Qualitative Interpretation of Load-Settlement Curves of Bored Piles

Transcription

1 ENGNEER - Vol. XXXX, No. 4, pp ,27 The nsttuton of Engneers, Sr Lanka Qualtatve nterpretaton of Load-Settlement Curves of Bored Ples H. S. Thlakasr Abstract: Statc load testng of ples yeld only the load-settlement behavour of ples. Whle t s useful to get the load-settlement behavour, the statc load testng yelds no other drect nformaton regardng the dstrbuton of load carryng capacty along the ple shaft and at the ple toe. However, varous researchers have shown that the shape of the load-settlement curve can be nterpreted to get such nformaton. Ths paper attempts to dscuss the reasons for commonly observed varatons of the shape of the load-settlement curves obtaned by testng bored ples and the use of the Elastc Shortenng Lne (ESL) n nterpretaton of the load-settlement curves. Load-settlement curves are categorzed usng the ESL and a mathematcal model, developed to smulate the load-settlement behavour of ples, s used to nvestgate the reasons for varaton of the shape of the load-settlement curves of dfferent categores. Accuracy of the commonly used Van Weele [8] method to estmate the skn frcton from the loadsettlement curve s also dscussed. Keywords: Bored ples, Statc load Test, nterpretaton. 1. ntroducton Ples are load tested to obtan nformaton related to load-settlement behavour, load carryng mechansm, types of materal present along the ple shaft and the ple toe, structural ntegrty of the ple. However, statc load testng of ples provde the load-settlement curve, showng the load-settlement response of the ple. Therefore, the geotechncal engneer should nterpret the load-settlement nformaton to obtan the other nformaton about the ple. Ths s especally mportant for a test ple as the nformaton gathered wll be used n desgnng the other ples. Varous researchers (Tomlnson [3], and Van Weele [8]) have shown that the shape of the load-settlement curve of a ple can yeld more nformaton than the drectly obtaned row data related to load-settlement behavour. More nformaton regardng the ple could be obtaned from dynamc load testng of ples usng Ple Drvng Analyser (PDA). However, nterpretaton of the results of the PDA test should be carred out by competent engneers and, n most cases, PDA results should also be verfed by statc load testng of some of the ples tested usng PDA. Therefore, the analyss or nterpretaton of statc load test results s extremely mportant. n ths research, t s amed at nvestgatng the methods of obtanng more nformaton from load-settlement data obtaned from statc load testng of bored ples. The tools, that are ntroduced here to obtan nformaton of the load tested ples, yeld qualtatve nformaton about the ple rather than the quanttatve nformaton. The qualtatve nformaton, together wth other nterpretaton technques presented n Thlakasr et al. [5] and [6], wll provde suffcent amount of nformaton regardng the tested ple. Even though most of the nterpretaton methods dscussed here wll be applcable to all the types of ples, the valdty of the developed methodologes wll be lmted to bored ples snce they are verfed only usng load-settlement behavour of bored ples. 2. Load transfer Mechansms of Bored ples t s very mportant to get an understandng about the load transfer mechansm of ples before nvestgaton of the load-settlement behavour. f the load actng on the ple s ncreased from zero to the ultmate capacty, followng three stages could be dentfed [1]:. Durng ntal stage of loadng most of the load carred by the skn frcton and less through end bearng and wth ncrease ple penetraton the skn frcton reaches the ultmate value; Eng. (Dr.)H. S. Thlakasr, C. Eng., PEng.(SL). MJE(SL), B.Sc. Eng. (Hons) (Moratuwa), M.Sc. (Land), PhD (USA), Senor Lecturer n Engneerng, Department of Cvl Engneerng, Unversty of Moratuwa, Sr Lanka. 61 ENGNEER

2 . Durng mddle stage of the loadng, the load capacty s the sum of lmtng skn resstance plus the pont capacty; and 11 -!).pl P- AE Where L, A, and E are length, cross sectonal area and Young's modulus of ple materals respectvely. Therefore, durng stages and, the ncrease n the settlement of the ple toe due to a loadng ncrement (.1.P) s the total settlement of the ple at the top mnus the elastc compresson of the ple shaft gven by Equaton [1]. Durng stage, the elastc compresson s less than the amount gven by equaton [l} for a gven load ncrement and the dfference s governed by the locaton of the skn frcton along the ple shaft and the shear stffness of the surroundng sol. t s obvous from the above dscusson that f the stffness and the capacty of the ple s to be nvestgated, t should be done by consderng the elastc compresson of the ple shaft as the reference lne. Therefore, a reference lne n the load settlement curve ndcatng the elastc compresson of the ple shaft, actng as a short column wthout any skn frcton, wll be a very valuable tool n nterpretaton of the load-settlement curves obtaned from statc load testng of ples. Ths reference lne s dentfed as the 'Elastc Shortenng Lne' (ESL) and t s mathematcally represented by equaton [2] n terms of the total force at the ple top (-) and the total settlement (p) rather than the ncremental values consdered n equaton [1].. Durng fnal stage, the load-settlement curve reaches vertcal asymptote ndcatng ultmate carryng capacty. The Fgure 1 llustrates the load carred by skn frcton and end bearng durng loadng through the above three stages. P, p"' P..., Pl,n.1 (a) (b) Pltn.t+AP (c) P-,,.u1, (d) Fgure 1: Load carryng through: (a) stage ; (b) end of stage ; (c) stage and (d) end of stage Settlement of the ple may be due to only two reasons: () Compresson of the ple shaft; and () settlement of the ple at the toe. Snce wthn normal loadng condtons, the ple materal s stressed wthn the elastc range the compresson of the ple shaft wll be the elastc shortenng of the ple due to the ncrement of the axal stress along the ple shaft. However, as the skn frcton vares along the ple shaft, the axal stress ncrement along the ple shaft s not unform. Durng stage, the axa load n the ple decreases wth the depth and the elastc compresson of the ple, per unt ncrease n the ple top force, ncreases wth the ple top force. Ths s manly due to the ncrease n the length of the ple nvolved n carryng the ple top force. At the end of stage (Fgure l(b)), the elastc compresson of the ple, per unt ncrease n the ple top force, reaches the maxmum and durng stage and (Fgure l(c)&l(d)) the same rate of elastc compresson of the ple contnues. The elastc compresson of the ple shaft ( p), correspondng to an ncrease of p of the ple top force, durng stages & can be mathematcally represented usng the onedmensonal Hook's law as gven n Equaton p= PL AE (2) 3. Classfcaton of the load-settlement curves based on the ESL Load settlement curves obtaned from feld testng of bored ples could be categorzed n to three groups, as shown n Fgures 2 to 4, based on the locaton of the ESL wth respect to the load-settlement curves obtaned. The categores are dentfed as gven below: Catagorey - ntal porton of the Loadsettlement curve s above the ESL (Fgure 2). Catagorey - ntal porton of the loadsettlement curve s parallel to the ESL (Fgure 3). Catagorey - ntal porton of the loadsettlement curve s below the ESL (Fgure 4). [1]. ENGNEER (1) 62

3 ocro men zmcn amen «mcro 4. Mathematcal smulaton of the loadsettlement behavor of bored ples emcn QCDQ,.,o"'=-:-: re A mathematcal smulaton presented n Thlakasr [7] s used to nvestgate the behavor of the above mentoned categores of ples. n ths mathematcal smulaton, ple s dvded nto n number of elements and equlbrum of each element s consdered after loadng. n descrtzaton of the ple, the porton of the ple above the ground surface s consdered as a sngle element and the porton of the ple below the ground surface s dvded nto n- number of elements wth equal length as shown n Fgure 5. 2cm 4.CD! 6CD aero mcro;=::::=============================----, -<>- Baltc Sutrrg Lre ""*"" Satlara1 v...ml Fgure 2: Category - ntal porton of the Loadsettlement curve s above the ELS QCD 1CDOCD zmco amm 4mCD smm emm 7tmCD (&'t- 6,.) zmm amco 4!DOO.,: ;s;:----'----'----'----'----, 2 V t (F',.2-F,,2) F',.1.1-Fs,1-1 F'.rFs, F'.,1+1-F,,1+1 F'. n-fs,n n Fgure 5, <5 and 81 are prevous settlement of the th element and the settlement ncrement of the same element due to ncrement of the load actng on the ple by F whereas F. and F s are the prevous skn frcton and the ncrement n the skn frcton due to the same load ncrement on the ple. The ple elements are connected to each other through ple sprngs and the skn frcton actng on each element s modeled as an elastc-perfectly plastc sldng sprng wth a maxmum resstance of F US,!. on the th element as shown n Fgure 6. Load(lcN) Ground level Fgure 5: Dscrtzaton of the ple wth deflecton and forces actng on each element. Fgure 3: Category - ntal porton of the loadsettlement curve s parallel to the ESL e.5. F Skn frcton Loadng Fus,; Fgure 4:Category - ntal porton of the loadsettlement curve s below the ES Dsplacement nvestgaton of the postons of the loadttlement curves relatve to the ESL wll be dscussed by smulaton of the ple load test as xplaned n Thlakasr [7]. A bref descrpton f the smulaton method s gven below. Unloadng Fgure 6: Load deflecton behavor of a sprng used to model the skn frcton on 'h element. 63 ENGNEER

4 The maxmum elastc deformaton of the sol sprng (Q) s referred to as the quake of the skn frcton of the 1h element. Smlar elasto-plastc sldng sprng can be utlzed to model the ground response at the ple toe wth maxmum force at the ple toe as Futoe and the quake at the toe as Q1oe. teratve procedure to check and lmt the maxmum force n the sol sprngs was adopted n the numercal procedure. (4) k,_,((o, +oh)-(o; +o,))=k,((o; +o,)-(a:., +o,.,))-k.(o: +o,)=o,j,:-1 + k-1-kl-tj{ + X+1.,-J,-1 + t,_1-t,-t,;>, + J """""''"'"'"""""""''" [A](o')+[Bl(o);(F) The stffness of the ple sprng, K; connectng 1h element and the + 111' element, can be taken as A,Et.L1 where A1 E and t.l1 are cross sectonal area, Young's modulus and length of the '" element respectvely. The stffness of the sol sprng for the l" element, k. can be obtaned from Fus,Q, and the same for the toe sprng, k,oe1 can be obtaned by F,11Q, ". The forces actng on the z7" element are shown n Fgure 7. =O (5) (6) A Mcrosoft Excel worksheet was developed to mplement the above descrbed numercal procedure. The developed program s capable of dvdng the ple nto 5 elements and smulatng the loadng stage of the ple durng statc load testng. S, 5. nvestgaton of the behavor of the ples usng the numercal smulaton The developed smulaton program was used to nvestgate the behavor of the three categores of ples dentfed n secton 3.. The smulaton was done usng a 2m long, 8mm dameter ple n a three layer medum as shown n Fgure 8. fls, t + fs, L3 ' F;,.. Fgure 7: Forces actng on the th element Consderng the equlbrum of the element, equlbrum equaton gven n equaton [3] can be wrtten: 1h.-F5,1.=O F1-1-F-F S,J ENGNEER 8mm Layerl L3 Layer 2 L3 Layer3 Fgure 8: Ple and the subsurface layerng used for the numercal smulaton,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,.,,,,,,, (3) The ultmate end bearng capacty of the ple, skn frcton capacty, Young's modulus of the ple materal are assumed to be 737 kn, 421kN and 35 kpa respectvely. The skn quakes of the three layers are assumed to be lrnm and the toe quake s assumed to be 8rnm as the base lne parameters. The parametrc study was carred out as gven below. The above equaton can be re-wrtten n terms of settlement and sprng stffness as gven n Equatons (4) and (5): Smlarly, n number of equatons can be obtaned for 11 number of elements and the stffness of the n" ple element, k, should be replaced wth the stffness of the toe sprng, k1oe. Consderng the above set of equatons, followng global equlbrum equaton [6], can be wrtten n matrx form for the equlbrum of the entre system. The system of equatons, gven by Equaton [6], could be solved to obtan the unknown dsplacement vector [S]. However, the maxmum sprng force n the sol sprng s k1q1 for the l" sol sprng and k, 'Q' ' for the toe sprng. Therefore, specal 5.1 Effect of the locaton of skn frcton on the load-settlement curve To nvestgate the effects of the locaton of the skn frcton on the load-settlement curve, the locaton of the skn frcton along the shaft vared whle keepng all the other parameters constant. Three cases were studed by assgnng 64

5 th' kn frcton of 421kN to three layers. The rt sutng load-settlement curves are as gven n l gure 9. Fgure 1 s generated by carryng out th ame study for the end quake of 16mm. nvestgated by changng the quake of the end bearng layer to 4mm and 16mm whle keepng all the other base parameters unchanged. The stffness of the bottom sprng s estmated as the rato between the ultmate end bearng capacty and the end quake. Therefore, halvng the end quake means doublng the end stffness and vce versa. The SF was appled at the layer 1 for all three cases. The effect of the changng the stffness of the end bearng layer on the loadsettlement curve s shown n Fgure e -8 c Cl) ECl) ;:; a - SF n layer SF n layer 2 -er- SF n layer 3 ESL ee c fgure 9: Effect of the locaton of the SF on the loadsettlement curve (End quake Bmm) Cl) E!... Cl) v, *--- ESL There s a very mnor reducton of the slope of the Stage of the load-settlement curve as the stffness of the end bearng layer ncreases. t doesn't have a sgnfcant effect on the loadsettlement curve. However, as the stffness of the end bearng layer ncreases the end porton of the load-settlement curve becomes flatter as expected l -+- SF n layer 1 End quake 4mm Fgure 11: Effects of the changng the stffness of the end bearng layer on the load-settlement curve. -4-t-- r >, --,----- E =::"."".--! E l. :_: -t >clll:----,' End quake 8mm ---&--- - = E () _! t-'1111\ SF n lay er 2 t-----'l!!l.'rl SF n Layer 3,,,,_ ESL Fgure 1: Effect of the locaton of the SF on the load-settlement curve baselne materal parameters (End quake 1 mm). 5.3 Effect of the magntude of the SF on the load-settlement curve L s evdent from the load-settlement curves h wn n Fgures 9 and 1 that the locaton of th skn frcton along the shaft has a sgnfcant Hect on the relatve locaton of the loadttlement curve wth respect to the ESL. Pr sence of the SF near the top of the ple makes th ntal porton of the load-settlement curve rt tter and as the SF s located n deeper layers ad-settlement curve becomes steeper. Effects of magntude of the SF s nvestgated by changng the SF to 211 kn and 844kN whle keepng all the other base parameters same and applyng the SF to the top most layer. The load settlement curves for the three cases are as shown n Fgure 12. t s clear from the varaton shown n Fgure 12. that the extent of the ntal flat regon of the load-settlement curve depends on the magntude of the skn frcton. The quake of the SF was kept constant and therefore, the stffness of the materal surroundng the ple shaft ncreases wth the SF. As a result, the slope of the.2 Effect of the stffness of the end bearng layer on the load-settlement curve The effect of the stffness of the end bearng t yer on the load-settlement curve was 65 ENGNEER

6 settlement curve as stated earler. Therefore, Category type load-settlement curves are resulted from the presence of the sol layer wth hgh shear stffness near the top of the ple. Category type of curves are obtaned by testng ples wth hgh stff sol layers near the mddle or toe of the ple. Presence of the small SF wth low stffness results n the Category type load-settlement curves _ SF=211 kn SF=422 kn E. -8 c: Q EQ EQ () -sf=844kn The factors affectng the Stage of the load-settlement curve -24 Fgure 12: Varaton of the load-settlement curve wth the SF. The most mportant factor that affects the Stage of the load-settlement curve s the stffness of the materal present below the toe of the ple. As shown n Fgure 11, hgher the stffness of the ple toe materal, less steeper s the Stage of the load-settlement curve. Thlakasr [4] usng the method proposed by Van weele [8] showed that the tangent lne drawn to the second stage of the load-settlement curve can be used to separate the SF and end bearng capactes, as shown n Fgure 13. ntal porton of the load-settlement curve reduces as the stffness of the sol surroundng the ple shaft ncreases. 6. Dscusson of the results of the parametrc study The results of the parametrc study clearly showed the exstence of Stage and Stage of the load-settlement curves as explaned n secton 2.. The results of the parametrc study can be dscussed under two sectons: () the factors affectng the Stage of the loadsettlement curve; and () the factors affectng the Stage of the load-settlement curve. Load 6.1. The factors affectng the Stage of the load-settlement curve ' The load-settlement curves shown n Fgures 9 and 1 clearly showed that the stage of the load-settlement curve s affected by the locaton of the SF. Presence of the SF near the top of the ple makes the Stage of the load-settlement curve flatter and the slope of the ntal porton of the load-settlement curve becomes steeper, when the SF s actng on the lower levels of the ple. Fgure 12 shows that the shear stffness of the SF affects the slope of the Stage of the loadsettlement curve. As the shear stffness of the layer provdng SF ncreases, the slope of the Stage of the load-settlement curve decreases as shown n Fgure 12. Fgure 12 also shows that the extent of the Stage of the load-settlement curve depends on the magntude of the loadsettlement curve. Moreover, t s evdent from the Fgure 11 that the magntude of the end bearng and stffness of end bearng Jo not sgnfcantly affect the Stage of the load- ENGNEER Fgure 13: Estmaton of the SF and end bearng capactes (Van Weele [BJ) f the Van Weele [8] method s appled to the three Load-settlement curves gve n Fgures 9 and 1, for presence of SF at dfferent locatons along the ple shaft, the SF estmated vares as gven n Table 1. Together wth the actual SF. Smlar varaton could be observed for other end stffness as shown n Table 1. t s seen from the SF values gven n Table 1 that the estmated SF from Van Weele [8] method underestmates the SF when the shear stffness of the SF layer reduces or when the SF s present at deeper levels of the ple. Moreover, as the stffness of the end bearng layer reduces, the SF estmated from Van Weele [8] methods devates more from the actual SF. 66

7 the ESL and the ple belongs to Category as shown by the parametrc study.. ble 1: SF estmated from Van Weele [8) method for dfferent end stffness of ples End uake Locaton of SF SF estmated Actual SF from Van (kn) along the ple shaft Wee le method (kn) Top thrd Md thrd Bottom thrd Top thrd Md thrd Bottom thrd Top thrd Md thrd Bottom thrd (mm) Axal force (kn) 1 S 2 3 1! l- --l 2 Fgure 15: Measured axal force along the ple shaft (Hussen and Camp [2]) 7. Feld Measurements 7.2 Case study 2 l>ata from, full scale statc ple load tests carred, >U t n the feld usng nstrumented ples, can be us d to strengthen the above explaned Matons of the load - settlement curves. Two 1.,. studes presented n Hussen and Camp(2 ( r presented here. A 3 mm dameter 42 m long nstrumented pres tressed concrete ple was subjected to statc load testng (Hussen and Camp [2]). The measured load - settlement curve and the axal force along the ples are shown n Fgures 16 and 17 respectvely. 7.1 Case study 25 mm dameter 24.lm long nstrumented pr -stressed concrete ple was subjected to statc 111,td testng (Hussen and Camp [2]). The 111 sured load - settlement curve and the axal ur e along the ples are shown n Fgures 14 and respectvely. j j Observed Load ESL Settlement 15 Axal force (k N) Settlement -e--esl 1 Fgure 14: Load -settlement curve (Hussen and Camp [21) S Mea ured axal force along the ple shaft clearly 111 ates that the skn frcton actng at the ple lrnost zero at the top porton of the ple as 4 s no apprecable reducton n the 1111,, ured axal force wth depth. The load 1 tll ment curve shown n Fgure 14 s below 45 1 Fgure 16: Load -settlement curve (Hussen and Camp [2]) 1 j 5 tlwr 2 --Measured axal load Fgure 17: Measured axal force along the ple shaft (Hussen and Camp[2]) 67 ENGNEER

8 References Measured axal force clearly ndcates that a consderable skn frcton s developed along the entre ple shaft. Therefore, the stffness of the sol along the ple shaft s hgh. The ESL s below the measured load - settlement curve and the ple belongs to Category, as shown by the parametrc study presented. 8. Conclusons t s shown n the paper that the three dstnct stages could be dentfed n the commonly obtaned load-settlement curves as observed by other researchers. t s shown that the Stage of the load-settlement curve depends manly on the skn frcton developed and the Stage depends manly on the end bearng. The mportance of the Elastc Shortenng Lne (ESL) of the tested ple s hghlghted and the feld load-settlement curves are categorzed nto 3 groups, as gven below, based on the poston of t wth respect to the ESL. Category - Stage of the Load-settlement curve s above the ESL. Category - Stage of the load-settlement curve s parallel to the ESL. Category - Stage of the load-settlement curve s below the ESL. 2 Hussen, M. M., and Camp, W., "Dynamc and statc tests on drven and cast n-place ples", nternatonal Conference on Desgn and Constructon of Deep Foundatons, Orlando Florda, USA, 1994, pp Tomlnson, M. J., Ple Desgn and Constructon Practce, 4th Edton, E & FN Spon, Thlakasr, H. S., "A Revew of the Desgn Practces of Bored and Cast n-stu Ples n Sr Lanka", Proceedngs ofannual sessons of'nsttuton 5. Thlakasr, H. S. and Slva, W. H., "nterpretaton of Compressblty Propertes of Sol Surroundng Bored Ples from Ple Load Test Results", paper submtted for the Annual Sessons oftze nsttuton ofengneers Sr Lanka, 27(a) 6. Thlakasr, H. S. and Slva, W. H., "nterpretaton of the End Bearng Condton of Bored and Cast n-stu Concrete Ples usng Statc Ple Load Test Results" accepted for publcaton n the Proceedngs of the Asan Regonal Conference of Sol Meezanes and Geotechncal Engneerng, December 27(b), Kolkata. t s shown that the most mportant factor that affects the Stage of the load-settlement curve s the stffness of the materal present below the toe of the ple. As shown n Fgure 11, hgher the stffness of the ple toe materal, s less steeper the Stage of the load-settlement curve. t s also shown that the estmated SF from Van Weele [8] method underestmates the SF when the shear stffness of the SF layer reduces or when the SF s present at deeper levels of the ple. The amount of underestmaton ncreases as the stffness of the end bearng layer ncreases. ENGNEER Bowels, J.E., "Foundaton Analyss and Desgn", 5th Edton, McGraw-Hll, ofengneers- 26, Sr Lanka. t s shown that the category of the ple manly depends on the SF. t s shown that Category type load-settlement curves are resulted from the presence of the sol layer wth hgh shear stffness near the top of the ple. Category type of curves s obtaned by testng ples wth hgh stff sol layers near the mddle or toe of the ple. Presence of the small SF wth low stffness results n the Category type load-settlement curves. Two case studes of the results of load testng of nstrumented ples are presented to verfy the fndngs of the parametrc study Thlakasr, H. S., "Analyss of the load settlement curves obtaned from statc load testng of bored and cast n-stu concrete ples n Sr Lanka", paper accepted for the nternatonal Conference on Sol and Rock Engneerng, August 27(c), Colombo, Sr Lanka. 8. Van Weele, A. A., "A Method of Separatng the Bearng Capacty of a Ple nto Skn-Frcton and Pont Resstance", 4th CSMFE, Vol. 2, 1957, pp