soil can then be seeded.

Grasscrete Paving Grasscrete slab with temporary

plastic formers

Some systems allow gravel, decorative aggregates or bark to be used to fill the cells in place of the more usual grass, and some manufacturers will supply cells ready-turfed if

required. Filled with a gravel, these plastic cells can bring stability to an inclined access way, and help minimise the inevitable drift of the gravel to the bottom of the slope. Plastic cell matrices (as they are sometimes known) are considered in more detail on a separate page.

Grass-filled plastic cells Gravel-filled plastic cells

The type of system chosen dictates the grass:hard-paving ratio, with some thin-walled plastic cells, offering up to 94% grass surface, rendering the structural elements virtually invisible, while others, such as the pre-cast concrete units that are available, make a feature of the cell shape, which is usually rectangular or hexagonal.

All of these cellular systems have been specially developed to ensure that the grass can and does survive, so much so that they can sometimes become overgrown if not tended regularly. Indeed, some systems are so successful at nurturing a natural overgrowth that they have been used on SSSI and sensitive heritage projects.

Grass paving at Stirling Castle

Points to consider when specifying a cellular reinforced grass system (From a discussion by RE Howden, MD of GC Group Ltd)

Precast Concrete block systems

The grass jointing is essential to the structural integrity of such systems. Where grass growth is absent or poor, the individual blocks have a lowered resistance to differential movement when trafficked. In regularly trafficked situations, this can result in sub-base pumping via the gaps between the individual units. Firm edge restraint is essential as any rooting through to the sub-base will be tenuous at best and the units will spread under load.

Castellated or studded systems can be difficult for pedestrians as the soil fill within the units settles under vibration or is washed from the surface, leaving a protruding 'stud' which presents a significant trip hazard.

GC Group's Grassblock

GC Group's Grassroad pavers, before and after

Plastic systems(HDPE and Polypropylene)

Only really suitable for low-speed occasional use. The relatively thin depth of these systems and their inherent flexibility can result in 'trampolining', where the units 'bounce' when trafficked. This can turn the surface into a quagmire in regularly trafficked areas, such as car-parks. The trade-off for a higher ratio of grass cover is a reduction in tolerance to wear and loading. Should not be used on gradients where slip resistance is essential.

In-situ castings

Each job can be individually designed according to traditional slab design methods to ensure competence and adequate drainage control. No differential settlement or lateral spread, so no need for edge restraint. The only system suitable for vehicular loads of 40 tonnes.

Other considerations

Gravel fill is acceptable in low-traffic situations, but hoggin should never be used as it will 'set' and kill off any grass.

Where reinforced grass paving is used for Fire Path access, special note should be given to the sub-base, which can quickly become saturated.

Erosion Control by GC Group

Reinforced TurfThis is a comparatively new development and, as far as I can ascertain, there have been only a few case studies of this type of technique, but it does have its admirers, and there will most probably be a small, but steady, market for it. The basic principal is that a specially selected grass is grown on a geo-membrane, which is then laid over a prepared bedding layer and sub-base. The sub-base provides the strength and stability, while the geo-membrane holds the grass layer together.

Manufacturers claim that this type of system can be used for most normal, low traffic-speed applications, although whether it will ever be developed and promoted to the sports industry or the general public remains to be seen. This system ensures 100% grass coverage, and the usual maintenance, such as feeding and mowing, will need to be undertaken on a regular basis. The life-time of this system is an unknown quantity at the time of writing, but should be good for 10 years.

There are also reinforced turfs developed specifically for erosion control, rather than trafficking, and are beyond the remit of this site. However, Some of the manufacturers listed on the links page also supply these 'erosion control mats', and their technical departments will be more than happy to help you with your queries.

Tensar Mat® by Tensar International

Mesh Protected TurfThis system is becoming quite popular with those responsible for overspill car-parking and pedestrian usage at occasional events, with groundsmen looking after areas of lawn, and with caravan/leisure parks, where the natural appearance of grass with the load-carrying ability of a pavement, all at a minimal cost and with little or no construction work required, is very appealing.

Turf Protection Mesh

by Tenax Ltd.

The basic premise is that a tough polypropylene or HDPE mesh is laid over an area of turf or grass, and this allows foot-traffic, low-speed cars and vans to use the area without completely ruining the grass. The mesh may need to be anchored to the ground at regular intervals by means of ground pegs, to prevent slippage and to help disperse the loads. The mesh works to protect the grass roots from being damage, to minimise 'pumping' of wet ground, and to avoid rutting of soft spots.

The soil type, the type of grass/turf cover and the level of protection required will determine the best mesh to choose for each application, and the better manufacturers have dedicated technical teams to help in identifying the best solution.

For lawned areas, the mesh is simply rolled out over the close-cropped turf, with adjacent rolls overlapping by 150-300mm, and anchored with ground pegs. Any dips, hollows, soft-spots etc, should be levelled out with grit sand or top soil before placing the mesh. Once laid, they can be left in-situ for the season, or even permanently, as most commercial mowers will ride over the embedded mesh without hindrance. For areas with longer grass, the meshes are placed after a mowing, and the grass is allowed to grow up and through the mesh, hiding it from view, yet providing sufficient stability to permit trafficking.

Grass mixturesall at approx 20-30 g per m²

General Parking Driveways

50% Perennial Ryegrass20% Slender Creeping Red Fescue25% Strong Creeping Red Fescue5% Browntop Bent

80% Chewings Fescue20% Browntop Bent

Verges and Picnic areas

35% Smooth Stalked Meadow Grass30% Slender Creeping Red Fescue25% Perennial Ryegrass10% Browntop Bent

Accessways

30% Hard Fescue20% Chewings Fescue20% Slender Creeping Red Fescue25% Strong Creeping Red Fescue5% Browntop Bent

IntroductionCellular paving is a rapidly expanding sector of the British and Irish paving market, as it is meeting a demand to provide a permeable, traffickable surface at a relatively low cost and with simple, low-skill installation techniques. It is also known, in some quarters, as "Matrix Paving" or "Geo-matrices".

Originally developed as a cheaper, lighter method of providing a Grass Paved surface, the potential uses for these moulded plastic systems are being expanded all the time. In our experience, the largest area of growth over the last few years has been their use with gravels. They stabilise a gravelled area, making it much more user-friendly to both pedestrians and vehicular traffic, as well as reducing scatter and drift, problems that have troubled gravels and other loose surface dressings for aeons.

Gravel filled

matrixby Hex-a-

path

Uses identified for cellular/matrix pavers include occasional and/or overspill car parks, access lanes, fire access roads, caravan hardstanding, amenity areas and picnic sites, event areas, rural/canalside paths, helicopter pads, golf course paths and parking, erosion control and stabilisation of earthworks, along with applications for residential paths and driveways.

   

TypesThere is a good selection of systems on the market, and choice of the most suitable systems for any particular project will need to take into account planned usage, anticipated loads, volume and type of traffic, existing ground conditions, drainage and, of course, cost.

Many of the most popular systems are based on a hexagonal cell, although square cells systems are available. The key points to note with any system is the open percentage, both at the top surface and at the base.

Grassway Geoblock

by WT Burdens

The open percentage for the top surface indicates just how much of the cell is visible once it's laid and filled. Most of the modern systems offer a minimum of 90% open surface.

The open percentage for the base affects the load-bearing capacity of the system. The manufacturers need to find a compromise between a fully open cell base, which offers best drainage but will be driven into the laying course when trafficked, and a fully closed cell base, which offers best load-bearing capacity, but cannot drain through. A typical 'compromise' for the amount of open base is in the range 40-70% and many of the systems on the market are capable of carrying loads well in excess of 150 Tonnes per square metre.

NetPave 50 from Netlon

Some systems offer accessories and other 'extras' such as bay marking caps and edge units, but one big drawback is that the various systems tend not to be compatible, so if a project is likely to be extended or amended at some future date, it's essential to ensure the range will be supported by the manufacture for several years to come.

It is not necessary for this site to catalogue and detail the various systems available – comprehensive information on the products is readily available from the manufacturers listed on the Links page, and they will also be able to provide details of their stockists and distributors.

Aco Groundguard with 10mm golden gravel

Nidaplast with 10mm quartzite gravel

ConstructionAlthough there are a wide variety of cellular systems available, the construction methods recommended by the manufacturers are fairly similar and a typical construction is detailed here.

Sub-Grade

The sub-grade is prepared by removing vegetation and other unacceptable material and then digging down to formation level, creating a reasonably level and compact sub-grade in the process. A drainage composite or separation membrane is often recommended and this should be installed as directed by the manufacturer at the sub-grade/sub-base interface.

Sub-Base

The sub-base layer is typically constructed from a selected free-draining material depending on the planned end use of the surface. Where vehicular traffic is expected, a

traditional Type 1 or 2 sub-base material may be used, often incorporating some form of integral drainage, but on those projects where sharp drainage is the prime requisite, the sub-base is more likely to be constructed from a gravel or an approved filter medium. Other projects may opt for a select fill.

Sub-base thickness will vary according to planned usage and ground conditions but is generally somewhere between 100mm and 200mm.

  

Kerbs/Edgings

Although kerbs and/or edgings are not always essential, they can improve the finished appearance of the paving by providing a neat and tidy edge. On projects subjected to regular vehicular traffic, then a kerb/edging is beneficial as they help restrain the matrices and prevent slippage or movement under turning forces.

Transverse restraining edgings are also a good idea when cellular matrices are used on gradients. As the name suggests, these are laid across the slope to prevent slippage of the units under load or because of gravity. The amount of separation between transverse restraints will vary depending on the gradient itself and the ground conditionsb but is generally somewhere between 4m and 10m.

Transverse restraints are typically a rigid edging, most often a flat-topped edging kerb (Type EF) laid on and haunched with concrete. Where only occasional traffic is anticipated, a securely anchored timber edging may also be used.

Almost any form of edging or kerb is suitable; where a rustic or natural look is required, a timber edging may be preferred, but for traffic schemes, a concrete edging may be a better choice. If edgings/kerbs are to be used, then they should be installed at this stage, ie, before placing and screeding the laying course material.

 

Laying Course

The laying course material for most cellular pavers is typically a grit sand. However, some manufacturers prefer to recommend a mix of sand and selected topsoil (usually 2 parts sand to 1 part soil) for use as the bedding layer when the cells are to be grassed.

The chosen bedding material may be laid over a separation membrane if there is a potential problem with the laying course material settling or being washed into the sub-base, or, in the case of grass paved cells, if there is the need to prevent the roots of any vegetation penetrating the sub-base.

The prime purpose of the laying course is to provide a well-profiled bed for the cellular tiles, and so thickness is kept to a minimum, generally 35-50mm, although this may be increased to 60-100mm or so where the cells are being used to support grass or other vegetation.

The laying course is prepared is the same way as any other screeded laying course and the methods used are described on the Screeding page.

 

Laying the Cells

Finally, the cellular tiles themselves are laid in accordance with the manufacturers' instructions. Some systems rely on small pegs or pins that are driven through adjacent units into the laying course and/or sub-base, tying the units together and anchoring them in place. Others feature a click-together system or lock-pins that connect adjacent units.

Laying Grassroad Pavers from GC Group

Full units are laid first and then the gaps are in-filled with units cut to size using a basic saw (hand saw, hacksaw or power saw). It is not usual for the unfilled units to be compacted: this is typically carried out once the cells are filled with the chosen material, be it gravel, soil or some other loose fill.

In the case of grass paving, the soil fill should be in accordance with the manufacturers' recommendations, which is normally a free-draining sandy loam type of soil. Care should alos be exercised in choice of grass seed to ensure a tough, hard-wearing mixture is selected.

Gallery

Gravel fill to Nidaplast Matrix

Gravel filled Hex-a-path garden path and steps

Car parking area for occasional use

Hexagonal cells with grass

Hauraton Recyfix system

Construction Diagram

Uses and Applications

Children's Playground

Shredded bark can be used to form paths where a very informal look is required, such as in the garden and it is also useful as a safer surface for children's play areas. Other applications are severely restricted as bark can only provide a surface dressing, not any form of structural load-bearing capability. Bark used to cover

trackway

There are several different types and grades of bark, and it can be worth shopping around to find the most suitable bark for a particular project. Factors that ought to be taken into consideration include:

o Colour - usually determined by source, but some bark is artificially coloured o Fire Testing (BS4790) o Particle size (10-50mm is a typical range for Play Areas) o Wood content - look for 10% or less o Dust/Fines content - ideally, should be less than 5% o Lifespan - 2 years should be a minimum

ConstructionVery simple to do. Decide where you want the path to run, and mark out with sand lines if required. Any surface vegetation should be scutched off to a depth of 35-50mm and disposed. Any slutchy or muddy patches should be excavated and backfilled with dry material such as crushed stone or sandy sub-soil. The bark can be laid directly onto the prepared sub-grade, ideally 50-70mm thick and trampled in to the surface soil. Depending on moisture content of the bark, this should give a coverage of around 20 m² per tonne.

For private play areas, such as in a family garden, it's usually recommended that the "loose-fill" be at least 300mm thick, and retained with an edging. Areas beneath swings and at the foot of slides are prone to having loose-fill kicked away, reducing effective cover, and requiring regular maintenance. Commercial or Local Authority Play Areas will typically need to comply with the relevent standards, and bark may not be acceptable as a surfacing.

See also....

Safety Paving Links to Play Equipment Manufacturers

Playground Bark Construction Detail

No edging restraint is required for a simple path, but the addition of such an edging can 'tidy-up' its appearance, giving a more formal look, and keeping the bark from straying too far. Bricks, lumps of stone, timber gravel boards or wooden poles normally used as stakes all make good edgings.

Permeable geo-membranes, also known as landscape fabrics, may be used beneath the bark to deter weed growth through the path, but the bark then has a tendency to be scuffed off the membrane, leaving it exposed and unsightly. These landscape fabrics often cost as much as, if not more, per unit area, than the bark surface dressing. They certainly have their uses in large-scale or commercial schemes, but their benefit to a small garden path is less obvious.

CoverageCommercially, bark is sold by the cubic metre, and 1 cubic metre = 1,000 litres

1 cubic metre will cover 20m² at a depth of 50mm, the usual depth for paths and tracks, or 3.3m² at 300mm, the usual minimum depth for playgrounds.

The density of bark varies according to source wood and type/size of chippings, but is generally around 400-550Kg per m³

Pros and Conso Very cheap to do and easy to remove if you change your plans. o Will need 'topping-up' at intervals as the older bark decomposes or blows away. o Can become soggy on waterlogged ground - See land drainage. o Price is highly variable, dependent on quantities purchased, as is coverage, which is

dependent on moisture content. o Source quantities of over 1 tonne via local landscape contractors, horticultural or

agricultural suppliers.

WoodchipsAn alternative, and often cheaper alternative to bark is processed woodchips. These are manufactured from reclaimed or scrap timber, such as old pallets, and can be impregnated with dyes to create weird, and some would say wonderful, colour options.

Because it is a manufactured product, the particle size of woodchips can be tightly controlled and it is less prone to degradation than bark. Larger particle sizes mean the product is less likely to be blown away by wind, and there are playgrade options available.