marcs paper

EARTH RETAINING STRUCTURES IN PERTH, WA

M A Woodward Soil & Rock Engineering

ABSTRACTThis paper provides a brief description of the ground strata and ground water conditions commonly encountered inexcavations in and around the CBD and residential areas near Perth, WA. The particular implications of theseconditions on the selection, design and performance of conventional and locally developed retaining wall systems areaddressed to provide guidance in the identification of the most appropriate system for future excavation projects. Asummary of the retaining wall systems used in Perth is presented, with comments relating to the advantages andlimitations of each wall type. Details of a number of case histories are provided to highlight the appropriate use of mostof the retaining wall types considered. Reference is made to a cautionary note relating to the inappropriate reliance, bysome sectors of the construction industry, on the weak cementing of the near surface Tamala Sands that occurs widelyover the Perth Coastal Plain.

1 INTRODUCTION A significant proportion of the land area developed for residential, commercial and industrial use, in and around Perth,WA, is located on the coastal plain where deposits of varying depths of sand over limestone predominate. These groundconditions are favourable for the support of vertical loads and accordingly, shallow foundations can frequently beadopted. Rapid rises in land value in the late 1990’s have led to the adoption of basement construction for increasingnumbers of developments. In particular, the use of basements has increased significantly for commercial developmentsin the CBD, and prestigious residential developments on both the Swan River and Indian Ocean fronts, as land valueincreases have outstripped construction costs.

The widespread presence of near surface sand deposits does not readily accommodate unsupported excavation,particularly when in close proximity to adjacent existing structures. This paper identifies some of the conditions thatare encountered in Perth, reviews how they impact on the use of “conventional” retaining structures such as diaphragmwalls, gravity limestone walls and “heavy sheet piles” and presents details of a number of systems, such as “light-weight” sheet piles, microfine cement grout, reinforced earth and soil nail walls, that have recently been developed tosuit the particular ground conditions that are encountered.

The near surface sands, widely intersected within the top 4 to 8 metres, are frequently weakly cemented and, for a shortperiod of time, can stand up with a near vertical cut face provided there is no vibration, water or mechanical impact todisturb the soil and cause the cementation to be lost. Unfortunately, the apparent ability of the weakly cemented sand tostand unsupported can lead to complacency and a lack of prior realisation of the nature and/or extent of retainingsystems that may be required in some circumstances. By presenting brief details of a number of projects in Perth,including the use of both conventional and some of the more recently developed systems, this paper provides details ofthe problems associated with ground retention that can be encountered, and some of the measures that have beenadopted to address these problems.

2 GEOLOGICAL & GEOTECHNICAL ISSUESA number of other papers presented in this publication address the geological and engineering nature of the coastalsands and limestone in great detail. When addressing geotechnical issues relating specifically to earth retainingstructures, it is relevant to note that weakly cemented sand, varying from a loose to a very dense state, over limestone,predominates.

The degree of cementing of the near surface sand can vary significantly but is often present in undisturbed strata in theTamala sand deposits. The Tamala sand, and underlying Tamala Limestone strata, are generally located within anominally 10 km wide coastal strip. Bassendean sands are located to the east of the coastal strip and are generally lesswell cemented. Inspection of the Perth geological map indicates that Perth city centre and the areas to the north andwest are mainly areas of coastal quartz sand and/or the coastal (Tamala) limestone.

Generally the strength of the cementing is sufficiently low that disturbance from vibration and/or excavation can breakthe inter-granular bonds and effectively remove the apparent strength returning the sand to a purely frictional material,i.e. the apparent cohesion, c′, is reduced to zero as a consequence of construction activity in the sand.

Australian Geomechanics Vol 38 No 3 September 2003 – The Engineering Geology of Perth Part 1 97

EARTH RETAINING STRUCTURES IN PERTH, WA M A WOODWARD

In low lying areas around the CBD, such as some parts of Northbridge and East Perth, and in most areas in closeproximity to the Swan River, recent alluvial deposits of soft clays and silty sands can be intersected. These materials,often encountered in association with near surface ground water levels, give rise to very different retaining wall andconstruction requirements.

It is relevant to note that the extreme seasonal variations that occur between the wet winter months and the long dryPerth summer can cause significant variations in the water table level in some areas. It is important to check the time ofyear that measurements of the ground water depth were taken, and the likely required timing and duration for anyretaining wall construction and performance, when assessing the possible implications of ground water on the wallbeing considered.

The coastal plain and central Perth are relatively flat areas with no significant topographical features. The few naturalslopes around Perth, such as the Kings Park Escarpment, or Mount Eliza, the South Perth foreshore and river frontagecliffs give rise to a particular and localized set of conditions that require careful consideration when constructionactivity encroaches into the top or bottom of these features.

3 TYPES OF RETAINING WALLS

3.1 GENERALIn order to effectively select and design an appropriate ground retention system, it is important to understand the natureof the retaining requirements, range of options that exist, and also to be aware of the potential benefits and limitations ofthese various ground retention systems. This section of the paper presents details of some of the ground retainingsystems that are available in WA. It is relevant to note that, in some circumstances, non-standard or hybrid solutions,combining a number of aspects of different conventional systems, can be adopted.

Ground retaining structures in and around Perth may be required to perform any of the following functions:

Requirements Options Comments

Enable steep excavationbelow the originalground level (Top down).

Embedded retaining walls (cantileveror propped), soil nail walls and insitugrouted gravity walls.

Basement constructions in the CBD, roadcuttings and below ground railway stations etc.Access, design life and allowable deflections areimportant selection criterion. In-situ groundconditions must be defined.

Retention of placed fillabove the originalground level (Bottom up).

Reinforced earth walls, gravity walls(limestone), Criblock walls andstructural (concrete and reinforcedmasonry) cantilever walls.

Residential developments, elevated carriagewaysand slope remediation. Fill properties must bedefined and controlled. Construction techniquewill strongly influence performance.

Slope Stabilisation ofexisting unstableslopes.

Soil nails, reticulated pile walls,Criblock walls and benching,reinforcement with vegetation.

Remediation of Mount Elisa and South Perthforeshore slopes. Existing slope geometry andsoil strata parameters must be well understood.Access and design life issues will dictate viableoptions.

Enable slopes to beover-steepened to arevised, more onerousprofile.

Soil nails, grouting, embeddedretaining walls, reticulated pile walls.

Developments along Mounts Bay Road and riverforeshore in Dalkeith and Mosman Park etc.Existing and proposed slope profile and soilstrata parameters must be known. Accessconstraints and construction methodology mustsuit slope profile.

98 Australian Geomechanics Vol 38 No 3 September 2003 – The Engineering Geology of Perth Part 1


3.2 EMBEDDED RETAINING WALLSThe most common form of ground retaining structure for the support of excavations is the embedded retaining wall.The structural wall element is formed in the ground strata from the existing surface and supports the retained soil as theexcavation is carried out. These structures are almost all vertical, and can stand in cantilever up to heights in the orderof 3.0 m to 3.5 m. Propped, braced or anchored walls are generally adopted where larger retained heights are requiredand/or where additional lateral support is required to ensure that wall deflections are limited to small values.

The following table presents brief details of a selection of some of the particular embedded retaining wall types that canbe considered for use in Perth:

Wall Type StructuralElement

Temporary/Permanent Advantages Disadvantages

ConventionalSheet Pile

Larssen orFrodingham Steel

Sheet Piles.T & P

High capacity elements, canbe reused, adequate water cut

off.

Durability, cost, noise &vibration.

Not widely used in Perth.

LightweightSheet Pile

Folded steel plateup to 6mm thick.Each sheet 800 to1100mm cover.

T

Cost, well suited for use withmechanical anchors. Systemeffectively developed in Perthand very well suited to work

in the weakly cementedTamala sands.

Limited capacity and depth,vibration. Limited water cutoff and declutching. Only

suitable for temporary walls.May need prebore in clays.

Soldier PileWall

Steel UC & Timberor Concrete

Lagging.

T

(can be P)

Cost, access, low vibrationand flexibility. Widely used in

Perth for temporaryexcavations.

Generally temporary andpotential poor alignment. Pre-

drilling required into basalLimestone.

ContiguousPile Wall

Bored or CFApiles. P

High capacity, low vibration.Can provide permanent

cantilever walls.

Cost, access for rig, no waterretention.

Not widely used Perth.

DiaphragmWall

Cast insitureinforced concrete

wall, 600mm to1200mm wide.

P

High capacity, can formpermanent structure, good

water cut off. Used for manydeep basements in

the Perth CBD

Cost, dimensional limits ofpanels. Large plant requires

clear access to an openworking platform.

Geocast/Echidna Wall

Continuous castinsitu reinforcedconcrete wall.

Typically 300mmwide.

P

Lower cost permanentcontinuous wall. Good

surface finish. Well suited toweakly cemented Tamala sand

strata.

New systems. Limited depthand only suitable to groundconditions above the water

table level.



Technical developments with specialised installation equipments and temporary mechanical anchors have resulted inwidespread use of lightweight sheet pile systems for temporary ground retention in and around Perth. The system isparticularly well suited to the weakly cemented Tamala sands in the coastal strip provided that underlying Limestonedoes not prevent installation to depth, and installation vibrations can be accommodated by adjacent structures.

Conventional retaining wall systems, as detailed in Figure 1, are rarely as cost effective as the light weight sheet pilewall system but are commonly used in Perth where one or more of the identified disadvantages of the lightweight sheetpile or soldier pile walls cannot be accommodated.

Recent developments of the Geocast and Echidna cast in situ trenched retaining walls have been made to accommodatethe ground conditions encountered in and around Perth.


Figure 1 Typical Lightweight Sheet Pile Wall Design


Figure 2: Geocast wall under construction, Northbridge, WA.

Figure 2 shows the Geocast system in use through coastal strip quartz sands in the Northbridge area immediately to thenorth east of the Perth CBD. Similar works have also been completed on projects in both East and West Perth.

These systems are being developed from deep trenching practice to enable the construction of permanent reinforcedconcrete walls for basements up to two stories in depth.

Projects completed in 2001 have shown that limitations with the ground water, buried obstructions and the level of thebasal Limestone must be addressed when considering this approach.

Both systems cut a temporary slot in the ground into which self compacting concrete is poured followed by verticalreinforcement. The systems are suited to use in undisturbed Tamala sands where the natural weak cementing providesthe required temporary slot stability before the concrete is placed.

Figure 3 shows the successful utilisation of a conventional soldier pile wall for an excavation into the Tamala sandsbelow the Perth Mint site to the east of the main CBD area.



Figure 3: Soldier pile wall, Perth Mint, Perth, WA.

3.3 REINFORCED CUT RETAINING STRUCTURES (TOP DOWN)The use of soil nails to reinforce the insitu soil as an excavated face is formed is now widely adopted around the world.The system can be used for temporary or permanent retaining structures and can offer a flexible and cost effectivesolution in many circumstances. As the soil nail wall involves the sequential excavation of the cut face, it is essentialthat the exposed soil strata can stand unsupported for a short period of time, as is commonly the case in the coastalsands around Perth.

For steep excavations in sand, temporary support, generally in the form of scrap corrugated iron roof sheets and starpickets, can be used prior to placement of the shotcrete facing. At shallower angles or in cohesive soils the temporarystability is not generally of concern. Recent developments, adopted by Perth based soil nail wall contractors to addresstemporary instability issues, have included the use of sacrificial lightweight sheet piles, a cast-in-situ grout membraneand pre-treatment of problematical areas with Microfine cement grout. All of these issues add to the standard costs of asoil nail wall but can enable the use of this solution in some circumstances that would otherwise require more costlystructural pile or diaphragm wall embedded retaining structures.

Caution must be exercised when adopting soil nail wall techniques below the water table. Temporary dewatering willbe required ahead of the excavation and provision must be made for weep drains through the wall facing to ensure thatwater pressures cannot build up behind the wall. The issues associated with dealing with these problems are oftensufficient to preclude the use of soil nails in areas where the excavation must extend below the level of the groundwater.

Slope stabilisation with soil nails can frequently be adopted without the need for any shotcrete facing. The nature of therequired surface stabilisation measures will be dictated by the slope angle and the cohesion of the soil strata. Systemssuch as surface vegetation, geogrids and matting have been used successfully on a number of projects. In arid climatesthe use of vegetation and the associated requirement for reticulation must be considered in terms of slope stability andcomponent durability. In Perth the selection of vegetation must be based around the use of native plants that can beassured of survival without the need for extensive reticulation.

Where the proposed soil nail wall is close to a site boundary, it is generally necessary to obtain permission to place thenails below the adjacent property. In some situations this can preclude the use of this otherwise suitable system.

3.4 REINFORCED EARTH FILL RETAINING STRUCTURES (BOTTOM UP)The reinforcement of placed and compacted soil fill, with some form of protective face or membrane, can provideflexible and cost effective retaining solutions. Geogrid and ‘deadman’ reinforced earth retaining systems areparticularly well suited to the Perth conditions due to the relatively low cost, or on site availability, of suitable cleansand fill. The design of any reinforced earth wall (REW) retaining structure must be based around the relevantparameters for the available fill material and proposed compaction methodology.



These systems are not generally pre-stressed and provide a relatively flexible or soft retaining structure. REW arefrequently constructed laid back at a small angle so that final deflections will, generally, result in a near vertical face.The wall facing is required to provide a durable membrane to prevent erosion and surface sloughing and at the sametime would be selected to give an acceptable aesthetic scheme.

Thiess utilised precast concrete facing panels with steel reinforcement strips to construct the bridge abutment walls forthe Kwinana Freeway Extension Project to the south of Perth. Geogrid reinforced limestone faced walls have been usedon a number of large scale residential developments where the walls were required to look like conventional limestonewalls but the geogrid approach offered a more cost effective solution (Figure 4).

Soft REW retaining solutions can often accommodate surface vegetation and curved sloping construction lines whichcan produce an environmentally acceptable, natural looking or even hidden retaining structure.

Figure 4: Geogrid reinforced limestone wall, Mosman Park, WA.

3.5 GRAVITY WALLSThe gravity wall provides lateral support from its self weight and bearing pressure at the base. Conventional limestonewalls are the most common example in WA. It is essential that the bearing pressure, imposed by the base of the wall onthe underlying founding soil strata, is acceptable and does not cause excessive rotation or settlement of the wallstructure. It is also essential that the wall geometry and mass provide adequate resistance to sliding as these walls donot typically incorporate any significant depth of embedment.

The introduction of Microfine cement grout in WA in the mid 1990s has enabled the insitu formation of substantialgravity retaining structures in clean Perth sands (Figure 5). The grouted block has properties not dissimilar tolimestone, i.e. unconfined compressive strengths (UCS) typically in excess of 2.0MPa, and can be formed prior toexcavation to provide a combined underpinning and ground retention function. It is essential that appropriate controlsare adopted on site as the grout block must be continuous and of the specified size and strength to perform adequately.The grouted option has the significant advantage that extensive over-excavation, as would generally be required toenable construction of a conventional gravity wall, is avoided.



Figure 5: Typical Microfine cement grout block detail.

One solution that is more widely used in other regions where limestone is not so readily available is the use of scraptyres to build walls. The tyres are placed in layers and filled with locally available fill material to form a gravity wall.Ties, running back to buried dead man tyre anchors, can also be utilised where increased lateral stability is required.This form of construction can consume large numbers of scrap tyres that are otherwise difficult and costly to dispose of.The very low material costs must often be offset against the increased construction costs that can occur with a labourintensive construction system.

4 LOCAL CAPABILITY, EXPERIENCE AND RESOURCESThe local capability and expertise, which has been developed by specialist contractors operating in WA, has evolvedspecifically to address the more commonly encountered ground conditions in and around Perth. The widespread use ofMicrofine cement grout, and the combination of lightweight sheet piles with mechanical anchors, both offer very costeffective retaining wall solutions in the Perth sands encountered below the coastal plains. More conventional systemssuch as soldier pile walls, contiguous pile walls, diaphragm walls and soil nailing are all offered by Perth basedcontractors and in many circumstances have also been tailored to suit local conditions.

The advice and recommendation of suitably experienced contractors should be sought early on in the selection processto ensure that the optimum solution is identified and that site specific issues such as access and environmentalconstraints are considered as well as the technical advantages and limitations of the possible solutions.

5 DESIGN ISSUES

5.1 GENERALThe design of any retaining structure can only be effectively addressed if the relevant potential modes of failure andunacceptable serviceability criteria, i.e. principally excessive deflection and/or settlement, are understood and defined.

AS 4678 – 2002, “Australian Standard – Earth-retaining structures”, presents a description of the fundamentalrequirements of the design of a retaining wall and also provides graphical details of potential modes of failure in boththe ultimate and serviceability limit state conditions. This standard does not however present details of acceptedmethods of analysis or design for specific retaining wall structures.



A number of established methods of retaining wall analysis exist that are based on the application of active (Ka) andpassive (Kp) earth pressures on the back and front of the wall. There are a large number of widely used publicationsdealing with the analysis and design of retaining walls and the selection of appropriate earth pressure coefficients. Asthe purpose of this paper is to be specific to Perth conditions these general issues are not considered further herein.

5.2 DESIGN PARAMETERS FOR PERTHThe widespread assumption, in the local construction industry, that most of the Perth area comprises of weaklycemented Perth sand over limestone, frequently leads to reliance on very limited geotechnical site investigations. Insome circumstances, no geotechnical investigation is carried out at all. In these situations it is necessary to adoptreasonably conservative design parameters and base the design methodology and analytical values on experience insimilar conditions. When the wall analysis is based on assumed parameters, it is particularly important that rigoroussite supervision and inspection are adopted to ensure that the conditions encountered on site are consistent with thedesign assumptions that have been adopted.

The design process will essentially require determination of destabilising loads or forces, i.e. overturning moments,horizontal forces and/or earth pressures on the structure and comparison with the restraining loads or forces that can besustained by the retaining structure. An adequate FoS against the ultimate limit state of collapse or failure must bemaintained. There are numerous publications which define appropriate design methods for the various retainingsystems. A number of these are presented below but cannot be addressed in detail here.

5.3 PREDICTION OF DEFLECTIONSSome specialised software packages produced for the analysis of embedded walls, adopt finite element analyticalmethods in an attempt to predict wall deflections during the specified construction sequence. The use of suchpackages,and adoption of reasonably conservative parameters for the Perth sands generally tends to ignore any potentialcementing, i.e. c′ would be set to 0.0 kPa and the soil stiffness would frequently be set at nominally 10.0 MPa/m depth,and so tends to overestimate wall deflections. Comparison between predicted and actual wall deflections on asignificant number of lightweight sheet pile walls in and around Perth indicates that in some circumstances the actualwall deflections are less than 50% of those predicted by WALLAP, one of the widely adopted wall analysis packages.Experience gained using WALLAP on wall analyses in Perth sands indicates that the adoption of higher soil stiffnessvalues, in combination with relatively low stiffness wall elements such as lightweight sheet piles or soldier pile walls,leads to numerical errors in the software and can prevent the analysis running properly.

The prediction of wall movements for gravity and reinforced earth retaining systems would require the use of finiteelement or finite difference, i.e. FLAC methods of analysis. These analytical approaches are rarely used for routinewalls as the value of the works is not large enough to justify the costs associated with the necessary detailed engineeringanalysis. Experience has shown that the following typical deflections can be assumed to represent a reasonablyconservative estimate of wall deflections for passive systems, i.e. where no pre-stressed anchors or pre-loaded struts areused:

Wall Type Maximum deflection NotesSoil Nail Wall Height x 0.003 Passive nails

Microfine Grout Wall ≤ 10 mm Base reaction in the middle third

The values presented above would not apply in non-standard conditions for example where the surface of the retainedground was at a steep batter and/or a significant surcharge load was applied to the retained ground by, for example, anadjacent building.

5.4 GROUND WATERA substantial proportion of all failed or problematical retaining walls are influenced by the action of ground water.Generally, retaining walls are not designed as water retaining structures and excessive deflections, collapse or loss ofstructural integrity can occur rapidly if the ground water is allowed to build up behind a wall. It is crucial that theselection, design, specification and construction of all walls are carried out to address any potential ground waterproblems.

The provision of catch or spoon drains on the retained surface above the wall are often very effective in avoiding anybuild up of water behind the wall that could result from infiltration of rain or down slope run-off water. Weep holedrains and toe drains at the base of the wall are a very simple means of preventing a rise in the water level behind thewall, provided that the system is durable and can be maintained, for example by clearing blockages etc.



As indicated on the WA Water Authority (WAWA) ground water atlas the depth to the ground water is significant, i.e.more than say 7.0 m in much of the area in and around Perth. However any developments in low lying areas that areclose to the river, or existing or backfilled lakes or swamps, i.e. some areas of Northbridge, East Perth and the GrahamFarmer Freeway corridor, must anticipate and accommodate interception of ground water at shallow depths. It is alsoimportant to note that projects located on steep slopes falling towards the river, such as Mount Elisa or the Kings ParkEscarpment, and foreshore areas, such as Dalkeith and Mosman Park, can intercept springs and elevated water tablelevels.

6 PROJECT DETAILS AND RELEVANT COMMENTSThe following brief project details are provided to indicate the locations and circumstances under which some of theretaining wall systems referred to in this paper have been used successfully in and around Perth. In order to avoidexcessive length the details presented have been limited to a brief description and the more relevant issues. Wherenecessary, further details for these and other similar projects should be sought by first contacting the Client forpermission, and then obtained from the specialist contractor and/or consultant responsible for the construction anddesign of these systems.

6.1 SERVETUS STREET SOIL NAIL WALL, SWANBOURNE, (TAMALA SAND – LIMESTONE)The first permanent soil nail wall used on a Main Roads Western Australia (MRWA) highways project (Figure 6). Thesoil nail wall was adopted as an alternative to the original cantilever RC wall as a substantial cost saving was achieved.The adoption of the soil nail system provided an efficient and geometrically flexible retaining wall that avoided theneed for significant, costly and environmentally damaging over-excavation that would have been required forconstruction of the conventional reinforced concrete walls. The two stage soil nail walls achieve an overall retainedheight of up to 10.5 m and adopt nails of up to 8.0 m in length. The excavation revealed soil strata varying from looseuncemented sand to medium strength limestone. The flexibility of the soil nail system, in combination with detailedinspection of each new area of excavation, enabled adjustment of the nail specification to suit the varying groundconditions and thereby ensuring optimisation of the wall.

Figure 6: Servetus Street Soil Nail Wall, Swanbourne.

6.2 MOTOROLA BUILDING, UWA, CRAWLEY, (TAMALA SANDS)The new Motorola research building on the University of Western Australia campus incorporated a two storeybasement. Interlocking, 9.0 m long, lightweight sheet piles were used, in conjunction with two rows of mechanicalanchors, to form the temporary perimeter retaining wall. After completion of the excavation and casting of the new pad



and strip footings the sheet pile wall was used as the back shutter to form the permanent reinforced shotcrete retainingwalls. Permanent lateral support of the completed wall was provided by the floor slabs of the new basement buildingthereby making the sheet piles and anchors redundant as soon as the below ground structure was complete (Figure7).

Figure 7: Lightweight anchored sheet pile wall, Motorola Building, UWA, Crawley.

6.3 ST JOHN OF GOD HOSPITAL, WEST LEEDERVILLE, (TAMALA SANDS)A conventional diaphragm wall with high capacity temporary grouted anchors was used for the permanent boundarywall on part of the development at St John of God Hospital. The diaphragm wall was required to form the permanentretaining structure for a two-three storey basement. This project demonstrates that conventional diaphragm walls canoffer cost effective solutions where permanent high structural capacity retaining walls are required. On this project, andas also well demonstrated on the Northbridge tunnel scheme, diaphragm walls can also offer the benefit of providingsignificant foundation capacity to support vertical loads imposed by buildings constructed on top of the basement wall.

6.4 MICROFINE CEMENT GROUT WALLS (VARIOUS LOCATIONS)Microfine cement grout injected retaining and underpinning walls have been used extensively in and around Perth toprovide temporary support for excavations up to 4.0 m in height (Figure 8). The system is particularly well suited tocircumstances where the new development requires excavation up to the boundary line and an existing building on theadjacent block must be supported. The use of a sheet pile, soldier pile or contiguous pile wall would generally result inlost width of 300 mm to 500 mm which may not be acceptable on a high value and/or narrow site.

Where the required excavation face is vertical and on the boundary line, it is not possible to optimise the grout blocklocation by placing it forward of the wall of the existing building. In these circumstances, and where the retained heightexceeds approximately 2.5 m, it is generally necessary and/or cost effective, to introduce lateral restraint and reduce thesize of the grout block. The lateral restraint can often best be provided with low capacity passive grouted anchors ornails with substantial head plates or waler beams to distribute the load into the grout wall. Raked props can easilyprovide the required lateral support but can be a significant inconvenience when building the permanent retaining wallin front of the grout wall.



Figure 8: Typical Microfine cement grout wall, Cottelsoe.

6.5 KWINANA FREEWAY EXTENSION, PERTH, (TAMALA & BASSENDEAN SANDS)Most of the new bridge abutments for the Kwinana Freeway extension and upgrade to the south of Perth wereconstructed from reinforced earth retaining structures. Precast reinforced concrete panels were utilised to form thepermanent abutment wall facing and deformed steel straps, supplied by The Reinforced Earth Company, were used toreinforce the sand fill. The use of single full height concrete facing panels provided a cost effective and visuallyacceptable abutment wall, but was more sensitive to total and differential settlements than a more flexible segmentalfacing system.

Concerns over potential problems associated with increased predicted settlements at the Mundijong Bridge site, due tothe poor nature of the founding strata, led to the adoption of a piled abutment to avoid assessed problems with thereinforced earth at this location.

6.6 240 ST GEORGES TERRACE, PERTH, (TAMALA SANDS)A combination of perimeter anchored secant pile walling, grouting, sheet pile walls, screw piles and a hybrid soldierpile - grout wall were utilised to overcome access restrictions and program constraints and enable the required retainingfunctions to be provided (Figure 9). One particular problem that required an unusual combination of methods was theunderpinning and ground retaining below an existing shop façade on Hay Street. Construction of the new basementrequired a two storey excavation immediately adjacent to the back of the shop façade that was to be retained. The roofand return walls of the original structure could not be removed before the underpinning, ground retention and wallbracing systems had been installed. The fragile nature of the existing structure, and severe access limitations imposedby the roof and side walls, precluded the use of bored piles and sheet piles for this section of the works.

In order to provide adequate support and restrict anticipated deflections to within required limits of less than 10mm, acombination of an anchored soldier pile wall with Microfine cement grout underpinning and arching blocks was usedbelow the main façade wall. Screw piles and flat jacks were used to form a prestressed underpinning framework tosupport the ends and returns of the main wall during demolition of the rear section of the building. The combination ofvertical underpinning of the façade wall with Microfine cement grout, and the incorporation of prestress loads in boththe wall anchors and underpinning screw piles enabled the excavation to be completed with negligible movement of thefaçade wall.



Figure 9: Hybrid anchored grout and soldier pile wall, 240 St Georges Terrace, Perth.

6.7 HAY STREET & ROBERTS ROAD, SUBIACO, (TAMALA SANDS)Lightweight sheet piles were driven to form the perimeter retaining wall for the two storey basement of thisdevelopment (Figure 10). Conventional mechanical anchors were used on three sides of the site but could not be usedon the fourth side as the close proximity of the Subiaco rail tunnel prevented the installation of anchors with therequired length. A hybrid sheet pile and soil nail wall system was adopted to utilise the sheet piles while achieving therequired stability with short nail lengths. The wall of the rail tunnel was located only 3.2 m from the sheet pile wallalignment and therefore restricted nail lengths to nominally only 3.2 m. In order to maintain adequate short-termfactors of safety, 90 mm nails were installed on a 1.0 m (H) by 1.0 m (V) grid through the sheet piles. The steel sheetsprovided an effective membrane in lieu of the shotcrete facing that would typically be adopted for a soil nail wall. Therelatively high density of short nails effectively reinforced the 3.2 m wide block of soil behind the sheet pile wall andacted as a mass gravity wall. Vertical excavation was achieved to a maximum depth of 5.5 m with recorded walldeflections of less than 8.0 mm.



Figure 10: Hybrid sheet pile and soil nail wall, Roberts Road and Hay Street, Subiaco.

7 SOME CAUTIONARY NOTESThe widespread expectation of some degree of cementing in Perth sands can lead to the adoption of inappropriateexcavation techniques. In the short term, excavations in weakly cemented sands can stand to heights in excess of 3.0 mwith a near vertical cut face. Factors such as vibration, inundation or revised loading conditions, can destroy the weakcementing bonds and cause instantaneous collapse. Any excavation over 1.0 m in depth must be dealt with in anappropriate manner as serious injury or death can result from sudden collapse if site personnel are working in the area infront of the excavation.

In order for an anchored or cantilever embedded wall to maintain adequate factors of safety it is essential that thespecified toe in depth is maintained. Driven walls, particularly with the less robust lightweight sheet pile system can besubject to refusal on limestone pinnacles, Coffee Rock and/or layers of cemented sands. In these circumstances it isessential that prebore, flushing or alternative measures are adopted to ensure provision of the required toe in depth.

The process of forming an insitu Microfine cement grout wall is heavily dependant on operator skill to achieve thepermeation required to provide a grout block of the required size, strength and uniformity. In particular, increased finesand/or organic matter content in the near surface sands, i.e. immediately below any existing footings to be underpinned,can severely restrict the flow of the Microfine cement grout. In extreme cases this can result in voids or gaps in thegrout block which must be re-grouted or repaired with sand cement mortar or concrete before bulk excavation proceeds.Organic material in the sands above the ground water level can delay or reduce the strength gain of Microfine cementgrout in some situations.

The formation of grouted anchors in potentially unstable granular soils can be problematical even when good groutingpractice and cased drilling techniques are adopted. It is essential that all grouted anchors in sands are formed in amanner that ensures continuity of the grout annulus.

Due to the highly critical nature of the role performed by anchors resisting lateral loads imposed by retaining walls, allanchors should be load tested before bulk excavation proceeds to the final design depth.



A retaining wall that has been designed and constructed with adequate factors of safety in terms of stability may exhibitunacceptably high deflections and therefore fail under serviceability criteria. In particular, it is important to note thatany cantilever embedded retaining wall must deflect forward to maintain equilibrium as the excavation is progressed.This situation should preclude the use of any cantilever wall system where the retaining wall is required to limitmovements to existing, adjacent, fragile or brittle structures. The use of pre-stressed or very stiff anchors and or struts,installed sequentially during staged excavation, can significantly reduce wall and associated soil movements.

8 CLOSUREIt is hoped that this overview of some of the specialised ground retaining wall systems that have been developed andused in and around Perth will be of assistance to engineers, architects, builders and developers addressing futureprojects in Western Australia. In particular, reference should be made to other papers in this publication which deal indetail with the anticipated properties of the soils that can be expected in the area. Optimism, based on the ability ofPerth sands to stand unsupported in the short term, must be avoided if collapse or excessive movements are to beprevented.


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