Pier Pressure in Natchez, MississippiPost Grouted Drilled ShaftsPass Test

by Mike Muchard, P.E.Applied Foundation Testing, Inc.

Introduction

Post grouting of drilled shaft tipsas a means of increasing capacity andquality assurance is gaining greatpopularity from designers and con-tractors alike. ADSC Associate Mem-ber firm, Applied Foundation Test-ing, Inc. has recently been involvedwith five such projects, all of which,including the Natchez Trace Park-way, have given drilled shafts a newcompetitive edge.

The team of Wilbur Smith Associ-ates and Hill Brothers, Inc. wasselected by the Eastern Federal Lands(EFL) Highway Division of the FederalHighway Administration (FHWA) todesign and build the project. Theproject entailed construction of thefinal 4.3 mile segment of the NatchezTrace Parkway. Seven new bridgestructures were required to completethe highway construction, one ofwhich was a 1,700 foot long bridgecrossing Catherine Creek and Melvin

Bayou. The foundations of thisbridge will potentially be subjectedto 30 to 65 feet of scour, requiring

the use of 6 foot diameter drilledshafts. ADSC Contractor Member,A.H. Beck Foundation Co., Inc., theteam’s drilled shaft specialty contrac-

tor, proposed theirpatented postgrouted drilledshaft process as acost saving andvalue added alter-nate to the speci-fied conventionaldrilled shafts.Applied Founda-tion Testing, Inc.provided turn-keypost grouted shaft

design engineering, load testing andpost grouting services.

The successful implementation ofthe alternate included performingpost grouted drilled shaft design cal-culations, installation and perfor-mance of a load test program, estab-lishment of a production groutingcriteria and quality assurance pro-gram.

Using a post grouted shaft designmethodology developed by The Uni-versity of South Florida and theFlorida Department of Transporta-tion, shaft length reductions as muchas 35 feet could be attained by grout-ing the shaft tips in the founding siltysoil materials.

To corroborate the design, an outof position test shaft was installed,

COVER FEATURE

Perhaps the greatest benefit of postgrouting drilled shafts is the unparal-leled quality assurance of knowing theside shear and end bearing capacity ofevery shaft to a level proportional to theapplied grout pressure.

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post grouted and load tested with a1,600 ton STATNAMIC axial com-pression load test. After the load testprogram, shaft tip elevations wererefined and grouting criteria wereestablished.

Perhaps the greatest benefit of postgrouting drilled shafts is the unparal-leled quality assurance of knowingthe side shear and end bearing capac-ity of every shaft to a level propor-tional to the applied grout pressure.

Soil Conditions

The regional geology consists ofalternating layers of sands, silts, andclays of the Catahoula formationoverlain by a loess formation. TheLoess formation is a deposit of windblown silt with SPT N-values gener-ally less than 25 bpf. This materialhad a unique characteristic of form-ing vertical faces when cut or eroded.The Catahoula formation underliesthe loess formation which consists ofdense to very dense sands, very stiffto hard clays, and both elastic andinelastic silts. SPT N-values rangedfrom 49 to 100+ bpf.

Post Grouted Shaft Design Method

The end bearing component ofdrilled shafts is not fully utilized in

most design meth-ods due to thelarge displace-ment required tomobilize ultimatecapacity. Conse-quently, a largeportion of the ulti-mate capacity nec-essarily goes un-used. In an effortto regain some ofthis unusable ca-pacity, mechanis-tic procedures tointegrate its con-tribution havebeen developedusing pressuregrouting beneath

the shaft tip. The post-groutingprocess entails: (1) installation of agrout distribution system duringconventional cage preparation thatprovides grout tube-access to thebottom of the shaft reinforcementcage, and (2) after the shaft concretehas cured, injection of high pressuregrout beneath the tip of the shaftwhich both densifies the in-situ soiland compresses any debris left by thedrilling process. By essentially pre-loading the soil beneath the tip,higher end bearing capacities can berealized within service displacementlimits.

The design approach for postgrouted drilled shaft tips developedby The Uni-versity ofSouth Flori-da makes useof commonparametersfor a conven-tional drilledshaft design.In this me-thodo logy,the availableside shear isan importantstep in deter-mining thepressure towhich the

grout can be pumped to the base. In-terestingly, the grouted end bearingcapacity is strongly dependent onavailable side shear. However, it isrelatively independent of theungrouted end bearing capacitywhen in sandy soils. As such, the endbearing in loose sand deposits can begreatly improved in both stiffnessand ultimate capacity. In silts andclays significant improvement instiffness can be realized resulting ingreater usable end bearing capacity.In all soil types, post grouting shafttips provides a capacity verificationto a level proportional to the appliedgrout pressure for every shaft grout-ed.

Foundation Description

The 6-foot diameter shafts weredesigned and constructed to resist adesign load of 675 tons beneath the100 year scour elevation. Due to theheightened quality assurance fromload testing and post grouting everyshaft, a safety factor of 1.8 wasapplied to the design load. Consider-ing all design conditions, post grout-ed shaft lengths ranged from 61 to100 feet. In addition, USF’s designmethod predicted grout pressuresranging from 275 to 325 psi toachieve an ultimate capacity of 1215tons in the scoured condition.

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Regardless of the post groutingimprovement with respect to axialload resistance, lateral capacitydemands for the design scour condi-tion dictated a minimum shaftlength. Therefore, the post groutedshaft length savings was limited bythe minimum tip elevation for later-al stability. Shaft reinforcement wasalso governed to resist lateral loadsand consisted of 24 - # 11 longitudi-nal bars with #5 shear hoops. It wasanticipated in the original designthat conventional shafts would haveneeded to extend 25 to 35 feet belowminimum tip to adequately resist therequired loads. Consequently, post

grouting resulted in a savings of 25to 35 feet per shaft.

Construction

A.H. Beck used much of their ownmanufactured equipment includinga crane mounted drilling rig fittedwith various custom 72 inch diame-ter drilling tools to excavate thesoils. Hole stabilization was main-tained with a Bentonite drilling slur-ry, mixed and processed with theirown manufactured slurry plant sys-tem. Slurry testing was routinelyperformed to ensure its effectiveness

consisting of density, viscosity, PH,and sand content. Because of the sta-ble nature of the upper Loess mater-ial, the 26 production shafts did notrequire use of temporary construc-tion casing. Only the test shaft uti-lized casing in the constructionprocess to isolate the scourable soilsduring the load test. After the shaftswere drilled to tip and mechanicallycleaned, a submersible pump wasused for final clean out. The cageswere lifted with multiple point picks.Once vertical, the grout plates wereinstalled on the bottom of the cage.After the cages were lowered to restfirmly on the bottom of the excava-tion, a 10 inch diameter tremie wasused to place the 9 inch slump con-crete via a pump truck. Concretequality assurance testing includedslump, air, unit weight, and com-pressive strength.

Load Test Program

To confirm the design and con-struction method, an out of positiontest shaft was installed, post groutedand load tested. The test shaft wasinstrumented with multiple levels ofstrain gages, telltales, crosshole soniclogging tubes, and a grout distribu-tion system at its base. The test shaftconstruction was performed usingthe same procedures as the produc-tion shafts with the exception of anisolation casing in the upper 40 feetto eliminate resistance from thescourable soil during the load test-ing. Shortly after construction of thetest shaft, it was post grouted.

The shaft response during postgrouting was measured with highprecision instrumentation and a dataacquisition system. Grout pressurewas measured with an electronicpressure transducer. Back up pres-sure readings were recorded from thecertified dial gage. Strain measure-ments allowed upward side shearand a lower bound base resistance tobe determined during post grouting.For measurement of upward shaft

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displacement during grouting, anindependent reference beam was setup over the shaft on which LVDT’swere mounted. Back up displace-ment measurements were made witha surveyor’s level. Grout volumemeasurements were made manuallyby recording the levels in the gradu-ated holding tank.

The shaft was then subject to a1,600 ton STATNAMIC axial com-pression load test. As is typical withthe rapid load test setup, load wasmeasured with a calibrated load celland displacement was measuredwith a photo-voltaic sensor triggeredby a stationary laser reference. Threemotion sensors provide redundantmeasurement of displacement andalso measure any eccentricity at theshaft head. Traditional survey wasperformed before and after the testto provide a check on permanentdisplacement. The strain gauges pro-vided a determination of base andside shear resistance. All instrumen-tation was monitored with a dataacquisition system which allowedimmediate results in the field.

Post Grouting the Shafts

Applied Foundation Testing, Inc.used a HANY grout plant suppliedby ADSC Associate Member, Ameri-can Commercial, Inc. The plant wasmade of three components includinga colloidal mixer, agitated holdingtank, and a hydraulic actuated pistontype pump capable of 1,500 psi groutpressure. Neat cement grout of TypeI/II Portland cement with awater/cement ratio of 0.5 waspumped to the base of each shaft. Onaverage each shaft was post grouted3 to 5 days after construction.Detailed documentation were madefor each production shaft including,at least four separate measurementsof grout pressure, grout volume, andshaft upward displacement that rep-resent the range of grout pressuresdelivered to the shaft.

SettingUp TheGroutingCriteria

Using theload test data,grouting cri-teria were de-veloped con-sisting ofthree maincomponents:1) GroutPressure; 2)Upward Dis-placement; and 3) Minimum GroutVolume. The grout pressure is themost important component since thecapacity improvement is directlyproportional to the applied pressure.Grouting of the test shaft was used asa basis for establishing the requiredpressure. Since the shaft began todisplace upward at an applied groutpressure of 310 psi, the maximumachievable pressure was limited bythe available skin friction. Althoughthe STATNAMIC load test provedthe shaft could support over 1,600tons in a scour simulated condition,it was thought that higherpressures may be achiev-able in the productionshafts which will haveadditional skin frictionfrom the scourable zone.For this reason, WilburSmith Associates specifiedthat the first productionshaft be instrumented withstrain gages and monitoredduring the grouting appli-cation.

Evaluation of the skinfriction behavior was alsonecessary to determine thetarget maximum upwardshaft displacement criteri-on. The grouting and loadtest results suggest that theskin friction was very pred-icable both in the upwardand downward loading di-rection. The upward friction

capacity during grouting, downwardfriction capacity during the load test,and the predicted friction capacityusing O’Neill and Hassan, 1994 werenearly identical. Further, it did notappear to degrade or go residualwithin a 1/2 inch of displacement.Therefore a conservative upward dis-placement limit of 0.25 inches wasset for production grouting. If thisupward displacement was met priorto achieving design grout pressure,grouting was to be stopped, the lines

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flushed, and the shaft was to be re-grouted after the data was evaluated.In three cases, stage grouting wasperformed and the shafts wereshown to have sufficient capacity.

Lastly, a minimum grout volumewas set to ensure that lines were notblocked and grout was reaching theshaft tip.

Summary and Conclusions

Post grouted drilled shafts weresuccessfully implemented as a costsaving and value added alternate tothe specified conventional drilledshafts. It was made possible througha systematic approach consisting ofpost grouted drilled shaft design cal-culations, installation and perfor-mance of a load test program, estab-lishment of production grouting cri-teria and a comprehensive qualityassurance program.

Using a post grouted shaft designmethodology developed by The Uni-versity of South Florida, capacities,tip elevations and grout pressureswere able to be predicted accuratelyfor estimation purposes. Incorporat-

ing load test data into the designmodels allowed even closer correla-tion.

Successful installation and perfor-mance of the load test program pro-vided validation of the grouting

method and confirmed shaft lengthreductions of 25 to 35 feet. Therein,required Grout Pressure, UpwardDisplacement, and a MinimumGrout Volume criteria were estab-lished for production shafts.

One of the most significant bene-fits of the post grouted drilled shaftconstruction method is the qualityassurance that is achieved. As such,the side shear and end bearingcapacity of every shaft can be assuredto a level proportional to the appliedgrout pressure. The enormous statis-tical reliability of knowing everyshaft’s capacity was merited with theuse of a 1.8 safety factor on the axialservice load. However, load testingand post grouting results suggestthat most of the shafts will have asafety factor of 2.0 or greater in thescoured condition.■

Reader Service #0745

PROJECT TEAM

Project Name: Natchez Trace Parkway, Bridge over St. Catherine Creek and Melvin Bayou

Location: Natchez, Mississippi

Owner: Federal Highway Administration, Eastern Federal LandsHighway Division

Project Engineer: Jeffery Schmidt

Design Build TeamGeneral Contractor: Hill Brothers Construction & Engineering

Project Manager: Sterling Akers

Designer/Geotechnical: Wilbur Smith AssociatesProject Manager: Michael Montgomery, P.E.

Drilled Shaft Contractor: A.H. Beck Foundation Co., Inc.*Project Manager: Gary PhillipsProject Superintendent: Tim Beavers

Load Testing & Post Grouting: Applied Foundation Testing, Inc.*Project Manager: Mike Muchard, P.E.

*Denotes ADSC Members.

Post grouted drilledshafts were successfullyimplemented as a costsaving and value addedalternate to the speci-fied conventionaldrilled shafts.