the role of geotechnical temporary works on infrastructure construction in ireland

Geotechnics on Irish Roads, 2000-2010 A Decade of Achievement

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THE ROLE OF GEOTECHNICAL TEMPORARY WORKS ON INFRASTRUCTURE CONSTRUCTION IN IRELAND

MARK PETERS, Byrne Looby Partners (Europe & MENA), Galway ANDY WILKINS, Byrne Looby Partners (Europe & MENA), Galway ANTHONY OBRIEN, Byrne Looby Partners (Europe & MENA), Galway

Paper presented to Engineers Ireland, Geotechnical Society of Ireland 11th October 2012

Cover photographs: (L) Structure S04-N7 Bowstring Bridge during construction, positioned on top of a reinforced earth approach embankment prior to bridge slide to final position. (R) Removal of temporary excavation support after staged excavation and replacement of soft ground at Bishop ODonnell-Seamus Quirke Road in Galway.

SYNOPSIS

By its nature, infrastructure construction in an urban context, particularly widening of an existing highway, can require significant phasing and sequencing of temporary works in order to deliver the permanent solution within the constrained environment. Byrne Looby Partners have worked on a variety of road schemes in Ireland including Phase I and II of the M50 Improvement Project and Bishop ODonnell - Seamus Quirke Road, Galway. A variety of ground conditions were encountered on these schemes, in conjunction with constraints such as traffic management, land acquisition, programme and restrictions imposed from adjacent infrastructure resulted in the development of a number of temporary works systems in order to permit construction of the permanent works solution.

On the M50 upgrade project, soil nailing was adopted as the temporary works solution at several slope locations permitting excavations for the construction of new bridges and temporary excavations adjacent to live highways and the Royal Canal in the vicinity of the improved M3 Interchange. At other locations, temporary excavations relied upon the undrained shear strength of the Dublin glacial tills. Elsewhere the construction works relied upon temporary dewatering measures to facilitate the construction of the permanent works.

Access to Monastery Road from the N7 required a new large bowstring arch bridge structure to be constructed. Due to extremely limited working space the new bridge had to be launched across the N7 into position from a temporary reinforced soil approach embankment. The bridge slide loading travelled across the reinforced soil earthworks imposing large transient loads as the bridge structure was manoeuvred into place. A soil-structure interaction model was developed using PLAXIS to understand the influence of the temporary loads on the reinforced soil / earth and predict the resulting ground movements and establish movement trigger levels. During the bridge slide, movements were monitored and compared to the predicted displacements.

At Bishop ODonnell - Seamus Quirke Road in Galway excavation and replacement of soft ground deposits was required in a phased solution adjacent live traffic. Temporary support was achieved using steel sheet piles which relied upon the support from the adjacent filled / in situ ground sections. Complete excavation and replacement of up to 6.5m of soft ground was achieved.


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INTRODUCTION

THE ROLE OF TEMPORARY WORKS IN HIGHWAY UPGRADE SCHEMES

A spell of increased / prosperous economic activity, such as that experienced in Ireland in the period between the mid-1990s and late 2000s, will result in increased traffic volumes, stressing the capacity of the existing road infrastructure. This stressing of the existing trunk road network leads to identification of requirements for both new highway infrastructure and the upgrade of existing highway infrastructure.

Upgrade of existing highway infrastructure involves the provision of additional traffic lanes to facilitate increased traffic flow and relieve congestion. Invariably, these roads are in and around urban areas or adjacent to significant development where acquisition of additional land is simply not possible or would require major public consultation. These spatial constraints generally result in the requirement for new traffic lanes to be constructed within the existing highway boundary or with minimal additional land take. By its nature, infrastructure construction in an urban context, particularly widening of an existing highway, can require significant phasing and sequencing of temporary works in order to deliver the permanent solution within the constrained environment. Typically, these constraints include:

Limited available working space for plant, machinery and labour resulting in logistical and health and safety constraints

Maintenance of existing traffic flows presents logistical and health and safety constraints

Prevalence of existing services and / or utilities resulting in logistical and health and safety constraints

Continuity with existing structures / earthworks resulting in technical constraints such as differential settlement, slip surfaces, cracking or seepage planes

Interaction with existing structures / earthworks resulting in technical constraints and / or future maintenance constraints

Existing / proposed landscaping or aesthetic considerations may impose constraints in terms of what is visually acceptable

The potential adverse effects of construction activities on sensitive adjacent structures

There are logistical constraints in moving plant, machinery and materials into and around tight urban sites

The cost of traffic delays during road widening

Unsuitable / poor ground conditions in the area of proposed widening

Incorporation of current highway alignment and geometric requirements when widening older motorways / trunk roads

A range of geotechnical solutions are available to mitigate and overcome these constraints. Consideration must be given to a range of potential solutions so as to ensure that the most technically and economically appropriate solution is derived. Where insufficient lands are made available to construct the additional lanes using normal construction practices, solutions can be broadly defined (non-exhaustively) into two categories with numerous sub-categories:

1. Soil / Rock Slope Strengthening / Steepening Strengthening of steepened cuttings using soil

nailing / rock bolting with appropriate facing system.

Widening of embankments by strengthened steepened slopes using geotextile reinforcement.

2. Retaining structures Embedded sheet pile, king pile or bored pile

walls In situ or precast reinforced concrete gravity

walls Modular gravity walls e.g. crib wall, gabions Reinforced earth walls.

Geotechnical temporary works are normally constructed with a view to implementing either one of the above solutions and may include the following:

1. Temporary cut slopes and temporary excavations e.g. to facilitate construction processes or installations.

2. Temporary strengthening and steepening of existing or modified slopes e.g. to facilitate construction processes or installations

3. Temporary construction platforms to facilitate plant, machinery or construction sequence / processes

4. Temporary de-watering or groundwater control measures


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5. Temporary retaining structures to facilitate construction processes e.g. excavation and replacement of soft ground

This paper presents a number of case studies, which are examples of temporary works solutions adopted to facilitate construction of permanent highway widening, focussing on how the geotechnical considerations and analysis methods informed the choice of solution and how particular project constraints were overcome.

Following this a discussion on geotechnical design of temporary structures is presented focussing on employing Eurocode 7 as the primary design code.

CASE STUDY 1

M50 UPGRADE PPP CONTRACT 2, S17-N3 BRIDGE PIER 1 TEMPORARY CUT SLOPE AND TEMPORARY SOIL NAILING

The 950m upgrade of the M50 motorway was completed in September 2010 and brought about significant improvements to the traffic condition around the greater Dublin area. Contract 2 was delivered through the M50 Upgrade PPP contract for the finance, design, construction and maintenance of the motorway. The NRA awarded the PPP contract to M50 Concession Limited, a consortium currently comprising Spanish infrastructure developers Global Via Inversiones, S.A. (GVI), Sacyr Vallehermoso, S.A. and Irish firm P.J. Hegarty & Sons Limited. The Contractor, M50 D&C Limited, comprised FCC Construccin Ireland Limited, Sacyr Ireland Limited and P.J. Hegarty & Sons Limited. Atkins Consultant Engineers and Eptisa designed the work on behalf of the Contractor, with Byrne Looby Partners acting as the Designers Sub-consultant. Roughan ODonovan Limited supervised the contract on behalf of the NRA.

The works delivered as part of Contract 2 comprised the following:

Upgrading 25km of motorway to dual three-lane from Junction 3 (M1 Airport Interchange) to Junction 6 (N3 Blanchardstown Interchange) and from Junction 11 (N81 Tallaght Interchange) to Junction 14 (R133 Sandyford Interchange).

Providing a fourth auxiliary lane along an 18km section.

Upgrading seven of the eight interchanges along this section of the M50.

Major upgrade of junctions at the M1, N2 and N3 from grade separated junctions to free-flow /partially free-flow interchanges.

70 principal structures including major crossings of railway lines, as well as the Royal Canal and the River Dodder.

As part of the N3 works, Byrne Looby Partners were employed by the Contractor to design the temporary works required to facilitate the construction of the Pier 1 base of Bridge S17-N3. The bridge carried a flyover section of the N3 and Pier 1 was constructed adjacent to the Royal Canal which posed the primary constraint to the construction of the Pier 1 pile-cap. A location plan is shown in Figure 1. The Royal Canal is elevated above the level of the pier base by approximately 4m and the proximity of the base is such that modification of the existing bank of the Royal Canal was required to facilitate the construction of the Pier 1 foundation.

GROUND CONDITIONS

The soil stratigraphy generally consisted of Made Ground/Fill (soft to firm brown sandy gravelly CLAY) overlying firm to stiff grey gravelly CLAY (Boulder Clay) and moderately strong to strong LIMESTONE. However, clayey medium dense and dense SAND and GRAVEL deposits were noted in the boreholes to be interbedded with the firm to stiff CLAY (Boulder Clay). The stiff CLAY strata are not consistent across the site. On this basis, a layer of the medium dense to dense SAND and GRAVEL was assumed to overlie the bedrock in all locations. In general the location of the top of the SAND and GRAVEL deposits was encountered at between 49.0mOD and 50.0mOD

Groundwater strikes were reported in the majority of the exploratory bores and generally the groundwater strikes occurred in the SAND and GRAVEL layers. Typical equilibrium groundwater levels were recorded at between 49.8mOD and 48.7mOD. The adjacent Royal Canal has lowest bank levels on the south side of the canal of between 51.3mOD and 51.6mOD in the vicinity of S17-N3 Pier Base 1. The corresponding bank levels on the northern side of the canal are at 52.1mOD. Accordingly, a design water level in the canal of 51.4mOD was adopted, which is at or above the minimum bank level on the southern side of the canal. This level would therefore correspond to a flood event


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in the canal. Typically the water level in the canal is 51.0mOD.

The proposed ground and groundwater engineering parameters, derived from the ground investigations are presented in Table 1. Given the proximity to the canal, emphasis was placed on deriving appropriate permeability characteristics for the soils. In the absence of in situ permeability testing, this was carried out based on Hazens method for estimating permeability (k) based on the D10 value (particle size at which 10% of the sample passes):

k = 0.1(D10)2 (m/s; D10 in mm)

The particle size distributions for the Glacial Till and Glacial SANDS and GRAVELS are presented in Figure 2 and Figure 3 respectively.

PROPOSED SOLUTION

The proposed formation level of the pier varied between 50.1mOD and 48.8mOD with the majority of the length of the pier (30m) at the higher level and the remaining length (16m) at the lower level. The plan arrangement of the pier is at a skew of approximately 4 i.e. approximately parallel to the canal. The distance between the canal edge and the proposed pier base varies from 2.9m at the western extent to 5.3m at the eastern extent.

For the majority of the pier base excavation, where the formation level was proposed at 50.1m OD (plus an allowance for blinding etc.), the proposed pier base formation was above the level of the SAND and GRAVEL stratum and groundwater level. In these areas, 45 slopes, assuming undrained behaviour of the Boulder Clay, were proposed.

For the remaining area of the pier base excavation, where formation level was proposed at 48.8mOD, the base of the excavation lay both below groundwater level and the interface of the SAND and GRAVEL strata. In this area, a temporary dewatering scheme was required in order to ensure a dry excavation and to maintain the stability of the 45 slopes over the duration of the temporary works. A photograph of the temporary slope is presented in Figure 4.

Furthermore, in the south-western corner of the temporary excavation, over a length of 26.8m, there was insufficient space between the excavation outline

and the Royal Canal to provide a 45 slope and the 2.0m minimum clearance. This length included the full extent of the lower formation level and a 6.1m length of the upper formation level. In this area, it was proposed to steepen the excavation side slope to near vertical (85), supporting the cutting with soil nails. A hard-facing was applied to the cutting using shotcrete. This solution had to be agreed, in principle, by Waterways Ireland.

The proposed soil nail solution is shown in Figure 5 to Figure 7. In order to limit the risk of soil nail installation adversely impacting on the integrity of the Royal Canal, a minimum clearance between the bed profile of the canal and the soil nails of 1.0m was maintained. In general, the soil nails were installed with an angle of inclination () to the horizontal of 12.5. However, the upper row of nails was inclined at 25 to the horizontal to maintain the minimum 1m clearance for nail length of 2.0m.

In order to account for the water level in the Royal Canal, and uncertainty regarding hydraulic conductivity between the canal and groundwater table, two design assumptions were made as follows:

Case 1: No hydraulic conductivity between the canal and groundwater level. Water level in canal modelled at 51.4mOD for canal lining material only. This case assumed no flow of water from the canal into the surrounding ground.

Case 2: Hydraulic conductivity/groundwater levels in the design sections determined from long-term seepage analysis allowing for flow from the canal into the surrounding ground, dependant on the permeability of the soils.

The groundwater analyses were carried out using the software program SEEP/W. For either analysis a 0.5m thick puddle clay liner was assumed between the bed profile of the Royal Canal and the underlying soils.

Stability assessments for the 45 slopes were carried out to calculate a global lumped factor of safety. Using undrained shear strength parameters for the soils, a minimum factor of safety of 3.0 was established. The effect of the assumption of hydraulic conductivity between the canal and groundwater table was adverse but only marginally.


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For the stabilised soil nailed cutting, design was carried out in accordance with the principles of BS8006:2005 and CIRIA C637. Stability analysis was facilitated by use of the software programme SLOPE/W. A minimum factor of safety of 1.65 was determined from the SLOPE/W modelling. The numerical difference between the two groundwater regime analyses described above was found to be marginal.

Given the sensitive nature of the adjacent canal and evaluation of the geotechnical risks, a monitoring scheme was proposed for the temporary soil nailed cutting. Soil nail slopes tend to move significantly as the nail reinforcement generates bond and start to pick up load. Six monitoring points were established along the slope and these were monitored during the two week construction period.

Amber and red monitoring trigger levels were set at 5mm and 10mm respectively for lateral movement. Figure 8 shows a plot of the slope displacement over the critical construction period. The displacements recorded that the amber trigger level was breached close to the top of the cutting. Monitoring frequency was subsequently increased but red trigger levels were not breached during construction.

Seepage analysis, undertaken as part of the temporary works design, had indicated that the rate of flow between the Canal and the groundwater table would be low, if present at all. However the concern remained that the temporary excavation could be destabilised by changes in the groundwater regime induced by the excavation.

In order to understand and control groundwater levels, particularly between the excavation and the Royal Canal, a groundwater monitoring system was also established. This system involved the installation of two standpipe piezometers in the footpath of the Royal Canal adjacent to the sections adopted for analysis. In addition the surface water level of the canal itself was monitored at three locations in the vicinity of the excavation.

Upper and lower design levels for groundwater were set in relation to the excavation level in the temporary works area at the time. Figure 9 shows a plot of the groundwater level readings over the construction period. Initially, ground level within the excavation was above groundwater level. However as construction progressed, the excavation level dropped below

groundwater level. The monitoring readings show that for penultimate excavation stage, which was the first to dip below groundwater level, there was a short period where the where the design levels were exceeded as the drainage in the base of the excavation was installed and commissioned. Once the drainage was installed and commenced working, the recorded groundwater levels tended to drop to between the design levels. This behaviour was repeated for the final excavation stage.

A pumping trial was undertaken from the drainage sump closest to piezometer P1 during the final two excavation stages to validate the pumping volumes predicted from the seepage analysis. The pumping trial caused a significant drop in the recorded groundwater level at piezometer P1 which recovered over a period of several days after the pumping trial was completed.

CASE STUDY 2

M50 UPGRADE CONTRACT 1, S04-N7 MONASTERY ROAD BOWSTRING BRIDGE SLIDE TEMPORARY REINFORCED EARTH EMBANKMENT

The M50 Upgrade Contract 1 was delivered through a Design & Build (D&B) contract by SIAC-Ferrovial M50 Joint Venture, a partnership between SIAC Construction Ltd. and the Spanish Construction Company Ferrovial Agromn S.A. The Contractors designer was a consulting engineering consortium of Hyder Consulting Ltd, PH McCarthy & Partners and Grontmij. Arup Consulting Engineers supervised the contract on behalf of South Dublin County Council and the NRA.

The works delivered under Contract 1 comprised the following:

The 8km section from Junction 7 (N4) through Junction 9 (N7) to Junction 10 (Ballymount) was upgraded from dual two lane motorway to dual three lane motorway, plus a fourth auxiliary lane in each direction between interchanges to facilitate traffic entering and exiting junctions.

New free-flowing interchanges were constructed at Junction 7, (N4, Liffey Valley Interchange) and Junction 9 (N7 Red Cow Interchange) together with a significant upgrade of Junction 10 (Ballymount).

A segregated corridor for LUAS was provided including a new 800m section of


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tramway, thus removing the need for LUAS and road traffic to cross each other.

12 new bridges were provided.

As part of the N7 works, Byrne Looby Partners were employed by the Contractor to assess the design of the permanent reinforced earth approach embankment (south) for the high, transient temporary loads associated with the installation (bridge slide) of the landmark N7 Bowstring Bridge.

The Bowstring Bridge connects the N7 Eastbound and Monastery Road (Clondalkin) with the Luas Red Cow Stop and Park & Ride Facility. The bridge forms part of the M50/N7 (Red Cow) Free-flow Junction plan removing the traffic lights at the former N7/Monastery Road Junction. The bridge has a suspended deck which accommodates two-way traffic, cycle-lanes and footpaths. It has a total span of 60m and is 20m wide.

The 1,550 tonne bridge was constructed on the approach embankments adjacent to the N7 and manoeuvred into place using specially imported heavy lift mobile jacking equipment during the night of 7th October 2008. As a result of the construction method and bridge slide, significant variable loading would be imposed upon the reinforced earth approach embankments. The concern was that these temporary loads, being much larger than the subsequent permanent loads might overstress the reinforcement, or cause significant deformation of the walls.

GROUND CONDITIONS

The ground conditions typically comprised up to 2m of Made Ground, overlying Boulder CLAY and LIMESTONE bedrock. The depth to LIMESTONE varied considerably over the site dipping in the north-south direction by as much as 10m. No groundwater strikes were recorded in any of the exploratory holes.

The Made Ground was described as consisting of brown gravelly CLAY with concrete, cobbles and boulders. The average SPT N value in this stratum was 40 varying between 15 and refusal.

Investigation of the Boulder CLAY proved difficult with cable percussion techniques beyond 4-5m owing to obstructions reported as possible boulders. Rotary techniques were employed to investigate the depth of the Boulder CLAY and level and quality of the underlying bedrock. SPT N values were reported

frequently as refusal (50 blows for less than 75mm penetration in any given drive).

The underlying bedrock was described as very strong, medium to thinly bedded, blue/grey/dark grey, fine-grained LIMESTONE. The quality of the mass however, was variable with an average rock quality designation of 25% in the upper 3-4m.

Based on the observed SPT results and considerable available engineering data for the Boulder CLAY in Dublin, a conservative friction angle () was adopted for the material of 35 for use in effective stress analyses. The corresponding undrained shear strength would be in the order of 75-100kN/m2 with some increase with depth.

PROPOSED SOLUTION

The basic construction sequence for the bridge is shown in Figure 10. The bridge superstructure is carried on a piled abutment and the approach embankments are formed using reinforced earth Hexlok walls. Furthermore, a Hexlok wall formed the southern face of the south abutment embankment in order to facilitate temporary traffic flow during the bridge construction because sufficient space was not available for traditional earthworks slopes. This was later replaced with a permanent embankment. The bridge itself was constructed offline and adjacent to the south approach embankment before being moved into position.

The reinforced earth walls were designed for permanent loads in accordance with the Coherent Gravity Method of BS8006:2005. However, Cl.6.6.5.3 of the code states the following:

Where the structure is of unusual geometry or supports concentrated loads which are not specifically covered in the code the local equilibrium method may not be sufficient and a global wedge stability check should be performed.

The temporary loads were large and transient relative to the permanent works. Therefore, a global wedge stability check had to be carried out for the temporary condition. The following loadings were considered in the assessment of the temporary works:

Bridge live load: 4,267kN Skid dead load: 589kN Skid slab dead load: 30kN/m2 applied


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The temporary loading was to be transferred via a ground bearing skid slab, 5.5m wide and 1.2m deep. The nearest edge of the skid slab was only 0.5m behind the wall facing. The resulting pressure beneath the skid slab was calculated to be 260kN/m2. A friction angle of 40 was specified for the Class 6I/J fill which was to form the reinforced earth approach embankment. The reinforcement elements comprised 50mm wide x 4mm thick, 12m long, galvanised steel straps, at 0.75m vertical centres.

The stability analyses were carried out using the software SLOPE/W. All of the BS8006 partial factors for material strength and loadings were included in the definition of design parameters except for the Ramification Factor. This factor is required, in accordance with BS8006, to be 1.1 where the consequence of a failure would be high. Therefore an overall FOS from SLOPE/W in excess of 1.1 was required to satisfy the Ramification Factor criterion.

In accordance with the guidance provided with the SLOPE/W software, the mobilised bond stress for the reinforcement straps has been defined as FOS-dependant. This means that the design bond stress for the straps is reduced by the calculated overall FOS. This methodology should be adopted where some movement is required in order to mobilise the bond stress in the reinforcement straps i.e. passive reinforcement such as straps, soil nails or geogrids.

The minimum calculated value for FOS was 1.19 thereby satisfying the stability criteria in accordance with BS8006. However, given the intensity of the temporary loading, it was considered prudent to undertake a more rigorous analysis using the finite element code Plaxis.

As part of the geotechnical risk strategy, a monitoring regime was installed in order to ensure that the design assumptions were validated and that the structure behaved as expected particularly as failure mechanisms for this type of structure tend to be brittle i.e. small movements / strains prior to failure. In order to develop the monitoring plan, an estimation of likely movements was required in order to assess how the structure was expected to behave. This was carried out using Plaxis. This analysis would provide predictions of wall movement under the temporary loading conditions which could then be compared to monitored wall movement in real time.

In the Plaxis model, the wall facing units have been modelled as individual beam elements with unrestrained connections (hinges) at the joints of the Hexlok panels. Interface elements have been applied to the reinforcement straps to model the interaction of the reinforcement and surrounding soil more closely. Mohr-Coulomb strength parameters were adopted for the soil strength and a conservative stiffness of 50,000kN/m2 with a Poissons ratio of 0.25 was adopted for both the Class 6I/J fill and the underlying foundation soils. A number of finite element sensitivity analyses were performed in order to understand the range and variation of wall movement and stresses developed in the reinforced earth system under the temporary loading. Additionally, the modelling of the facing elements was explored, either by modelling as a full height panel or as unrestrained, hinged, discreet elements.

Figure 11 shows predicted horizontal movements from the Plaxis analyses. The Plaxis analyses indicated that horizontal movements in the order of 6-8mm could be expected with significantly more movement around the mid-height of the wall. Based on these movement predictions, movement monitoring positions were set up towards the mid-height of the wall. The movements were monitored during the bridge slide process, with measurements in x, y and z directions. The proposed trigger levels are presented in Table 2. Vertical displacements (settlements) were expected to be small. Additionally, load readings were taken from the jack system at each of the temporary support locations to confirm loadings were as predicted. The loading had been conservatively assessed for the design and in reality the loads were approximately 30 40% less than anticipated.

During the bridge slide, surveying targets were monitored regularly. A maximum recorded movement of 9mm was recorded, breaching the amber limit. A contingency plan to place fill in front of the reinforced earth wall or to return the bridge to its initial start position, dependent on the bridge slide phase had been prepared in keeping with the philosophy of the Observational Method. Monitoring frequency was increased however, no further significant movements were recorded and the contingency measures were not implemented.

Over the duration of the bridge slide, on the Hexlok wall forming the southern face of the south approach embankment, the range of observed lateral movement


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was 1.8mm to 8.9mm with an average of 5.2mm demonstrating slightly better performance than indicated in the Plaxis analyses. A maximum vertical displacement of 2.0mm was observed adjacent to the permanent Hexlok wall. Based on the observed results, the Plaxis analyses were deemed to be validated.

The case history demonstrates the need to utilise modern analysis techniques in order to fully understand expected behaviour for complex situations. Adhering to the British Standard calculation methods alone would have indicated an adequate factor of safety but would not provide the level of knowledge required to validate expected behaviour against observed movement monitoring.

CASE STUDY 3

BISHOP ODONNELLSEAMUS QUIRKE ROAD WIDENING SCHEME TEMPORARY SUPPORT FOR SOFT GROUND EXCAVATE AND REPLACE

Bishop ODonnell Road / Seamus Quirke Road is part of regional route R338, which links the R336 and the Western Distributor Road the main approach roads to Galway from the suburban and coastal areas west of the city and the N6 across the Quincentenial Bridge. The road passes through the largely residential areas of Rahoon, Shantalla and Westside, which have grown significantly in recent years. The area also supports shopping, commercial and social services and sporting facilities, all of which access Seamus Quirke Road directly or indirectly.

In accordance with the Galway County Borough Development Plan 1999, the following provisions were set out as part of the Seamus Quirke Road Widening Scheme:

The provision of a cycle way from the roundabout at Bishop ODonnell Road (Western Distributor Road) to the roundabout at Corrib Park.

Improvements to road junctions at Seamus Quirke Road / Circular Road / Rahoon Road with the possibility of a roundabout at this location.

The provision of bus lanes on Seamus Quirke Road and the Western Distributor Road from the Corrib Park roundabout to the Cappagh Road.

The overall aims of these provisions are to provide increased traffic capacity between the suburban and coastal areas west of Galway and the city centre and to provide improved cycle and pedestrian facilities on the route with a view to facilitating the achievement of the aim for greater use of more sustainable modes of transport. This case study will focus on the excavate-and-replace solution for the area of relatively soft ground at Bishop ODonnell Road-Rahoon Road Junction, the position of which is shown in Figure 12.

GROUND CONDITIONS

The area at the Bishop ODonnell Road-Rahoon Road Junction had been characterised by substantial depths of soft ground. Substantial historical settlements have been noted in the area as shown in Figure 13. As a result of this, the area was investigated with geophysics using MASW and seismic refraction methods. The interpretation (refer Figure 14) indicated that, locally, substantial deposits of overburden material were prevalent in depths of between 6-7m. These geological features were identified as ancient infilled river channels.

The overburden material itself comprised up to 2m of made ground generally composed of clayey gravelly SAND with many cobbles and boulders and road construction materials. This overlay a relatively thin layer of very soft organic fibrous PEAT which in turn overlay 3-4m of very soft creamy / grey shelly MARL. The depth at which the CPT probe refused correlated well with the interpreted bedrock level of the geophysics. Bedrock in this area was proven to comprise very strong pink / white fine-grained GRANDIORITE.

The prevalence of MARL across the area is not uncommon in Galway. The deposit is characteristic of a lake or water-filled channel depositional environment known as an aquatic / limnic environment. Its distinctive colour is derived from the nature of the depositional waters which were highly basic (containing calcium and magnesium).

Trial pitting was carried out in order to assess the extent of the soft soil deposits and to assess the likely engineering behaviour of the soft soils. Figure 15 shows cone penetration test (CPT) results in the vicinity also. The very soft layers of PEAT and MARL between 2m and 5m depth realise a sleeve friction (fs) of between 10kN/m2 and 30kN/m2 and a tip resistance (qc) of


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between 0.5MN/m2 and 2MN/m2. The average value of undrained shear strength has been calculated as 25kN/m2 based on the relationship presented below which is also related to the plasticity index (Bowles, 1997):

K

vCu N

qc

=

where:

)2(50

5.513 += PK IN

This correlated well with observations of undrained shear strength made on-site during the trial pitting and subsequent excavations. For the purposes of design, moderately conservative values of undrained shear strength of 15kN/m2 for the PEAT and 20kN/m2 for the MARL were adopted.

PROPOSED AND ALTERNATIVE SOLUTIONS

In order to remove the problem of historical settlement in the area, a piled slab was initially proposed to form the road foundation. This would have involved installation of driven precast concrete or steel piles thought the soft overburden deposits to found in the underlying bedrock. A reinforced concrete slab would then be cast on top of the piles which would carry the road construction and imposed traffic loading. The loading would be transmitted to the underlying bedrock via the piles. In addition to supporting the road construction, the slab would also facilitate / support the existing services traversing the area, thus its depth would be approximately 2.5m from the finished road level meaning that up to 3m of material would have to be excavated anyway. Furthermore, given the extensive services within the area, a pilot hole would have had to be excavated for each individual pile to ensure no risk of striking a service or utility line prior to driving.

The Contractor submitted an alternative proposal which involved full excavate-and-replacement of the soft ground. This solution would reduce cost and construction time significantly. However, the primary constraint to the proposal was maintaining a two-way traffic flow during the works. A solution was developed utilising a line of temporary sheet piles which would be placed centrally on the carriageway as a means of support to the live lane of traffic while the excavation and filling operations were carried out over the adjacent lane. The sequencing of implemented solution is presented in Figure 16.

Primarily, the solution involved use of a waling beam to provide support to the sheet piles over the upper wider section of the excavation. The waling beam would derive its support passively from the ground either side of the excavation. Below this, the sheet piles would rely on membrane action to be self-supporting in the area where excavation proceeded to bedrock. A small component of passive resistance was also assumed from the underlying bedrock as experience had shown that some bite of the sheet pile into the upper slightly weathered bedrock would be realised. An allowance was also made for a hydrostatic head differential behind the sheet piles. In order to avoid overstressing of the sheet piles, a strict limit was placed on the maximum width of excavation proposed. The excavation proceeded incrementally requiring the waling beam to be moved incrementally also. Restrictions were placed on the time allowance for open excavations and cleaning of the in-pans of the sheet piles to ensure all soft material was removed. The formation bedrock was verified either visually where the excavation was dry or by probing where groundwater was present. The sheet piles were inspected visually during excavation for the presence of split clutches which may have compromised the solution.

The backfill comprised Class 6A selected granular fill to be placed underwater and Class 6F2 selected granular fill above this. Plate bearing tests were carried out on the Class 6A fill after placement of a blinding layer of fill. On completion of one side of the carriageway, the process was repeated on the other side. The replacement fill level was left roughly 1m lower than final fill level to allow for placing of the final compacted fill layer and road construction after removal of the sheet piles. This negated the need for settlement monitoring during excavation however, lateral movement of the sheet was monitored to ensure that the wall was behaving as predicted. Settlement of the final road construction will be monitored on an on-going basis in accordance with the works requirements.

The sheet pile wall was analysed using the Oasys FREW software for the analysis of embedded retaining walls. Figure 17 shows output from the FREW model (SLS Case) and Figure 18 shows a photograph of excavation progressing near the base of the sheet pile wall. Undrained soil conditions were assumed for the PEAT and MARL. The analysis required an iterative procedure to ensure that the membrane action was correctly modelled and that the artificial strut stiffnesses, which modelled the membrane action and


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waling support, were consistent with the amount of movement calculated by FREW.

The stiffnesses of the artificial struts were calculated as a function of the axial stiffness of the sheet pile wall as it went into tension over the width of the excavation at the level considered. The strut stiffnesses therefore increased with depth as the width of the excavation decreased. The additional stiffness offered by the waling beam was calculated as a function of the passive resistance of the soil which would provide the support either side of the excavation. The calculated strut loads (as shown on Figure 17) would then have to be carried structurally (as a tensile force) across the sheet piles and distributed passively into the supporting soils either side of the excavation. This stiffness was iteratively increased / decreased with increasing wall movement such that the magnitude of passive support was consistent with the expected wall rotation. Consideration was given to provision of welded diaphragms between the pans of the sheet piles to allow greater structural capacity and hence a wider excavation however, it was considered more practical to proceed with the lesser width of 1m at the base of the excavation. The structural design of the sheet piles considered the tension carried across the clutches to ensure that overstressing did not occur which may have resulted in unzipping of the sheet piles.

Fundamentally, the analysis followed the principles of CIRIA C580 in deriving design section forces however, in this instance, it was more critical that the amount of wall movement was estimated to analyse the membrane action in the sheet piles and to provide confidence that the assumed passive resistance would be realised.

The assessment of the feasibility and subsequent development of the alternative solution was possible because of the additional ground investigation (geophysics and trial pitting) which defined the extents of the problem spatially and provided confidence in the engineering parameters adopted for design.

The success of the solution relied heavily on the strict construction control procedures regarding excavation width and verification of the excavated level. Non-generic or bespoke solutions such as this require a high level of commitment and diligence from the Contractor in order to ensure that the assumptions of the geotechnical analysis are realised and that the construction control procedures specified by the designer are implemented.

CURRENT BEST PRACTICE IN GEOTECHNICAL DESIGN OF TEMPORARY WORKS -DISCUSSION

This section presents a discussion on the implications for the geotechnical design of temporary works in accordance with Eurocode 7.

TEMPORARY CUT SLOPES

Temporary cut slopes or embankments are frequently used in infrastructure projects to enable construction of the permanent works. Where ground conditions are suitable, slope angles as steep as approaching near vertical may be employed in order to facilitate construction processes, installations and temporary widening of access routes.

It is worth noting the advice offered in Eurocode 7 which states (cl.11.5.1) In analysing natural slopes, it is generally an advantage to make a first calculation using characteristic values, to get an idea of the global factor of safety, before starting to design. Experiences with comparable cases including investigation procedures should be applied. This implies that the partial factor approach adopted by Eurocode 7 should be used with caution as modification of soil strengths in stability analyses may result in unrealistic or incorrect failure mechanisms. An initial unfactored analysis should inform the design.

Traditionally, slopes were analysed with a view to establishing a lumped factor of safety. Distinction has been made between an appropriate lumped factor of safety for temporary conditions as opposed to permanent conditions (Trenter, 2001) giving due regard to environmental and economic risk.

Eurocode 7 design procedures, using the partial factor approach, verifies overall stability by way of identification of ultimate limit states (GEO and STR) where the applied partial factors are defined in the National Annex. It is noted in Cl. 2.4.7.1(5) that less severe values [of partial factors] than those recommended in Annex A ([Partial and correlation factors for ultimate limit states and recommended values] or the National Annex) may be used for temporary structures or transient design situations, where the likely consequences justify it. However, no guidance with regards to the choice of partial factors for


11

temporary works is provided (except for temporary anchors).

TEMPORARY SOIL STRENGTHENING FOR STEEPENED CUT SLOPES USING SOIL NAILING

Soil nailing and soil reinforcement are not expressly considered in Eurocode 7 and current best practice (as advocated in the UK National Annex) is to design these elements in accordance with BS8006-1:2010 (reinforced earth) and BS8006-2:2011 (soil nails). The standard has recently been developed into two parts covering both elements.

In distinguishing between temporary and permanent conditions, BS8006-2:2011 Cl. 4.1.5 states: The partial factors given [for actions, material properties and soil resistances] do not explicitly take account of whether the works are of a temporary or permanent nature; this should instead be reflected in the selection of appropriate characteristic values for the material properties and characteristic soil nail resistances. Emphasis is placed on the derivation of the design bond stress reflected in the factor k for determining the characteristic bond stress (bk) from ultimate values. The k factors in BS8006-2:2011 have been selected to result in equivalent experience with lumped factors between 1.5 and 3.0 on ultimate bond resistances (and micropile / ground anchor designs). The range given for k is to reflect whether nails are used in a temporary or permanent application and the degree to which full dissipation of pore pressure is relevant. Therefore, a clear and well-defined distinction is made between temporary and permanent works for soil nail design. However, where checks on external rotational failures are carried out, as required by BS8006-2:2011, there is no guidance regarding the appropriate partial factors to be applied in a temporary condition.

TEMPORARY SOIL STRENGTHENING FOR STEEPENED EMBANKMENTS (REINFORCED EARTH)

BS8006-1:2010 advocates the use of best practice guidance for the design of temporary (as opposed to permanent) reinforced embankment structures which can be found in BR470 (specifically for piling platforms) and CIRIA SP123. High temporary reinforced earth embankments are generally rare in practice. Typically, the expense of constructing a high reinforced earth embankment is such that it is uneconomical unless it is incorporated into the

permanent works. In the temporary condition, design may take account of the short-term strength of the reinforcement. Geosynthetics display time-dependent behaviour (creep). The short-term strength of the reinforcement can be taken into account in a temporary loading condition as appropriate. The permanent condition must subsequently take creep into account and lower strengths will be adopted to ensure the level of strain is such that the structure remains serviceable for its design life.

The foundation soils beneath strengthened embankments must be given due consideration particularly where soft silts and clays are prevalent. Where the founding stratum is a fine-grained soil and undrained behaviour is expected, application of a rapidly applied load, e.g. from embankment construction, will result in an increase in porewater pressure (undrained response). The design situation envisaged when loading soft clays is a rapid undrained failure and collapse.

TEMPORARY RETAINING STRUCTURES

Temporary retaining structures are frequently employed during construction to provide lateral support to excavations. The complexity of the temporary support may vary from a simple trench box to support an area of excavation to remove soft soil to large diaphragm walls supporting a cut-and-cover tunnel section. For routine excavations, existing good practice guidance, such as CIRIA R97, should be used. However, for more complex structures, detailed design in accordance with an appropriate code of practice will be required. In the UK and Ireland, Eurocode 7 is now the recognised standard for design of retaining structures.

In the case of embedded walls, a recent study by Markham (2012) highlights some of the short-comings of using Eurocode 7 in design of temporary support. These include:

Different interpretations of Eurocode 7 can produce dramatically different results for example, passive earth pressure can be treated as a resistance, a favourable action or an unfavourable action each of which produces a different result in an analysis.

Similarly, water pressures can be treated in several different ways.

Use of total stress parameters on the passive side of the wall may result in the analysis


12

becoming ill-conditioned and a sensitivity analysis may be required.

In the UK, CIRIA C580 is cited as non-contradictory complementary information (NCCI) however, parts of CIRIA C580 are, in fact, contradictory to Eurocode 7.

Some recommendations are made in the study with a view to relieving the opposition to use of the code and promoting its acceptance among temporary works engineers. These include:

Use of the single source principle which removes some of the complexity of applying different partial factors to different parts of the earth and water pressure diagrams.

A partial rewrite of the code is recommended to ensure that NCCI is, in fact, non-contradictory.

In all cases, a check on external (rotational) stability should also be carried out for embedded walls. The choice of soil parameters for use in analysis, drained or undrained, should be tempered with experienced judgement and reference to appropriate case histories where available.

CONCLUDING REMARKS

Upgrade of existing highway networks present formidable challenges, particularly in the urban environment. A wide variety of constraints may influence the project and affect the choice of earthworks measures employed in developing the upgrade. Slope stabilisation and strengthening measures such as soil nailing, reinforced soil (slopes) and reinforced earth (walls) are commonly used to assist in providing adequate working area inside of the lands made available. Temporary retaining structures are also commonly employed to assist with ground support and the control of excessive ground deformation during construction. Optioneering should be undertaken in order to determine the most appropriate, technically feasible and commercially attractive solution.

There are a wide variety of geotechnical factors to consider for highways upgrade and widening schemes. Primarily the stability of existing assets must be considered. This can be influenced adversely by proposed construction and existing geological and groundwater conditions. A detailed examination of these is required to assess the proposed concept.

Furthermore, the behaviour of the proposed geotechnical structure must be considered giving due regard to potential adverse effects such as differential settlement, slip surfaces, cracking or seepage planes.

Groundwater can play a significant part is assessing the suitability of temporary works solutions and Case Study 1 demonstrated that groundwater regimes should be given careful consideration.

Case Study 2 demonstrates the usefulness of more advanced analyses in developing appropriate monitoring regimes. Using finite element or finite difference codes to inform the design and, in particular, the anticipated behaviour of a soil structure is to be encouraged provided that it is tempered with experienced engineering judgement

Case Study 3 demonstrated a bespoke non-generic temporary sheet pile wall solution. An estimate of wall movement, using soil-structure interaction software, was key to analysis of the membrane action of the sheets and providing confidence that the assumed passive resistance was realised. Additional ground investigation was crucial in assessing the feasibility of the alternative solution and, subsequently, in informing the development of the solution. The additional information allowed the problem to be defined more clearly in extent and provided confidence in the engineering parameters to be adopted in the geotechnical design of the solution.

All of the case studies presented here demonstrate the importance of understanding ground movement and understanding the difference between large and small strain problems e.g. Case Studies 1 and 2 respectively, and also the difference between passive and active ground response e.g. Case Studies 1 and 3 respectively. This correct anticipation of ground movement and behaviour will inform the designer in determining the appropriate trigger levels and monitoring regimes. Use of movement monitoring is a fundamental and necessary requirement as a means of reducing risk, validating expected engineering behaviour and ensuring safe delivery of geotechnical solutions.

As with all temporary works solutions, the success of the solution relied heavily on the strict construction control procedures. Non-generic or bespoke solutions such as this require a high level of commitment and diligence from the Contractor in order to ensure that the assumptions of the geotechnical analysis are realised


13

and that the construction control procedures specified by the designer are implemented.

Traditional design methodologies have made the distinction between temporary and permanent works generally allowing less onerous factors of safety to be associated with temporary conditions. The design of temporary works, as opposed to permanent structures, is not expressly considered in Eurocode 7 (except for temporary anchorages).

Some distinction has been made between temporary and permanent conditions in the more developed / specific partial factor codes (for example BS8006-2:2011 for soil nailing) and this is to be welcomed. However, there are still short-comings when referencing Eurocode 7 for external stability checking in a temporary situation which need to be addressed. Further to this, there are a number of other short-comings identified by the temporary works / geotechnical community associated with the interpretation of the code and correlation with non-contradictory complementary information which are a source of consternation to engineers.

The geotechnical designer relies upon previous experience and understanding of short term or temporary ground behaviour to inform decision-making within the context of tried and tested codes of practice. Current best practice, in the form of Eurocode 7, will influence the future trends of discussion between designer and reviewer with regard to design of temporary geotechnical solutions.

Some guidance on appropriate partial factors for use in geotechnical design of temporary works would be welcomed among the geotechnical and temporary works engineering community.

ACKNOWLEDGEMENTS

The authors would like to acknowledge M50 D&C, Siac-Ferrovial and Coffey Construction Ltd. for their kind permission to publish the data contained within the case studies of this paper. The views expressed in this paper are the sole views of the authors and do not represent the views of any third parties.

The authors gratefully acknowledge the assistance and support given by many colleagues at Byrne Looby Partners both during the projects and in the production of this paper.

REFERENCES

Bowles, J.E. (1997) Foundation Analysis and Design Fifth Edition. McGraw-Hill

BS8006:1995 Code of Practice for Strengthened / Reinforced Soils

BS8006-1:2011 Code of Practice for Strengthened / Reinforced Soils and Other Fills British Standards Institute

BS8006-2:2011 Code of Practice for Strengthened / Reinforced Soils Part 2: Soil Nail Design. British Standards Institute

Building Research Establishment (2004) Working platforms for tracked plant: Good practice guide to the design, installation, maintenance and repair of ground-supported working platforms. BRE Press (2004) Report No. BR470 London 2004

Department of Transport Highways, Safety and Traffic, Departmental Advice Note HA 43/91 Geotechnical Considerations and Techniques for Widening Highway Earthworks

Gaba, A.R., Simpson, B., Powrie, W. and Beadman, D.R. (2003) Embedded retaining walls Guidance for economic design. Construction Industry Research Information Association (CIRIA) C580, London 2003

Irvine, D.J. and Smith, R.J.H. (1992) Trenching Practice. Construction Industry Research Information Association (CIRIA) R97, London 1992.

Jewell, R.A. (1996) Soil reinforcement with geotextiles. Construction Industry Research Information Association (CIRIA) SP123, London 1996

Markham, P.D. (2012) The design of temporary excavation support to Eurocode 7. Proc. ICE Geotechnical Engineering, Vol. 165, No. 1, pp. 3-12

Phear, A., Dew, C., Ozsoy, B., Wharmby, N.J., Judge, J. and Barley, A.D. (2005) Soil Nailing Best practice Guidance. Construction Industry Research Information Association (CIRIA) C637, London 2005

Trenter, N.A. (2001) Earthworks: A Guide. Thomas Telford


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Figure 1 - Location plan for S17-N3 Bridge Pier 1 adjacent to the Royal Canal


15

STRATUM (kN/m3)

() c/cu

(kN/m2) Avg D10

(mm) k

(m/s) MADE GROUND/BOULDER CLAY

(Drained Behaviour) 20 34 0 MADE GROUND/BOULDER CLAY

(Undrained Behaviour) 20 - cu = 75 0.02 4 x 10-6

SAND AND GRAVEL 20 35 0 0.2 4 x 10-4

ASSUMED PUDDLE CLAY CANAL LINING 17 - cu = 10 - 1 x 10

-8

LIMESTONE

21 35 500 - 1 x 10-7

Table 1 - Ground engineering parameters for S17-N3 Pier 1 temporary works

Figure 2 - Particle size distributions for Glacial Till

Figure 3 - Particle size distributions for Glacial GRAVELS


16

Figure 4 - Temporary 45 slope adjacent to the Royal Canal

Figure 5 - Section through proposed temporary soil nailed slope adjacent to the Royal Canal


17

Figure 6 - Elevation on proposed temporary soil nailed slope adjacent to the Royal Canal

Figure 7 - Photograph of as-constructed soil nail stabilised steepened embankment prior to casting of Pier 1 base


18

M50 PPP UPGRADING CONTRACT 2

-0.010

-0.009

-0.008

-0.007

-0.006

-0.005

-0.004

-0.003

-0.002

-0.001

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

13/08/09 14/08/09 15/08/09 16/08/09 17/08/09 18/08/09 19/08/09 20/08/09 21/08/09 22/08/09 23/08/09 24/08/09 25/08/09 26/08/09 27/08/09 28/08/09Date

Mo

ve

me

nt (m

)

M1 Lateral

M1 Vertical

M3 Lateral

M3 Vertical

M5 Lateral

M5 Vertical

TEMPORARY WORKS AT S17-N3 PIER 1SIDESLOPE TOTAL DISPLACEMENTS - MONITORING RESULTS

Red Trigger Limit

Amber Trigger Limit

Red Trigger Limit

Amber Trigger Limit

Figure 8 - Movement monitoring of soil nailed cutting adjacent to Royal Canal

M50 PPP UPGRADING CONTRACT 2

49

50

51

52

25/07/2009 01/08/2009 08/08/2009 15/08/2009 22/08/2009 29/08/2009Date

Can

al Le

vel

(m

OD

)

Upper DesignLevel - P1

Lower DesignLevel - P1

Upper DesignLevel - P2

Lower DesignLevel - P2

Piezometer 1

Piezometer 2

TEMPORARY WORKS AT S17-N3 PIER 1PIEZOMETER AND EXCAVATION - MONITORING RESULTS

Initial baseline readings Passive dewatering during initial excavations

P1 pumping trial

P1 recovery after pumping trial

Recovery from instrument installation

Figure 9 - Groundwater monitoring of soil nailed cutting adjacent to Royal Canal


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Figure 10a - S04-N7 Bridge Slide


20

Figure 10b - S04-N7 Bridge Slide


21

Figure 11 - S04-N7 anticipated horizontal movements from Plaxis

Table 2 - S04-N7 proposed monitoring trigger levels


22

Figure 12 - Location map showing the position of Bishop ODonnell Road-Rahoon Road Junction

R338 Bishop ODonnell Road

R338 Seamus Quirke Road Bishop ODonnell-

Rahoon Road

Galway City Centre


23

Figure 13 - Settlement of wall in the vicinity of Bishop O'Donnell Road-Rahoon Road Junction prior to improvement works

Observed wall settlement / distortion


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Figure 14 - Interpreted rock levels from seismic refraction / MASW correlation


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Figure 15 - Cone penetration results for the area around Bishop O'Donnell Road-Rahoon Road Junction


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Figure 16 - Temporary sheet pile solution for excavate-and-replace at Bishop O'Donnell Road-Rahoon Road Junction


27

Figure 18 - Excavation near the base of the sheet pile wall

Figure 17 - Output from FREW (SLS) for temporary sheet pile wall

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