Dynamic Prediction of Pile Static Bearing Capacity - vulcanhammer ...

More documents

Recommendations

Info

) ?.larch, 1963 SM 2~~~.c~nl~loycci \\,as co~;lpnrcd to the pile ultimate static bearing cxpncity.!s conlparison 1s tile 1)rlnclpal object of the paper. It is notd that theoret-? niatcliin:: of rc.';ponsr re'sults is valid only up to the point in tlmc forich velocity passcs through :I zero value.CO.MPARISON OF THEORY AND CXPERIAIENTFig. 2 ;)resents top force versus time as nleasured for all experimentales in the present study. Fig. 3 shows results for Pile 1, a full-scale steele pile 74 ft long. Theoretical and ex~erimcntal rcsults bear close resemnce,and the ;~redicted static bearing capacitv is Co = 225 kips ns opposed214 kips measured in a static load test. C, is thus Iiigh by 5%.REDUCED SCALE PILE NO. IPC, -9.9 KIPSvi;X'in this instance. This example typifies pile load capacity increase with lime.Bccat~se of thc cxcellc~~cc of thcdyn:ln;ic prediction Inthis case it also becomesa good object for test of tfjc sensitivity of thc prctltction method to thevalue of C(, chosen. Fig. 7 illrlstrntes this, where values of C, = 232, 242. and252 kips have beer1 tried, with corrcspnding theoretical dlsplaccments comparetito me;isurcd tcst results. it is clear from this that close displacementniatcliing sharply delineates the proper Co vali~e under the present method.The elastic and rigicl Body contri6utions to pile veloclty may be separatelycalculated. The elastic result contains alnlost all of the 'oscillatory" portionof the reslmnse, whereas the rigid body contribution is one of almost astraight-line decreasing velocity, suggesting the action of a rigid bmiy underconstant deceleration from constant soil resistance.Nore can be saici of the new information revealed by an:~lyses of this type.This will be reserved to n later point, alter n simplified dynamic pllc calculationhas been suggested.Fig. 8 illustrates the clyn;tnlic prediction method for a cnsc of a very shortpile (12 ft). The match acliic~ed is the worst of all cases treated as far ascurve shape is concerned, though general trends are preserved. Ifowever, theprcdictcd alti~nate stntic be:lring cnp:tcity is CC, = 9.9 kips as op;msccl to ameasured 9.5 kips, the prctlictiol~ being high by 4%.The cnlculated rigicl body nrai elastic contributio;~s for this case are presentedin Fig. 9. h'ere, in this short pilc, the c1:istic contributir-, is seen 13be reiatively much less import:int. As seen irom this figure, the mCthcwl underuse clearly delineates thc diifercnt roles of bo!h elastic nnd rigid bc~iymotions in the action of piles.In Ref. 9 ?. rigid pile m&iel u.as cmployecl. Briefly, !l!c :\nnl:;sis methodused therein was as iol:o\vs:Let .I/ bc tIlc nlass of t!le l~ile 2nd S ils :~cccl~r:ctinti; then the force b:11-\ Tlh'E IN MILLISECONDS'ig. 4 s!lows results lor pile 11, another lull-sc:ile stcel plpc pilc 74 ft:. Iiere :he n~atch of tlleoretical and expcrin~cntal curves is less satisorybut the stntic capacity prediction remains lair: C, = 290 kips asnst a measured ultinlate static value o[ 252 kips, tlic dynamic prcdictionlr here being 15%, high.'ig. 5 shows results for Pilc 111, upitti theory bcing a good match as to~e stupe in displ:~cemcnt,: velocity, and acceleration. T!le stntic caj~acity!icted is C, = 220 kips as opposed to a mensured ultimate value of 204, the dynamic result being high by 8%..'ig. 6 presents rcsults for Pilc ITIn, the snmc pilc as 111 but re-drlvcn fori:?>ic tcst .zfter a ground *set-up" period of two \vecks. Tt!c tlynnrnlc 1)l.eionof ultim;~te s!ntic cnlncity coincides with the actu:~l st:rtic tcst result
" which i;, is ti\., ::nil rcsis!nncc per unit !c:I::~I,. u:?dcr t ~ .;lssur?~l~!inn, ~thnt,oil rcsistnncc hnr the simple formRI,L = C,, + n,S + nlSa + . . ..._......_._......~ . (4)I>? \,nlue of C, mny bc.detcrn?irlcd esperirn~r.tn!!~ 11 F(,) .? nrc Lnoupn nthe instant when thc vclocit?. .I' = 0, at which insta::: LJ; ,, = C,!.Usc O! this schcnic docs In fact ylcld rcnsonablc olxfc~-~f-mn~nitu(i~ reultsfor C, xs indlcaled in Xef. 9.!!owl*er, vnrlous causes act to rcducc t]lc accur2cy of tI:is nlet:ld, thehief ones being the elastic action of the pile and the tvpicnll~ moscillntor)-"r 'hashye- appennncc of the usual scc~lcrox~:~~ record, \r)lich :nust be rend3 ol)hin :I? thc pint wllcrc its tin12 int~::~~] ,? pnssr.5 t,h:.ot!c!l zcro. Thcarticularly rapid local var-iations of s in :lie rc.c:ioil of .Y = 3 render thee.?tfin,c of X there quite uncertain of fntcri?~-c~2tinll.One very esefu! schcnic for the cor~.cctior: 0: tllc..;c! iii:~ccarncies is !o plotilr from xq. 3 for :ill timcs I of ir;tcrcst to 2 ::ivc:, )::::l;,::cr. !,leu' (i.c. scv-:![ tens of lililiiscco~is). .( typicnl rcsuit of tllis 1:iritI is s!~ou.n in >-16. 10.::?IT, l?cgirlning xpl)rosir.iatcly at ti;:lc :, .: an:! c~::!i::iz nl;~)ro?:imntcly)en S pnsscs throilgh zero, or for :i:iy c!:nsl~~l s!:ortc~ tin!(. in!crvni, tile;:c nver:lge ol I,iiL(1) is obtained. 7'liis :ivcr-:lge ]~!.ovcs'to bc r!uitv closc tothcr the nicasurccl or the dynnmic:iliy cnlc~:l::tcc! \.;lli:c of C,, csl)r?ci:tlly ifover nn intcrv:il in tllc \?ci!iitj. of I,, u.licrc ,? pnsses throt~gh zcro. 'i,ticsired tilne a\.eragc cnn often bc n~ndc cjuitc tccil by r:;c bj. r;iml)ly fnirii~g :Irnight line through thc plot of LnL(t).This schctnc is, ififact, littlcc!i!:crerit ill c!fcct from nncltllcr vnrlnnt whicll1;. xlso be used: a best fit straight line is inl:.cii 111:-ough tnc dcsccndingrtlo~: of t1:e velocity cilrvcs like thosc in Figs. 3 through G, anc! t!ic slope of3 Ilne is considcrcd to be the rlgicl bcxty dccclerntio~l; then uslng Eq. 3, rrirch the value of F~(!) Is, rend off 21 thc time v.,hcrl the :.:ireti straight vc-:ity line pxsscs through zefo, yiclds 2 vnluc for LR,.--Thnt such xpproxlnnte schemes mny work qultc well is nttcstcd to by thesi11ts pr!rayed In Tnblc I. Thc flrst column Iderltiflcs the 11ilcs dfscussctlcviously under thc clynnmic nnalysis iiicthcai. Colunln 2 t:lvcs C, ns lountl 11y: ni:ltcllinp nlell,od. Column 3-~ivcs t!~c rcsisl:~ncc calcuInttxi from only t11r:Id M y portion of the prcdlctctl dyn:tlnlc rcslmtisc :it tlic pint wllcrc thevrlocft). pn!;scs t!:r.nul:h zero. Colun~n 4 s)?ni'.s rc!jlll(s of sinlplificxl nvcr:~[:ingin th" ncir;11!x~?l?ncxi of I n Irn::tyf on I;;". 3. Colilnln 5 pl-c~cilt~ nrtlrni stntfc testrcsults for oltir:i:ltc Ionti bcarlng cnpncity.I! nl;t!.. fr-nr I1rr:sr rpsrllts, ~rprrd !ha! t!:~ .:cry prncticnl fr:ll>hlc:~ln\$crnt:i~l[; mc:hfi-Is st1ggcstcd, do in f:ict, ytc!c! ~ocd rcstl:ts in thc cnscs consii!crrtiand render the nlorc elaborate computer ~r~ilysls urulcccssnry CXccl'tns n rcsc:lrch tcoi. l')lls is 3 decided ndl.nnlngc, suggestin[:!he posslbili!?.of rnpid cncincering ~ s t i ~ ~ of : pllc ~ t i static ~ ~ bra!-lng capacity In thc fieldtll:oLlgh the Use of instlumentcd dynnmic tests.V:!li!c cnrlicr ni:alyticnl 1;lcthods )lave nlrcxdy 1)ccn prcse~lted ill tlie litcrnturefor nn:llyzing pile drivirl~ "by the \i.nvc cquntion." the ~Ircscnt ni~thodc!ircctly cn~nbincs and cnl::;,nrcs tllcory \vi!i~ corrcsmndlng clynxillic t2.\:l)crimclit.'1%~ nlcthoc! offers t!~c possibility of tl:;n:lr:lic ill p!:~cc of sL?tic londkcstir.!: for pllcs, :lt Ic?.st in ccrt:\in ty])cs of soil.In tlic prcsc11t co:!tr:., 11:~ rllc:lsul.rd }i:Lll)n~,-:. fi):.~~: is tnkc:) ns illl~~t, ':-7 -I']"ci::j n:lt[ dis*,ker-lio~ri..,- ~ ~ ~ ~ i d ns f rn c ; d ~ 'Tlrc u n;~l:,-tic;\l mc:]~ri(i :>-1s!7.l!ntrs - t!-,(. s:i:l!i. in:izt :I;;qt. Lt'licn thr-n~:itch bct\vi:e:: :~cLun! :\::il ~,rt.c!ictcd ot!!ir,~t is r:cior!, t!ie !)rciIhdb~ 0: st:iticsoil resist?.nce nlso nl~!:en:-s tobe good. l e i s \rork:t co:,tinuous pile morici-&her tnnn 2 set oj fi~c~.ctc ~~)rir.g-~ilnsscs--\i.ns LI!;CC!. \?'hi!r ~!IC con!inilOuSco;lsta:,:-sccmn r,iie :nC of ti!r \JCI!I:!:; 11: ~III. 5ttltIy is 1Il:rt this 11;:ly1 , ~ ~;os.c.ii>lc fordyn:!rnic tcsts just :IS It IS for sl:i:ic I~sstS. 'l'n u-11;lt cxtcn: :hisI>rovcs to I)e 1:uc for :kli typcs of soil is :k si~l)jcct for co~:ti::uin;; study. It mnyt.vc11 1-cquirc morc tlctnlicd rci:ntxl lo tlic clcfinitio:i of tl:~' ;lssurnrcl resis::lr:ccis :ilo~ll: tlic ]>llc. It is !101)~d that ll[:li! nl:ly l>c :;!!I:'! oj:~i:i this i~y t11e esj~cricnccof otliers nnc! tll:tt const~-\~ctivc co.rn11rcnt Illnx l)c clicltccl on this point.One of tile nddltlon:il qucstlo~~s is Ihnt of tllc pi;(: tnr ~.rsist:incr. For pointbc:lringpiles a form for lhls, FIJ(!), milst bc :~ssanirltl. Thc 1,rcscnt stuc!)' h:isslrccccdcd to ;! rcnsonnblc cxtc:1t \l:ith n \+cry si:l~l)!c o:cr-;~ll ~.c.sistnncc ln\rp.Continuin!; stutly rilny in:l)rovc [his Inuv.I'hc nttcml>l hcrc to :lc!lIcvc sim)~lc tlyr!:tniic ~:stir:i:\tc~s of bc:~riny: cnlxicltj'tl~rou::li :ilorr rn:;lIy np!,iic.cI critc~.l:~ 1~1s :\]so :,-lcldctl c.llcour;\gill!; results. Jn]);it-lfcular, the s:ltlcn: cilnr;ictcris!lcs o[ tllc 11ilc :lctlvity urltlcr tire it:tmn~cr-.. . !!~~?i:;g.. .I' ' .: i;:..?i: :. h;. , .!: , 1,:. g,:;. # 6--. v;:,:!;i-:b. ;, . 2.7:.. ,,.- 2: ;/r:?* 7, . f.:.. x:ic:.':. , f.!..E 41:. - f. 1ti:ff '
Page 3 and 4: tI5114 f b hTarch, 1969 Shl 2TITE T
Page 5: w-cipal objcctivc of the prcsent wo
Page 9 and 10: tile cove rning cquation becotnes .
Page 11: hlarch, 1969the hamnier force unifo

Dynamic Prediction of Pile Static Bearing Capacity - vulcanhammer ...

Create successful ePaper yourself

Delete template?

Save as template?