Piled-Raft Foundation Behaviour on Consolidating Soft Soil

Transcription

1 Piled-Raft Fundatin Behaviur n Cnslidating Sft Sil Sumya Ry, PhD Schlar f Civil Engineering Department, Jadavpur University, Klkata, West Bengal, India. Dr. Bikash Chandra Chattpadhyay, Ex. Head and Prf. f Civil Engineering Department, Bengal Engineering & Science University, Shibpur, Hwrah, West Bengal, India. Abstract In areas where sft sil f lw strength and high defrmability extends ver cnsiderable depth frm grund surface, deep fundatin like piles are adpted by the designers fr cnstructin f high rise buildings, resulting high cst f cnstructin. Hwever, in such cases, a relatively cheaper fundatin system like raft if adpted can cunteract the pssibility f shear failure due t huge super structural lads n the sub sil but the resulting settlement wuld be t large t be permitted. Thus, t have an ecnmic fundatin system, a raft is prvided ver the base f the prpsed building and sme piles are installed at specified lcatin belw the raft t increase the lad carrying capacity f the cmbined system with reduced resulting settlement. But in a cnslidating sft sil, the interactin between sil, raft and pile becmes time dependent. In this paper, a simple design methdlgy fr pile raft system is prpsed. An attempt has been made t incrprate the effect f time dependent cnslidatin settlement, calculated frm the evaluated cnslidatin characteristics f the subsil frm sil explratin. Results f the pile lad test data cnducted at the site were cnsidered fr determinatin f pile capacity. It is shwn that fr a chsen piled-raft system in sft clayey subsil, cnsideratin f cnslidatin effect f the sub sil gives mre ratinal evaluatin f the interactin prblem f a piled raft fundatin. Keywrds: Piled-raft, cnslidatin, time dependent, sft clay, lad test. Page 1 f 11

2 1. Intrductin In sft clayey subsil, perfrmance f a fundatin is very much affected by time dependent sil defrmatin. Time dependent behaviur f sil results frm prperties f cnslidatin which has certain nn linear characteristics. In sft grund, piled raft fundatin are widely used and emplyed in cnstructin f high rise buildings fr their lw verall and differential settlement with higher bearing capacity. The design f piled raft is based n the sil structure interactin between the cnstituting elements and this is achieved thrugh different methd prpsed by Puls (2001), Katzenbach et al. (2000), Randlph (1994), Franke (1991). Hwever, the piled raft subsil interactin prblem is highly cmplicated as it depends n large number f parameters like pile-raft gemetry, pile spacing, sub sil characteristics etc. Especially, lad defrmatin f sft sil may becme nn linear under high stress level. In case f submerged cnditin sft grund displays lw strength, sensitive thixtrphy and high cmpressibility. Hence, in realistic design f piled raft fundatin system time dependent behaviur f sil defrmatin and cnslidatin characteristic f the funding grund shuld be given due imprtance. In this paper, an attempt has been made t frmulate a design methd fr calculatin f lad carrying capacity f piled raft system in a sft cnslidating underlying sil stratum. Pile elements are used t cntrl r restrict the average settlement t a permitted value and at any pint f time, the raft shares a prtin f lad s that the piles carry the remaining super structural lad. The effect f nging cnslidatin settlement is cnsidered in design. The time effects f interactin f piled raft and sil is numerically mdelled. As a result, the time dependent behaviur f interactin f the piled raft and the sil is investigated in this paper by incrprating the effect f cnslidatin f the sub sil. Prpsed methd includes the field perfrmance f pile frm rutine pile lad test cnducted at the cnstructin site in Eastern part f Klkata city in a very unique sft clay depsit extending frm 2 m t 16 m (mre r less) belw grund level. 2. Literature Survey f Analytical studies n Piled-Raft In the analytical field, pineering wrk was started by Butterfield & Banerjee (1971) and thereafter imprtant develped mdels are Strip-Spring mdel by Puls (1991), Plate-Spring mdel f Clancy and Randlph (1992), Bundary element methd by Sinha (1997), FEM applicatin in raft and Bundary element fr pile by Hain and Lee (1978), Franke et al. (1994), FEM analysis invlving plain strain & axisymmetric prblem by Hper (1974); Prks & Kulhawy (2001), 3D FEA by Zhang et al. (1991). All these methds d have specific bjectives in studying the verall & differential settlements, raft bending and parametric effects. Time effects in sil structure analysis was first cnsidered by Wd et al. (1975) n the basis f 1D Terzaghi s mdel f cnslidatin by virtue f finite difference methd. Then the time dependent respnse f the piled-raft-sil interactin system under vertical lading was analysed by Cheng et al. (2004) using 2D FEM based n Bit's thery f cnslidatin. The linear creep Page 2 f 11

3 mdel was incrprated by Viladkar et al. (1993) int FEM in interactin analysis and it is fund that bending mment, cntact pressure and differential settlement vary with time. A simplified rhelgic element mdel was used by Xia (1994) t evaluate the distributin f raft cntact pressure n visc-elast plastic sil. A three dimensinal FEM is prpsed by An et al. (2001) t predict the creep settlement f fundatin n elast visc plastic sil. The interactin analysis cnsidering time effects induced by bth viscsity and cnslidatin was cnducted by Wang et al. (2001) in which a clsed frm fundamental slutin f stresses f saturated visc elastic sil underlying raft under vertical lading is derived. Hwever, a critical study f Puls (2001) shwed that results frm such mdels shws large scatter frm each ther. The literature survey n piled raft fundatin design discussed abve shws that, mst f the previusly frmulated design cases verlk time-dependency f sil defrmatin and may give rise t inaccuracy in evaluatin f interactin behaviur and unreliability in design f structures. Till date n apprach has been made t frmulate a relatively simple and accurate methd fr designing a piled raft fundatin in cnslidating fundatin bed. Actual site cnditins are t sme extent simulated as istrpic. Settlement riented design methdlgies are als limited. 3. Prpsed Design Methdlgy Fr the present wrk, t understand the raft sil, raft pile and pile sil interactin f cmpsite pile raft fundatins sme practical assumptins have been made fr the stress strain behaviur f the pile, the subsil and raft. The interactin f the pile and sil respnses is restricted in linearly elastic regin. Such assumptins have resulted in satisfactry utcme in the piled raft researches based numerical mdel f Ry and Chattpadhyay (2011) and n finite element mdels f Mssallamy et al.(2009), Jeng et al. (2003). The prpsed methd is frmulated basically t determine the time required, iteratively, by the piled raft cmpsite fundatin where lad sharing and cnsequent lad transfer between the pile and raft reaches an ptimum balanced state fr a super structural lad n it in a sft cnslidating sub sil. Study was als dne t evaluate the separate individual lad carrying capacities f raft and pile. As piles take huge lad n a very small amunt f settlement, crrespnding lad sharing and settlement f the raft is als studied. The time settlement relatinship fr the raft is als btained taking int cnsideratin cnslidatin prperties f the existing sil prfile. This is dne t recgnize time effects in interactin f piled raft and sub sil as it has gt a practical significance as Chun-yi Cui et al. (2005) thrugh his EVP sil mdel has shwn that reactins and defrmatin f pile raft fundatin varies with time in cnslidating sft grund cnditin. Capacity f pile is determined thrugh lad test. The ttal settlement and lad settlement characteristic f pile is determined thrugh lad settlement curves btained frm the rutine lad test. Page 3 f 11

4 3.1 Capacity f piled raft fundatin Fr a raft, prpsed design apprach starts with evaluatin determinatin f its bearing capacity frm bth the shear failure criteria and permissible settlement limits fr existing subsil prfile. The safe lad fr the raft is finalized fllwing the mst critical cnditin f the abve tw criteria. Nw frm the rutine pile lad tests the lad that culd be safely taken by the pile is evaluated thrugh lad settlement curves. Thus fr a chsen settlement f, if raft carries a lad, R and pile carries a lad, P, then the capacity f piled raft fundatin, PR can be expressed as PR = R+ P (1) Here settlement takes care f bth immediate and cnslidatin settlement f the subsil prfile. Figure 1 illustrates the schematic presentatin f pile lad test result and piled raft lad sharing. (a) (b) Example Figure 1: Schematic diagram (a) Lad settlement curve f rutine pile lad test; (b) Lad taken by raft and pile at chsen settlement,, frm pile lad test curve. 3.2 Calculatin f P The value f settlement f the cmbined pile-raft system, can be taken and adpted as per prject requirement r subsil cnditin and crrespnding lad n pile i.e. P can be btained directly frm the lad settlement curves f the cnducted rutine lad test n pile. The value f can be varied t btain required lad sharing mechanism between the piles and the raft within linear zne f the lad settlement curve f the pile. Page 4 f 11

5 3.3 Calculatin f R As mentined previusly, in the prpsed apprach is the ttal settlement i.e. sum f bth initial and cnslidatin settlement fr the raft. The value f R fr raft is calculated cnsidering bth cnslidatin settlement and immediate settlement f the existing subsil prfile. At a cnslidatin settlement f c, let the lad taken by the raft be Rc. At that lad f Rc, crrespnding immediate settlement, I is calculated. Rc can be derived frm the cnslidatin equatin. C c c H lg10 1 e p Rc p (2) Frm the abve equatin, c CeH Rc can be written as Rc [ 10 ] p p (3) Where, p is the initial verburden pressure. Ce Cc 1 (4) e H = height f cmpressible strata. C c = cmpressin index and e = initial vid rati f the cnslidating layer. Finally, is t be re checked as c I (5) S that the ttal settlement f the raft and pile becmes almst identical and hence R can be written as R Rc (6) Fllwing the abve equatins, a prgram is develped t study the lad transfer mechanism between the piles and raft. Nw, iteratively is selected and crrespnding lad n piles and raft is calculated. Figure 2 illustrates the schematic representatin f time settlement curve f raft, lad settlement f the raft and gradual cnslidatin settlement prcess f the piled raft fundatin respectively. In additin, the time settlement f raft is incrprated t btain the ptimum time required t reach the balanced state f the piled raft fundatin where lad Page 5 f 11

6 Stratum thickness (m) NMC (%) transfer and ttal settlement f piled raft becmes almst negligible and full lad carrying capacity f the piled raft fundatin is mbilised. (a) (b) (c) Example Figure 2: Schematic diagram (a) Time settlement curve f raft; (b) Lad settlement f raft at chsen settlements; (c) gradual cnslidatin settlement f pile raft cmpsite. 4 Results and Discussins T elucidate the described principle f design f pile raft fundatin n cnslidating sft sil, an example prblem f 15m by 15m square raft with 4 piles f length 30 m and diameter 1000 mm at a spacing mre than 3 times its diameter is symmetrically arranged. A site in the eastern part f Klkata city is selected fr the present study. In the primary design stage, a large raft size was avided keeping in view the sft subsil cnditin and better understanding f the prpsed methdlgy. The subsil prfile cnditin f the site and detail f pile lad test is shwn in Table 1 and Fig. 4 respectively. 4.1 Site Cnditin A site in the eastern part f the Klkata city, where a cmmercial cum residential buildings are prpsed, is selected fr critically analyzing the present design technique. Klkata falls under typical deltaic regin. The existing subsil prperties and ther details are indicated in Table 1. Example Table 1: Sil prfile with design sil parameters Descriptin f N Shear C m c v in m 2 /kn x 10-4 parameters sil 1 e Range (kpa) C (kpa) ( ) I (0 t 2) Light grey, fly ash fill II (2 t 8) Silty clay with grey patches (kn/m 3 ) Page 6 f 11

7 Stratum thickness (m) NMC (%) Example Table 1 cntinued Descriptin f sil III (8 t 16) IV (16 t 22) V (22 t 30) Silty clay with decmpse d vegetatin Bluish medium silty clay Medium dense silty sand with mica Yellwish dense sand N (kn/m 3 ) Shear parameters C (kpa) ( ) Cc 1 e m v in m 2 /kn x 10-4 Range (kpa) VI (30 t 33) Bre stpped Water table was fund t be at 0.8 m belw existing grund level during sil explratin and is assumed t be belw 2.0 m during dry weather cnditin. 4.2 Allwable lad n Raft Ultimate bearing capacity f the chsen raft was first calculated frm the existing sil prfile fr the shear failure case. The crrespnding safe bearing capacity was fund t be 7.45 t/m 2. The value is quite satisfactry fr the prpsed site cnditin. But the ttal settlement at this capacity was fund t be 225 mm which is beynd the permissible settlement fr raft. Allwing the cnslidatin settlement f 100 mm, allwable capacity fr the chsen raft becmes 4.0 t/m 2 i.e. 40 kpa fr the present site cnditin, whereby the raft wuld be able t withstand quite a high lad withut suffering excessive settlement. 4.3 Calculatin f PR PR calculatin as per the prpsed methd can be described as fllws. Any super structural lad mre than 40 kpa is nt permissible fr the said raft as cnslidatin settlement wuld be beynd safe value. As mentined previusly, adpting the said raft t be supprted by 4 piles arranged symmetrically shwn in fig. 4(a) is cnsidered and a pressure f 50kPa i.e tn is applied n the piled raft fundatin system. The capacity f pile frm fig. 3(b) is 70 t crrespnding t a settlement f 0.5 mm. Thus ttal lad taken by 4 number f pile is 280 tn. At this pint f settlement f 0.5 mm taken as 1, the lad taken by raft cmes t abut 845 tn i.e. it is subjected t a pressure f kpa. At a small settlement f 0.5 mm pile can take quite a large lad. Cnsidering this as the nset f the lad sharing prcess between the raft and the pile and taking it as 1 st iteratin, fllwing eqn. 3 t 6, a prgram is executed and results are tabulated in Table N. 2. Fr calculatin f the ultimate time required by the pile raft fundatin t reach a balanced state f lad sharing, time settlement relatinship fr the adpted raft at a pressure range f 0.25 kpa t 0.5 kpa is pltted frm the cnslidatin characteristic f the existing sub sil btained frm sil explratin and is given in fig. 3(a). The individual lad carried by raft and piles fr gradual cnslidatin settlement are als given in Table N.2. Frm fig 5 (a) and (b) stress time Page 7 f 11

8 relatin and stress settlement relatinship respectively fr the piled raft fundatin culd be bserved. The curves reveals that apprximately after a time span f 7.5 years f full lad mbilisatin, lad transfer between piles and raft fr the adpted piled raft reaches a balanced state. After this time perid further cnslidatin settlement diminishes and attains a final value. If nly piles are prvided t carry a lad f 1125 tn, frm figure 3(b), required number f piles wuld have been 10 and if nly raft is prvided fr bearing such a lad, it wuld have suffered a settlement beynd a permissible value. Thus, pile raft fundatin is prved t be mre ecnmic cmpared t individual pile r raft fundatin and in additin verall settlement is cntrlled. (a) (b) Example Figure 3: (a) Time settlement curve f 15x15 sqm raft; (b) Lad settlement curve f 1000 mm diameter pile having length 30 m and cut ff at 2.0m belw G.l. Example Figure 4: Adpted piled-raft gemetry. Example Table 2: P and R values fr gradual cnslidatin settlements Page 8 f 11

9 Iteratin N. Increment in cnslidatin settlement (mm) Ttal settlement (mm) P (tn) R (tn) Stress n raft (kpa) Setting time in year PR (tn) 1125 (a) (b) Example Figure 5: (a) Stress-time curve f pile supprted raft; (b) Stress versus settlement fr adpted piled-raft fundatin. 5 Cnclusins Fllwing cnclusins can be drawn frm the prpsed simplified apprach fr the piled raft fundatin n sft cnslidating subsil cnditin. In rder t ecnmize the design f a piled raft system, raft must be allwed t share sme part f super structural lad. In case f under lying sft sub sil prfile t minimize the differential and verall settlement in piled raft fundatin due cnsideratin must be given cnslidatin characteristics f the funding stratum. The present methd by virtue f its prcedures, includes the all the sil-structure interactin effects f pile, raft and cmpsite piled raft fundatin system as the methd is slely based n the determinatin f all the engineering characteristic f a site physically and capacity f pile is directly calculated frm the rutine lad tests. Page 9 f 11

10 In this prpsed methd the pile dimensins, raft dimensins, different suitable methds f pile grup arrangement culd be incrprated in tentative designs t make a mst cst effective and efficient fundatin system fr a prttype fundatin system. The prpsed methd can help the designer in the first design stage t check the ratinality f a piled raft fundatin and t investigate bth the serviceability requirements as well as the ultimate limit state f the fundatin required fr a specific prject and site cnditin. References An G.F. and Ga D.Z. (2001) 3D FEM Applicatin t the Predictin f Creep Settlement f Sft Clay Cnsideratin Elastic-Visc Plastic Cnslidatin, Jurnal f Tngji University, 29(2): Butterfield, R. and Banerjee, P. K. (1971) The Prblem f Pile Grup- Pile Cap Interactin, Getechnique, 46(2), Cheng, Z.H., Ling, D.S. and Chen, Y.M. (2004) Time Effects n Pile Raft Fundatin n Vertical Lading, China Civil Engineering Jurnal, 37(2): Chun-yi Cui, Ma-tian Luan and Ying-hua Zha (2005) Time-dependent Behaviur f Piled Raft n Sil Fundatin with Reference t Creep and Cnslidatin, EJGE, Vl.14, Bund. A, Clancy, P. and Randlph, M. F. (1992) Analysis and Design f Piled Raft Fundatins, Research Reprt N. G 1062, Department f Civil Engineering, University f Western Australia, Perth, Australia. Franke, E. (1991) Measurement Beneath Piled Rafts, Keynte Lecture, ENPC Cnf., Paris, Franke, E., Lutz, B. and El-Mssallamy, Y. (1994) Measurements and Numerical Mdeling f High Rise Building Fundatins n Frankfurt Clay, Getechnical Special Publicatin, ASCE, 40, Hain, S. J. and Lee, I. K. (1978) The Analysis f Flexible Pile Raft System, Getechnique, 28(1), Hper, J. A. (1974) Observatins n the Behavir f Piled Raft Fundatin n Lndn Clay, Prc. Institutin f Civil Engineers, Part 2, 55, Jeng, Gy-Sung and Chi sik-kyung, (2003) Design Charts f Piled Raft Fundatin n Sft Clay, Prc. 13 th Int. Offshre and Plar Engg. Cnf., Hnululu, Hawaii, USA, May, 2003, Page 10 f 11

11 Katzenbach, R., Schmitt, A. and Turek, J. (2000) Piled Raft Fundatin Prjects in Germany, Design Applicatin f Raft Fundatins, (Ed.) J. A. Hemsley, Thmas Telfrd, Lndn, Puls, G. Harry. (1991) Analysis if Piled Strip Fundatins, Cmputer Methds and Advances in Gemechanics, (Eds.) Beer, G., Bker, J.R. and Carter, J. P., Balkema, Rtterdam, Puls, G. Harry. (2001) Piled Raft Fundatin: Design and Applicatin, Getechnique, 51(2), Prks, W. A. and Kulhawy, F. H. (2001) Cntributin t Piled Raft Fundatin Design, Jurnal f Getechnical and Geenvirnmental Engineering, ASCE, 127(1), Randlph, M.F. (1994) Design Methds fr Pile and Piled Rafts, Prc. 13 th Int. Cnf. n Sil Mech. Fund. Engg., New Delhi, Ry, S. and Chattpadhyay, C. B. (2011) A Simple Prcedure fr Design f Piled Raft System, Prc. 3 rd Indian Yung Getechnical Engineers Cnference, 25-26, March, 2011, Sinha, J. (1997) Piled Raft Fundatins subjected t Swelling and Shrinking Sils, Ph. D. Thesis, University f Sydney, Australia. Viladkar, M.N., Ranjan, G. and Sharma R.P. (1993) Sil-Structure Interactin in the Time Dmain, Cmputer and Structure, 27(2): Wang. J.H., Chen, J.J. and Pei, Jie. (2001) Interactin between Super structure and Layered Visc- Elastic Fundatin Cnsidering Cnslidatin and Rhelgy f Sil, Jurnal f Building Structures, 35(4): Wd, L.A. and Larnach, W.J. (1975) The Interactive Behaviur f Sil-Structure System and its Effect n Settlements, Sympsium n Recent develpment in Analysis f Sil Behaviur and their Applicatins t Getechnical Structures, University f New Suth Wales, Australia, Xia, Z.Z. (1994) Calculatin f Cntact Pressure Distributin n Elast-Visc Plastic Sil Medium, China Civil Engineering Jurnal, 27(2): Zhang, G. M., Lee, I. K. and Zha, X. H. (1991) Interactive Analysis Behavir f Raft-Pile Fundatins, Prc. Of Ge cast, 91, Ykhama, Page 11 f 11