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1、TECHNICAL NOTEEffect of Compressive Load on Uplift Capacityof Cast-Insitu Bored PilesAmit ShelkeN.R.PatraReceived:2 June 2010/Accepted:11 June 2011/Published online:1 July 2011?Springer Science+Business Media B.V.2011AbstractField tests were conducted to study theeffect of compressive loading on the
2、 uplift capacity ofsingle piles embedded in silty sand.The test programconsists of four instrumented cast in situ axial pile loadtests in compression,pure tension and tension with 25and 50%of compressive load of ultimate capacity incompression.The experimental results indicate that thenet ultimate u
3、plift capacity of single pile decreases withincrease in compressive load.The shaft friction is nonlinearinnature.Itobservedthatasthecompressiveloadincreases the shaft friction along the length of piledecreases.KeywordsCompression?Field test?Friction?Pile?Uplift1 IntroductionSome structures like tran
4、smission towers,mooringsystems of ocean surface or submerged platforms,tallchimneys,jetting structures,etc.are constructed on pilefoundations,which have to resistuplift loads.Theupliftresistance of pile mainly depends upon limiting skinfriction between soil and the surrounding soil(Downsand Chieurzz
5、i 1966;Meyerhof and Adams 1968;Dasand Seeley 1975;Levacher and Sieffert 1984;Chatto-padhyay and Pise 1986).Most previous studies on pileuplift capacity do not consider the effect of the priorhistory of compressive loading on the pile.The founda-tionsaresubjectedtosuperstructureloadingandwiththeconst
6、ruction of superstructure the compressive load onthe foundation increases gradually.Construction is agradualprocessandloadbeingtransferredtothestratumthrough the foundation system is incremental in nature.The weight of the superstructure will often imposesignificant compressive loads on the piles pr
7、ior toapplication of uplift loads.The full static compressiveloadcomesonfoundationatservicecondition.Extensivetheoretical and experimental investigations have beencarriedouttostudythebehaviorofsinglepilessubjectedto compression and uplift load.However,studies oneffect of static compressive load on u
8、plift capacity ofpilesarelimitedandthattolaboratorymodeltests(Dashand Pise 2003;Joshi and Patra 2004;Krishna and Patra2006).In the present study an attempt has been made tostudytheeffectofthemagnitudeofcompressiveload(0,25 and 50%of ultimate capacity in compression)on thepull out resistance of cast-
9、in situ bored single piles.2 Experimental Setup and Testing Program2.1 Plan for TestingThe program of pile load test was performed at thesite located In situ field testing laboratory,IndianA.Shelke?N.R.Patra(&)Department of Civil Engineering,Indian Institute ofTechnology Kanpur,Kanpur 208016,Uttar P
10、radesh,Indiae-mail:nrpatraiitk.ac.in123Geotech Geol Eng(2011)29:927934DOI 10.1007/s10706-011-9423-zInstitute of Technology Kanpur,India.A layout ofthe plan is shown in Fig.1.Pile(P1)was tested incompression and piles P2,P3,P4 were tested in pureuplift and uplift subjected to 25 and 50%compressiveloa
11、d,respectively.2.2 Soil Condition and ClassificationThe sub surface profile was explored by augur boringand samples were recovered and standard penetrationtests were performed at 1.5 m interval.The summaryof the soil conditions is shown in Fig.2,the soilstratification,moisture content,SPT along the
12、depthof the borehole.The average value of SPT recordedas 19 which measured to the depth of 6 m.Theaverage density of soil was 19.6 kN/m3.No groundwater was present in the boreholes.The soil mainlyconsists of silt with 69%of silt and 30%of sand.2.3 Details of Pile Casting and InstrumentationThe bored
13、 piles employed for testing were 0.4 m indiameter by 4.0 m in length.For installation of piles,boreholes were made by an auger.The holes weredried up.During boring,no casing was used as anycollapse or caving did not take place up to the depthof 4 m.Piles were first installed by placing 12 mmreinforc
14、ement steel bar in the center and filling thehole with M-20 grade of cement.Total number of 6bars used in the pile cage,diameter 0.32 m with a Fe-415 graded steel.In case of uplift test 1 m steel rodswas protruded in where as in case of compression testthe proper pile head was prepared to transfer t
15、he loadon to the pile.For pile load test,ten reaction pileswere used in which A1A4 were used anchor pile(0.4 m diameter and 3.5 m depth)and A3A10 wasused as reaction piles.In each pile,total numbers of 5strain gauges were installed at the depth of 1 m eachfrom the top and two rods were instrumented
16、whichare diametrically opposite to each other.The averagevalues of the strain measurement were used forcalculation of load distribution and shaft frictionalong the pile length.2.4 Procedure for Pile Load Test2.4.1 Compression TestThe schematic diagram of the test arrangement forthe pile in compressi
17、on is shown in Fig.3.In the1.P-Main pile 2.A-Reaction pile Geotechnical Engineering laboratory In situ Geotechnical field engineering laboratory Main pile Anchor pile 2.82 m2.82 m P1 P2 P3 P4 A6 A1 A2 A3 A4 A5 A7 A9 A8 A12.82 m Way toair strip4.65 m N Fig.1 Location of test site928Geotech Geol Eng(2
18、011)29:927934123experimental set up a loading jack was placed on thetop of the pile head over-laid by a reaction beam.Onthe reaction beam a loading platform was created toapply the static load through the loading beam.Thereaction beam was fixed rigidly to the pile head totransfer the reaction load.T
19、he experimental test upfor the compression test is shown in Fig.4.Theanchor piles were spaced at a distance of 2.5 timesthe diameter away from the main pile.The pile head,where compressive load to be applied,was chippedout and with the help of plain concrete and a perfectplan head was prepared.A ste
20、el bearing plate wasplaced on the head of the pile.The loading on the pilewas applied with the help of hydraulic jack ofcapacity 1,961 kN.The jack was calibrated beforeuse.The displacement was measured on either side ofthe pile head.2.4.2 Uplift TestThe testing arrangement of single pile under pureu
21、plift is shown in Fig.5.The loading on the pile wasapplied with the help of hydraulic jack of capacity1,961 kN.The jack was allowed to rest on the I-beamwhich in turns rest on the two cross anchor pile.Aspecial design of loading frame was fabricated whichis used to carry the uplift load.The base of
22、thehydraulic jack rests on the beam which pushes it indownward direction,but due to end restrained the pilewas pulled out.2.4.3 Uplift Test with Compressive LoadFigure 6 shows the testing arrangement of the pileunder uplift loading with 25%of compressive load.0123456010203040SPT-N Blows/1.5 mdepth S
23、ilty sand Silt Silt Test pile Borehole log D e p t h(m)Silt wl%k/m329 21.1 18.9 2519.2 252519.47 Fig.2 Soil condition at the test site-IITKGeotech Geol Eng(2011)29:927934929123In the testing arrangement two cross girders wereplaced perpendicular to each other.The first girderrested on the pile head.
24、Above the first girder,aloading platform was created.The sand bags ofknown weights were placed over the loading plat-form.The weight of each sand bag was carefullymeasured before placing over the platform.On thetop of the loading platform with sand bags,secondLoading beam Reaction beam Pump unit Pil
25、e head Uplift load LoadingcolumnStatic compressive load Fig.3 Schematic sectional view of experimental setupReaction beam Loading beam Fig.4 Test setup arrangements for pile load test(P1)undercompressionJack Loading beam Reaction beam Fig.5 Test setup arrangements for pile load test(P2)underpure upl
26、ift930Geotech Geol Eng(2011)29:927934123girder was placed.On the second girder hydraulicjack was placed over which steel bars from the pilehead was anchored.After application of compressiveload,the side supports were removed and theminimum clear spacing of 5 cm was kept so thatentire compressive loa
27、d comes directly on the pilehead through the girder.Subsequently,the uplift loadwas applied in increments of 98 kN with the help ofhydraulic jack resting on the top of second girder.Each load increment was maintained for not less than15 min each until displacement ceases or rate of thechange of the
28、displacement was less than 0.02 mmper minute.At each load increment the dial gaugereadings and strain reading were recorded.Similarprocedure applied for pile under uplift subjected to50%of compressive load.3 Results and Discussions3.1 Load Displacement Behavior3.1.1 Pile Under Axial CompressionThe l
29、oad displacement response of piles under axialcompression is shown in Fig.7.The load incrementwas maintained as 98 kN in each step about 20%ofthe expected failure load.The double tangent tech-nique was employed to calculate the ultimate capac-ity of the pile in compression as 491 kN.In doubletangent
30、 technique two best possible tangents aredrawn asymptotically to the initial and final section ofthe loaddisplacement curve to obtain the point ofaxial compression capacity(Poulos and Davis 1980).3.1.2 Pile under Uplift Subjected To 0,25 and 50%Compressive LoadThe load displacement response of the p
31、iles underuplift subjected to 0,25 and 50%compressive loadsis shown in Fig.8.The gross ultimate uplift load istaken as the load at which the load displacementcurve exhibits a peak or asymptotically approaches tothe displacement axis at a constant load.The grossultimate uplift loads subjected to 0,25
32、 and 50%Staged loadingJack loadingLoading platformFig.6 Test setup arrangement for pile load test(P3)underuplift subjected to 25%compressive loadFig.7 Load versus displacement curve for single pile undercompressionFig.8 Load versus displacement curve for single pile(0,25and 50%static compressive loa
33、d)Geotech Geol Eng(2011)29:927934931123compressive load are 304,353 and 441 kN,respec-tively,and the corresponding pile head displacementsare 0.005d,0.00195d and 0.003125d,respectively,.The net ultimate uplift load on a pile was computedby subtracting the compressive load,weight of pilesand weight o
34、f other accessories(girder,jack etc.)from the gross ultimate load.The net ultimate upliftcapacity of piles under pure uplift and uplift withcompressive loads(25 and 50%)are 292,219and184 kN,respectively.3.2 Variation of Net Ultimate Uplift Capacitywith Percent of Ultimate CompressiveCapacityThe vari
35、ation of gross uplift capacity and net upliftcapacitywithpercentofultimatecompressivecapacityis shown in Fig.9.The gross uplift capacity increaseswiththepercentincreaseinthecompressiveloading.Itis observed that the net uplift capacity decreases withincrease in compressive loading.The gross upliftcap
36、acity of the pile increases due to presence ofcompressive loads and also due to localized densifica-tion of granular material around the periphery of thepile.Also due to static compressive loading and thenuplifting the pile,the failure zone is initiated and thestaticfrictionisovercomewhichrelatestot
37、heloweringof net uplift capacity.3.3 Load Distribution Along the Pile ShaftThe strain gauges were placed at required positionalong the pile shaft to find the strain distributionalong the length of the pile.The modulus of elastic-ity E of composite material is calculated E=EsVs?EcVc.The Esand Ecare m
38、odulus of elasticityof steel and concrete and Vsand Vcare the volumefractions of the steel and concrete,respectively.The value of modulus of elasticity of concrete is29E6 kN/m2.The axial force along the length of thepile shaft is calculated as qi=eiEAi.Where qiis thepile axial load at the location o
39、f strain gauge,E isModulus of elasticity of concrete,e is the strainmeasured,A=cross sectional area at the location ofstrain gauge.3.3.1 Pile Under Axial CompressionThe load distribution along the shaft of the pileunder axial compression is shown in Fig.10.Themagnitude of load distribution along the
40、 pile shaftdecreases with increase in depth.The decrease inload distribution is marginal up to a depth of 0.25times the length of pile and maximum load iscarried by top region only.At the bottom end of thepile(0.75 L)the load distribution remains practi-cally constant.Fig.9 Variation of gross uplift
41、 capacity and net upliftcapacity with static compressive loadingFig.10 Axial load distributions during compressive testing onsingle pile932Geotech Geol Eng(2011)29:9279341233.3.2 Pile Under Pure Uplift and Upliftwith Compressive LoadThe comparative study of load distribution along thepile shaft subj
42、ected to 0,25 and 50%of compressiveload for 294 kN load is shown in Fig.11.It isobserved that the load distribution along the pile shaftdecreases with increase in depth.Again it increaseswith increase in compressive load(i.e.050%).3.4 Shaft FrictionThe shaft friction of the pile was calculated from
43、thestrainmeasurementsduringthe pileloadtest.Thetotalload transferred to the surrounding soil between twopointsiscomputedasthedifferencebetweentheforcesderived from the strain readings at these points and thecorresponding weight of the pile.Hence,the averageshaftfrictioncanbeexpressedasfijqj?qi?WijSi
44、j;wherefij=average shaft friction between station i and j,Sij=surface area of the pile between stations i and j,Wij=weight of the pile between i and j.The compar-ison of shaft friction of pile subjected to 0,25 and 50%of compressive load is shown in Fig.12.The shaftfriction isnon linear innature.Iti
45、sobservedthatas thecompressiveloadincreases,theshaftfrictionalongthelength of pile decreases.4 Coefficient of Lateral Earth PressureBased on the empirical relationship listed in Salgado2008,coefficient of lateral earth pressure has beencomputed for pure uplift loading with 0%compres-sive loading.The
46、 coefficient of lateral earth pressureis calculated from Eq.1.K 0:7K0exp 0:0114?0:0022lnr0v=pa?Drno1where K0is the static earth pressure and r0vis theeffective earth pressure.The coefficient of lateral earth pressure evaluatedfrom Eq.1 is plotted in Fig.13.Coefficient of lateralearth pressure for 25
47、 and 50%is not calculated as anynumerical or theoretical treatment of the problem isabsent in the literature.With the increase in depththe coefficient of lateral earth pressure changednonlinearly.Fig.11 Comparative study of load distribution along the pileshaft(0,25 and 50%compressive load)Fig.12 Va
48、riation of shaft friction along the pile length(0,25and 50%compressive loading)Geotech Geol Eng(2011)29:927934933123Therefore,with the aid of experimental results thecoefficient of lateral earth pressure is back calculatedasqSL Kr0vtand2where qSLis the limit unit shaft friction.From Eq.2the coeffici
49、ent of lateral earth pressure is calculatedand overlaid in Fig.13.However,the coefficient oflateral earth pressure calculated from empiricalcorrelation(Eq.1)is not in agreement with thevalues from experimental investigations.Therefore,an extensive theoretical investigation has to becarried for reaso
50、nable explanation of experimentalresults.5 ConclusionsField tests were carried out to study the effect ofcompressive loading on net ultimate uplift capacity ofthe cast in situ bored piles.The program consisted ofpile under axial compression,axial tension and pileunder uplift with 25 and 50%compressi