Lindquist_Secant Pile Shoring - Developments in Design and Construction (DFI2011)

8/10/2019 Lindquist_Secant Pile Shoring - Developments in Design and Construction (DFI2011)

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SECANT PILE SHORING DEVELOPMENTS IN DESIGN AND CONSTRUCTION

Eric S. Lindquist, Berti-Lindquist Consulting Engineers, Inc., 920 Country Club Drive, Suite 2A, Moraga,California, USA

Rob Jameson, Malcolm Drilling Company, Inc., 3503 Breakwater Court, Hayward, California, USA

High groundwater levels combined with variable soil and rock profiles presentchallenges to the design and construction of deep excavation support systems.Increasingly, secant piling techniques are the preferred technical solution forshoring in these cases. Recent developments in drilling equipment, tooling andprocedures allow cost-effective construction of deep, overlapped pile systems totight tolerances in extremely difficult ground conditions.

This paper summarizes critical design considerations for secant pile shoringsystems, detailing recent advances in installation procedures and verificationtechniques that allow secant piles to be used for an expanding range of projectconditions. Technically- and commercially-viable secant pile shoring designsrequire that stringent drilling tolerances be achieved in order to successfullyprovide combined structural support and groundwater cutoff systems, particularlyfor unreinforced circular secant pile shafts acting in ring compression. Moderndrill rigs and tooling can advance fully cased shafts in geotechnical conditionswhich range from cohesionless soils below the groundwater table to hard rock,within a single drillhole. Down-hole instrumentation provides verification thattolerance requirements are met and allow secant pile shoring to be used at sitespreviously considered unacceptably risky.

In California, secant pile shoring systems are being used for excavation depths ofover 100 ft (30 m) on projects with challenging site conditions. Two case studiesare presented detailing severe geotechnical conditions and excavation depthswhich represent the regional state of practice in secant piled excavation support.These projects provide real world examples of the design, construction and

construction verification techniques that permit secant pile shoring to be used inmore demanding geotechnical conditions and to greater depths than ever before.

Introduction

Secant pile walls are formed by constructing aseries of overlapping primary and secondaryconcrete-filled drill holes. The primary piles areconstructed first, followed by secondary piles,which are cut into the previously placed primarypile concrete. The amount of overlap requiredbetween the adjacent piles is a function of thestructural design requirements and the

installation tolerance that can be achieved.

Secant pile walls can be used for bothexcavation support and as a relativelyimpermeable cutoff wall. Although they tend tobe more expensive than either sheet piles ordeep soil mix (DSM) walls, secant pilingtechniques can be employed in geotechnicalconditions which preclude these other types ofshoring systems.

Design Considerations

The success of a secant pile system requiresthat the individual piles be structurally soundover their full length, and that adjacent piles beconstructed to tolerances that maintain areasonable overlap between the primary andsecondary piles.

Secant pile walls for excavation support aredesigned to function in one of two ways. Thetypical design approach develops the structuralcapacity of the wall in the vertical direction. Thewall spans between its points of support (e.g.,cross-lot bracing or tiebacks) as a vertical beam.For this type of wall system, the primary pilesare unreinforced, and the secondary piles arereinforced with either a wide flange section or arebar cage. The primary piles are designed to


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act as lagging between the structuralsecondary piles, and the secondary piles aredesigned to have sufficient flexural and shearstrength as beams in the vertical direction.

Secant pile walls can also be designed as ringcompression structures. In this case, the pilesare installed in a circular configuration and loadsare resisted in circumferential compression.This type of structure is structurally-efficientbecause unreinforced concrete has significantcompression capacity. Soil, groundwater andsurcharge loads are resisted by thrust(compression) forces acting in the ring. Thedesign compression strength of the unreinforcedring can be computed using ACI 318 provisionsfor unreinforced concrete, and, although rarelycritical, the buckling capacity of the ring shouldalso be evaluated.

For unreinforced ring compression structures thequality of the pile installation is particularlycritical. The thickness of the effectivecompression ring that can be safely assumedfor design purposes is directly related to the pileinstallation tolerance that can be achieved.Failure to achieve the minimum design overlapbetween the adjacent piles will compromise thestructures ability to perform as designed. Forthis reason, excavation depths for which secantpile compression ring structures have beenemployed have typically been limited to around60 feet.

Recent improvements in drilling equipment andmethods, combined with instrumentation toverify actual installation tolerances, are allowingdeeper excavations to be supported usingsecant piles. These current practices arediscussed in more detail with case historyexamples in the following sections of this paper.

Secant Pile Construction

The requirements of cost and performance mustalways be evaluated when selecting

construction methods for shoring systems. Awide array of techniques is offered within thefoundation industry; however, in many casesexcavation depth and geotechnical conditionslimit the options that can be employed atchallenging sites. The cost for secant pilingtypically exceeds that of deep soil mix, sheet pileor similar watertight shoring systems; however,the versatility offered by secant piling allowseffective construction through highly variable

ground profiles ranging from saturatedcohesionless soils to hard rock, includingadvance through cobbles and boulders andeven man-made obstructions.

For secant piling, installation tolerance is criticalto project success and directly related to cost.When tight tolerances are maintained, pilespacing is maximized, reducing the number ofpiles required along an excavation perimeter.Once the permissible tolerance is defined, thepile installer must select construction proceduresand equipment to satisfy this controllingperformance criterion, while minimizing unitconstruction costs, typically by maximizingproduction rates.

Tolerance is evaluated in terms of both layoutcontrol and drilling verticality. Significant overallcosts savings can be realized by using a guide

trench and templates in order to ensure pilelocation is controlled at ground surface. Forexample, pile quantity can be reduced by 8% to10% in a 40 ft (12 m) diameter shaft if locationtolerance is controlled to within +/-1 inch (25mm) in plan, compared to an industry standardof +/-3 inches (75 mm) for layout based onsurface staking.

Figure 1: Guide Trench

Verticality tolerances of 0.5% (1 in 200) orstricter are typically necessary for secant pilingprojects, compared with standard requirementsof 1% to 1.5% (ACI 336.1) for drilled piers. Pilespacing is maximized for economy, andtherefore successful secant piling projectsrequire exceptional attention to drilling


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procedures, equipment, and quality control toensure overlap is maintained. Drilling methodsand equipment selection are integrally linked inthe construction process.

A range of techniques, including augercast andaxial soil mixing, can be employed to installsecant piles, provided plan layout is controlledand initial rig verticality is confirmed and verified.These high productivity systems combinetechnical performance, fast installation, and lowcost and are suitable for excavations in soil andsoft rock to depths of up to 45 ft (14 m). Pilediameters are limited to 24 to 36 inches (610 to915 mm) for these installation methods, butpotential installation rates are in the range of500 to 1000 lineal feet (150 to 300 m) per shift.The double rotary system of cased augerdrilling can increase viable secant piling depth toaround 60 ft (18 m) while retaining the

competitive benefits of augercast drilling.

For deeper excavations, layout control anddrilling tolerance becomes increasingly critical.Increased pile diameter and improved verticalitycontrol can both increase the viable depth of asecant pile excavation support system.Diameters of 36 to 48 inches (915 to 1220 mm)are typically employed for 50 to 100 ft (15 to 30m) deep shoring systems. However, due toincreased unit costs associated with largediameter piles, the construction methods mustaim to optimize verticality in order to minimize

pile quantity and corresponding overall projectcosts.

Sectional heavy wall drill casing, advancedconcurrently with the drill tool, performs the dualfunction of maintaining boring stability incohesionless or unstable ground and stiffeningthe drill string in order to limit deviation at depth.Kelly drilling methods allow a range of soil androck tooling to be employed within the casingssuch that different tools can be utilized toaccommodate variations in ground type as thedrill hole is advanced. Casing teeth

configurations can be modified between or evenwithin project sites to suit actual groundconditions.

Top drive rotary crawler drills are ideally suitedto secant piling and can rapidly advance drilltools concurrent with 24 to 48 inch (610 to 1220mm) diameter casing while maintaining strictverticality tolerances. Numerous commerciallyavailable rigs with torque in the range of 150 kip-

ft to 195 kip-ft (200 to 250 kN-m) are suitable forpile depths up to 60 ft (18 m). Greater piledepths require enhanced equipment capabilitiesfor efficient advance rates and handling ofcasing and drill tools. Machines with torque inthe range of 260 to 295 kip-ft (350 to 400 kN-m)are usually employed for drilling to depths of 100ft (30 m). Production rates can be in the rangeof 150 to 300 lineal feet (45 to 90 m) per shift.

In parallel with the evaluation of rig torque, drillselection should consider the weight of casingthat can be efficiently handled by the machine.For depths exceeding 100 ft (30 m), anddiameters in the range of 42 inches (1065 mm),oscillator attachments are required to assistcasing advance and extraction due to the self-weight of the drill string. Oscillators increase therange of constructible pile diameters and depths,but limit production rates with resultant cost

implications.

Figure 2: Drill Rig and Casing Oscillator

Secant piling systems are rarely employed forexcavation support at depths exceeding 100 ft(30 m). In order to maintain overlap whileaccounting for drilling deviation at this depth,large pile diameters are required, and in turnsecant piles become prohibitively expensive.However, modern drilling equipment such as the

Bauer BG50, with 350 kip-ft (470 kN-m) oftorque and 130 kip (600 kN) winch capacity, canconstruct piles to a depth and diameterexceeding any conventional commerciallimitations. The recent Transbay Terminal testprogram in San Francisco demonstrated thecurrent limit state of practice by constructing 7.2ft (2.2 m) diameter secant piles to a depth of 230ft (70 m). The sectional casing was advancedusing a rotator system, with 150 ton excavation


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and support cranes employed to achieve aproduction rate of approximately one pile perweek.

Installation Verification

Until recently, there was no ready means ofevaluating the verticality of a drill hole prior to itbeing exposed in an excavation. Therefore, itwas necessary to make reasonably conservativeassumptions about the verticality tolerance onwhich a design could be based.

Downhole survey techniques that permitmeasurement of both the diameter andplumbness of a drill hole are now available. Forexample, the Sonicaliper is a sonar device thatprovides a 360 degree profile of the drill hole atany depth. It can be used for surveying drilledshafts as well as slurry wall panels. The

instrument is able to make measurements in adry hole, under water, or even through polymeror bentonite slurry. Survey readings can betaken in a cased or uncased hole.

Figure 3 is an example Sonicalipermeasurement at a depth of 70 ft (21 m) in a 36-inch (900 mm) drill hole. The dashed green lineand light crosshairs illustrate the theoreticalposition and diameter of the hole. The surveyedhole location and diameter are shown by theblue circle and dark crosshairs. In this example,the downhole measurements indicate that the

hole is offset from its theoretical location by 1.9inches (48 mm).

Figure 3: Sonicaliper Measurement

The Inclination Measuring Instrument(inclinometer) manufactured by Bauer providesan alternate means of surveying cased drillholes. The instrument is similar to aninclinometer probe that is used for measuringlateral ground movements in a small diameterslotted casing, although at a much larger scale.

Downhole surveys permit tighter installationtolerances to be used for design because theas-built pile locations can be confirmed prior toexcavation. If the surveys indicate a potentialproblem, holes can be redrilled or otherproactive corrective action can be taken at aspecifically targeted location.

Recent Projects

Mormon Island Auxiliary Dam Key Block

The Mormon Island Auxiliary Dam (MIAD),located about 20 miles (32 km) northeast ofSacramento, is a 4800 ft (1460 m) long, 110 ft(34 m) high earthfill dam that helps impound the

American River to form Folsom Lake. TheUnited States Department of Interior, Bureau ofReclamation has concluded that some of thedownstream foundation soils are susceptible toliquefaction during a large earthquake. TheBureau designed a Key Block to mitigate thepotential problems resulting from thisliquefaction. The Key Block is a 55 ft (17 m)wide by 900 ft (274 m) long area at the toe of the

existing dam from which the foundation soils willbe removed down to and keyed into bedrockand then replaced with lean concrete andengineered fill. The Key Block excavation isspecified to be performed in segments notexceeding 150 ft (46 m) in length with no lessthan 300 ft (91 m) of unexcavated length inbetween. Hence, the Key Block will beconstructed in a total of seven excavation bays;a test bay followed by six longer productionbays.

The soil conditions at the Key Block consist of

dredged alluvium overlying a thin layer of clayeycolluvium, which in turn overlies variablyweathered amphibolite schist bedrock.Historically, the alluvium was dredged a numberof times for its gold content. The majority of thedredge tailings are poorly graded to silty sandand gravel with cobbles and occasionalboulders; however, finer-grained silty sand withsome gravel and clay is found near the base ofthe deposit. Bedrock is anticipated to be


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encountered at depths ranging from about 52 to78 ft (16 to 24 m) below grade. Thegroundwater table at the Key Block is near theground surface, and the project design criteriarequire the shoring system be designedassuming groundwater at grade.

Previous attempts at improving the seismicperformance of the downstream foundation soilsincluded the installation of thousands of stonecolumns, and a test section in which an attemptwas made to treat the soils with jet grout. Thestone columns and jet grouted mass will beencountered during the construction of the KeyBlock.

A reasonably dry, shored excavation is requiredso that the moderately weathered bedrock at theKey Block subgrade can be cleaned andinspected prior to the placement of the lean

concrete. Further, the excavation shoringsystem must accommodate excavation of up to8 ft (2.4 m) into bedrock. Therefore, the shoringwall is required to penetrate into bedrock forboth toe support and groundwater cutoff. Thetwo diaphragm wall options that wereconsidered feasible for the challenging soil androck conditions at this site are secant piles andslurry walls. A secant pile wall was determinedto be the most cost-effective solution for thisproject.

The shoring wall design is based on 3.28 ft (1 m)

diameter secant piles installed at either 2.25 ft(690 mm) or 2.0 ft (610 mm) center-to-centerspacing, depending upon the depth to bedrock.The design assumes that a verticality toleranceof 0.5% is achievable. The 2.0 ft (610 mm) pilespacing will be employed in locations where thedepth to bedrock exceeds 70 ft (21 m) to allowfor drilling tolerance and to increase thestructural capacity of the wall where theexcavation will be deepest. The primary pilesare unreinforced 3000 psi (21 MPa) concreteand the secondary piles are reinforced withW24x94 sections over their full depth. The

design considers the wide flange beam actingcompositely with its surrounding concrete.

The design loads for the shoring system are soilpressure (including the substantial lateral

surcharge from the adjacent dam), groundwaterpressure, construction surcharge, and a seismicincrement based on an earthquake with a peakground acceleration of 0.20g.

Up to five levels of crosslot bracing will beinstalled in each excavation bay. The bracingframes have bolted wale splices and strut-to-wale connections so the frames to be easilyassembled and disassembled and thecomponents can be used in different size bays.Figure 4 is a typical cross-section through thefull depth Key Block excavation.

Work on the secant pile wall at the test baystarted with the construction of a cast-in-placeconcrete template in order to accurately definethe geometry of the secant pile wall at theground surface. A total of 96 secant piles wereinstalled at the test bay. A Bauer BG40 drill rig

advanced flush bolted sectional casing intobedrock. Concrete was placed using the tremiemethod, and then casing was extracted. TheW24 reinforcing beams were wet set into theconcrete-filled secondary piles.

Drilling conditions proved to be extremelychallenging at the test bay. Cobbly soil, existingstone columns, and hard rock conditionsresulted in exceptional levels of wear on the drilltooling. Casing through the cohesionless,saturated alluvial soils was essential.

At the test bay, bedrock was encountered atdepths of between 53 and 61 ft (16 to 18.5 m),and the secant piles were drilled to a total depthof 71 to 80 ft (21.5 to 24 m).

Excavation commenced after the secant pileconcrete was allowed to cure for a minimum oftwo weeks. Figure 5 shows the test bayexcavated to a depth of 52 ft (16 m) with thefourth level of bracing in the process of beinginstalled. The as-built secant pile wall hasproven to be relatively watertight, with onlyminor local weeping at the interfaces between

adjacent piles. As the secant piling is exposed,the accuracy of drilling and layout control hasbeen confirmed by observation and performanceof the system.


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Figure 4: Typical Section at Key Block

Figure 5: Key Block Excavation at 52 ft (16 m)

New Irvington Tunnel Vargas Shaft

The 3.5 mile (5.6 km) long New Irvington Tunnelwill provide a seismically sound alternate to theexisting conveyance tunnel connecting the San

Francisco Public Utility Commissions (SFPUC)water sources in the Sierra Nevada and

Alameda County to the Bay Areas water supplysystems. The project includes a 41 ft (12.5 m)inside diameter, 115 ft (35 m) deep temporaryshaft to create access for 13 ft (4.0 m) diametertunnel drives in two directions. The shaft isadjacent to the I-680 freeway at Vargas Road inFremont, California.

The soil profile at the shaft consists of 20 to 35 ft(6 to 11 m) of fill and colluvium (sand with graveland cobbles and medium stiff clay) overlying

bedrock. The groundwater level was anticipatedaround 14 feet below the top of shaft elevation.Bedrock was expected to be intensely tomoderately fractured, weak to moderatelystrong, with shear zones.

Ground support for the shaft was originallyenvisaged as secant piles penetrating a few feetinto bedrock, below which rock dowels andshotcrete were to be installed in a top-down


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manner. However, when the first secant pilewas drilled it became evident that the installationof rock dowel and shotcrete support would bevery challenging to a depth of approximately 95ft (29 m). The shaft support design wasmodified to extend the secant piles intocompetent bedrock, essentially drilling to the fulldepth of the shaft. The resultant use ofunreinforced secant piles as a stand-alonecompression ring shoring system for a 100 ft (30m) deep excavation is unprecedented.

A total of 76, 3.28 ft (1 m) secant piles at 1.83 ft(560 mm) center-to-center spacing form acompression ring as shown in Figure 6. Thesecant piles are unreinforced 3000 psi (21 MPa)concrete. A 1.5 ft (460 mm) minimum thickeffective compression ring is required to providesufficient capacity to resist the specified shaftdesign loads. This requires a minimum overlap

between the adjacent piles of around 5 inches(130 mm). To ensure sufficient pile overlap at adepth of 80 ft (24 m), the piles were specified tobe installed within 1 inch (25 mm) of their

theoretical location at the ground surface andwith a deviation from plumb of no more than0.5%. Even tighter tolerance needed to bedemonstrated in order for the shaft support tofunction beyond a depth of 80 ft (24 m).

A guide trench was provided to set the secantpile locations at the ground surface. A BauerBG40 drill rig was used to drill 115 ft (35 m)deep holes that were cased to a depth of 100 ft(30 m). The BG40 proved to be up to the task ofcutting through the structural concrete primarypiles during the installation of the secondarypiles. The concrete mix was modified during theinstallation process in order minimize earlystrength gain while maintaining the specifiedminimum compressive strength.

Every hole was surveyed using the Sonicaliperat depths of 60 and 100 ft (18 and 30 m). The

downhole surveys indicated that all the holeswere well within the specified verticalitytolerance.

Figure 6: Vargas Shaft Secant Pile Layout


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The secant piles were tremie concreted with thecasing progressively extracted as the concretewas placed. The volume of concrete placed ateach hole was typically around 45 cubic yards(34 cubic meters), which is about 25% morethan the theoretical volume of a 115 ft (35 m)deep, 3.28 ft (1 m) diameter hole.

During excavation, observations of the as-builtpiles indicated that, for the most part, installationtolerance was outstanding. The shaft wassuccessfully supported to a depth of 100 ft (30m) using secant piling without supplementalsupport.

Figure 7: Vargas Shaft at 90 ft (27 m)

Recent construction at the Deephams SewageTreatment works in England employed 4 ft (1.2m) secant piles to form a 62 ft (19 m) diameter

shaft to a depth 79 ft (24 m) (Hayward, 2011).The Deephams project included reinforcingcages within secondary piles, and supplementalcast-in-place concrete compression rings withinthe shaft, in contrast to the stand aloneunreinforced pile system constructed at theVargas Shaft. Both shafts were constructedusing the same type of drilling equipment, aBauer BG40 rig working with sectional heavywall drill casing.

Conclusions

Recent project experience has proven thatappropriate drilling equipment and methods canachieve installation tolerances that allow secantpile shoring systems to be used in geotechnicalconditions and for excavation depths previouslyconsidered to be infeasible. Modern drill rigsand tooling allow cost effective installation ofsecant piling excavation support systemssuitable for depths of up to 100 ft (30 m). Stateof the art equipment enables construction ofpiles with diameters and depths that far exceedlimitations of currently commercial viableprojects. Downhole survey techniques allowdesigners to confirm critical tolerances are met,refining design efficiency and enhancing cost-effectiveness of secant piling solutions.

At the Mormon Island Auxiliary Dam Key Block

project, secant piles were installed successfullythrough saturated, cohesionless soils withcobbles, boulders and into hard rock. The workcompleted on that project to date demonstratesthat secant piling can be utilized in extremegeotechnical conditions which precluded the useof almost all other foundation techniques. At theVargas Shaft, an unprecedented excavationdepth was supported using an unreinforcedsecant pile compression ring withoutsupplemental support.

References

ACI 318-08, 2008. Building Code Requirementsfor Structural Concrete.

ACI 336.1-01, 2001. Specification for theConstruction of Drilled Piers.

Hayward, D., 2011. Pushing the boundaries,Ground Engineering, February, pp. 10-11.

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Lindquist_Secant Pile Shoring - Developments in Design and Construction (DFI2011)