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4

G. G. Goble, Chairman Depa r tmen t of Civil, Environnrental and Architectural Engineering

University of Colorado, Boulder, Colorado 8 0 3 0 9 Garlend Likins. President

Pile Dynamics, Inc., Cleveland, Oh io 4 4 1 0 6 W. Teferra, Senior Engineer

Pet ro Dynamics, Inc., Lafayette, Louisiana 7 0 5 0 2

For presenta t ion a t Associated Pile 81 Fitt ing Corp. PILETALK Seminar

Miami Beach, Florida, March 1978

PROF. G O R L E is Cllairmun, Dept. o f Civil, Environmentai & Architectural Engirlecrirlg orrci Director o f tile Pilitlg Research Laboratory, L!niversiry o f Colorado. Wllile Proji.ssor utltl Chuirn~an o f ' t h e Civil Engineering Departnlent at Cuse IVestcrn Rescrvc Utliversi~y, George Goble developed a me thod o f Pile Capacity Dcternlirroriorr from Dytratrlic hIeasurement that is gaining world wide acccpturlce ailcl rise. Norv at the CJrliversity o f Colorado. und head o f a j'orrndutiorrs cotrs~rltitrg orgot~i~ut iotr as well, I-'roj:.Goblc is actively corttitlrting dc~~elupttrctrts in [Iris J'icld. A native o l Idaho with h1.S. artd Ph.D. degrees fro171 t l ~ e University 01' IVushing:on, ,\lr. Goble runs a Fullbright Grantee for advanced s tudy it1 (;crnictl)'. He worked ~citlr the U. S. Air Force, Oregotl DOT, and u cor~s~~lru t r t h e j t r e startirig a teaching curerr o f Case it1 1961. f l e n r o ~ ~ e d to [Ire U n i v c r s i t ~ ~ o f Colorado it1 early 1977. Proj: Gohle is active in nrutrv profc~ssior~al grolrps c ~ r ~ d 110s \%.on Ironors in wclding and sfructurul design cotrlpetit iot~. l l e wrires and lect~lres e x t r n s i v e h o n pile jout~datiotls.

I i G A R L/IIVD I,IKIi\'S is prcsitlent oj' Pile Dynumics oJ'Clevelatld and a consult-

ant t o Gnhle & Associufes. f l c srudied urlder atid rc~o.rked with Prof. Goble o n cleveloprrrc~t~~ oj' pilc cupucitv mon i tor i r r~ eqrripn Ile holds a B.S. {rum

I I C'usc, a11 ;If. S. jbom C(I.TC ItJestcjrr~ Rcsc~rve Urii~'ersity utld is a PI1.D. candidare I . , rl1t.r.c..

It1. Tt'f;f:'fZ K.4 rvorkctl oti rlrc O/liO D O T ~ ~ r o j e c f r ~ ~ h i l c a graduure srudcnt at Cusc, IiJestcr~l. rlrcrr cot11 itllied rvifll (,'ohle 11 ssociures. I{e is )lo tv Setlior f f trgitrc~~~r rvitlr l'c~tro Dytlurrrics, w h o ( l o ojj'shore work with the Pile .4 nalyzer.

In 1 9 7 2 the Ohio Departrnent o f Transpor ta t ion cllanged the i r pile driving specification t o read, essentially, tha t H-piles driven t o rock should be driven to a b low c o u n t o f 7-0 Bt'l (b lows per inch), indepcndent o f h a n l n ~ e r size o r any o t h e r considera t ion . Since this change in specification was controversial, a research project was under taken t o investigate it at Case Western Reserve University unde r t h e sponsorship of t he Ohio Depar tment of Transpor ta t ion and t h c 1:rderal I l ighway Adminis t ra t ion .

'l'hc t>;isic goal of' this stlicly was Lo esamino 'tlte performance o f steel ti-piles ~ l r ivcn ii~corcliilg to tile specific;ition u~lcler ;I variety o f conditions. -. I w o sites were selectri l : v n c Iloving a so f t shallow overburden underlain by a

. . . d u * . - L - L , UL1,hL U ~ L . I ~ U l U L I 1 gldull '-3 so f t rock tha t became harder with dep th (Cleveland). 11 sod p r o f ~ l e fo. L. is given in Fip. 1 and 2 -.

V ..riety of ilarnmers were used at each location and t h e piles were aU , driven t o t h e 2 0 BPI cri terir . During driving extensive d y n a ~ n i c m e a s u r e ~ n e n t s

were made. ~ f t e r cornpietion o f driving most o f t h e piles were load tested statically and then ext rac ted fo r visual examinat ion .

T h e data obta ined in these tests were qu i t e vo lun~ inous . It will orlly be possible here t o surninarize the results with emphasis o n t h e conclusioils. A m u c h m o r e comple t e presentation o f t h e results is con ta ined in Reference I .

F I G U R E 2 Soil Profile, West Cleveland (W92)

Sanduskv Tests-Driving t o Hard Rock

All of t h e test piles were 1 0 I-IP 42. This section was selected in o rde r tha t load tests c o l ~ l d he run t o failure at a reasolqable force magni tude . Table I gives t h e hammers used and t h e piles driven a t t h e si te near Sandusky, Ohio. O n e vertical pile and o n e pile battered at o n e hor izonta l t o fou r vertical were driven t o a b low coun t o f a t least 20 BPI o r unti l t h e pile had obviously been extensively damaged at its t ip. Vertical piles with APF points were driven with each h a m m e r except t he MKT 9B3. T h e 9B3 was n o t used fo r t h e piles with points since this hamm,er was though t t o be t o o small t o damage the piles w i thou t points so t ip damage was n o t considered t o be a problem. O n e addi t ional pile was driven by the Kobe K-7-5. Th i s pile designated K25-VE, was driven p l u m b wi thou t t i p reinforcement and was used t o a t t empt t o drive a pile with a large h a m m e r with n o tip re inforcement and not i n d ~ c e ' d a r n a ~ e . T h e h s m n ~ e r was immediately shut d o w n when i t was observed tha t t h e t ip had rcachcd rock.

During driving, b low coun t was recorded as well as cornpletc set-rebound records and , fo r diesel hammers s t roke o r bounce chamber pressure. In addi-

l i V 8 r . lulcr dl lu d ~ L C s L l d ~ l O l l W d b I I I L ~ S U I L L d l i t l ~ ~ I I L . L ' I iULLbhCCI oy the Plle Capacity Analyzer. Analog records o f each r b low were

X X X X X X X O o o u m m w a N N N N W ~ . ~ W ~ ~ N N N ~ D V I L n W W W U L J O O O W W I I I I I

. m 7 < m & & & & 1 1 1 1 1 1 ' " " " <;

recorded o n magnetic tape . Thet ransducers and recordin; >ern are shown in Fig. 3 . Due t o space l imitation this recording and processing sys tem will no t be discrissed fu r the r since it has been described extensively elsewhere (Ref . 2). T h e Case Method pile capacity is glven for each pile in Tahle I.

P v Z H

0 cj

t' > v m m H e P m n cn H

FIGURE 3 Case Method analyzer and recording sys tem with transducers o n t h e pile in t h e foreground.

Af ter driving, I 2 o f t h e piles were loacl tested statically. The capacities n1e:rsured arc also rcpor tcd in Tal>lc I. All load tests were at Constant Rate o f I'enetratiorl; t he Capacity was evaluated using Davisson's procedure.

T h e Sandirsky s i te was almost ideal in supplying t h e desired condi t ions o f :I soft overburden over hard rock. In t he overburden soil t he blow coun t s were very low and in every case rock was rcachetl within, o n e foot o f t h e sarne dep th . Driving c o r ~ t i n u e d in an atternpt to reach the desired b low coun t . In Inany cascs fo r pilcs wi thout pile t ips the hot torn o f t h e pile prompt ly buck- led ant1 fur ther apparent penet ra t ion o f t h e pile was due t o addit ional pile danlage. Since the ovcrhurilen soil is a h o r ~ t 2 2 ft deep , tile ; ~ n ~ o u n t that t he pilcs have heen s h o r ~ c r i e ~ l can he clctcrmincJ hy sui>tractinp that alnourlt.

Duririg driving t h e performance o f ail of t h c pilcs with tips was the same. Sliortly a f t e r r s a c h ~ n g rock tha t portion o f the pile extentling above the

FIGURE 4 Sandusky Pile 9B3-V

F,IGURE 6 Sandusky Pile 520-P

FIGURE 5 Sandusky Pile 520-V

FIGURE 7 Sandusky Pile 520-8

grounu lallccl In gross buckhng. Ut course, these huckle were cllt o t f before performing ttie load tests. Also pile 08-V bu'cki y a t t h e t o p af ter reaching rock, probahly d u e t o p o o r driving sys tem ai,,;nent.

S o m e o f t he ext rac ted piles are shown in Fig. 4 through 12. T h e condi t ion o f t he 520-P pile in Fig. 6 was typical o f all o f t h e piles tha t had point rein- forcement. When the piles were ext rac ted 3 back-hoe was used t o excavate t o within abou t 4 ft of tile t ip t o avoid adt l i t iond pile damage du-ing ext rac t ion . Even this dep th o f soil was sufficient t o hold t h e t ip and pull of f s o m e o f t h e previously damaged sec t ion , as seen in Fig. 1 0 and 11. Some fur ther com- ments are appropriate. Note tha t even though pile 520-B was badly buckled and probahly was shor tened b y a t least 3 f t , it still carried a static load o f 354 kips whicti is associated with a stress o f 28.5 ksi. T h e smallest failure stress was 8.6 ksi bu t t ha t pile was fur ther loaded t o a stress o f 12.4 ksi prior t o d iscont inui l~g the test. Damage o f t h e t y p e s h o w n in Fig. 4 did no t affect the pile capacity.

Soft Shale in West Cleveland

The soil a t t he second site graded gradually f rom a dense sand.y silt t o a decomposed shale t ha t increased in s t rength with dep th . T h e general proce- dure used iii driving and testing was the same as a t t he Sandusky site. It was not possible t o obta in exact ly t he s ame Ilarnrners. T h e Dslrnag D-15 replaced the Kobe K-13 and the Delrnag D-5 was added. A Link Belt 4 4 0 was used in rcstrike testing.

In :he soil o f this si te driving was qu i t e different. All piles refused wi thout an:; sign o f damage except for o n e pile tha t was dainaged at t he t op , probably due to h a m m e r inisalignment. A s u m m a r y o f t he piles driven is given in Table TI.

F IGURE 8 Sandusky Pile 08-V F I G U R E 9 Sandusky Pile 0843

FIGURE 13 W 9 2 Pile K 2 5 V

13. For the rock condi t ion of w5athered shale gradually becoming more f i r n ~ with dep th , it was fount! that t he largest pile capacities were obtained from the deepest pile penetrations. Similarly, t he lowest capacities c o n e - sponded t o the sliallowest penetrations.

-1 I u 2 i i i &- I =c -z 1 w r.. Lo i 7 . All piles drive11 t:, t hz hard l imestone were at o n e t ime capable of sup- -1 c: a ~ i 0 ~ 0 ~ ~ ~ 0 ~ 0 3 ~ 0 0 ~ 0 0 0 3 H i % > . = z ; . z 2 : , . Z Z > . Z z i . Z Z z Z 1 porting loads similar in size t o the pile yield load. These maxirnum pile L ;n = I capacities were observed b y either Case Method testing o r by static load tests.

3 &- I I 8 . Contiriued driving in the a t t empt t o ob ta in several inches of pcnetra- y ; - i > a n ' a m > a O > a a , a m > a O ! t ion in to t h e hard rock only 1ed.to s t ructura l pile damage, confirmed b y elec- c: i < d

5 w 3 ( A A , - ! , " I 1 I I ~ 1 1 , 1 , , , ,

0 0 0 tronic measureinents and pile ext ract ion. This s t ructura l damage was some-

kl r- m ! n L n m m m

2 O m Q m m ' n ~ ~ ~ . m m m 4 d , c , , N w m

! " m m n ~ ~ ~ ~ m o o o n n ~ x y y t imes responsible for large reduct ions in load capacity. - -.. 9. Larger hanimers ( 0 8 , K1-5) clearly damaged t h e Sandusky piles before

n Z the 20 BPI (b lows per inch) 1 9 7 2 Ohio DOT driving specification was satis- d u

U 0 fied. If piles were n o t excessively driven (08V where driving was s topped w Wo? 0 0 C 3 0 o

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W 0 . -4 0 eaiiy due t o local t o p damage o r K25VE which was s topped intentionally

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o after only one blow on rock) then good static load test performance was

1 achieved. I 10. Pile tip protect ion prevented t ip damage a t t he hard rock site. Piles then i I

failed struclurally above ground in gross co lumn buckling during driving. This u above-ground colurnn failure did no t adversely affect t he compressive static

m 0 0

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0 0 0 0 4 0 11. Best results for driving piles a t t he Sandusky site would not follow a

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blow coun t criteria. Blows per inch is meaningless since real rock penetration N N m

-4 was no t achieved. The blows per inch gave on ly a n indication o f how effective

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the hammer was in damaging the pile structurally. Driving beyond 3 0 BI'I f o r ,, - 9

. - the 5 2 0 and K13, and beyond 6 BPI fo r t h e 0 8 and K25; for o n e inch was an P invitation for s t ructura l pile damage. E 12. T h e dynamic field ins t rumentat ion did an excellent job in Sandusky o f 2 b

a determiriing when t h e pile first had sufficient capacity o r when the pile was m being damaged. >

I .E" an .4 VI !n E Y 2 4 (0 C n cl > cl N

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4 * Different in Weathered Shale

. ~ c r ".""rnefi twlore the 10 131'1 1972 specificati(,n was ~ a t i s f i e d . ~ h l - .en by larger hamrners also had higher capacities. e largest ha~nrne r s (K75 and 0 8 ) dnrnaged the pile tops at t he Cleve- -- A.

. land test site before the 1972 driving criteria o f 20 BPI was achieved. 16. Although n o pile tip sustained severe structural darnare which woulti

reduce load test capacity, the flange t ips o f several o f the piles were spread apart . This flange distortion was largest for t he large Ilammers ( o r largest pile penetrations).

17. While capacities a t the end of driving were adequa te a t W9; ? for a 9 ksi design and safe ty fac tor of' 2.0 except for piles tiriven by the DS and 9B3, static testing t w o weeks later revealed a signicant loss in static capacity. At this t ime only t h e piles driven by the 0 8 ant! K25 still had sufficient capacity.

18. Dyn'arnic testing o n x i t r i k e at W92 of t l ~ e S20V and O8V piles af ter t he static tests also showed a loss of capacity since the t ime of initial driving. Comparison o f t h e CAPWAP analyses for these piles reveals t ha t t he loss o f capacity was due to resistance losses in the shale. A small set u p resistance was observed in the solid overburden.

19. Due t o the substanti;il strength loss with t ime in the shale at W92 , piles driven by the 5 7 0 and D l 5 also n o longer met the design load with a safety fdctor o f '2.0.

20. In every case at W 9 2 , the 1972 driving specification was n o t satisfied. Either the piles had insufficient static capacity for the 9 ksi design load and a safety fac tor o f 2% ( D 5 , 9133, 5 3 0 and D15) o r t l i e pile was damaged due t o excessive stresses before the 20 BPI was reached (05 , K 2 5 ) .

21. Pilc t ip protect ion had little, if any , effect o n static pile load perform- ance at t h e W?;! site. It is interesting t o , n o t e that t he effect of t h e sof t rock was to prevent t ip damage. I t is hypothesized that since the resistance on ly developed gradually as the pilc penetrated, t he rock provided lateral restraint sufficient t o prevent load buckling.

Con~par i sons a t Hard and Soft Rock Sites

22. These t w o sites probably represent limiting condi t ions fo r t he range of rock s t rengths o f interest .

23. It is interesting t o no te that the pile stresses were s t~bstant ia l ly influ- enced b y the rock stiffness ant! soil overh~lrden. Cross buckling of the pilc in the 6 t o 8 ft of unrestrained colurnn length above the ground bccurred o n all tip reinforced piles a t the hard rock Sandusky site. No'pile failed b y gross buckling a t t he so f t rock W92 site.

24. Major pile t ip damage is much more likely when the rock is hard and t h e pile will no t penetrate. Penetration in to . the soft rock prevents this s tn lc- tural damage.

25. T h e soil strength of the overburden is also impor t an t in determining the Wcelihood o f damage. Large skin tesistance forces tend t o reduce t b e down- ward traveling compression wave with t h e result tha t t h e max imum force a t t h e pile t ip is reduced. This smaller tip force is less likely t o cause tip damage. This was t h e s i tuat ion at W92. Inspection of the n laximum spring forces in CAPWAP shows a reduction in maximum forces with d e p t h d u e t o the rela- tively large skin friction. F o r the piles at Sandusky with l i t t le skin resistance, the inpu t compression wave travels unchanged t o the pile t ip. If tip resistance is small, t he wave reflects as tension and the net force is small at t he tip. If t ip resistance is large, however, the compression wave reilects in coniprcssion. The t w o waves superimposed are then likely t o cause damage.

*". . during load testing. At the Sandusky site t h e so11 a h o ck was very

soft and the rock was unusually level. Therefore , th . ?em does not \

appear t o be a serious o n e for pile design. -

27. One of the primary consideratiolls in pilc d e s ~ g n must be the magnitude of the 1o:ld to be carried. If t h e ~ t r u c t u r e loads are small, then high design stresses should no t be used. It may be substantially more cost effective to use lower design stresses and m o r e piles in sorne cases.

28. Basetl o n the test results discussed here , we recomnlend that pile driving of H-piles to so f t rock be controlled in t h e same manner as is t he case for o ther pile types.

In general, load tests are unnecessary and driving can probably be governed by a formula. f4amrners should be selected in the same fashion as is t h e case for friction piles. In unfavorable soils o r o the r critical cases tile Case Method can be usetl for capacity evaluation. T h e loss o f s t rength o f the shale at W92 should be of serious concern. Piles driven in to shale should therefore b e restruck with the longest possible wait tirne and blow coun t s s i~ou ld be mea- sured with great care at t he end of driving and the beginning o f restrike. These blow counts should then provide a satisfactory const ruct ion control inechanisrn. F o r soil and rock condi t ions similar t o Sandusky, piles could be driven with small hamrners !vith l i t t le concern fo r damage. If large ham- mers are used, i t is difficult t o avoid darnage a t the pile t ip.

For piles driven t o hard rock, particularly when the overburden soils are soft , a b low coun t criteria is of limited usefulness. A penetration criteria

may be more desirable and the use of 1:lrgc hammers should be carefully con- trolled, particularly if the piles are shor t . In such cases, i t is more desirable to simply drive until rock is reached as identified b y blow coun t and ham- mer performance. If l a rge 'han~mers are used, t h e inspector should be cau- tioned not t o overdrive.

For hard rock, t he use of t ip reinforcement in t h e form o f pile points was shown to be very effective. We recoinmend tha t they always be used on

hard rock jobs.

29. Dur t o Ilrnited da ta , we canno t define t h e line between hard and soft rock. Untll more da ta is a v a r l ~ b l e o n a m u c h broader range of rock types, this definition must rcrnaln subjective.

ACKNOWLEDGMENTS

The tests described were spor~sored b y the Ohio Departmenr o f Transpor- to [ion wirh participarion of the Federol High way Admiriistrorion. The

authors want to especially acknowledge the assistance of' Raymond A. Grover, Foto~dorions Engineer, ond Leon Talbert, Engineer o f Research and Develop- ment wirh the Ohio Deportmerit of Transportatiot~.

L . B. Foster, Inc. supplied the Kobe hummers. Associa:ed Pile & Fitting C'orp. jiir?~islrcd rhe Prlryrl IJoints und the It-piles t o which tlley were 0:tached ro supplemerrf rhe tesrs on tirrprotected piles.

The opiniotrs and eoncltrsions expressed are ~ h n s c o j [he uuthors; rhry do nor necessuri1.v represenr the views o f f h e Ohio D07 '0r FIIWA.

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