Influence of Hydraulic Hysteresis on Effective Stress in Unsaturated Clay

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1 Intentionl Jounl of Envionmentl nd Eth Sciences 1:1 21 Influence of Hydulic Hysteesis on Effective Stess in Unstuted Cly Anuchit Uchipicht Abstct A compehensive pogm of lbotoy testing on compcted kolin in modified tixil cell ws pefom to investigte the influence of hydulic hysteesis on effective stess in unstuted soils. The test dt e pesented on nge of constnt suction she tests long wetting nd dying pths. The vlues of effective stess pmete χ t diffeent mtic suction wee detemined using the test esults. The effect of hydulic hysteesis phenomenon on the effective stess ws obseved. The vlues of effective stess pmete χ obtined fom the expeiments wee comped with those obtined fom the expessions poposed in litetue. Keywods Unstuted soils, Hydulic hysteesis, Effective stess S I. INTRODUCTION EVERAL elsto-plstic models fo unstuted soils hve been developed using effective stess ppoch [1]-[5]. The effective stess convets soil with multiple fluid phses to n equivlent single-phse mteil with single-stess stte continuum. Thus, the elsto-plstic constitutive equtions of solid phse cn be pplied using the effective stess to unstuted soils. Anothe dvntge of the effective stess ppoch is tht soil popeties cn be detemined using conventionl lbotoy equipment with few modifiction fo testing unstuted soils. The effective stess fo unstuted soils cn be expessed s function of the extenlly pplied stesses nd the intenl fluid pessues nd defined s [6], ( p u ) + ( u u ) p = χ (1) whee p is the men effective stess, p is the men totl stess, u is the poe i pessue, u w is the poe wte pessue, u uw is the mtic suction, nd χ is the effective stess pmete ttining vlue of unity fo stuted soil nd zeo fo dy soil. The pmete χ is stongly elted to the soil stuctue [7] nd suction tio, defined s the tion of mtic suction ove the i enty suction [8], A. Uchipicht is with the Civil Engineeing Deptment, Vongchvlitkul Univesity, Nkhontchsim THAILAND (coesponding utho to povide phone: ; fx: ; e-mil: nuchitu@yhoo.com). w [ s / s ] Ω e fo s se χ = (2) 1 fo s se in which, s = u uw is the mtic suction, s e is suction vlue mking the tnsition between stuted nd unstuted sttes, nd Ω is mteil pmete, with bestfit vlue of.55. Fo the min wetting pth, s e = sex, nd fo the min dying pth s e = se, in which s ex is the i expulsion vlue nd s e is the i enty vlue. Futhemoe, the use of the degee of stution s pmete χ hs been pplied in few models [1], [3]. Thus, the influence of hydulic hysteesis on the effective stess hs been intoduced though the pmete χ. The min objective of this ppe is to pesent effect of hydulic hysteesis on effective stess in unstuted soils though esults fom compehensive pogm of lbotoy testing on compcted smple of kolin in tixil cell. The test dt e pesented on nge of constnt suction she tests long wetting nd dying pths. II. EXPERIMENTAL EQUIPMENT A. Equipment Peption The tests wee pefomed using conventionl tixil cell with few of modifictions fo testing unstuted soils s shown in Fig. 1. The modified cell ws cpble of independent mesuement nd contol of poe i pessue nd poe wte pessue t the top nd the bottom boundies of the specimen. The poe i pessue ws contolled though cose poous stone plced t the top of the specimen. The poe wte ws contolled t the bottom of the specimen though high i enty vlue cemic disc. The cemic disc ws ttched to the pedestl bse using epoxy glue to pevent flow of i to the wte comptment though its suoundings. The suction in the smple ws contolled using xis-tnsltion technique [9]. Pio to ech test, the cemic disc nd the poe wte contol system wee stuted using technique simil to tht used by [1]. The empty cell ws filled with de-ied wte nd pessuized to cell pessue of 6 kp. The wte ws llowed to flow though the cemic disc into the wte comptment while mintining the wte pessue t the bse of cemic disc t the tmospheic level. Afte collecting cm 3 of wte flow, the dinge vlve ws closed fo t lest 2 hous to dissolve ny i tpped within the cemic disc. The 2

2 Intentionl Jounl of Envionmentl nd Eth Sciences 1:1 21 ddition flushing pocess ws equied to ensue tht the cemic disc ws fully stuted. Axil foce TABLE I INDEX PROPERTIES OF KAOLIN Popety Vlues De-ied wte Poous Stone Composite ubbe membne O-ing To poe wte pessue contol system Specimen Fig. 1 modified tixil cell Cell Wll High i enty disc B. Specimen Volumetic Stin Mesuement A digitl imge-pocessing technique ws used fo mesuing volumetic stins of the specimens. The imges of the specimens wee tken duing testing using digitl imging equipment. A Nikon D5 digitl single lens eflex cme with 8 pixels by pixels esolution ws use to cptue the imges of specimens. A Nikon AF-Nikko 85 mm f/1.8d lens ws used togethe with the Nikon D5 cme. To poduce cle specimen boundies on the imges, the specimens wee lighted to povide sufficient illumintion fo thei shdow es. The pocess of detemintion of specimen volume fom the digitl imges ws pefomed using technique simil to tht of [12]. The specimen height ws mesued long the xis of specimen ppeing in the digitl imges. Fo dimete mesuement, the specimen ws ssumed s seies of stcked discs with vition in dimete. The dimete of the specimens ws obtined though veging ll ssumed discs. Finlly, the specimen volume nd the volumetic stins wee clculted fom the equivlent cylindicl shpe of specimen with the mesued height nd the vege dimete. III. SAMPLE PREPARATION AND SET-UP To cell pessue contol system To i pessue contol system A. Smple Popeties nd Peption The expeiments wee cied out on lbotoycompcted-kolin. The index popeties of test soil e given in Tble 1. Liquid limit (%) 52 Plstic limit (%) 31 Specific gvity 2.72 Mximum dy unit weight * (kn/m 3 ) 14.1 Optimum moistue content (%) 27.5 * Obtined using stndd Pocto test. To mke identicl specimens with mtix menble to stiffening with incesing mtic suction, the smples wee peped dy of optimum s in [11]. The kolin smple with wte content of 25% ws stticlly compcted to dy unit weight of ppoximtely 11.8 kn/m 3. Pio to compction, the kolin smple ws cefully wetted to wte content of 25% nd cued fo 24 hous in seled plstic bg fo moistue equliztion. The compction ws cied out in nine equl lyes in gesed split mold of 38 mm dimete. The soil-wte chcteistic cuve (SWCC) of the compcted specimens, pesented in Fig. 2, ws obtined by keeping the compcted specimens in the modified tixil cell t the diffeent vlues of constnt mtic suction fo 3 dys. Fo wetting potion of the cuve, the mtic suction ws fist incesed to kp nd then educed to the tget vlue. The vlues of i enty ( s e ) nd i expulsion ( s ex ) obtined fom the SWCC e 8 nd 35 kp espectively. B. Specimen Set-up The specimen ws coveed by two ubbe membnes. To minimize diffusion of i though the membnes, the silicon gese ws plced between the two membnes. The coveed specimen ws then plced on the cemic disc nd seled by fitting O-ings ound the bse pedestl nd the top cp s shown in Fig. 1. Afte ssembling the pptus, the cell ws filled with de-ied wte nd n xil lod of bout 5 kp ws pplied to secue the specimen in plce. The fee i in the specimen ws flushed though the top cp by pplying the wte pessue of 5 kp though the cell bsed while mintining the pessue t the top t the tmospheic level. To pevent filue of the specimen, cell pessue of 75 kp ws pplied thoughout the flushing pocess. Afte flushing fee i in the specimen fo thee dys, the top dinge line ws closed fo 24 hous to equlize poe wte pessue within the specimen. IV. TEST PROGRAM AND PROCEDURE A totl of 12 she tests wee pefomed using the modified tixil equipment nd wee divided into two goups fo stuted nd unstuted specimens. The min pupose of the tests on stuted specimens ws to detemine the slope of the citicl stte line in the plne of devito stess ginst men effective stess. Fo the tests on unstuted specimens, the pupose ws to investigte the effect of hydulic hysteesis 21

3 Intentionl Jounl of Envionmentl nd Eth Sciences 1:1 21 on stess-stin chcteistics. All the test esults wee cefully nlyzed nd utilized to detemine the pmete χ. Degee of Stution S ex = 35kP Wetting S e = 8kP 1 1 Mtic Suction (kp) Dying Fig. 2 Soil wte chcteistic cuve of compcted kolin A. She Tests on Stuted Specimens A conventionl consolidted dined tixil tests pocedue ws followed in expeiments on stuted specimens. Thee specimens wee consolidted to diffeent isotopic effective stess of, nd kp. At the end of consolidtion, the xil stess ws pplied while mintining the cell pessue constnt. B. She Tests on Unstuted Specimens The she tests wee pefomed on both died nd wetted specimens. Pio to ech she test, the specimens wee consolidted to n isotopic stess of kp. Then the specimens wee unloded to the isotopic stess of kp. Thus, ll specimens in this testing goup wee subjected to identicl stesses stte befoe pplying mtic suction. The specimens wee then subjected to the diffeent vlues of mtic suction nged fom to kp. Fo the died specimens, the mtic suction ws incesed fom kp (stuted stte) to the testing vlue. Fo the wetted specimens, the mtic suction ws fist incesed to kp nd then educed to the testing vlue. The dined she tests wee then pefomed by incesing xil stess while mintining cell pessue, poe i pessue nd mtic suction constnt. V. RESULTS AND DISCUSSION The test esults in tems of stndd nottions fo tixil tests e pesented thoughout. Two pis of the wok conjugte vibles e men effective stess nd volumetic stin ( p nd ε v ), nd devito stess nd she stin ( q nd ε s ). These vibles e defined s, in which, stesses. σ + 2σ p = (2) 3 q = σ σ ε v = ε + 2ε 2 ε = ε ε 3 s ( ) σ nd σ e the xil nd dil effective ε nd ε e the xil nd dil stins. Compessive stesses nd stins e ssumed positive. The tests esults e pesented in Figs. 3 to 6. It cn be seen tht the citicl stte, in which the devito stess nd the volumetic e constnt nd independent of the she stin, ws ppoched in ll the tests. The stess-stin cuves nd stess pth fo the stuted specimens subjected to diffeent vlues of isotopic stess e shown in Figs. 3 nd 4, espectively. The slope of the citicl stte line in the plne of devito stess ginst men effective stess of the stuted specimen is.88. The stess-stin cuves fo the died nd wetted specimens t diffeent vlues of mtic suction e shown in Figs 5 nd 6. The stin-hdening esponse ws obseved in ll the mtic suction vlues. Futhemoe, t the sme level of mtic suction, the stess-stin cuves of the died specimens lwys lies bove tht of the wetted specimens. Mking use of the obsevtion of the citicl stte lines t diffeent mtic suction by [13], the pmete χ cn be detemined by ssuming tht the slope of the citicl stte line in the plne of devito stess ginst men effective stess is independent of mtic suction. Thus, using the effective stess expession in (1), the pmete χ cn be expessed s, q f ( p f u ) M χ = (3) u u whee p f nd q f e the men totl stess nd the devito stess t the citicl stte, M is the slope of the citicl stte line in the plne of devito stess ginst men effective stess. The vlues of the pmete χ clculted using eltion (3) nd the she test dt t the citicl stte t the diffeent vlues of mtic suction e shown in Figs 7 nd 8. It is obvious tht the hydulic hysteesis phenomenon ffects the effective stess though the pmete χ. Moeove, the expession in (2) fo pmete χ poposed by [8] gives good tend ove the nge of testing mtic suction on both dying nd wetting pths. The esults lso show tht the use of degee of stution s pmete χ is not suitble fo w 22

4 Intentionl Jounl of Envionmentl nd Eth Sciences 1:1 21 clculting the effective stess. 35 Devito Stess (kp) p' o = kp p' o = kp p' o = kp Devito Stess (kp) 25 u -u w = kp (died) kp (died) kp (died) 5 kp (died) kp (stuted) She Stin Fig. 3 Consolidted dined tixil tests on stuted specimens She Stin Fig. 5 Suction-contolled tixil she tests on died specimens 35 u -u w = kp (died) 5 Devito Stess (kp) M =.88 Ciicl Stte Line Devito Stess (kp) 25 kp (wetted) kp (wetted) 5 kp (wetted) 25 kp (wetted) kp (stuted) Men Effective Stess (kp) Fig. 4 Effective stess pths fo tixil tests on stuted specimens She Stin Fig. 6 Suction-contolled tixil she tests on wetted specimens 23

5 Intentionl Jounl of Envionmentl nd Eth Sciences 1: χ = S hysteesis on the stess-stin chcteistic ws obseved. The vlues of effective stess pmete χ t diffeent mtic suction wee detemined using the test esults nd comped with those obtined fom the expessions poposed in litetue. χ χ Khlili et l. [8] Mtic Suction (kp) Fom expeiments Fig. 7 Pmete χ fo died specimens Fom expeiments 4 Mtic Suction (kp) χ = S Khlili et l. [8] Fig. 8 Pmete χ fo wetted specimens VI. CONCLUSION An expeimentl pogm of she tests ws conducted on compcted kolin using conventionl tixil equipment modified fo testing unstuted soils. The modified tixil cell ws cpble of independent mesuement nd contol of poe i pessue nd poe wte pessue t the top nd the bottom boundies of the specimen. The effect of hydulic ACKNOWLEDGMENT This wok ws suppoted by the Thilnd Resech Fund unde Gnt MRG5828 REFERENCES [1] R. W. Lewis, nd B. A. Schefle, The Finite Element Method in the Defomtion nd Consolidtion of Poous Medi, John Wiley nd Sons, UK, [2] Y. Kohgo, M. Nkno, nd T. Miyzki, Theoeticl spects of constitutive modelling fo unstuted soils, Soils nd Foundtions, Vol. 33, No. 4, 1993, pp [3] D. C. Sheng, S. W. Slon, nd A. Gens, Finite element fomultion nd lgoithms fo unstuted soils. Pt I: Theoy, Intentionl Jounl fo Numeicl nd Anlyticl Methods in Geomechnics, vol. 27, no. 9, 3, pp [4] B. Loet, nd N. Khlili, An effective stess elstic-plstic model fo unstuted poous medi, Mechnics of Mteils, vol. 34, 2, pp [5] D. Gllipoli, A. Gens, R. Shm, nd J. Vunt, An elsto-plstic model fo unstuted soil incopoting the effects of suction nd degee of stution on mechnicl behviou, Geotechnique, vol. 53, no. 1, 3, pp [6] A. W. Bishop, The Pinciple of Effective Stess, Teknish Ukebldk, Nowy, 1959, pp [7] J. D. Colemn, Stess-stin eltions fo ptly stuted soils, Geotechnique, Vol. 12, No. 4, 1962, pp [8] N. Khlili, F. Geise, nd G. E. Blight, Effective stess in unstuted soils, eview with new evidence, Intentionl Jounl of Geomechnics, vol. 4, no. 2, 4, pp [9] J. W., Hilf, An Investigtion of Poe Pessue in Compcted Cohesive Soils, Technicl Memondum 654. Denve, CO: US Bueu of Reclmtion, [1] D. G., Toll, The Behviou of Unstuted Compcted Ntully Ocuing Gvel, PhD Thesis, The Univesity of London, Impeil College of Science nd Technology, London, UK, [11] Y. J. Cui, nd P. Delge, Yielding nd plstic behvio of n unstuted compcted silt, Geotechnique, vol. 46, no. 2, 1996, pp [12] E. J. Mci, J. K. Pke, nd N. C. Costes, Mesuement of volume chnges in tixil tests using digitl imging techniques, Geotechnicl Testing Jounl, Vol. 2, No. 1, 1997, pp [13] A. Uchipicht, Expeimentl Investigtion nd Constitutive Modelling of Themo-Hydo-Mechnicl Coupling in Unstuted Soils, PhD Thesis, The Univesity of New South Wles, NSW, AUSTRALIA, 5. Anuchit Uchipicht ws bon in Nkhotchsim, Thilnd. He obtined Phd in Geotechnicl Engineeing fom the Univesity of New South Wles, Austli in 5. He is cuently n ssistnt pofesso t Vongchvlitkul Univesity in Thilnd. His mjo fields of study e unstuted soil behvios, gound impovements nd geoenvionmentl engineeing. 24