i ec rivinc cons Tuc:ioncon:roOy: ae CaSe me:~

by C.-J. GRAVARE', MSc, CE, G. G. GOBLE>, PhD, FRANK RAUSCHEf, PhD, at GARLAND LIKINS, Jr.,t MSCE

IntroductionPILES ARE FREQUENTLY the best solu-tion to foundation problems. However,high safety factors are commonly em-ployed to obtain a sufficient confidenceleva'I in the foundation, given the uncer-tainties about hammer, soil and pile pro-perties. By testing the integrity and bearingcapacity of the pile, these uncertaintiescan be eliminated. This will give a moreeconomical foundation design.

Static load testing provides one methodby which a pile's integrity and bearingcapacity can be verified. However, staticlioad testing is time consuming and ex-pensive, thus allowing for only a smallnumber of tests on one site. To only alimited degree, therefore, can variabilityof soil, hammer and pile be taken into ac-count by the static load test method.

Observations of ram stroke and pile settaken during pile driving have been usedin a driving formula for quantitative analy-sis of 'bearing capacity for at least thelast century. Concurrent with the morerecent development of automated compu-ting capabilities for wave equation ana-lyses", electronic measurement techni-ques during pile driving were devised.Thus, the scope of pile driving observa-tions dramatically widened and includedaccurate electronic measurements duringthe pile impact. The Case method, namedafter Case Institute of Technology, Cleve-land, Oh'io, USA, consists of measuringpile top forces and accelerations. Field orlaboratory processing then provides pilebearing capacity, pile stresses and ham-mer energies.

In this article, the development of theCase method will be described. Its poten-tials will be discussed and an illustrativeexample will be given.

Electronic component developmentAfter the idea of using electronic mea-

surements on impact driven piles had beenconceived in the early 1960's, two distin-ctly different problem areas were investi-gated. The first was the development of atransducer system for the measurement ofhigh frequency impact events. Difficultieswere encountered in building equipmentthat withstood the rugged environmentalconditions on typical pile driving sites. Anadditional requirement was that the instal-lation procedure should be quick and sim-ple for routine usage.

The most successful transducer systemproved to be (see Fig 1) piezoelectricaccelerometers with internal amplifier, highresonant frequencies and high g levels,and reusable, lightweight, bolt-on straintransducers built with resistance straingauges. Both transducers easily adapt to

—Strain— Pile Dnving Analyser

gauge

1

OOM I c I'0 p h 0 r1 c

Oscilloscope

Fig. 1. Schematic layout for Pile DrivingAnalyser measurements and recordingsystem

any pile shape or material. In preparationfor transducer attachment, holes must bedrilled into the pile —for H piles, clear-ance holes; for pipe piles, tapped holes;and for concrete piles, holes with anchorinserts must be prepared.

Additional development work designedand constructed a field signal processor.This unit, now commonly referred to as a"Pile Driving Analyser" provides signalconditioning and amplification, and con-verts strain to force using the pile mater-ial and size properties. The analyser in-tegrates acceleration to velocity, inte-grates the product of force and velocityto yield energy, and evafuates the forceand velocity for bearing capacity. In ad-dition, the analyser determines the ex-

tremes of all phenomena obtained such asacceleration, force, veloicity and energy.The results are displayed and printed indigital form

The first analyser successfully used un-der field conditions was completed in1970. With improvements in electronicsand interpretatioin techniques, additionalmodels were built including the most re-cent as shown in Fig. 2.

Theoretical developmentThroughout the research project a met-

hod was sought to reliably evaluate pilebearing capacity. The initially proposedidea of treating the pile as a rigid bodywas abandoned because of the sometimesrather strong effects of pile elasticity.

Derivation of the best approach wasbased on closed form one-dimensionalwave solutions. Important equations areexplained in Appendix A. In effect, themethod uses the measured force andvelocity at the time of impact and thepile response values of the same quanti-ties when the stress waves returned to thepile top after reflection at the bottom.Based on travelling wave theory, the re-sistance force is then determined utilisingdamping parameters, empirically determin-ed as a function of soil grain size to ac-count for losses due to soil viscosity.

During the research project, correlationdata was gathered for comparison of dyna-mic predictions with static load test re-sults. Originally, data was obtained onOhio Department of Transportation con-struction sites.

In 1970 the project was extended tocover additional states. Th'is diversificationprovided data on many different pile types,sizes and materials in a wide variety ofsoil conditions. A summary of the efforts

vManager, Goteborgs Betongpalar AB, Gothen-burg, Sweden

lichairman, Dept, of Civil, Environmental andArchitectural Engineering and Director of PilingResearch Laboratory, University of Colorado, atBoulder, USA

f President of Goble and Associates Inc., Cleve-land, Ohio, USA

tPresident of Pile Dynamics Inc., Cleveland, Ohio

20 Ground Engineering

Fig. 2. Pile Driving Analyser with strain and acceleration transducers attached to pipe

and correlations'f dynamically predictedand statically measured pile bearing ca-pacities is shown in Fig. 3.

To date, the authors are still striving toextract additional results from pile topforce and velocity measurements. For ex-ample, it has become possible to evaluatepile structural damage regarding its locationand its seriousness at locations belowgrade. This technique'onsiders the ham-mer impact as a sonar test. Using theforce and acceleration records, reflectionsfrom the point of damage are clearly ap-parent. Quantitative evaluations are pos-sible because two records (force and ac-celeration) are taken.

Another technique determines the maxi-mum tension stresses at locations otherthan the pile top where the measurementsare taken. These stresses are of great im-portance for a safe concrete pile installa-tion. Another newly developed techniqueprovides pile bottom or rock stiffnessvalues for end-bearing piles.

Probably the greatest insight into thepile and soil behaviour responses is ob-tained when the pile top force and velocitymeasurements are analysed using CAPWAP(Case Pile Wave Analysis Program, seeAppendix A, Part II). In contrast to theCase method, CAPWAP is not dependenton a knowledge of soil type, thus provid-ing damping factors for construction siteswhere no experience or static load testresults exist. A digital computer and there-fore office evaluation of the data is nec-essary if either CAPWAP or the end bear-ing resistance method are requested.However, tension stresses and pile dam-age can be evaluated in the field,

Field test procedureField testing is done in either of two

ways. First, the pile can be instrumentedand monitored while it is being driven.In this way, pile bearing capacity can beobtained as a function of penetration. In

addition, stresses and hammer efficiencycan be monitored for a safe and efficientpile installation; corrective measures canbe taken, if necessary, and driving criteriaestablished.

The second method woiuld test the pilesome time after installation. The pife isrestruck with a relatively small number ofblows. This second method has the ad-vantage of providing bearing capacity val-ues which include set-up/relaxation of soil".r ngth changes after dr',ving.

The instrumentation is attached near thepile top. The signals of force and accelera-tion are led through a common signal cable(Fig. 1) to the pile driving analyser. Forceand velocity are monitored on an oscillo-scope, thus providing pile damage criteriaand a check on data quality. Accelerationand force are recorded on a magnetic taperecorder together with voice submittedobservations.

For completeness, pile penetration, blowcount and, if possible, hammer data suchas stroke, air or bounce chamber pressureshould be recorded. Although these ob-servations are not an absolute necessity,they will aid in the establishment of areasonable pile-driving criteria for pilesnot subjected to the Case method dynamictest.

Test backgroundPiles were driven for the extension to

the Gravesend sewage works in Kent, UKin September 1978. The design load forthese piles was 850kN with a safety factorof 2.0. The contractor, Balken Piling Ltd.,used its precast concrete pile system con-sisting of regularly reinforced 275 x 275mm (A = 756cm') cross-sectional seg-ments. The total pile length was 25m. Allpiles were driven with a Banut 400 whichis a hydraulically powered rig and hammersystem. The 3000kg ram is lifted by a hy-draulic piston and the height of the com-pletely free drop can be varied between100 and 500mm.

600

22 400o0

E

~ 200

200 400 600Static test, kips

Fig. 3. Correlation of Case method capac-ity predictions with static load testresults

SOIL DRIVING LOGDESCRIPTION BLOWS/258 MM

6 188 266 388 466 5868

CAPINAP RESISTANCE DISTRIBUTION RESULTSRES/ELEMENT KN FORCE IN PILE KN

8 58 166 158 266 8 588 1886 1588 2888

Soft siltyclay 5

18- 166 Firm silty

c I ayi—CLLUcs

15- 15

Clayey sandsenti y silt

Gravel withsome sand,silt

r

26 ~900

25-

Cliafk withPritty chalk

Enil of l>oringRP950

I

i

I

Fig. 4. Driving summary with soil description and CAPWAP results at the end of driving

22 Ground Engineering

In addition to the regular piling installa-tion, the contractor drove an additionalpreliminary test pile which was tested dy-namically during driving and then with astatic proof test. This testing was perform-ed as part of a research project sponsoredby the Royal Swedish Academy of Sci-ence, The Building Research Board of Swe-den and Balken Piling.

The soil profile (Fig. 4) consists of siltyclay to a depth of 16m overlaying a layerof sand to a depth of 18.3m. Below was amixture of gravel, flint and sand to a depthof 21.3m and then chalk to at least thebottom of the borehole at 24.4m. Only afew blows were necessary to drive the pileto 10m. The pile driving was at a ratheruniform low ulow count (Fig. 4) until thegravel layer was reached. Blow courtsjumped dramatically due to the increasedtip resistance in this layer. Blow countsincreased further when the chalk was en-countered. Refusal was met at a penetra-tion of 1.5m in the chalk.

The standard Case method measure-ments of force and acceleration were madethroughout the entire driving procedure.Transducers were attached 1m from thetop of the pile. Results from the pile driv-ing analyser were printed on paper tape;transducer signals were also stored onanalog magnetic tape with an FM instru-mentation recorder for further analysis.

This project is intended to demonstratethe power of the Case method electronicmeasurements during pile driving, It wouldbe impossible, however, to show the fullcapabilities of this system on any onepro,'ect; the measurements presented aretherefore intended as only a sample. Thedynamic records do not indicate the pres-ence of any significant pile structural dam-age or potentially harmful tensile stresses.Therefore, readers interested in these topicsmust look elsewhere".

ln addition to the standard Case methodpi'le top measurements, strains were alsorecorded at a distance of 1m from the piletoe both during driving and during thestatic test.

The pile was statically load-tested tofailure one day after installation and loadswere measured at the top as well as at thetoe. In this way, skin friction was sepa-rated from end-bearing loads.

Test resultsAlthough results and comparisons were

made in the field with the pile drivinganalyser, the analog tape of force andacceleration at the pile top was submittedto Pile Dynamics Inc. for further analysisby CAPWAP. Other information such assoil borings, driving logs, load test inform-ation and results of pile toe strain read-ings was withheld until after the CAP-WAP analysis had been completed sothat results obtained would be as inde-pendent as possible.

The data analysed is from the end of theinitial driving to represent the service con-ditions of the pile at that time. Compari-sons made with the static test must bemade bearing in mind the soil's strengthchanges with time. For example, the ulti-mate capacity of the pile could even in-crease further after the load test.

The pile had a measured material wave-speed, c, of 3428m/s and modulus ofelasticity E = 28.8kN/mm'. Typical mea-sured force and proportional velocity (vel-ocity x the pile impedance EA/c = forceas long as no reflections from cross-sec-tion changes, pile end or skin friction resis-

2 000—

QJ 1 000-

00

(a> VxEAi

2 000-

.-1 OOO-

ns sac-"x0 1 2 3 4

— ((

Fig. 5 (a) Measured pile top force andproportional velocity with CAPWAPcomputed top force: (b) Measured piletoe force with CAPWAP computed toeforce

tances are present) are shown in Fig. 5a.Using the CAPWAP procedure (AppendixA) with the measured acceleration/velo-city as input, several static soil resistancedistributions, damping constants and qua-kes were tried, each trial producing acomputed pile top force curve, The soilparameters were varied until the match ofcomputed and measured forces could nolonger be improved within the accuracyof the lumped mass model, elastic-plasticsoil resistance law and the linear viscousdampers employed. The final force matchin Fig. 5a is of fair quality.

The pile was converted into a 16-elementlumped mass model with element weightsand spring stiffness values given in Table I.Also, Table I presents the soil resistancevalues used to produce the final forcematch, the quakes, element resistances andelement damping constants. In addition, thesum of the element resistances are given;this sum is equivalent to the force distri-bution in the pile when the pile is at its

predicted ultimate load. The JS and JT arenon-dimensional equivalent Case dampingfactors for the skin and toe, respectively.The JT toe damping constant may be con-verted to the dimensioned value J in thetable by multiplying by the pile impedanceEA/c. Similarly the JS skin damping con-stant, when multiplied by EA/c and dis-tributed among the elements in proportionto the element static resistance, will givethe element J values. The equivalent Smithdamping parameters are 0.12 and 0.21 s/mfor the skin and toe, respectively. TheSmith damping approach is commonly usedin conventional wave equation analyses.

Since a complete dynamic analysis isperformed in CAPWAP, the computed topforce is not the only function which canbe obtained; the forces in any spring andthe motion (acceleration, velocity and dis-placement) of any lumped mass are alsodetermined as a function of time. Thus, acomplete stress history of the pile is ob-tained including absolute stress minima(tension) and maxima. Since the strain atthe pile toe was measured independently,the CAPWAP computed toe strain waslater compared (Fig. 5b) with the mea-sured one.

The CAPWAP analysis determined anultimate failure load of 1 590kN with 950kNof this load in end-bearing The remaining640kN of skin friction was found to bemore or less distributed uniformly over thelower half of the pile.

The resistance distribution was usedwith the lumped mass pile model to pre-dict the load displacement curve duringa static load test. Secondary settlementsare ignored in this program. This computedload test curve is shown in Fig. 6a.

A static load test was performed thefollowing day. The pile was rapidly loadedin 100kN increments and each load incre-ment held for 16 minutes before proceed-ing to the next load. The maximum loadwhich could be applied was 1 800kN. Usingthe strain measurement at the pile toe, aload of 910kN of end-bearing was foundat the maximum 1 800kN load. The re-maining 890kN was then carried inskin friction; this represents a 250kN in-crease in skin friction from the end ofdriving and may be the result of a gain ofsoi'I strength in the silty clay layers. Theresults of the static test are shown in Fig.6a both with and without (as the pre-dicted CAPWAP static test had assumed)creep effects, Fig. 6b shows the skin fric-

Depth,m

Quake,mm

RES, SUM RES, JkN kN kN/m/s

Weight,kg

StiffnesskN/mm

1

23456789

10111213141516

Pile toe

1.53.04.56.07.59.0

10.512.013.515.016.518.019.521.022.524.0

2.52.52.52.52.52.52.52.52.52.52.52.52.52.52.52.5

3.1

1020202020304040405050505060

140

159015801560154015201500147014301390135013001250120011501090950

0

0.01.82.22.22.22.84.04.65.75.75.75.75.76.97.5

17.1

286286286286286286286286286286286286286286286286

9830983098309830983098309830983098309830983098309830983098309830

TABLE I. GRAVESEND CAPWAP RESULTS

JS = 0.12, JT = 0.30

2 000-

With creep

oo 1 000o.0

I—

PWAP

0i0 20 30 40

Oisplacement mm

2 000-

oQJ 1 000-0o.0,

earing

0 1000 2000((s) Pile iop loacl, kN

Fig. 6 (a) Measured static load testcurve —with and without creep effects—and CAPWAP predicted load testcurve: (b) Static test skin fri ction/end-bearing load distribution

tion/end-bearing relationships as a func-tion of total load.

The dynamic results reported abovewere obtained from laboratory analysisof the measured force-acceleration datawith a digital mini-computer and a rathersophisticated analysis routine. Dynamicmeasurements taken were analysed duringthe actual pile driving with the pile driv-ing analyser. Using a simpler model, theanalyser predicted 1 730kN ultimate cap-acity, using an assumed Case dampingconstant of J = 0.2.

The analyser also determined a maxi-mum driving stress of 0.168kN/rn'm', arather safe value regarding dain'age due todriving stresses. The maximum measuredenergy transferred to the pile was 10.8kJ;assuming a drop of 450mm, the potentialenergy is 13.5kJ. Under this assumption,the efficiency (ratio of energy transmittedto the pile top with respect to theoretic-ally available energy) of the Banut 400driving rig was 80%. This is an excellentratio considering that conventional air/steam cabled drop or diesel hammerstransmit usually 50% and rarely as muchas 70% of their rated energy into the pile.

The particular data analysed and pre-sented in this article is only part of a largerconstruction project of Balken Piling.Further test details of this project or ofthe dynamic testing equipment are ob-tainable from B'alken Piling Ltd., Birch-woo'd Way, Cotes Park West, Somer-cotes, Derbyshire DE55 4PY.

APPENDIX ACase and CAPWAP procedures

1. Case method (named after Case West-ern Reserve University)

Beginning in 1965, research was con-

March, 1980 23

ducted at Case Western Reserve Uni-versity in Cleveland, Ohio, to develop amethod using electronic measurementstaken during pile driving to predict pilebearing capacity. Pile top acceleration, a,and pile top force, F, were measured andthe pile was originally assumed to be arigid body of mass, m. The resistanceforce of the soil using Newton's Law wascalculated as

R = F- (m)a

where F and a are functions of time. Inorder to eliminate resistance force com-ponents dependent on pile velocity, F anda were chosen when the pile top velocity,v, found by integration of acceleration,became zero.

Further studies including longer piles(more than 60ft) showed that the pileelasticity could not, in general, be neglec-ted. Assuming uniform piles and idealplastic soil behaviour, the following equa-tion was derived from a closed formsolution to the one-dimensional waveequational

R = '(F(t,) + F(rz))

mc+ (v(t,) —v(ts) ) ... (2)

2L

where tz = tt + 2L/c and t, ... is aselected time during the blow,L is the pile length,v, the velocity of the pile top, andc, the wave transmission speed onthe pile material.

Of course, the resistance, R, was againdependent on the choice of time t, . In-itially t, was also chosen as the timewhen the pile top velocity became zero.Later the method used tt as a function ofsoil property in terms of a 0'action of2L/c (time delay method).

Today the Case method models the soilresistance, R, as the sum of a static, S,and a dynamic component, D,

R = S+ D (3)

The "damping force", D, is obtainedapproximately as

D=jxvt„, (4)

where J is a damping constant, andv,„, the pile toe velocity.

It can be shown from wave theory thatthe pile toe velocity can be calculated as

v =2v ——Ri up iutimc

(5)

where v is the pile top velocity attime t,.

It should be noted that t, is chosen atthe time of the maximum velocity of thepile top (time of impact) and that J isoften used in dimensionless form afterdivision by mc/L. Of course, J is depend-ent on the soil type.

Eqn, 5 is approximately correct for thefirst 2L/c after the initial arrival of thestress wave at the toe, The static soilresistance, S, is then easily obtained bysubtracting the calculated damping force,D, from the total driving resistance.II. The CAPWAP method (Case PileWave Analysis Program)

Either pile top force or pile top velocity

can be used in a dynamic analysis as aboundary value (both together would notlead to satisfactory results). An analysiscan then be performed either in closedform or in a so-called wave analysis pro- I

cedure, i.e. in a discreet form, Of courseit is then necessary to describe the soilresistance forces.

The soil reaction forces are passive andup to now it has been found sufficientlyaccurate to express them as a functionof pile motion only, It is furthermoreassumed that the soil reaction consists ofa static (elasto-plastic) and a dynamic ~

(linear damping) component. In this waythe soil model has at each point threeunknowns (elasticity, plasticity and vis-cosity) .

The dynamic analysis is performed inthe CAPWAP method after the procedure,that was introduced by Smith. This pro-cedure divides the pile into a number ofmass points and springs. In this way thereare three times as many unknown soilparameters as pile elements. First, a reas-onable assumption is made regarding thesoil parameters, and then the motion ofthe pile is assumed using the measuredpile top acceleration as a boundary value.Output results are not only the pile ele-ment motions and soil resistance forces,

'utalso the computed pile top force, allas a function of time.

The computed and the measured piletop force will in general not agree witheach other. It is necessary to improve thismatch iteratively by changing the assumedsoil resistance parameters. Finally, a com-puted pile top force will be obtainedwhich cannot be further improved. Thecorresponding parameters of the soilmodel are then considered the correctvalues. The results of the CAPWAP ana-lysis then are the magnitude and locationalong the pile of both static and dynamicresistance forces. Static computations canbe used to predict the static load testcurve of the pile.

In 1970 a program was written that per-formed the necessary computations anddecisions automatically. This program re-sulted in satisfactory solutions for pileswhich were not more than 75ft (33m) inlength. For longer piles computation timesbecame excessive. A recent program per-forms the computations "interactively". Inthe interactive mode one analysis is ob-tained using a minicomputer, and then theengineer determines the necessary changesof soil parameters for the next analysis.This method uses a machine with approx-imately 16k core memory. Of course, onealso needs a plotter to draw the measuredand the predicted pile top force curves.Even for longer piles it is usually suffici-ent to analyse 10 to 20 times.

References1. Smith E. A L (1960); "Pile driving analysis

by the wave equation", Proc. ASCE, August2. Goble, G, G. & Rausche, F. (1976): "Wave

equation analysis of pile driving, WEAP pro-gram", Vole. 1-4, Report No. FHWA-1P-76-14.1,National Technical Information Service, Spnng-field, Virginia 22161, July.

3. Gable, G, G., Likins, G, E. & Rauscha, F(1975): "Bearing capacity of piles from dyna-m'cs measurements", Final Report, Depart-ment of Ciwl Eng'nearing, Case Western Re-serve University, Cleveland, Ohio, March

4. Rausche, F. & Gob/a, G. G. (1978): "Deter-mination of pi'e damage by top measure-ments", ASTM Sympos'um Behaviour of DeepFoundations, Boston, Massachusetts, June.

5. Likins, G. E (1979): "Determination of piletension stresses by top measurements".Seventh Pile Measurements Seminar, Cleve-land Ohio February

Pile design-the 'similitude'ethod

(continued from page 19)developed and introduced by the authorof this Paper'.

It is the responsibility of the Engineerto monitor the site construction of thepile and consequently to decide the valueto be assumed for the ultimate bearingcapacity of the large diameter pile. Themaximum is indicated by the curve on theright-hand side of Fig. 10, the minimumby the curve on the left-hand side.

It is advisable to make the pilot pileof a substantial length, in order to ensurethat at a certain depth the instrumentsindicate no stress, thus separating thepile into an "active" part and an "inert"part. If this is done the length of the pileto be designed may be limited to the act-ive length.

The computations, which start in thiscase from a base bearing capacity P„"=0, lead to a final load-bearing capacity P"relying on skin resistance only. In sucha case the possible contribution of thetoe resistance can be considered as anadditional safety factor. Contrary to this,it is not possible to use the pilot pile forthe design of large diameter piles whoselength is shorter than the active lengthof the pilot pile.

Limits of accuracyThe accuracy which can be expected

by using the above calculations dependson the number of instruments insertedalong the pilot pile shaft The more thisnumber, the more accurate is the finalnumerical result for P and e.

Testing programmeThis Paper outlines the philosophy of

design using the congruence equationssuggested by the author A programmeof pile tests is currently being carriedout and the results will be reported in asubsequent Paper.

ConclusionsThe experimental design method des-

cribed in the Paper, based on a mechani-cal similitude derived from a strict con-gruence relationship, does not require anyprevious knowledge of the geotechnicalcharacteristics of the soil,

The soil is defined through theinstruments.The factor f represents the 'soil'n

each section limited by twoconsecutive instruments.

By assuming the settlements at theI toe are equal, the settlements of the

large diameter pile are smeller along theshaft and at the top in comparison with

,the corresponding values of the pilot pile.

~This is on the safe side.

Through the analytical calculation, it ispossible to separate the base resistancefrom the skin resistance of the pile. Thisenables the Engineer to assess its perfor-mance specification (allowable settle-ments, safety factors, etc.)

References1. Cambefort, H. (1964): "Essai sur le comporte-

ment en terrain homogene de pieux isoles etde groups de pieux" Annales de I'InstitutTechnique du Bgtiment et des Travaux Pub'ics.Paris, Decembre.

2. Meyarhof, G. C. (1975): "Bearinq capacityand settlement of pile foundations". XI Ter-zaghi I ecture, ASCE Annual Convent. on, Den-ver, Colorado, November 6.

3. Lizzi, F. (1976): "Pieu de fondation k cellulede precharge" Construct'on, No. 6, Paris, Join.

4. Peck, Hanson, & Thornburn (1973): FoundationEngineer.'ng

March, 1980 25