Driving Stresses in Concrete Piles Journal/1976/Januar… · the diesel hammer. There is a great...

Driving Stresses inConcrete PilesG. G. GobleProfessor of Civil EngineeringCase Western Reserve UniversityCleveland, Ohio

K. FrickeGraduate AssistantCase Western Reserve UniversityCleveland, Ohio

G. E. Likins, Jr.Research AssistantCase Western Reserve UniversityCleveland, Ohio

K. Fricke G. E. Likins, Jr.

70

D uring the summer of 1973 exten-sive measurements were made

during the driving of piles for a bridgein Dade County, Florida. These mea-surements raise serious questions re-garding the validity of the ram-pileweight ratio as an index for specifica-tion of pile driving hammers if a dieselhammer is used.

During the past several years a re-search project on pile driving has beenunderway at Case Western ReserveUniversity sponsored by the Ohio De-partment of Transportation and theFederal Highway Administration. Somefinancial support has been obtainedfrom several other state highway de-partments and a few private organiza-tions including the Prestressed ConcreteInstitute.

The primary purpose of this projecthas been to develop means for deter-mining static pile capacity from dynam-ic measurements made at the pile top.Considerable success has been achievedand the resulting approach known asthe Case Method is now being imple-mented with increasing frequency inpile design and construction control.

Summary ofPrevious Work

A complete discussion of this researchproject, covering a 10-year period, isbeyond the scope of this paper.1 -4

However, it will be reviewed briefly sothat the Miami tests can be placed incontext.

The Case Piling Research Project hascomprised five phases of activity.

First phaseThe first and most important part of thework has been the development of in-strumentation for making dynamic mea-surements at the pile top during driv-ing.

Since most of the measurement ex-

Synopsis

Dynamic tests `"'were per-formed in Miami, Florida, onthree 18-in. square, 60-ft longprestressed concrete piles.Each pile was driven with adifferent size open end dieselhammer. Strains during driv-ing were measured at themidlength of the pile and atthe pile top. Set rebound andram stroke were also mea-sured. Since some currentspecifications include pile-

ram weight ratios for the pur-pose of limiting drivingstresses in concrete piles,this test was useful to investi-gate how successful the coderestrictions were in limitingpotentially damaging stress-es due to various size ham-mers. Measured tensile andcompressive stresses werecompared with hammer size,pile net displacements, ramstroke, and hammer throttlesetting to determine condi-tions which were most likelyto produce damaging tensileor compressive stresses. Itwas found that pile-ramweight ratio is an unsatisfac-tory parameter for controllinghammer selection for openend diesel hammers. In fact,the use of this ratio can leadto excessive pile stresses.Finally, alternate proceduresfor selecting driving equip-ment are presented.

PCI JOURNAL/January-February 1976ҟ 71

Fig. 1. Data acquisition system at-tached to a concrete pile.

perience has been obtained during on-going construction projects, a primaryconsideration in the development of theequipment was that it be rugged anduseable in the rather difficult condi-tions common on pile driving jobs.

The resulting system is air portableand has been used throughout most ofthe United States as well as in Canada,Mexico, and Europe.

The field data acquisition system isshown in Fig. 1 attached to a pre-stressed concrete pile during the DadeCounty tests. It consists of a straintransducer, the diamond-shaped deviceshown in the extreme upper right inFig. 1.

These devices are bolted to oppositesides of the pile. They measure thedeformation over a 3-in, gage lengthand have been calibrated both staticallyand dynamically with measurementstaken from strain gages directly at-tached to the pile.

The other transducer next to thestrain transducer in Fig. 1 is a piezo-electric accelerometer. These devicesare also attached on opposite sides ofthe pile. For concrete piles the trans-ducers are attached with expansionbolts set in holes drilled in the pile sur-face. This system has now been wellproven by many tests on concrete piles.

The ouptut from the transducers iscarried through a single cable to a port-able analog tape recorder and associat-ed signal conditioning. After returningto the laboratory the tape recorded datais automatically converted to digitalform using a minicomputer controllingan analog-to-digital converter.

The data can then be stored on digi-tal magnetic tape and can be automati-cally plotted and processed. A sample ofsuch plotted data is shown in Fig. 2This system makes it possible to recordand process hundreds of hammer blows.

Second phaseThe second phase of the research pro-gram was the development of a meansof reliably predicting pile capacity fromthe above dynamic measurements. Thisgoal was the original motivation for theproject and a series of prediction meth-ods have evolved where successivemethods have represented improve-ments and refinements.

This approach, known as the CaseMethod, has been correlated with over70 piles which were also statically test-ed to failure and has recently also beenproven analytically. 4 The Case Methodcomputations involve the measuredforce and acceleration records and arecomputationally very simple.

Third phaseA third phase of the project has beenthe development of computational de-vices to perform the Case Method com-putation in the field in real time and im-mediately display the results.

The current equipment will not only

72

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perform this task but also calculate theenergy delivered to the pile and displayextreme values of force, velocity, andacceleration generated during impact.Thus, the system could be used in thefield to modify the driving system basedon measurements of actually generatedstresses.

This equipment has been acquiredby state departments of transportation,the Federal Highway Administrationand private foundation consultants forroutine use in pile design and construc-tion control.

Fourth phase

The fourth phase of the piling researchprogram has been the study of dynamicpile analysis. An analysis procedure hasbeen developed which uses the mea-sured force and acceleration as inputand determines the soil resistance forcesand their distribution.

A computer program, known asCAPWAP, performs a wave equationtype analysis on the pile using the mea-sured acceleration as input. The resis-tance forces are adjusted iteratively un-til the pile top force computed from theinput acceleration matches the mea-sured force. An application of this pro-gram is described in this paper.

Fifth phaseThe fifth phase of the research activityhas been the study of hammer perfor-mance characteristics. This has particu-larly involved diesel hammers and onesuch study is presented here.

Most of the research activity hasbeen sponsored by the Ohio Depart-ment of Transportation. In order thatthe available measurements be broad-ened beyond pile and soil types avail-able in Ohio, tests were sponsored byother highway departments. As part of


that research program three sites werescheduled for testing by the Florida De-partment of Transportation. 5-7

At about the time of these tests achange was made in the pile drivingspecification used by the Florida DOTwhich would have required that thepile-to-ram weight ratio not exceed ap-proximately four. This type of limitationis common in pile driving specifications.

The requirement is based on the factthat in the simple impact of a mass on aslender rod the length of the inducedstress wave is determined by this ratio.Heavier rams cause longer stress waves.

On the other hand, the induced stress(at least until after reflection from thetip, neglecting small side friction forces)is directly proportional to the ram im-pact velocity.

In the case where a light ram is used,inducing a short stress wave, the com-pression wave is reflected as a tensionwave back on the oncoming compres-sion wave, during the easy driving por-tion when there is little tip resistance.

If the compression wave is short,then the net tension can be sufficientlylarge to exceed both the prestress forceand the tensile strength of the concrete.The traditional solution to this prob-lem has been to use heavy rams withthe resulting long stress waves so thattension stresses are limited.

This approach was supported by stu-dies conducted at Texas A and M Uni-versity using a wave equation computersimulation of the pile driving system.This computer program, reported in1963, 9 and other similar ones havefound increasing use in pile driving.

However, the program was devel-oped to model air-steam hammers anddid not include either the thermody-namic or mechanical characteristics ofthe diesel hammer. There is a greatdifference between a pile driving sys-tem and the idealized rigid mass im-pacting the elastic rod.

Contractors using light, high impact

velocity diesel hammers have claimedconsiderable success in driving con-crete piles, and at the time of the Flori-d.. specification change, they voicedsome concern. Since the contractor onthe Dade County test site was planningto use a diesel hammer it was possibleto expand the scope of the tests tomake measurements to check the actualstresses generated.

Field Tests

Extensive soils data at the site wereobtained and made available by theFlorida Department of Transportation.In general, the soils could be describedas a mixture of limerock and sand. Splitspoon blow counts showed considerablevariability as could be expected fromthese soils. The site was ideal for evalu-ating induced tension stresses since thepile could be expected to break throughhard layers during driving.

For the purpose of the hammer tests,three piles were driven. They were all18 x 18-in solid sections 60 ft long. Allaspects of the driving systems wereidentical except the hammers.

Three DELMAG hammers wereused; the D-30, the D-22, and the D-12. These three machines gave a rangeof pile-ram weight ratios from 2.97 to7.18. Dtails of the hammers are givenin Table 1 and the piles in Table 2.

The DELMAG hammers are classi-fied as open end diesels; that is, the ramis free to float in the cylinder and thestroke will vary depending on the driv-ing conditions and, for the D-30, on thethrottle setting.

The following measurements weremade for every hammer blow on eachof the three piles:

1. Strain at the pile top was recordedon magnetic tape.

2. Acceleration at the pile top wasrecorded on magnetic tape.

3. Strain at pile midlength was also

74

Table 1. Miami hammer information.Performance data D12 D22 D30

Ram weight (piston), lb 2,750 4,850 6,600Impact block weight, lb 810 1,600 1,600Energy per blow, ft per lb 22,500 39,700 23,870-54,200Number of blows per minute

(rated) 42-60 42-60 39-60Note: Pile Helmet—MKT, 24 in.-sq, with an 18-in. filler. Total assembly

was 2650 1b. No cushion was in helmet assembly.

recorded until that point on the pilepassed below the ground surface requir-ing the removal of the transducers.

4. Set-rebound was measured at theground surface on a piece of paper at-tached to the pile by drawing a pencilacross a straight edge supported on theground. This gave a good measure ofboth dynamic displacement and finalset for each blow. An example of such ameasurement is shown in Fig. 3.

5. Ram stroke was measured for eachhammer blow by visually observing theram top against a measuring rod at-tached to the hammer cylinder and re-cording each stroke on an audio cassetterecorder.

In excess of 2500 hammer blowswere recorded.

Laboratory Processing

After returning to the laboratory atCase Western Reserve University, alltape recorded hammer blow data wereconverted to digital form and stored onmagnetic tape. It was then possible toassociate the strain and acceleration rec-ords for a given blow with the measuredhammer stroke and the set-rebound rec-ord. The acceleration record was inte-grated to obtain velocity and integrateda second time to displacement. This dis-placement value could then be com-pared with the one obtained from theset-rebound record.

Strains were converted to force usingthe pile area and material dynamic

Table 2. Basic Miami pile information.Pa rameter Test Pi le #1 Test Pile #2 Test Pile # 3

Length (total), ft 60.0 60.0 60.0Length below gages 58.5 58.5 58.5b, in. 18 18 18Area, sq in. 324 324 324Weight, kips 19.62 19.81 19.74Hammer D30 D22 D12 & D30Cushion 3-in. pine 3-in. pine 3-in. pine

Tes t dates

Dynamic test 7-11-73 7-12-73 7-12-73Date cast 7-3-73 N.A. 7-3-73E, ksi 4425 4473 4586c, ft per sec 11,695 11,740 11,907Notes:E = modulus of elasticityc = wave speed in concrete


Fig. 3. Sample set-rebound measurements.

modulus as determined from a fieldmeasurement of the wave speed. Alsothe known proportionality betweenforce and velocity was checked auto-matically.

Selected hammer blows were thenautomatically plotted on a digital drumplotter. Due to the time involved it wasnot feasible to plot each record and acareful visual examination of all 2500blows would have been impractical.

Important parameters were deter-mined and printed for each hammerblow. They included maximum acceler-ation, maximum velocity, maximum dis-placement, maximum energy deliveredto the pile (ENTHRU), final energy(that portion of the energy delivered tothe pile which does not go back to thedriving system as rebound), maximummeasured compression stress, maximummeasured tension stress, and the CaseMethod of static capacity.

Test Results

Test Pile No. 1 was 60 ft long and wasdriven with the D-30 hammer. Drivingand testing started after the pile hadbeen set in a predrilled hole about 7 ftdeep.

A total of 572 hammer blows wererecorded and analyzed. These datawere recorded in six sets in order to di-vide the data into blocks to simplifyprocessing.

Unfortunately, the electronically re-corded data from Set 6 produced mean-ingless results apparently due to com-pression failure at the pile top so thelast 45 blows were lost. However, theram stroke and set-rebound data wereavailable from this set. A strain trans-ducer was located at the pile mid-lengthduring the first three sets (233 blows).

The soil conditions proved to be idea]

76

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since the driving resistance built upsomewhat and then dropped dramati-cally as the pile apparently brokethrough a limerock layer. The stroke,driving record, and Case Method capac-ity prediction plotted versus blow num-ber is shown in Fig. 4.

Easy driving was experienced for thefirst 60 blows. At the point where thepile apparently broke through the lime-rock layer the set increased to almost 5in. per blow. It is important to noticethat at the same time the strokedropped from 6.5 to 5.0 ft. As the resis-tance again increased, the stroke alsoincreased.

For Test Pile No. 1 the ultimate ca-pacity according to the Case Methodclimbed rapidly to a peak value of near-ly 800 kips at about Blow No. 100 andthen decreased steadily throughout theremainder of the driving to a final valueof 400 kips.

Maximum measured compression

stress at the pile top and maximummeasured tension stress at the pile mid-length are also shown plotted againstblow number in Fig. 5. Maximum ten-sion occurred at about Blow 65. Thispoint in the driving record is not asso-ciated with particularly easy drivingconditions (½-in. set).

The tension stresses were much lowerat the maximum set condition at Blow41. The maximum measured tensionstress was 2.16 ksi.

A possible explanation for thechanges in stress, particularly tensionstress, when driving was interrupted, isthat the resistance distribution changedas pore water pressure declined.

Driving was stopped on this pilewhen it exhibited signs of damage atthe top. This damage was probablywhat rendered the top strain transduc-ers ineffective during the last data set.No visible sign of tension cracking ap-peared in spite of the large tension

TEST PILE NO.1

DELMAG D-30 HAMMER

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Fig. 4. Driving data for Test Pile 1, D30 hammer.


TEST PILE NO.1

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Fig. 5. Maximum measured stresses for Test Pile 1, D30 hammer.

stresses recorded.Test Pile. _No. 2 was 60 ft long and

was driven by a D-22 hammer about 27ft from Test Pile No. 1. Driving andtesting began after the pile had been setin a predrilled hole about 11 ft deep. Atotal of 741 hammer blows were record-ed and analyzed.

Again, the data was broken into sixsets, primarily to facilitate processing.All of the recorded data proved satisfac-tory, but inadvertently no ram strokedata were recorded during Set 5. Astrain transducer was located at the pilemidlength for the first two sets (201blows).

The stroke, driving record (set), andCase Method capacity prediction areshown in Fig. 6. Easy driving was en-

countered between approximatelyBlows 15 and 45 in the first data set.The set reached a maximum value ofabout 4 in., apparently after breakingthrough a hard layer of limerock.

Note that at the same time the ramstroke decreased from 4.5 to 4.0 ft. Asthe resistance again increased (i.e., setdecreased) the ram stroke values imme-diately increased to 5.0 ft and thengradually climbed to 5.8 ft as the setreached a low of 0.055 in. per blowduring the last 100 blows.

This amounts to a blow count of 218blows per ft as compared with only 3blows per ft at about Blow 32 of thefirst set. During the easy driving portionthe Case Method Capacity indicatedabout 75 kips and this increased to 130

78

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DELMAG D-12 HAMMER

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kips at the end of Data Set 1.By the end of Data Set 2 the capac-

ity was 350 kips and it continued to risesteadily until it reached a value of 880kips at the end of driving. This is quitea different performance than that ex-hibited by Test Pile No. 1 and is a dis-turbing characteristic of the soil in-volved.

Maximum measured compressionstress at the pile top and maximummeasured tension stress at the pile mid-length are shown plotted against blownumber in Fig. 7. The top compressionstress shows a gradually increasingtrend but is considerably lower thanthat recorded on Test Pile No. 1.

Unfortunately, tension stress datawere obtained for only the first data set.The transducer malfunctioned duringthe second set and then had to be re-moved because of the ground surface.Tension stresses are considerably lowerthan measured on Test Pile No. 1.

Test Pile No. 3 was 60 ft long andwas driven by a DELMAG D-12 ham-mer at a location an additional 22 ft be-

yond Test Pile No. 2. Testing was be-gun after the pile had been set in a pre-drilled hole about 11 ft deep.

A total of 1330 hammer blows wererecorded and analyzed. The data werebroken into 12 data sets. A strain trans-ducer was located at midlength for thefirst nine data sets (1075 blows).

Note in Fig. 8 that as the pile brokethrough a hard layer (around Blow No.300), the set increased rapidly until itreached a maximum value of 0.6 in. perblow at about Blow No. 350 in Set 2.

At about this time, the stroke de-creased from 5.0 ft to about 4.6 ft andthen went back up to 5.0 ft as the re-sistance increased.

Note that the Case Method capacityprediction also decreased as the drivingresistance went down (as would be ex-pected). The stroke remained at 5 ftthrough about Blow No. 800 and thenslowly increased to 5.8 ft at the end ofdriving.

This is shown in Fig. 8 along with thedriving record (set), and Case Methodprediction versus cumulative blow num-

80

TEST PILE NO.3

DELMAG D-12 HAMMER

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200ҟ400ҟ600ҟ800ҟ1000 1200CUMULATIVE BLOW NUMBER

Fig. 9. Maximum stresses for Test Pile 3, D12 hammer.

her. Near the end of Set 11 the blowcount climbed up to 400 blows per ftand then fell back to 267 blows per ftduring Set 12.

Maximum top compression stress andmidlength tension stress are shown as afunction of cumulative blow count inFig. 9. The largest tension stress occurswith the largest sets but it is muchsmaller than those measured with theD-30.

Analysis of Results

To get a general view of the large vol-ume of data obtained, several importantparameters were examined to obtaintrends. In Fig. 10 the stroke for each ofthe hammer tests is shown as a func-tion of pile set.

The data were plotted and from themthe bands shown were obtained as werethe average lines. For both the D-22and the D-30 the same characteristic isshown. The stroke was constant for setvalues greater than about 1 1/2 in. For

smaller sets the stroke increases toabout 6 ft for the D-22 and 8 ft for theD-30.

Still greater strokes would probablyhave been achieved in the D-30 test ifdriving had not been stopped due to thepile top damage. The D-12 hammershowed the beginning of the same trendbut due to its small size only rathersmall sets were achieved.

Important is that the stroke achievedby the smaller hammers was smaller.Apparently, the short stress wave in-duced did not transmit rebound energyback to the ram as effectively as did thelonger stress wave associated with theheavier ram.

In Fig. 11 the maximum top com-pression stress is shown as a function ofram stroke. Since it is known that stressis proportional to impact velocity, thetrend of the data is as expected.

It should be explained that the D-30data are shown as two blocks since veryfew blows occurred with strokes be-tween 61/z and 71/2 ft. However, thedata trend is consistent with the other

PCI JOURNAL/January-February 1976 81

D-22D-12'

2

4

SET - INCHES

Fig. 10. Measured ram stroke versus pile permanentset showing increase in stroke as driving resistance

increases.

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hammers.In Fig. 12 the maximum top com-

pression stress for each hammer blow isshown as a function of pile set. Sincethe stress and the stroke are closely re-lated these curves have the same shapeas the stroke versus set results. TheD-30 data show a greater increase instress with decreasing set.

In Fig. 13 the maximum tensionstress is shown as a function of set. Thebehavior is more complex here. TheD-12 results show an increase in tensionstress with increasing set but only overthe very short available range of set.

The D-22 results are constant over awide range of sets with an indication ofincrease at the smallest available valuesof set. It is unfortunate that some datawere lost in exactly this region.

The D-30 results probably are themost general and can be explained asfollows:

As the set decreased from 4 in. toabout 1½ in., the maximum tensionstress remained approximately constant.In this range the ram stroke was aboutconstant inducing a constant maximumcompression stress. Since there was verylittle tip resistance a constant maximumtension was also produced by the re-flected tension stress wave. As the setdecreased below 1 1 in., the stroke be-gan to increase substantially producinga much greater compression stress.However, the tension stresses increaseat a slower rate because the tip resis-tance has also increased allowing lesstension to be reflected. As the tip resis-tance increases further it finally causesthe maximum tension stress to becomesmaller.

It should be noted that changes in re-sistance distribution may have a sub-stantial effect on the magnitude of theinduced tension stresses.

82

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Fig. 11. Measured relationship between maximumtop compression stress and ram stroke.

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Fig. 12. Maximum measured pile top stress versuspile permanent set. Since the stroke increases asthe set decreases, the maximum top stress also in-

creases.

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Fig. 13. Maximum measured pile midlength tensionstress versus permanent set. With decrease in settension stress increases until the tip resistance is

great enough to reduce the tension reflection.

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Fig. 14. Predicted versus measured midlength stress versus time curves.Maximum tension stress is accurately determined from top measurements

using CAPWAP.

84

Computation ofMaximum Tension Stress

It is not always possible or conve-nient to measure tension stresses alongthe pile length. Therefore, the determi-nation of maximum tension stressesusing the CAPWAP program combinedwith measurements at the pile top wastested. This technique uses the mea-sured acceleration at the pile top as aninput, and adjusts the soil resistances sothat the calculated force at the pile topmatches the measured force.

The primary reason for developingthis capability was to determine the soilresistance and its distribution from mea-surements of force and acceleration atthe pile top. In this case, it can also beused to obtain a complete stress distri-bution in the pile during a single ham-mer blow.

After determining the resistance mag-nitude and distribution from the mea-sured top acceleration and force, thecalculated forces at midlength could becompared with the measured value. Asample result is shown in Fig. 14.

Probably further refinement of thecomputer program will produce stillbetter correlation. These results, how-ever, indicate that the CAPWAP pro-gram can be a useful tool in evaluatingdiving stresses.

Reduced Throttle Tests

After completion of the driving of TestPile No. 3 to refusal with the D-12,the D-30 hammer was used to drive thepile further using a variety of throttlesettings.

This machine is equipped with athrottle having 10 discrete settings,each of which gives a metered quantityof fuel. About 30 blows were recorded

at each of six different throttle settings.

The resulting mean measured valuesof ram stroke, maximum top compres-sion stress, and pile set are shown inFig. 15. The throttle is seen to providea convenient means of controlling in-duced stresses, but it does this with apenalty on productivity.

Conclusions

1. It is feasible and practical to makeroutine measurements of accelerationand force under the hammer during piledriving. Furthermore, large volumes ofthis data can be examined using cur-rently available real time data process-ing systems.

2. The CAPWAP program can beused to evaluate induced tensionstresses from measurements made atthe pile top.

3. The use of pile-ram weight ratiosin controlling tension stresess in pilesduring driving is quite unsatisfac-tory when applied to diesel hammers.While these conclusions are basedon tests at only one site, it is just thesoil conditions present at this site whichthe limiting pile-ram weight ratio issupposed to control. In fact, the highestmeasured tension stresses were generat-ed by the heaviest ram. The reason forthis performance is described in the pa-per. If current pile-ram weight ratiosare used for selecting driving equip-ment, damage problems will be morefrequent and severe. While tests at asingle site can hardly be used to justifyreplacement limitations, they do proveconclusively the inadequacy of the pile-ram weight ratio.

4. Throttle controls on diesel ham-mers can provide a convenient means ofcontrolling induced stresses, but onlywith a penalty on productivity.


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1 2 3 4 5 6 7 8 9 10HAMMER THROTTLE SETTING

Fig. 15. Ram stroke, pile set, and maximum pile top compression stressversus hammer throttle setting for the DELMAG D30 hammer. As thethrottle is reduced, maximum top compression stress also decreases as

does system efficiency.

Recommendations

The Dade County tests have shownconclusively that pile-ram weight ratiois not a satisfactory parameter for se-lecting hammers to avoid tension crack-ing. The measurements made there aresupported by experience at other siteswhere less comprehensive testing wasdone.

However, it is inadequate to statethat a particular parameter, such aspile-ram weight ratio, is unsatisfactorywithout offering alternatives for theconsideration of the profession.

These alternatives are based on themost extensive field measurement pro-gram yet reported, now involving wellover 100 sites with many piles tested oneach site. Unfortunately, the recommen-dations must be more complicated that

just a simple ratio.

The best procedure for equipmentselection is the use of field measure-ments on either a predesign test pro-gram or on the first production piles.

Since electronic equipment is now at astage where it can be used routinely it ispossible to measure both maximumcompression and tension stresses in-duced during driving. The drivingequipment and procedures can be mod-ified so that satisfactory stress limits aremaintained.

If field tests are not possible or pos-sibly as an adjunct to them, the drivingsystem can be checked by wave equa-tion analysis. However, such an analysisis meaningful only if the driving systemand soil conditions, as modeled in thecomputer, realistically reflect the actualconditions.

86

Soil modeling is inexact at best. Fur-thermore, wave equation programswhich are generally available do not ac-curately model the thermomechanicaloperation of diesel hammers. Resultsobtained from wave equation analysiscannot be expected to be better thanthe program used.

If neither of the above procedures isavailable, equipment selection becomesdifficult. Further recommendations arepresented recognizing that much moredata are necessary to provide a reliable,comprehensive answer.

In dealing with a phenomenon ascomplex as pile driving it is unlikelythat a single set of guidelines will applyin all cases.

The recommendations presented be-low are based on preventing pile dam-age while maintaining the highest pos-sible driving system efficiency. Theremay be other considerations which willalso govern equipment selection.

Piles less than 40 ft longNo special requirements must besatisfied. A softwood cushion load-ed across the grain and having aminimum thickness of 2 in. shouldbe used. If damage occurs at thepile top, system alignment shouldbe checked and if damage still oc-curs the cushion thickness shouldbe increased.

It. Piles over 40 ft longA. Air-steam hammers

The pile-ram weight ratio must notexceed 4.0 and a softwood cushionof at least 4 in. thickness should beused. As the pile length increases,the cushion may have to increasein thickness. Probably for pilesgreater than 60 ft long, at least6 in. of cushion should be used.

cushion. In easy driving reducethe throttle to the minimumsetting which causes the pile tomove and the hammer to run. Asthe driving resistance increases,open the throttle. If damage oc-curs at the pile top, increase thecushion thickness or reduce thethrottle.

2. Open-end hammers

(a) For hammer energy-pileweight ratios' less than 2.0,use 2 in. of softwood cushion.

(b) For hammer energy-pileweight ratios greater than2.0, use at least 2 in. ofsoftwood cushion and reducethe throttle so that the ham-mer energy-pile weight ratioof 2.0 is not exceeded forblow counts less than 30blows per ft. For higher blowcounts the throttle can beopened to achieve more pow-er. If top damage occurseither reduce the throttle orincrease the cushion thick-ness. The hammer energy-pile weight ratio limitationcan be satisfied by increasingthe thickness of softwoodcushion used. The manufac-turer's rated energy is de-creased by 10 percent of therated value for each P/a in.of softwood cushion added inaddition to the 2 in. mini-mum.

In the application of a dynamic for-mula the manufacturer's rated energyshould be decreased by 10 percent ofthe rated value for each 1 r in of soft-wood cushion (of course, this reduc-tion should be applied to the 2 in. mini-mum cushion also).

B. Diesel Hammers

1. Closed-end hammers°The hammer energy-pile weight ratio is de-

Use at least 2 in. of softwood fined as the manufacturer's rated energy in foot-pounds divided by the pile weight in pounds.

PCI JOURNAL/January-February 1976 87

7.References

1. Goble, G. G., and Rausche, F., "PileLoad Test by Impact Driving," Pa-per presented to the Highway Re-search Board Annual Meeting,Washington, D.C., January 1970.

2. Goble, G. G., Rausche, F., andMoses, F., "Dynamic Studies on theBearing Capacity of Piles, PhaseIII," Report No. 48, Division ofSolid Mechanics, Structures and Me-chanical Design, Case Western Re-serve University, 1970.

3. Rausche, F., Moses, F., and Goble,C. G., "Soil Resistance Predictionsfrom Pile Dynamics," Journal ofthe Soil Mechanics and FoundationsDivision, ASCE, Vol. 98, No. SM9,Proceedings Paper 9220, Septem-ber 1972, pp. 917-937.

4. Goble, G. G., Likins, G., andRausche, F., "Pile Capacity by Dy-namic Measurements," Departmentof Civil Engineering, Case WesternReserve University, Cleveland, Ohio,March 1975.

5. Goble, G. G., and Likins, G. E., Jr.,"A Static and Dynamic Pile Test inWest Palm Beach, Florida," Depart-ment of Solid Mechanics, Structuresand Mechanical Design, Cleveland,Ohio, August 1973.

6. Goble, G. G., and Fricke, K. E., "AStatic and Dynamic Pile Test in Tal-lahassee, Florida," Department ofSolid Mechanics, Structures, andMechanical Design, Cleveland,Ohio, December 1973.

Discussion of this paper is invited.Please forward your discussion toPCI Headquarters by June 1, 1976.

Goble, G. G., Fricke, K. E., and Li-kins, G. E., "A Static and DynamicPile Test in Dade County, Florida,"Department of Solid Mechanics,Structures, and Mechanical Design,Cleveland, Ohio, March 1974.Lowery, L. L., et al., "Pile DrivingAnalysis—State of the Art," Report33-13 (Final). Texas TransportationInstitute, Texas A & M University,January 1969.

9. Samson, C. H., et al., "ComputerStudy of Dynamic Behavior of Pil-ing," Journal of Structural Division,ASCE, Vol. 89, August, 1963.

Acknowledgment

The tests reported here were sponsoredthrough a grant to Case Western Re-serve University by the FoundationEquipment Company, Newcomerstown,Ohio.

The study was performed in conjunc-tion with another pile testing programsponsored by the Florida Departmentof Transportation.

The opinions, findings, and conclu-sions expressed in this paper are thoseof the authors and not necessarily thoseof the sponsors.

The basic research project withinwhich the equipment and methods weredeveloped was sponsored by the OhioDepartment of Transportation and theFederal Highway Administration.

Financial assistance was also re-ceived from the Prestressed ConcreteInstitute.

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