Observed installation effects of vibro replacement stone columns in soft clay

Transcription

1 Technical Pape R1 Obseved installation effects of vibo eplacement stone colmns in soft clay D. Egan & W. Scott Kelle Gond Engineeing, Ryton-on-Dnsmoe, Coventy, UK B. McCabe Depatment of Civil Engineeing, National Univesity of Ieland, Galway, Ieland ABSTRACT: The installation of vibo eplacement stone colmns in any soil cases significant changes to the stess egime aond each pobe location. In soft clays, the pincipal effects ae cased by a cavity expansion mechanism as the pobe is intodced into the soil and the stone colmn constcted. This pape pesents obsevations of changes in soil volme and poe wate pesse ding stone colmn installation. The implications fo the most sitable colmn constction techniqes and the soil stength limiting colmn constction aising fom the obsevations ae discssed. The data pesented ae also of inteest to those validating nmeical models of stone colmn installation, and isses elevant to the pactical application of nmeical modeling ae consideed. INTRODUCTION This pape consides the obseved effects of both single and mltiple stone colmn installation in soft clay and demonstates that sccessfl gond impovement can be achieved with this techniqe. A detailed explanation of the vaios Vibo Replacement and Compaction techniqes available is povided by Sondeman & Weh () and is not epeated hee. This pape concentates on stone colmns installed sing the dy bottom feed method, which is the most widely sed techniqe in the UK fo otine pojects on soft clay sites, often pefeed nowadays to the wet method. A significant advantage of the dy bottom feed method ove the wet method is that a spply of wate flsh is not eqied with the attendant eqiement of handling and disposal of the wet spoil. Nmeos case stdies illstating the se of the dy bottom feed method fo a wide ange of pojects have been pblished, fo example, Coope & Rose (1999), Weh (), McNeill (7), Casto (7) and Egan (8). Thee aspects of the installation pocess ae consideed in this pape: (a) the gond esponse ding the installation of a dy bottom feed stone colmn, (b) changes to the soil stess field and poe wate pesse ding the eqalization peiod following stone colmn constction and (c) the inteaction effects eslting fom the constction of both small gops and lage gids of colmns. THE CONCEPTUAL MODEL FOR INSTALLA- TION OF A SINGLE COLUMN An example of a moden bottom feed poke is shown in Fige 1. The main body of the poke is boadly cylindical with a nmbe of fins attached which conteact the tendency fo the poke to otate ding colmn constction. The vibating motion is geneated by an electic poweed otating weight hosed within the poke casing. Fige 1 a typical bottom feed poke

2 Technical Pape R C (kpa) Amps (A) As the poke is loweed into the gond the soil is mainly displaced lateally as a cavity is opened p. Some vetical smeaing of the soil adjacent to the poke may occ, althogh thee is little evidence that this is significant, and some vetical heave of the gond sface can eslt. The magnitde of heave is highly dependent on the aangement of colmns being constcted as discssed below. Depth.... Fige. Poke cent demand compaed with ndained shea stength The stone is fed to the base of the poke body and compessed ai flsh is sed to assist the stone delivey. The ai flsh, which is channeled to the bottom end of the poke, assists in ceating a small annls between the poke and the gond, edcing fictional dag. The ai flsh also conteacts the sction that the poke wold geneate as it is withdawn fom the base of the colmn back p the boe, and which cold lead to the sonding soil moving back into the peviosly ceated void. The electical cent dawn by the poke ding initial penetation of the gond and sbseqent compaction of the stone povides an indication of the stength and stiffness of the gond. Fige compaes the meased daw of electical cent plotted against depth ding the constction of a.m long colmn in soft allvial clay. Also shown is the pofile of ndained shea stength fom the site (estimated fom cone penetation test data). The incease in cent daw with incease of the ndained shea stength of the host soil is clea. As thee ae a nmbe of othe factos which can affect the cent daw of the poke it is not possible to develop a simple and nivesal elationship with soil stength. Howeve the ability to mease and ecod the cent consmption does offe a qalitative indication of the qality of colmn constction in soft soils whee ovewoking of the gond (a sign of poo wokmanship) shold be avoided. This is one of the easons why moden dy bottom feed igs, fitted with advanced instmentation, can be sed whee hitheto the wet system wold have had to be adopted. Depth (m.... HEAVE Most expeienced pactitiones ae awae that the installation of stone colmns into soft clay can geneate a degee of sface heave, paticlaly if constction contol is inadeqate and ovewoking of the gond takes place. Some degee of sface heave ding the installation of stone colmns sing the dy bottom feed method is inevitable and demonstates that the stess egime aond the colmn is being alteed and impovement of the gond is being achieved and is not detimental to the final fondation pefomance. Fo this eason heave is aely meased (indeed it is not necessay to do so). Nonetheless the few case stdies whee heave was ecoded ae of inteest as they show that the amont of heave is a fnction of the colmn size, spacing, extent and constction method. Casto (7) epoted a conical zone of sface heave with a maximm vale of 9mm following the installation of a test gop of seven colmns. Fige. Stone colmn and instmentation layot epoted by Casto (7)

3 Technical Pape R1 The colmns wee.8m in diamete, 9.m long installed at.8m centes thogh soft to fim allvial clay by Kelle Tea in Spain. Fige shows the colmn layot and the locations of piezometes installed pio to the stone colmn constction. It is nclea if the heave epoted was a local maximm and the oveall volme of heave was not epoted. Watts et al () epoted diffeent amonts of heave fo diffeent colmn densities in the soft Case clay at the Bothkenna test site in Scotland. (Details of this site ae flly discssed in Geotechniqe, Volme, 199). The colmns wee installed to a length of m sing a Bae HBM bottom feed ig. The installation pocede followed the now dated pactice of amming a lage volme of stone into the base of the colmns ceating an enlaged toe of p to 1.m diamete. Extensometes evealed that most heave was geneated by the constction of the blb, emphasising that this pactice is to be avoided if heave is to be contolled. No heave was ecoded fo two colmns located at 1.m centes (althogh 1mm of lateal displacement was meased by inclinomete.m fom the colmns). Thee colmns at.m centes podced an aveage local incease in gond sface of 17mm; fo colmns at 1.m centes podced an aveage of 1mm of heave. A maximm of mm heave was ecoded de to installation of colmns m long to sppot an 8.1m sqae concete test aft. The total volme of heave ecoded fo these colmns eqated to 7% of the stone intodced into the gond. Measements of heave have also been made ding a few contacts pefomed by Kelle Gond Engineeing in the UK. In Contact A in Lincolnshie, an aveage of aond 1mm of sface heave was meased ding the installation of a lage gid (8m x 7m) of m long mm diamete colmns in soft allvial clay (kpa<c<kpa, %<w<7%). The colmns wee installed on a m by m sqae gid and the volme of heave eqated to 7% of the volme of stone in the colmns. In anothe contact (Contact B), thee sepaate ows each compising five colmns (mm diamete and.m long) wee installed in Case clay. Contos of the aveage magnitde of heave, estimated fom a gid of levels taken aond each colmn gop, ae shown in Fige. The aveage maximm heave was aond 7mm along the centeline, diminishing with distance fom the colmns. In this case, the volme of heaved gond was estimated to be appoximately % of the volme of stone installed in the colmns. While the phenomenon of heave is occasionally efeed to in the liteate, vey little qantitative gidance is available to estimate how mch heave can be expected fo any given colmn aangement. Defining the Peimete Ratio P and Heave Ratio Hp accoding to eqations [1] and [] below appeas to allow a elationship to be developed, and vales of P and Hp ae shown in Table 1 fo the case histoies discssed above. P Colmn peimetes in a gop Extenal peimete of the gop (1) Hp = Volme of heaved gond () Volme of stone colmns Table 1. Data on obsevations of heave ding colmn installation Refeence Nmbe of colmns in gop Colmn length Aveage colmn diamete Aangement Peimete atio, P Heave pecentage, Hp m mm % Casto (7) m spacing 1. 1 Watts et al () 8.1m aft Colmns Colmns Colmns m centes aond edge of aft in cente Kelle Contact B. Single line 1.m centes. Kelle Contact A Infinite Infinite m sqae gid. 7

4 Technical Pape R Heave Ratio Sface Heave Ratios 8% 7% % % % % % 1% % Peimete atio Small gops Lage gid 8.1m Raft Fige. Contos of heave aond a line of stone colmns The data fom Table 1 ae plotted in Fige and while it is ecognised that the database is limited the data shown an inteesting tend sggesting some sot of elationship between colmn layot and amont of heave exists. Fthe collection of field data is ecommended so that this elationship can be sbstantiated. Heave Smmay It is clea fom a nmbe of obsevations that the pocess of constcting stone colmns cases some lateal and vetical soil displacement, and that the popotion of lateal to vetical stain is a fnction of the lateal confinement of the host gond and adjacent stone colmns. Single colmns, ows and small gops of colmns exhibit less vetical heave than lage gids of colmns. It also appeas that that the close the colmn spacing fo any given colmn aangement the geate the magnitde of heave. In consideing these data, vaiations in gond conditions and installation pactice between diffeent sites is not consideed and at this stage and the tends shown in Fige shold be consideed indicative athe than definitive. TOTAL STRESS AND CAVITY EXPANSION The penetation of the vibo poke into the gond and sbseqent chaging and compaction of the stone aggegate may be modelled mathematically by the theoy of cylindical cavity expansion, sch as that developed by Gibson and Andeson (191). As the pocess of stone colmn installation is elatively qick in satated soft clays, ndained cavity expansion is most applicable. Cavity expansion theoy comes in vaios foms, taking accont of (i) the eqied igo in modelling soil behavio (elastic, elastic-plastic o citical state soltions), (ii) whethe the medim is finite o infinite, and (iii) the effect of vaios initial adii on the cavity stesses. Using Gibson and Andeson s (191) elasticplastic theoy, the zone of soil closest to the cavity is in a plastic state and the soil beyond emains in an elastic state. The adial extent of the plastic zone, R, may be calclated as: R ae () whee p is the cavity pesse is the adial total stess at adis, c is the ndained shea stength and a is the adis of poke. Cavity expansion is effectively infinite in the case of a penetating poke (stating fom zeo initial adis and expanding to adis a), in which case it is easonable to assme that the pesse in the cavity will have eached the limit pesse (plim), calclated accoding to: p Fige Gond sface heave atios fo stone colmn installation. lim p c h c c E (1 ) () whee ho, E and ae the fee field hoizontal total stess, Yong s modls and Poisson s atio espectively. In this instance, the adial stesses in the plastic (<R) and elastic (>R) zones will be given by eqations [] and [] espectively: R h c h c c ln R () ()

5 Technical Pape R1 Stess Distibtion Aond an Expanded Cavity in Undained Cohesive Soil 1 1 s distibtion thoghot sq distibtion thoghot Mean nomal stess in hoizontal plane,sm Raploch Data Seies 8 Casto fomlation Stess (kpa Radis (m) Fige. Stess field aond a stone in soft Case clay The above eqations wee sed to calclate the stess field at m depth fo Contact B. At this site the clay was vey soft with an ndained shea stength, c, of aond 1kPa at this depth. The stess field is shown in Fige, fom which it can be obseved that the adis of the plastic zone is 1.m, beyond which the mean hoizontal stess is nchanged fom the initial (pe-colmn installation) vales. PORE PRESSURES AND CAVITY EXPAN- SION Obsevations Ding Colmn Constction Kisch (), Gab et al (7) and Casto (7) meased the poe pesse esponse to the installation pocess in thee sepaate stdies. Fige 7 epodces the data shown by Casto (7) as an example of the geneal tends. In all thee cases the poe pesse ose extemely apidly as the poke fist penetated the gond geneally eaching a peak when the pobe achieved the same depths as the piezomete. This is in keeping with geneal tends obseved fo displacement piles (i.e. McCabe et al 8). Shap and significant flctations in eadings chaacteise the cves ding colmn constction and ae attibted to a ange of cases inclding contact pesse flctations as the poke is aised and loweed into the gond (low fe- Fige 7. Poe wate pesses meased ding stone colmn constction (Casto (7) qency), cyclic vibations fom the poke (high feqency ~Hz), possibly effects fom escape of ai flsh into the gond and geneal consction vibation. In all cases the poe pesses eached a qasi static level, albeit at an elevated level, immediately afte completion of each stone colmn. Sbseqent installation of adjacent colmns leads to a fthe incease in the qasi static levels. Dissipation of the excess poe pesse is obseved, occing at diffeent ates, govened by the pemeability chaacteistics of the host soil. Kisch () and Casto (7) do not povide dissipation data beyond the end of constction of the colmn gops and it shold be noted that the colmns appea to have been constcted in a high silt content soil so may not be diectly compaable to stone colmns constcted in soft clay. Casto (7) pesents a finite diffeence analysis to estimate the ate of dainage, howeve the meased Pesse (Pa) 8-Ag -Sep Colmn constction 11-Sep Kelle Raploch Contact Site Footing B Tial Constct footing 18-Sep -Sep Load footing -Oct Date Fige 8 Piezomete esponse to colmn loading Kelle Case clay site 9-Oct 1-Oct -Oct -Oct Footing B m depth Footing C m depth Footing A m depth Footing C m depth

6 Technical Pape R1 dissipation was appoximately 1 times faste than pedicted by the theoy. This was attibted to the pesence of gond factes cased by the penetation of the ai flsh ding colmn installation ceating pefeential dainage paths, bt the actal easons wee not conclsively poved. Following Colmn Constction Fige 8 shows the dissipation of meased poe pesses fom Kelle Gond Engineeing Contact B. The piezometes wee BAT-System instments (spplied by BAT Geosystems AB) and compise a poos filte tip which is inseted into the gond independently of the measing senso which can be inseted and emoved fom the filte tip many times and withot casing any gond distbance. To pevent possible damage to the measing sensos, they wee emoved fom the filte tips ding stone colmn constction, being eplaced immediately constction was completed. The poe pesses meased theefoe ae those pevailing jst afte completion of the stone colmns. Fige 8 also shows the esponse of the poe wate pesses ove the following two months. It shold be noted that a concete footing was constcted and then loaded as shown in Fige 8, befoe fll dissipation of the installation excess poe wate pesse had occed. Oedomete tests caied ot on samples of the Case clay indicated that.<cv<1.7m /y and fom insit measements 1.<Ch<7.m /y. Watts et al (1) epot fo the installation of colmns fo a 8.1m sqae aft at the Bothkenna test site, a maximm 1kPa ise in poe wate pesse ding colmn constction, fom a piezomete.m deep installed midway between colmns 1.m apat. The elevated pesse appeas to have dopped back to aond kpa within hos and had etned to pe-teatment levels afte abot months. Howeve some elevated pesse appaently emained between two colmns at a depth of m afte fo months, althogh it is not clea why this was the case. Relevance of Obsevations to Cavity Expansion Theoy Casto (7) applies the Randolph et al (1979) fomlation of cavity expansion theoy applied to the installation of stone colmns, sggesting that the incease in poe pesse at a adis, is elated to the ndained shea stength c, and adis of the plastic zone (R) by: c R ln( ) (7) A vey good coelation with meased peak inceases in poe wate pesse at vaying distance ding the installation of a single stone colmn is shown. Installation of sbseqent colmns did not follow the theoy at all well. It is pobable that the changes in the soil stess field and poe wate pesse distibtion geneated by installation of the fist colmn violate the assmptions govening the cavity expansion theoy fomlation applied fo installation of sbseqent colmns. The cve fom eqation 7 is plotted in Fige 7 aising fom the expansion of a mm diamete cavity.m long plotted with meased poe wate pesses fo Kelle Gond Engineeing Contact B. A easonable coelation is obseved at a depth of.m midway between two colmns. Implications of Poe Pesse Changes The incease and then decease of poe pesses aising fom colmn installation indicate that the gond is sqeezed lateally as the colmns ae installed, casing an immediate ndained incease in poe wate pesse which then dissipates as the clay consolidates and gains stength. Fom the pblished evidence the consolidation pocess appeas to take within a peiod of a few weeks following colmn installation, ding which time the gond between the colmns inceases in stength. This pocess will sally be complete befoe constction of the sppoted stcte stats, o at least have conclded ding the ealy stages of constction the evidence being sccessfl completion ove many yeas of a lage nmbe of vibo eplacement pojects in soft clays. IMPLICATIONS FROM OBSERVED BEHAV- IOUR FOR NUMERICAL MODELLING While the installation of boed piles can be simlated in Finite Element packages sch as PLAX- IS, thee is cently no satisfactoy pocede available to model the lage lateal displacements associated with pile diving o stone colmn installation. Some simple attempts to capte expected changes in gond popeties have been poposed, althogh thee is geat ncetainty egading how the gond esponds as high qality field data that is sfficiently extensive is limited. Some appoaches that have been taken inclde: (i) inceasing K in the vicinity of the colmns to eflect the pemanent incease in total stess

7 Technical Pape R1 (ii) Getif et al (7) model installation in PLAXIS -D by defining a cylinde of dmmy mateial with pely elastic popeties and a nominal Yong s Modls (E=kPa). The dmmy mateial is then expanded fom an initial diamete of.m to a final diamete of 1.1m, befoe the popeties of the dmmy mateial ae conveted into those of the colmn mateial. Howeve, this appoach appeas to be nsitable fo PLAXIS -D as its basis is in small stain theoy and nmeical instability/inaccacy may aise fom lage element distotions. (iii) Pompted by expeimental eslts, Kisch () stats with a cylindical block having the popeties of stone, befoe applying a adial expansion. He also sggests that the stess incease may be modelled by applying a tempeate gadient to the stone colmn having a coefficient of themal expansion, o the application of a stain o stess field to the finite element mesh. Howeve the cavity expansion appoach was deemed to be speio fom a nmeical stability point of view. The key isse in modelling cavity expansion in a FE famewok is the amont of adial stain that mst be applied in the model to eplicate field conditions, and impotantly, whethe this amont of expansion violates the assmptions in the FE theoy sed. Thee is consideable amont of field wok and model development that needs to be caied ot in tandem befoe installation effects can be inclded in otine design. CONCLUSIONS The obseved effects of stone colmn installation indicate that they impove the stength and stiffness of the host gond by a combination of intodcing ganla elements of geate stength and stiffness than the host gond. Additional benefit is deived as a fnction of inceasing the hoizontal stesses and sbseqent consolidation and incease in the stength of the host gond. Simple closed fom eqations based on cavity expansion ae sefl fo modeling the inceases in hoizontal stess and poe wate pesse ise de to the installation of a single colmn. Whee gops of colmns ae installed inteaction of the stess fields fom each colmn occs in a way that can not, at pesent, be descibed by simple closed fom eqations. While thee has been faily widespead se of nmeical modelling fo both eseach and poject pposes, few stdies have addessed the installation effects. Within the few nmeical stdies that have addessed installation effects diffeent modeling appoaches have been adopted and at the pesent time thee does not appea to be a nivesally accepted, igoos, means of acconting fo the installation effects. What is clea is that installation effects can inflence the pefomance of vibo eplacement stone colmn pojects, and as is the case with most geotechical pocesses, constction contol by expeienced pactiones emains the key to sccessfl implementation. REFERENCES Casto 7. Poe pesse ding stone colmn installation, Poceedings of the 18 th Eopean Yong Geotechnical Enginees Confeence, Ancona: Italy Coope, M.R. and Rose, A.N Stone colmn sppot fo an embankment on deep allvial soils. Poceedings of ICE Geotechnical Engineeing, Vol. 17, pp 1-. Egan, D. 8 CPT Testing fo the design and Implementation of vibo eplacement gond impovement. Poc. d Intenational Confeence on site chaacteisation. Taipei: China. Gäb, M., Schweige, H.F., Thne, R. and Adam, D. 7. Field tial to investigate the pefomance of a floating stone colmn fondation. Poceedings of the 1 th Eopean Confeence on Soil Mechanics and Geotechnical Engineeing. pp Madid: Spain. Geotechniqe, Volme, 199. Gibson, R. H. and Andeson, W. F 191. In-sit measement of soil popeties with Pessemete. Civil Engineeing and Pblic woks eview. Vol.. No. 8, May. Getif, Z., Boassida, M. and Debats, J.M. 7. Impoved soft clay chaacteistics de to stone colmn installation, Comptes and Geotechnics, Volme, Isse, Mach 7, pp Kisch, F.. Vibo Stone Colmn Installation and its effect on gond impovement. McCabe, B.A., Gavin, K.G. and Kennelly M. 8. Installation of a edced scale pile gop in silt, Poceedings of the nd BGA Intenational Confeence on Fondations, Dndee. McNeill, J. A 7. Impovement of Soft Gond by vibo stone colmns. Poc. Confeence on soft gond engineeing. Long, Jennings & Rtty (ed) Geotechnical Society of Ieland. Dblin: Ieland. Randolph, M.F. Cate, J.P. & Woth, C.P Diven piles in clay the effects of installation and sbseqent consolidation. Geotechniqe 9 (): pp1-9. Sondemann, W & Weh, W,. Deep vibo techniqes. Gond Impovement. nd Edition Ed. Moseley and Kisch. Watts, K. S, Chown, R. C & Seidge, C. J. 1. Vibostone colmns in soft clay: A tial to stdy the inflence of colmn installation on fondation pefomance. Poc 1 th Int Conf SM&GE. Vol 1: pp Weh, J. The ndained cohesion of the soil as a citeion fo the colmn installation with a depth vibato. Tansvib. Genin, Holeyman & Roche-Lacoste. (ed) LCPC. Pais: Fance.