Deep Basement ConstructionAn Auckland Case Study

J Grant MurrayKingston Morrison Ltd, New Zealand

Abstract

Parklands Apartments is a sixteen storey hotel development on the fringe of Auckland’s CBD with thirteenstoreys above ground and three basement car park floors. Within the constrained site there was a steep cross fallsuch that the excavation on the road frontage was the full three storeys deep whilst at the back of the site theexcavation was limited to a single storey.

To facilitate the construction of the basement and satisfy the concerns of adjacent property owners threedifferent construction methods were adopted; bottom-up, top-down and open excavation. Soil nailing wasutilised on two elevations to provide temporary support as the excavation progressed to the basement formation.On one elevation soldier piles were installed and the upper basement floors constructed to provide a proppingaction in advance of the excavation. The final, least critical elevation, was formed in open cut with slopesbattered to provide temporary stability during construction.

The paper describes the preliminary investigations and basement construction techniques adopted. Load testresults on prototype and production soil nails are presented. The influence of installation methods and theperformance of the nails in terms of load displacement and wall deformations are discussed.

Keywords:Deep basement, soil nailing, ground deformations

Deep Basement ConstructionAn Auckland Case Study

J Grant MurrayKingston Morrison Ltd, New Zealand

Summary.

1. INTRODUCTION

The purpose of this paper is to describe thegeotechnical engineering experience gained in thedesign and construction of a deep basement incentral Auckland.

Parklands Apartments is a 16 storey developmentwhich includes three basement levels of car park.The maximum excavation below existing groundlevel to the basement formation is some 10m. Onthis constrained site three different techniques wereadopted to retain the stability of the excavation.

The paper briefly describes the project history andthe restrictions placed on the development by it’slocation and relationship with neighbouringproperties. A summary of the ground investigationsundertaken is presented with the development ofthe concept and detail design of the basement wallretention system. The performance of the soilnailing solution adopted is discussed with someobservations on the implication of the installationtechniques.

2. BACKGROUND

The market for high rise apartment blockdevelopments in Auckland has been quite volatile.In 1995 Kingston Morrison provided some conceptarchitectural and structural advice to a developerwith a prime city centre site.

Over a period of two years KM continued toprovide structural and foundation engineeringdesign advice and even prepared concept drawingswhilst investors were sought and a commitment topurchase apartments was obtained, primarily fromthe Asian market.

In mid 1997 a commitment to proceed was made andthe client group invited tenders in a modified designbuild format. Contractors were asked to preparebids based on the concept designs and KM werenovated as Design Engineers.

3. SITE DESCRIPTION

The site is located on the east side of Grey’sAvenue in central Auckland and backs on to Myers

Park (Fig 1). The site, occupied by a hostel andassociated car parking, sloped steeply from front toback with an overall fall of approximately 5-8m.Grey’s Avenue also slopes from south to north,therefore the highest ground level was on the frontelevation, adjacent to a Jewish Community Schooland Synagogue.

The geology of central Auckland is dominated bythe presence of the Waitemata Series, a Miocenedeposit comprising interbedded siltstone andsandstone with a distinctive weathering profile.Overlying this in places are late Pliocene andPleistocene deposits of generally firm to stiff siltyclay and clayey silts which are often difficult todistinguish in borelogs from the underlying residualsoils of the completely weathered Waitemata’s. Inmany places the Auckland Volcanic Formationunconformably overlies these deposits but not inthe immediate vicinity of the site.

4. GROUND INVESTIGATION

When the project was committed to proceeding ageotechnical investigation was undertaken on thesite comprising three simple rotary cored boreholesput down in the accessible corners of the site.

During the drilling in situ SPT’s and undisturbedsamples were taken in representative strata. Handheld shear vanes were performed in the end of thecore barrels (although, due to extreme sampledisturbance, this provided an unreliable measure ofshear strength).

The ground conditions were largely as anticipatedand the laboratory work was restricted to simpleclassification tests. Budget constraints preventedan exhaustive or rigorous laboratory testing regimebut a single triaxial extension test was attempted in asample recovered from the residual soils and aprofile of UCS values was obtained by testing corerecovered from the weathered profile of theWaitemata’s (Table 1).

Although the ground investigation data wasavailable no detailed foundation or retaining walldesign was undertaken in advance of the tendernegotiations.

Table 1Material Su

kPac’kPa

∅’ UCSMPa

Unit I (Crust) 90-150

5 22.5 N/A

Unit II (ResidualSoil)

50-120

10 25 N/A

Unit III (CWWaitemata Sandst)

0 0 35 N/A

Unit IV (Weath’dWaitemata)

0 15 35 N/A

Unit V (Waitemata) N/A N/A N/A 0.5-2.0

A simple pile design concept was developed for thepurpose of obtaining a competitive bid from thetenderers. The tenderers were also advised that thepreferred basement retention system would involvea general arrangement of soil nails in a bottom-upconstruction sequence. In other words, excavate tobasement formation, provide temporary support bysoil nailing and then build the structure from thebasement - up.

The contract was ultimately awarded to HartnerConstruction Ltd (HCL) and their soil nail sub-contractor Construction Techniques Ltd (Contech).The contractors opted to put down some additionalinclined bores from within the site boundary to adepth of 15m below the road on the Grey’s Avenueelevation. Their objective was to check the soilconditions outside the site boundary into which thesoil nails would be anchoring.

5. BASEMENT CONSTRUCTION

The proposed development occupies the fullsection and in order to maximise the number of carpark spaces the basement wall construction neededto be as slender as possible.

Although there were three basement levels on theGrey’s Avenue elevation, due to the sloping natureof the site, the depth of excavation on the MyersPark elevation was barely the equivalent of one level(≈2.5m).

Following some negotiation with Auckland CityCouncil Parks Department it was agreed that theMyers Park excavation could be formed in open cutby obtaining permission to batter back a temporaryslope into the park grounds.

In order to proceed with the excavation andfoundation construction a Resource ConsentApplication had to be lodged with the AucklandCity Council. At that time it became clear that therewas a significant objection to the construction ofthe basement from the Jewish Community thatowned the property immediately to the south.

The school and synagogue were large brick builtstructures founded on piles and built in the 1980’s.The owners appointed an Engineer who advisedthem that it was a relatively brittle structure thatwould be susceptible to damage should there beany lateral ground movement resulting from thebasement excavation.

The Jewish Community objected to the use of thesoil nails on the basis that, in principle at least, theywere a “passive” system which mobilises capacityas the ground yields. Their Engineers expressedsome concern over the performance of the soil nailretention system and therefore two otheralternatives were presented for their considerationin order to expedite the consent process.

The alternative retention systems proposed for thesouth elevation were:-

• A variation on the bottom-up constructiontechnique using soldier piles and excavating tobasement formation providing tensioned groundanchorages as tie backs.

• Top-down construction using soldier piles andexcavating in stages leaving a sufficient soilbuttress or berm in front of the wall until suchtime as the permanent floor slabs provide thepropping action necessary to support the wall.

The second option was favoured by thecommunity’s Engineer on the basis that it avoidedthe installation of any temporary works oranchorage system under their property.

To maximise the basement parking areas it was stillnecessary to retain the excavations on the north andwest elevations by temporary soil nails. Beingforced into this mix of top-down and bottom-upconstruction caused considerable planning andprogramming difficulties.

6. SOIL NAIL DESIGN

The soil nail design was favoured because itenabled the excavation to be taken out to theproperty boundary. The reinforced spray concretefinish was also incorporated into the design of thepermanent works as the outer face of the basementwalls.

Although the nails were only required to providesupport during construction and were primarilydesigned as temporary works it was consideredprudent to check their capacity to support theexcavation long term in the event that the projectwas delayed at some point after constructioncommenced.

A limit equilibrium stability analysis was carried outon critical sections of the excavation using the soilprofiles and groundwater conditions identified fromthe boreholes put down on the site perimeter.

The undrained shear strengths of the Pleistocenedeposits and Residual Soils were stronglyinfluenced by the moisture conditions. Thedesiccated near surface soils were stiff to very stiff(su ≈ 100 - 150 kPa) but with depth, immediatelyabove the more distinctive weathered WaitemataSeries, the Residual Soils had a higher degree ofsaturation and were generally only firm (su ≈ 50 - 75kPa).

The upper unit of the completely weatheredWaitemata Series was characterised by a persistentlayer of sandstone which, although retaining thefabric of the parent rock, was logged in theboreholes as a medium dense to dense uniformlygraded fine to medium sand. Since this unit wouldbe exposed at the base of the excavation there wassome concern about the local stability effectsparticularly since the sands were believed tosupport a perched water table.

The stability analysis identified critical slip surfacesand using the SLOPEW software package it waspossible to incorporate into the section analysismultiple anchors representing the soil nails.

Using an assumed value of the grout/ground bondit was possible to identify the length of nailsrequired to provide the required stability (Fig. 2).

As would be expected the effective stressconditions dictated the length of the nails. TargetFS values of 1.5 were defined as the acceptancecriteria based on the NZ Building Code (Ref. 1).This was considered to be conservative since theconstruction of the basement would be completeand the nails would be redundant long before thenegative pore pressures set-up by the excavationwould be dissipated.

It was considered that attempting to debate with theconsenting authorities the use of a less onerousstability criteria for a temporary works situation wasnot a viable option when the subsequent delay inreaching an agreement would have a greater impacton programme and budget than would have beensaved in the design or construction costs.

A variety of hand calculation checks wereundertaken to validate the general arrangement ofthe soil nails using wedge analysis (Ref 2) andguidelines identified in BS8006 (Ref 3) and SoilNailing Design by Jewell (Ref 4).

Due to the presence of buried services in thefootpath on Grey’s Avenue the top row of soil nailswas replaced with a row of deadman anchors. Atthe proposed nail head positions a tensioned cablewas fixed to embedded anchor plates and tied backto timber piles at 3.0m centres on the kerb line.

7. SPECIFICATION AND TESTING

The specification for the soil nails was performancerelated. There was no prescriptive definition of theinstallation technique to be adopted, materialspecifications were not included and the effectivenail diameter was left to the contractor’s discretion.

The basic requirement of the specification was toprovide a nail of specified length with a minimumbond criteria (design working load) of 15kN permetre length of nail.

The design drawings described the generalarrangement of the nail heads, shotcrete thickness,reinforcement mesh and drainage details.

The specification called for the contractor to installat least one prototype nail using his proposeddesign and production techniques and demonstrate,by testing the nail to failure, that the bond criteriawould be satisfied. During construction thespecification called for the contractor to proof load10% of the production nails. Any production nailswhich failed the proof load test would be down-rated and replaced with new nails at mid span whichwould also be subjected to proof load tests.

The soil nailing sub-contractor, Contech, wereconfident of obtaining the minimum capacity fortheir proposed nailing system comprising a 25mmdeformed MS rebar grouted into an effective borediameter of 150mm. This was perhaps optimisticsince in order to meet the specification the requiredminimum unit grout/ground bond stress (τ) for thissystem is approximately 65kPa. Commonly quotedvalues of τ = 0.3 to 0.6su (Ref 5 & 6), therefore thenails would have to be wholly embedded in a stiff tovery stiff residual soil to meet the design criteria.

The contractor elected to install three prototype testnails but proceeded with the installation ofproduction nails in advance of testing theprototypes.

The results of the pull-out tests on the prototypenails are presented in Figure 3 as load displacementgraphs. The tests were deemed unsatisfactory sincethe required ultimate capacity of the prototype nailswas 90kN (Fs = 2) and the contractor was required toundertake further trials.

Proof load tests on production nails which hadalready been installed suggested that the requirednail capacity was not being achieved.

The finger of suspicion was pointed at the “washboring” method of installation adopted by thecontractor to aid production rates. In the opinion ofthe author, drilling fluid pushed down the hole toassist in the recovery of the clay cuttings wasseverely disturbing the grout contact surface andleaving a residue of very low strength material onwhich to bond.

Without wanting to abandon their preferred drillingmethod Contech elected to improve the nail pull-outcapacity by post-grouting. Test results onprototype and production post-grouted soil nailsare presented in Figure 4 and demonstrate theimprovement in load displacement performance forsimilar length nails.

Grout take volumes during installation weremonitored by Contech. Based on the submittedconstruction records it was possible to establish atheoretical average effective diameter for the soilnail over it’s known length.

Effective diameters varied from 170mm to 320mm(mean effective dia. = 200mm cf theoretical bore dia.≈ 150mm) and reflects the degree of disturbance thatmay been experienced by the soils during drilling.

8. WALL/NAIL DISPLACEMENTS

The specification (and Consent Conditions) calledfor the excavation to be monitored. The limits ofacceptable lateral displacement were set at 0.2% ofthe retained height or excavation depth.

The contractor opted to monitor ground movementsby accurate surveys on the nail heads. This led tosome discrepancies since the nail head weresurveyed and then, in some instances, subjected toproof load testing.

The installation of the soil nails started on the northwall elevation. During the installation of the secondrow of nails on this wall a failure occurred whichwas the result of a combination of circumstancesincluding high rainfall and the presence of aservices trench in the anchoring zone. As a result,the displacement records on the north wall were lessreliable than the those on the west wall which was,in any event, the more critical elevation.

Table 2 presents a summary of the displacementmonitoring results which indicates borderlinecompliance with the displacement criteria.

Table 2West Wall Measured Allowable % >

Location disp. disp. AllowableW1/9 22 20 10W0/12 25 18 39W0/15 24 18 33W1/15 21 18 17W3/15 18 18 0W0/18 21 17 24W1/18 22 17 29W3/18 17 17 0W1/21 23 16 44W0/24 18 16 13W1/24 17 16 6W0/27 14 14 0W1/27 18 14 29

It is possible that additional lateral displacementwas observed as a result of the installation effects.Two possible mechanisms are:-

1. There was a zone of softened soil around thegrout annulus which yielded more to mobilisethe nail capacity than would have been expectedfrom an intact or less disturbed stiff clay.

2. The post grouting caused a local bulb or bell ofgrout around the nail column which mobilisedpull out capacity by bearing rather than shaftadhesion.

Despite the apparent wall displacements there wereno obvious or visible signs of movement or distressin the wall or retained ground surface.

9. PROJECT OBSERVATIONS

In hindsight the conclusions which can be drawnfrom the basement construction experience at Grey’sAvenue can be summarised as:-

1. It is important that the consent of all propertyowners on adjacent sites is obtained well inadvance of preliminary design.

2. It must be appreciated that in negotiations withproperty owners and statutory authorities theirdecision making processes may not be governedby rational technical argument.

3. The installation techniques for soil nails willhave a significant impact on capacity. In stiffhigh plasticity residual soils, significantdisturbance should be anticipated.

4. At least three prototype nails should be testedto failure in advance of the installation of anyproduction nails and all production nails shouldbe proof loaded.

5. Displacement criteria based on 0.2% of the wallheight is unreasonable since the apparentmeasured displacements in excess of this criteriahad no impact on the wall or adjacent ground.

10 ACKNOWLEDGMENTS

The efforts of Hartner Construction Ltd andContech Ltd in handling the difficulties of workingon such a constrained site and being forced intoadopting a complex construction sequence areacknowledged. Their contribution to providing thefactual data used in the preparation of this paper isappreciated.

Thanks are also due to Simone McDonnell who putmuch effort into the collation of information andsupervision of the soil nail and foundationinstallation.

11 REFERENCES

1 BIA, (1993) New Zealand Building Code,Verification Method B1/VM4

2 Dept. of Transport (1994), Design Methods forthe Reinforcement of Highway Slopes byReinforced Soil and Soil Nailing Techniques,Design Manual for Roads and Bridges : Pt 4,HA 68/94. HMSO

3 BSI, BS8006 : 1995 Code of Practice forStrengthened/reinforced soils and other fills.

4 Jewell (1989), Reinforced Soil: Constructionand Specification, Soil Nailing Design.University of Oxford CPD Programme.

5 BSI, BS8081 : 1989 British Standard Code ofPractice for Ground Anchorages.

6 Barley (1988), Ten thousand anchorages inrock. Reprint from Ground Engineeringeditions Sept, Oct, Nov 1988, TTL