performance specs for cfa

8/3/2019 Performance Specs for CFA

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PERFORMANCE SPECIFICATIONS FOR DRILLED PILES

Dan Brown, Auburn University, Alabama, USA

Robert Thompson, Dan Brown and Associates, Sequatchie, TN, USA

Silas Nichols, Federal Highway Administration, Washington, D.C., USA

Deep foundation contracting for transportation projects in the UnitedStates has typically followed the design-bid-build model in whichcontractors bid on a specific work product according to relativelyinflexible prescriptive specifications. With continuous flight augerpiling and the many types of proprietary drilled displacement augerpiling, there appear to be advantages to contracting this workaccording to a different model. This paper provides a discussion ofthe use of a performance specification for this work in which thecontractor has greater freedom to choose the most appropriatefoundation system and also greater responsibility for the

performance of the completed foundation. With public agenciessubject to open bidding, this type of contractual arrangementpresents opportunities to encourage innovation. There are alsochallenges for implementation in public works projects, notably theselection of qualified contractors and the verification of performancerequirements on production foundations.

INTRODUCTION

Continuous flight auger (CFA) piling have ahistory of use in the U.S. commercial market,but have been used infrequently on publicworks transportation projects. This under-

utilization of a viable technology is at leastpartly the result of perceived difficulties inquality control on the part of transportationagencies. In addition, the proprietary systemsused for the installation of drilled displacement(DD) piles are not easily incorporated intotraditional design-bid-build delivery systemsfor public works projects.

This paper outlines a process for utilizing aperformance-based specification for drilledfoundation piling in which the contractor isresponsible for the final determination of pilelength. The approach requires that the

contractor provide the quality control andperformance measurement parametersnecessary to ensure that the owner is providedwith the foundation capacity that is required forthe job. The key for the owner is that thespecifications require measurements thatprovide a reliable indication of performance.With reliable performance indicators, thisapproach can allow contractors to exerciseingenuity and seek the most cost-effective andtimely solutions to achieve the project

requirements. The procedures outlined in thispaper are incorporated into Federal HighwayAdministration guidelines for the use of CFAand DD piling on U.S. transportation projects.

DRILLED PILES IN U.S. PRACTICE

Most CFA piling installed in the U.S. for thelast 30 years have utilized crane-mounteddrilling equipment with a sand-cement groutpumped through the hollow stem of the augeras shown in Fig. 1. These crane-mounted rigshave no means of applying downcrowd ontothe auger, and therefore have limited ability tocontrol the rate of penetration and the potentialfor soil mining, i.e., removal of excess soilduring drilling. Piles installed in this mannerare typically described as “augercast” piles,“augered pressure grouted (APG)” piles, or“augered cast-in-place (ACIP)” piles.

Recent years have seen a trend toward newdrilling technologies replacing the old. MoreAmerican contractors have obtained theequipment and expertise to install continuousflight auger piling using European typehydraulic equipment, and these more powerfuldrilling systems have generated increased useof alternative types of drilled piles. Theseinclude conventional CFA (Fig. 2), drilleddisplacement (DD) piles (Fig. 3) intended toinstall a cast-in-place pile with full



displacement and no spoil, and partial orintermediate drilled displacement (IDD) piles(Fig. 4) which may displace some soil but notact as a full displacement pile. Manycontractors and equipment builders havepatented various components of the drillingsystem, most typically some part of the tooling.

Another new technology which is gainingincreasing acceptance is the use of on-boardcomputer monitoring of drilling installationparameters as a function of depth, includingdrilling speed, applied torque, grout orconcrete pressure, and delivered volume.

Such systems offer the potential for improvedverification of the quality of the constructedpile.

U.S. public transportation agencies often wishto utilize the economy and efficiency of drilledpiles in appropriate circumstances, but thesemyriad of different and proprietary systems aredifficult to incorporate into the conventionaldesign-bid-build project delivery used on mosttransportation works. The result is that most

project designers stick with driven or boredpiles with prescriptive specifications controllingevery aspect of the work. The open biddingprocess is very effective in achieving thelowest possible unit cost, but may not achievethe greatest overall economy if manyalternatives are excluded.

Preparation and enforcement of contractdocuments are important critical steps in theintroduction of new technologies.Performance based contract specificationsoffer the potential to encourage the use of newtechnology, and public agencies have utilized

this approach in recent years for micropileconstruction. In order to encourage localtransportation agencies to exploit newtechnologies to achieve improvements inspeed, economy, and reliability, the FederalHighway Administration (FHWA) of the U.S.Department of Transportation is working todevelop a performance based guidespecification for CFA and drilled displacementpiles.

Figure 1 Crane Attachment for ACIP Pile Figure 2 Hydraulic CFA Rig

Figure 3 Partial or IntermediateDisplacement Pile Rig

Figure 4 Full Displacement Pile Rig



PERFORMANCE vs PRESCRIPTIVESPECIFICATIONS

The use of prescriptive contract specificationshas a long history for driven and bored piles inthe U.S. transportation industry. Publicagencies are required by law to use open

bidding for selection of contractors in order toavoid any possibility of favoritism (political orotherwise) in the selection process. The resultis that transportation projects typically utilizean extensive set of rigorous specificationscovering all aspects of design andconstruction. These range from the AASHTO(American Association of State Highway andTransportation Officials) design specifications,to individual state design and constructionspecifications to project-specific specialprovisions. Contractors may have someoptions for construction methodology, e.g., theuse of casing or drilling slurry with bored piles,

but the specific requirements associated witheach type of construction methodology aretightly controlled by the specifications.

Advantages and Limitations of PrescriptiveSpecifications for Piling

The current practice of using prescriptivespecifications for bored and driven piles hasmany perceived advantages. Withconventional design-bid-build, the designerassumes all the risks of performance (i.e., loadresistance) but also exercises direct controlover the selection of pile quantities.

Construction of a quality product is thought tobe assured by the rigorous specifications andinspection. The competitive bidding processobtains the lowest possible unit cost for thepile as designed. This system has workedwell in many respects for these maturetechnologies, as failures are extremely rareand contractors have great incentive to controlprices.

However, with emerging or rapidly developingtechnologies, the use of prescriptivespecifications has significant limitations. Thedesigner has little incentive to assumeperformance risk with innovative systems withwhich he or she has little experience. Theopen bidding process generally precludes thedesigner from specifying a single proprietarypile system, and there is little incentive for thedesigner to attempt to provide a wide range ofalternates. Likewise, contractors have littleincentive to improve the product or to producea quality product to any level above theabsolute minimum standard established by thespecifications and the owner-specified quality

assurance system. The development ofrigorous specifications for design andconstruction is a time-consuming processwhich cannot respond to rapidly changingtechnologies in a timely manner withoutextraordinary effort. Although performancefailures have been rare, claims and delays are

all too frequent.

A Performance-Based Guide Specificationfor Piling for Public Agencies

As a part of the development of GeotechnicalEngineering Circular No. 8 for the U.S.Department of Transportation, FederalHighway Administration (FHWA), a guidespecification has been developed for use byU.S. public transportation agencies and anyother interested parties. This specificationprovides for an owner-controlled designwherein the owner (public agency) provides

preliminary plans that show the pile designloadings, footing/cap design, and pile layoutfor each footing/cap location. The owner alsoprovides related design criteria andrequirements, subsurface data, site limitations,construction material and testingspecifications, and required contractorsubmittals and review requirements. The pre-qualified CFA pile contractor designs theindividual piles and pile cap connections andselects the CFA construction process andequipment.

During the pre-bidding process, the pre-

qualified CFA pile contractors prepare apreliminary CFA pile design and a firm costproposal based on the owner’s preliminaryplans and specifications. If the CFA pileportion of the project is to be subcontracted,general contractors will receive bids from thelist of pre-qualified CFA pile contractors andinclude the best offer and name of theselected CFA pile subcontractor in their bidsubmittal. Once the contract is awarded, theselected CFA pile contractor prepares detailedCFA pile design calculations and workingdrawings and submits them to the Engineer forreview.

Advantages and Limitations ofPerformance Specifications for Piling

Performance based specifications offer someadvantages for new technologies, and in theU.S. transportation industry, micropiles haveenjoyed successful implementation in thismarket by using performance specifications.With new technologies, designers may beencouraged to accept innovation because the



risks associated with performance areassumed by the contractor. Rather than usingrigorous prescriptive specifications, a rigoroustesting program is used to verify performance.Rather than using the lowest unit cost for aspecified pile type to obtain competitive bids,contractors are provided with incentive to offer

the most competitive pile system to achievespeed and economy. This more flexiblesystem encourages innovation and is wellsuited to rapidly changing technologies. In theU.S. transportation construction market, drilledpiles (including CFA, DD, IDD) are anemerging technology which can offer benefitsto owner agencies from this alternateprocurement method.

There are difficulties and limitations withperformance specifications. Although thedesigner does not directly assume the risk ofperformance, the end result is dependent upon

a clear and concise expression of thedesigners expectations of performance. Thelevel of testing and monitoring to assureperformance must be defined in order toprovide the quality assurance necessary toprotect the owners interests. Because agreater burden is placed onto the contractor, itis even more important that a qualified andexperienced contractor be chosen; pre-qualification of contractors is an issue withwhich many public agencies struggle. Ifcontractors propose new and innovative pilingsystems, the design and inspection team willhave a challenge to familiarize themselveswith the features of the system so thatverification of performance can be assured.

REQUIREMENTS OF PERFORMANCESPECIFICATIONS

Performance specifications include someimportant features that are general to anygood specification for piling works and somerequirements that are specific to CFA, IDD,and DD piling.

General Requirements

Some of the general requirements of a goodperformance specification for foundation pilinginclude:

• well defined provisions for prequalificationof subcontractors,

• clearly defined performance requirements,

• high quality geotechnical information,

• requirements for contractor submittals,

• testing requirements for verification ofperformance.

• mandatory preconstruction meeting

Contractor prequalification is a critical issuebecause of the increased responsibility for thefoundation performance to be placed on thecontractor under this type of contract.Prequalification requirements focus on thedocumented experience of the contractor and jobsite supervisor as well as the licensed

design engineer (who may be either anemployee or a separate consultant partneringwith the contractor). Enforcement of thisprovision is important, as inexperienced andunqualified contractors can lead to delays,inferior work, potential claims, and increasedcosts.

Another factor that project owners/designersmust consider is that the performanceexpectations must be clearly conveyedthrough the contract documents. Theperformance requirements include axial and/orlateral load resistance and limiting

displacements for service load conditions. Ifreinforcement and/or flexural capacity forbending are part of the owner design,minimum tip elevations and minimumreinforcement lengths may be specified. It isworth noting that some types of drilleddisplacement piles may enhance lateral loadperformance and there may be provisions toinclude this performance characteristic in thecontractor design component. However, thedesign of the connection to the pile caprequires that the pile cap details be clearlyindicated on the contract documents.

Performance requirements are inherently tiedto geotechnical conditions, and it is critical thathigh quality geotechnical information must beincluded in the contract documents. Reliablegeotechnical information is also necessary inorder to minimize the risk of claims anddisputes. For routine projects, it is notpractical or efficient for contractors to obtaintheir own geotechnical information forpreliminary design. There are oftenconstraints of time, permits, right-of-way, andpractical efficiencies of allocating scarceresources for multiple contractors to beduplicating the site investigation efforts inorder to bid a job.

Contractor submittals after the contract hasbeen awarded must include complete designcalculations and working drawings for reviewand approval by the project engineer. Thesesubmittals should be signed and sealed by thecontractor’s design engineer, who had beenpreviously pre-qualified to perform this work.Working drawings must include all theinformation necessary for the construction and



quality control of the piling. A pile installationplan must be submitted to document theequipment, step-by-step description of theinstallation procedures, sequence, mixdesigns, equipment and procedures formonitoring critical parameters duringconstruction, and protection of nearby

structures. A conformance testing plan shouldalso be submitted along with the pileinstallation plan, and this plan should describeboth the pre-production load testing programand the post-installation testing to verifycapacity and/or pile integrity.

General testing requirements for verification ofperformance must be spelled out in thespecifications. In general, the total number ofload tests on any specific project may dependon such factors as the size of the project, thesoil conditions and variability, the required loadresistance, sensitivity of the structure, and site

access conditions. However, the minimumload testing requirements for the specificproject should be specified. The contractdocuments should include an item foradditional tests above the specified minimumnumber so that the project engineer has ameans to direct that additional testing beperformed without change orders. It isimportant to note that the pre-production testsare used not only to demonstrate theperformance of the test pile, but also toestablish installation criteria for the productionpiles.

A final general requirement is to include amandatory preconstruction meeting. Such ameeting can help to establish communication,bring attention to any special concerns, andhelp to minimize the risk of misunderstandingslater.

Specific Requirements for Drilled Piles

Besides the general requirements forperformance-based specifications outlinedabove, there are some items specific to drilledpiles that should be included. These includespecific items pertaining to automatedmonitoring of installation and concretingoperations, to integrity and verification testing,and to the correlation of these controlparameters to those established duringconstruction of pre-production test piles.

Automated Monitoring

The use of automated monitoring equipment isa key element of quality assurance for drilledpiles. In fact, the availability of this QA/QC

technology is a major factor in the acceptanceof continuous flight auger piles for public worksprojects, given that many public agencies havehad reservations about their ability to

adequately verify the performancecharacteristics of production piles. Some ofthe required components of an automatedmonitoring system include:• rate of penetration of the tool during

drilling,

• torque and down pressure from the rig,

• concrete (or grout) volume duringextraction of the tool,

• concrete pressure during pumping.

The rate of penetration of the tool may be acritical parameter for performance, dependingupon the type of pile to be installed and the

geotechnical conditions. The torque and downpressure from the rig are generally recordedalong with the rate of penetration as anindication of the effort extended duringinstallation. The axial capacity of conventionalCFA piles in non-cohesive soil can be quitesensitive to penetration rate, and groundsubsidence can occur if excessive soil flightingis permitted during installation. For partial orintermediate displacement piles, the amount oflateral soil displacement affects capacity andwill be related to the rate of penetration. Inthese conditions, the penetration rate duringdrilling must be tied to the production control

parameters for QA/QC.

However, the axial capacity of a displacement-type pile is generally insensitive to the rate ofpenetration of the tool, since the resulting soilimprovement around the pile is produced bythe shape of the tool and flighting of soil doesnot occur in any case. In cohesive orcemented soils, the capacity of a conventionalCFA pile may be relatively insensitive to therate of penetration, since these non-caving

Figure 5 Automated Monitoring forQC/QA



Figure 6 Proof Testing of ProductionPiles with Statnamic (Rapid Load Test)

Device at a project in Florida

soils tend to stand without flighting even if thetool is over-rotated. Examples include thecemented limerock soils in Miami andweathered shales in Texas. In theseconditions, the emphasis on penetration rateduring drilling may be relaxed as a productioncontrol parameter for QA/QC.

Volume and pressure of concrete or groutduring casing are critical parameters forQA/QC to ensure that a pile of good structuralintegrity is achieved. Only with an automatedmonitoring system that records these controlsas an incremental function of length along thepile can the owner be assured that the pile iscast correctly. These systems also providethe real time feedback that allows the operatorto control the process with precision. TheFHWA guide specification requires the use ofthese systems on all CFA projects for bridgesand other critical transportation structures.

Integrity and Verification Testing

A performance-based specification mustinclude testing to provide the owner withassurance that the performance requirementsare achieved. With CFA piles, there are twomajor components to the testing requirements:1) integrity testing for assurance that the pile isstructurally sound and 2) verification testing forassurance that the pile has the requiredgeotechnical capacity.

Several types of integrity tests have been usedfor CFA piles, but the most widely used andeconomical test method is the sonic echo test.Sonic echo is performed by striking the pile topand observing the echo trace returned to aninstrument from the pile toe or any changes inimpedance along the pile; this technique iswell documented in the technical literature andrepresents a relatively mature technology.The advantages are the speed and economyof this test method compared to alternatives(which might include one or more accesstubes within the pile). A limiting factor is theattenuation of the signal with long slender pilesand the fact that any irregularities in geometrywill complicate the return signal and make adefect in the pile more difficult to detect.

Defects in terms of structural integrity arepotentially the most serious when they occurnear the top of the pile, and a defect near thetop would be most easily detected using sonicecho. Additionally, the uppermost few metersare often the most vulnerable to constructionrelated defects because of the potential forinstability at the top of the hole, the lack of

pressure in the fluid concrete or grout, and thepotential for contaminants to fall into the fluidconcrete or grout during rebar placement.

Sonic echo testing of each pile during the testpile program and early in the production pileschedule can help to identify any problems inthe construction techniques being used at theproject site and allow correction before a largenumber of defective piles are constructed. Atypical performance specification shouldrequire some minimum percentage of piles tobe tested, perhaps even 100% for a difficultproject.

Verification load testing on production pilescan now be readily accomplished usingdynamic (DLT) or rapid (RLT) loading

techniques (as illustrated on Figure 6). Withthe advent of relatively mobile testingequipment, a significant number of productionpiles can be selected by the engineer orowner’s representative for verification testing.These load testing technologies provide agood indication of load resistance, particularlyif “calibrated” against conventional static loadtests during the pre-production pile loadtesting program.



Pre-production Pile Load Testing

A pre-production pile load testing program iscritical for performance based contracting ofCFA piles. The load test program not only isused to verify the contractor’s designmethodology, but also to establish control

parameters for construction based on theautomated monitoring program. The controlparameters (penetration rate and/or torqueand downcrowd if appropriate, concretevolume and pressure and over-consumptionratio of concrete or grout) set by the contractorand measured during construction of the testpiles are, within some allowable range, thetarget requirements for production piling.Without the link between the controlparameters and measured performance, itwould be quite easy for the rig operator to verycarefully install the load test piles with an eyetoward optimizing capacity, only to see a later

change in emphasis toward optimizingproduction after the test pile program is over.By establishing control parameters based onthe load test piles, a consistency can beestablished that will provide assurance to theowner that the performance criteria are met.

CHALLENGES FOR IMPLEMENTATION

While the advantages of performancespecifications for CFA piles are apparent,there are significant challenges toimplementation of this approach. Thesechallenges include enforcement of pre-qualification requirements, owner/engineerreview of contractor submittals in a timelymanner, recruitment and training of inspectorsfor CFA pile construction.

Prequalification of subcontractors can be achallenge for public agencies because of thehistory and public demand for open bidding inthe U.S. The public agency must devote thetime and energy to establish appropriaterequirements for contractors and to set up areview process that will be recognized as fairand impartial. Several state departments oftransportation have successfully establishedprequalification of subcontractors for micropileand other specialty construction works, sothere are precedents to follow in this regard.

For a performance based design, thecontractor must make a submittal withcalculations, drawings, and a description of theconstruction plan. CFA piles have beenrelatively uncommon in construction oftransportation works, and therefore manyengineers within public transportation

agencies have relatively little experience withthese piles. In addition, new techniques andequipment are rapidly emerging in theindustry. There is a distinct need for educationand training of engineers in order that criticalevaluation of proposed designs can beachieved in a timely manner.

Besides the need for education and training ofdesign engineers, the inspectors and technicalsupport staff require recruitment and training.Training for construction inspection is neededacross all types of deep foundation works, butthe additional testing requirements and thedifferent approach represented byperformance based specifications for CFApiles add to the challenge.

Perhaps it could be concluded that the biggestchallenge for implementation is at the nationallevel, for the FHWA to implement education

and training programs to address thechallenges facing the individual state a localagencies!

Summary and Conclusions

CFA and drilled displacement piles representa potentially useful technology fortransportation works due to the speed andeconomy of these piling systems. The widevariety of different piling systems can beutilized via a performance based approach tocontracting for these foundations. A numberof features characteristic of performance

specifications are described in this paper, withparticular emphasis on the need for QA/QCusing automated monitoring systems and theneed for an appropriate testing program toassure the owners that the performancecriteria are met. There are challenges forimplementation of this approach relating toprequalification of contractors and educationand training of engineers and technical staff.However, the adoption of a performancebased specification offers the promise ofencouraging contractors to be innovative andfocused on performance aspects of theirfoundation installations.

References

Armour, T., Groneck, P., Keeley, J., andSharma, S. (2000). “Micropile Design andConstruction Guidelines ImplementationManual”. Report No. FHWA-SA-97-070.

Brown, D., Dapp., S., and Thompson, R.(2006). “Geotechnical Engineering Circular 8:Continuous Flight Auger Piles,” (draft final).

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