COMPARISON OF DIRECT AND INDIRECT MEASURED SOIL- WATER CHARACTERISTIC CURVES FOR A SILTY SAND

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1 Interntionl Journl of GEOMATE, Nov., 217, Vol.13, Issue 39, pp.9-16 Geotec., Const. Mt. & Env., ISSN: , Jpn, DOI: COMPARISON OF DIRECT AND INDIRECT MEASURED SOIL- WATER CHARACTERISTIC CURVES FOR A SILTY SAND T. Aeykoon 1, R.S. Udukumurge 2, *C. Gllge 3 nd T. Uchimur 4 1 Fculty of Engineering, University of Mortuw, Sri Lnk; 2,3 Science nd Engineering Fculty, Queenslnd University of Technology, Austrli; 4 Deprtment of Civil Engineering, University of Tokyo, Jpn *Corresponding Author, Received: 19 My 217, Revised: 3 My 217, Accepted: 2 June 217 ABSTRACT: It is time consuming nd needs sophisticted testing pprtus to determine unsturted soil properties such s hydrulic conductivity nd sher strength. Therefore, engineers hesitte to use unsturted soil properties in economicl geotechnicl prolem solving. To promote the use of unsturted soil properties in geotechnicl engineering designs, numers of methods hve een developed to estimte/predict unsturted soil properties. The Soil-wter chrcteristic curve (SWCC) defines the reltionship etween the soil suction nd wter content. During pst few decdes, different mesuring nd estimtion/prediction methods hve een developed y reserchers to scertin SWCC of soils. Among them, direct nd indirect methods re widely used to mesure the SWCC of soil. Indirect methods such s xis-trnsltion technique re commonly used in the lortory to determine SWCC. It is importnt to understnd how well the indirectly mesured SWCC is relted to ctul soil-wter retention properties of soil during drying nd wetting process. Bridging tht reserch gp, in this study, Soil wter chrcteristic curve (SWCC) of sndy soil ws mesured using oth indirect (Axis trnsltion method using Tempe Pressure Cell) nd direct methods. The direct mesured SWCCs were otined y sujecting the instrumented soil column nd model emnkment to wetting nd drying cycles. When compring SWCCs mesured y direct nd indirect methods, it ws found tht the indirect method provides very close greement with the outcomes of direct methods. This ensures tht the SWCCs mesured in the lortory y using indirect methods cn e used in Geotechnicl Engineering prctice. Keywords: Soil wter chrcteristic curve, Unsturted soil, Tempe pressure cell, Mtric suction, Instrumented soil column, Instrumented model emnkment 1. INTRODUCTION The mesurement of soil prmeters for unsturted soil condition hs lwys een time consuming nd exoritnt pproch nd s result, geotechnicl engineers finding it difficult to incorporte prevlent knowledge of unsturted soil mechnics into routine geotechnicl designs nd prolem solving. In ddition, lortory experimenttion of this nture requires rigorous process nd technicl expertise tht might e imprcticl for quick decision mking tsks. To encourge the utiliztion of unsturted soil properties in geotechnicl designs, numerous methods hve een proposed nd developed within the pst few decdes [1,2]. The soil wter chrcteristic curve (SWCC) defines the constitutive reltionship etween the soil-wter potentil nd wter content of unsturted soils which cn e utilized to ridge the gp etween the sturted nd unsturted soil prmeters. The complex unsturted soil ehvior cn e investigted through this conceptul frmework nd vst mount of reserch hs een crried out in this regrd [3,4]. Key elements of these reserch were to comprehend the unsturted soil ehvior ccording to the chnge in soil wter content (i.e. grvimetric, volumetric wter content or degree of sturtion). Volumetric wter content, θ w θ s Air entry vlue (AVE), ψ ψ w (ψ r, θ r ) Soil suction [kp] Desorption (drying) curve Asorption (wetting) curve Fig. 1 Generl SWCC for soil [5] Furthermore, soil- wter chrcteristic curve is centrl to the ehvior of n unsturted soil nd cn e relted to other properties descriing the ehvior of soil, such s unsturted coefficient of permeility nd the sher strength [6-8]. Therefore, in geotechnicl engineering prctice, unsturted soil mechnics theories in routine 9

2 Interntionl Journl of GEOMATE, Nov., 217, Vol.13, Issue 39, pp prctice nd numericl models, sed on the SWCC nd sturted soil properties, hve een developed to predict unsturted permeility function nd unsturted sher strength properties. Currently, there re well-estlished direct nd indirect methods to mesure SWCC for prticulr soil. Direct methods include pressure plte, Buchner funnel, tensiometers, nd pressure memrnes. These methods mesure the porewter pressure in the soil or impose known ir pressure to soil nd llow the wter content to come to equilirium with the imposed ir pressure. Among these methods, conventionl pressure plte is the most common method. Indirect methods include filter pper nd het dissiption sensors. These methods use mesurements or indictors of wter content or physicl property tht is sensitive to chnges in wter content. The direct methods re recognized for higher ccurcy of the outcome, however, ltnt prolems of the direct method re the costly nd time-consuming nture of the pproch which encourges users to follow simple indirect methods to determine soil specific wter retention curve (Fig. 1). The suction vlues re mostly deemed s mtric suction (U -U w ) nd seldom use of totl suction cn e seen in literture. Mostly, the soil moisture content is represented s volumetric wter content (θ), yet grvimetric wter content (ω) is lso eing used in unsturted soil mechnics to determine SWCC. The sorption nd desorption curves (Fig. 1) refer to the wetting nd drying process, respectively, in which the difference in wter content t sturtion etween drying nd wetting is the residul ir content. In Fig.1 the SWCC during wetting process is not the sme s drying process, which is referred to s hysteresis, i.e., the soil s ility under the similr suction to hve two different wter contents when the soil is eing wetted or dried. For specified suction vlue, the soil eing wetted hs less wter content thn the soil eing dried [9] & [1]. In this study, SWCC of snd hs een determined using oth direct nd indirect lortory methods in order to investigte the degree of greement etween SWCCs derived from ech method. A Tempe pressure cell ws used to conduct the indirect pproch y repeting three drying-wetting cycles wheres direct methods were pproched through instrumented soil column nd model tnk. 2. TEST MATERIAL Edoski snd from Irki prefecture in Jpn ws used s the test mteril throughout the investigtion. Wet sieving nd hydrometer nlysis were performed on selected representtive smples conforming to the JGS (Jpnese Geotechnicl Society) stndrd test methods. The fine mteril (percentge finer thn.75 mm) of Edoski, mounts to 16.4% nd the grin size distriution for the forementioned soil is shown in Fig. 2 elow. Aprt from grin size distriution, the other sic soil properties such s specific grvity, minimum & mximum void rtios, compction nd Attererg limits were conducted in ccordnce with JGS stndrd test methods nd the results re given in Tle 1. According to Unified Soil Clssifiction System (USCS), the test mteril ws clssified s silty snd. Tle 1 Property tle of Edoski soil [3] Fig. 2 Grin size distriution of Edoski snd [3] 3. TEST APPARATUS 3.1 Apprtus Used for Indirect Mesurement of SWCC A Tempe pressure cell (Fig. 3) ws used to otin wter retention curve for Edoski snd 1

3 Interntionl Journl of GEOMATE, Nov., 217, Vol.13, Issue 39, pp using indirect mesurements. This pprtus comprised of rss cylinder (Φ = 5 mm nd h= 6 mm), se plte, high ir-entry porous cermic disk (3 kp) nd top cp. A representtive soil specimen is plced on the cermic disk which is emedded on top of the se plte nd the wter flow through the specimen is controlled y tue connected to the se plte. A tue connected to the top plte enles to regulte the ir pressure t n pproprite level during the experiment. Fig. 4 Modified pressure trnsducer sensitivity to slinity nd temperture effects [7] & [8]. ADR proes utilized in this study hve very short response time (1-5 seconds) up to n ccurcy of ± 1% with soil specific clirtions. 4. METHODOLOGY 4.1 Indirect Mesurement of SWCC Fig. 3 Schemtic digrm of Tempe pressure cell [5] 3.2 Apprtus Used for Direct Mesurement of SWCC A soil column nd soil ox tests instrumented with pressure trnsducers nd wter content mesuring sensors (Thet proes) ws conducted on Edoski snd iming to mesure drying nd wetting soil-wter chrcteristic curves directly Pore-wter pressure trnsducers Fig. 4 depicts the strin guge type pressure trnsducers with the cpcity of kp. Moreover, trnsducers were modified y ttching cermic cups in order to mesure oth positive nd negtive pore-wter pressures (-9 kp to + kp). The cermic cup consists of n AEV of kp nd sturted wter permeility of cm/sec Moisture content sensors ADR (Amplitude Domin Reflectometry) proes were used to mesure the volumetric wter content of the Edoski snd nd the previous reserch hs estlished tht ADR proes hve lower Prior to commencing the study, the high AEV cermic disk ws sturted nd susequently it ws verified y the procedure illustrted y Gllge et l. (21). In order to persist the sturtion of the system, wter tnk ws connected fter sturtion of the cermic disk. A soil specimen (dry density = 1.35 g/cm 3, grvimetric wter content = 1%) ws plced in the rss cylinder such tht trget density ws chieved nd then specimen ws sturted s depicted in Fig 5. The weight of the ssemly ws constntly monitored to identify the point of sturtion t which the djcently mesured weight ecomes constnt. From periodic oservtions, it ws noted tht the time tken for sturtion ws 2 to 3 dys. The Tempe pressure cell ws connected to system s depicted in Fig 3. It should e noted tht wter level of the wter tnk ws constntly mintined t hlf smple height nd vented to tmosphere nd therey conserve zero pressure (U = Uw = kp) in the specimen. Once the smple weight ws constnt, the mesured weight ws recorded for the corresponding zero suction vlue (U - Uw = ). Following this, ir pressure (U) ws incresed to nother vlue (i.e.,.5, 1., 2., 3., 5., 7., 1., 2., 5.,. nd 2. kp) through the ir supply inlet, resulting specimen wter to drin out (i.e. nlogous to drying process) to the wter tnk through se plte until the specimen moisture equilirtes (i.e. constnt weight of the ssemly). During the weighing process, inlet nd outlet tues were 11

4 Interntionl Journl of GEOMATE, Nov., 217, Vol.13, Issue 39, pp remined closed. Then the weight of the specimen ws recorded for the corresponding mtric suction vlue (U = Uw = 5 kp) nd the procedure ws repeted for ll the suction vlues. Fig. 6 Drying nd wetting SWCC for Edoski snd 4.2 Direct Mesurement of SWCC Fig. 5 Sturtion of the specimen [5] The wetting process ws crried out through reverse pproch y dropping the ir pressure t the inlet of the top plte from 2 kp to kp. After U dropped down to kp, the specimen ws tken out nd oven dried to mesure the corresponding grvimetric wter content of the smple. This wter content together with previous chnge in weight of the ssemly ws used to ck-clculte the wter contents corresponding to the other suction vlues. The suctions were then plotted ginst their corresponding wter contents to otin the SWCCs. The Fredlund- Xing eqution (Eq. (1)) ws used to determine the estfit curves (Fig. 6) for the otined experimentl dt during drying nd wetting cycles [11]. θ ( ψ,, n, m) = C( ψ ) s n [ e + ( ψ ) ] { ln } m Where, ln(1 + ψ / ψ r ) C( ψ ) = 1 ln[1 + ( / ψ ) θ = Volumetric wter content ψ = Suction (kp) ψ r = Sunction corresponding toresidul wter content θr( kp) θs = Sturted wter content, m, n = Fitting prmeters ( hs the unit of pressure kp) θ r (1) Column test with direct mesurements of wter content nd suction The column ws sujected to three cycles of wetting nd drying during which soil wter contents nd pore-wter pressures were continuously recorded t four different depths in the column. This section descries the experimentl setup including sensors used to mesure pore-wter pressure nd wter content nd the preprtion of the soil column. () Pore-wter pressure trnsducers Initilly, cermic cups were sturted y immersed in the wter nd followed y pplying vcuum condition for 24 hours period prior to emed in soil. Fig 4 shows the cermic cup nd pressure trnsducer ssemly. The pressure trnsducers were re-clirted for the lortory working pressure rnge of -6 kp to +6 kp to compre the clirtion fctors with the mnufcturer defined curves. Fig 7 depicts the mnufcturer defined clirtion chrt for the pressure trnsducers. 12

5 Interntionl Journl of GEOMATE, Nov., 217, Vol.13, Issue 39, pp Sme procedure ws crried out for different for different wter contents to otin clirtion chrt specific to Edoski snd (Fig. 8). (c) Column test nd its mesurements Fig. 7 Clirtion chrt for the pressure trnsducers () Moisture content sensors In order to soil specificlly clirte ADR proes, oven-dried soil ws mixed with distilled wter nd pcked into plstic cylinder of 8 mm in inner dimeter nd mm in height s uniformly s possile y mnul compction into five equl lyers up to the full volume of the cylinder. The smple ws then weighted y using n electronic lnce to otin the wet weight of the smple. ADR proe ws verticlly inserted to the soil nd output ws connected to the dt logger in order to red output voltge. The output of the ADR proe ws oserved in the computer screen nd the vlue ws noted once it ws stle. The soil smple ws oven-dried to otin the grvimetric wter content, m nd ulk dry density of soil, ρ d. The corresponding volumetric wter content, θ, cn e otined s follows, ssuming the density of wter to e 1 g/cm 3. As shown in Fig. 9, cylindricl column which hs n inner dimeter of 2 mm nd height of 6 mm ws filled y compcting wet Edoski snd (initil wter content = 14 %) to chieve dry density of 1.35 g/cm3. During the snd column preprtion, four ADR proes (M) nd four modified pressure trnsducers (P) were instlled to mesure volumetric wter content nd suction of soil, respectively. P4 nd M4, P3 nd M3, P2 nd M2, nd P1 nd M1 were instlled in the soil t the depth of 6, 18, 3, nd 42 mm from the top soil surfce, respectively. A grvel lyer ws plced t the ottom of the column to fcilitte drining of wter. The soil column ws wetted y pouring wter in to the top of the column nd the drying ws then llowed nturlly in the room environment. During the period of out 66 dys, the column ws sujected three cycles of wettingdrying to investigte the sensor responses s shown in Fig. 1 & M4 M3 M2 P4 P3 P2 (2) M1 P Grvel lyer 2 Fig. 6 Schemtic digrm of the soil column nd the sensor rrngements Model tnk test with direct mesurements of wter content nd suction Fig. 8 Clirtion chrt for ADR moisture sensors The tnk used in the model tests is shown in Fig. 12. This tnk hs length of 22 cm, width of 8 cm, nd height of cm. The wlls of the tnk re mde of steel pltes except for the front side which is mde of cryl g lss for esy oservtion of the deformtion process. 13

6 Interntionl Journl of GEOMATE, Nov., 217, Vol.13, Issue 39, pp Pore pressure sensor [P] with wter content sensor [M] 2 4 Pore pressure sensor only (All dimensions in mm) P1M1 P2M2 P3M3-1 P4M4 P5 P6M5 P7M6 P Time [Hours] 15 P9M7 P1M Fig. 1 Responses of pore pressure trnsducers emedded on soil column Fig. 12 Schemtic digrm of model tnk temperture. Susequently, wter content (nturl) ws mesured nd excess mount of wter ws dded to chieve the pre-determined initil wter content (i.e. 16%). Once the soil ecme moisture M1 M2 M3 M4 uniformed, the model emnkment ws prepred y wet compction. Pre-determined soil density ws mintined throughout the process y keeping 5 mm soil lyers for compction. The sme procedure ws repeted until the full height of the soil model ws otined. Further, pressure trnsducers nd moisture sensors were emedded t specific loctions s the soil plcement progressed. Susequent to the successful setting-up of the model tnk, the soil ws sujected to wetting for 19 hours y pplying n rtificil rin of 4 mm/hr which ws followed y 24 hours nturl drying period. Afterwrds, rinfll of 8 mm/hour ws pplied for 2.2 hours durtion s the second wetting cycle nd then gin llowed to dry it for Time [Hours] Fig. 11 Responses of the wter content sensors emedded on soil column The ox is divided in to three sections, i.e., the centrl portion which is 197 cm long nd used for construction the slope nd the left nd right chmers ech 11.5 cm wide for collection nd dischrging wter, respectively. These three sections re divided y perforted wlls with metl meshes ttched to them to llow esy movement of wter without wshing out the soil grins. For these model tests, the inner wll of the left chmer is mde impermele y ttching thin cryl sheet Preprtion sensor stiliztion[21.6 hrs] 2-Rinfll-1(4 mm/hr) [19.1 hrs] 3-Drying-1 [27.8 hrs] 4-Rinfll-2 (8 mm/hr) [2.2 hrs] 5-Drying -2 [43.9 hrs].1.5. Edoski snd ws oven dried for 48 hours under constnt temperture of 1C nd the soil lumps were crushed mnully once the temperture of dried mteril ws reduced to room 4 2 Volumetric wter content [m3/m3] Volumetric wter content,θw Perforted wll 5 2 P1 P2 P3 P4-2 8 Pore-wter pressure [kp] Time [hrs] M1 M2 M3 M6 M7 12 Fig. 13 ADR sensor responses with time 48 hours. Figure 13 & 14 depict, the temporl distriution of the pore wter pressure trnsducer nd wter content sensor responses, respectively. 14

7 Interntionl Journl of GEOMATE, Nov., 217, Vol.13, Issue 39, pp Pore-wter pressure [kp] Fig.14 Pressure trnsducer responses with time Time [hrs] 5 P1 P2 P3 P7 P9 present similrity prt from slight difference tht my rise due to incongruity of pproch. Volumetric wter content, θ w.5 P4-M4 (mesured - column test) Drying (mesured - Tempe cell) Best-fit(drying)-FX Wetting (mesured - Tempe cell) Best-fit (wetting)-fx h j g c f i d --c: Wetting-1 c-d-e: Drying-1 e-f-g-h: Wetting-2 h-i-d-e: Drying-2 e-j-h: Wetting-3 h-i-d-e: Drying-3 Suction, u -u w [kp] e 5. RESULTS AND DISCUSSIONS Using the Tempe pressure cell nd the ssocited test procedure explined in this pper, oth drying nd wetting SWCCs for the test mteril (Edoski soils) were mesured in the lortory. Fig. 6 depicts the mesured SWCCs for the Edoski soil specimen t the initil dry density of 1.35 g/cm 3, in which the suctions were then plotted ginst their corresponding wter contents to otin the SWCCs nd the Fredlund- Xing eqution ws used to determine the est-fit curves (Fig. 6) for the otined experimentl dt during drying nd wetting cycles. Column test nd model tnk tests were employed s the direct mesurements of wter content nd suction. To otin SWCCs from the results of the column test, the mesured volumetric wter content ws plotted ginst mesured suction t ech level. As ltntly depicted in Fig. 15, wter retention curves otined from soil column test susequent to series of wetting nd dying cycles present close greement with indirectly mesured (tempe pressure cell) SWCC, irrespective of slight over estimtion of drying curve in the indirect method. The SWCCs mesured in the lortory using Tempe pressure cell nd emnkment model were then plotted on the sme grph s shown in Fig. 16. It cn e seen from these grphs tht gret portion of directly mesured SWCCs (min drying, min wetting, nd scnning curves) lie within the indirectly mesured drying nd wetting SWCCs. Further, the hysteresis etween the corresponding drying nd wetting curves of the direct (model emnkment) nd indirect (tempe-cell) methods Volumetric wter content, θ w Volumetric wter content, θ w.5 P3-M3 (mesured - column test) Drying (mesured - Tempe cell) Best-fit(drying)-FX Wetting (mesured - Tempe cell) Best-fit (wetting)-fx i f e j g h --c: Wetting-1 c-d: Drying-1 d-e-f: Wetting-2 f-g-d: Drying-2 d-h-i: Wetting-3 i-j-d: Drying c d Suction, u -u w [kp] Fig. 15 SWCCs otined from Tempe-cell nd soil column test.5 g P3-M3 (mesured - model test) Drying (mesured - Tempe cell) Best-fit(drying)-FX Wetting (mesured - Tempe cell) Best-fit (wetting)-fx c f e --c: Wetting-1 c-d-e: Drying-1 e-f-c-g: Wetting-2 g-d-e:drying-2 Edoski snd ρ d =1.35 g/cm Suction, u -u w [kp] d 15

8 Interntionl Journl of GEOMATE, Nov., 217, Vol.13, Issue 39, pp Volumetric wter content, θ w.5 P6-M5 (mesured - model test) Drying (mesured - Tempe cell) Best-fit(drying)-FX Wetting (mesured - Tempe cell) Best-fit (wetting)-fx g d --c-d: Wetting-1 d-e-f: Drying-1 f-c-g: Wetting-2 g-e-f:drying-2 Edoski snd ρ d =1.35 g/cm Suction, u -u w [kp] Fig. 16 SWCCs otined from Tempe-cell nd model emnkment test 6. CONCLUSION c In this study, soil wter chrcteristic curves (SWCC) for Edoski snd ws otined using oth direct nd indirect methods. As for direct methods, instrumented soil column nd model tnk were sujected to wetting nd drying cycles under controlled lortory conditions nd sensor responses were plotted t ech sensor loction to determine SWCC for snd. Keeping ll the soil prmeters nd conditions constnt, SWCC of Edoski snd ws experimentlly otined y Tempe pressure cell s simple indirect method. It is ltntly found tht results of the indirect method provide very close similrity to the outcomes of the costly, complex nd time consuming direct methods (i.e. instrumented soil column nd model tnk). However, the slight vrition of the SWCC in forementioned methods my due to the experimentl flws nd differentil sensor outputs cused y insignificntly minor environmentl impcts, yet cn e disregrded when compred to the cons of direct methods. Tken together, these findings imply the prcticlity of the indirect methods to determine soil specific SWCC when compred to rigorous direct methods. 7. ACKNOWLEDGEMENTS Authors would like to thnk Interntionl Journl of GEOMATE for pulishing this pper. 8. REFERENCES e f [1] Fredlund, D. G., Xing, A., Fredlund, M. D., & Brour, S. L. (1996). The reltionship of the unsturted soil sher to the soil-wter chrcteristic curve. Cndin Geotechnicl Journl, 33(3), [2] Zpt, C. E., Houston, W. N., Houston, S. L., & Wlsh, K. D. (2). Soil wter chrcteristic curve vriility. In Advnces in unsturted geotechnics (pp ). [3] Brour, S. L. (1998). Nineteenth Cndin Geotechnicl Colloquium: The soil-wter chrcteristic curve: historicl perspective. Cndin Geotechnicl Journl, 35(5), [4] Nm, S., Gutierrez, M., Dipls, P., Petrie, J., Wyllce, A., Lu, N., & Muñoz, J. J. (21). Comprison of testing techniques nd models for estlishing the SWCC of rivernk soils. Engineering Geology, 11(1), 1-1. [5] Gllge, C., Kodikr, J., & Uchimur, T. (213). Lortory mesurement of hydrulic conductivity functions of two unsturted sndy soils during drying nd wetting processes. Soils nd Foundtions, 53(3), [6] Gllge, C. P. K., & Uchimur, T. (21). Effects of dry density nd grin size distriution on soil-wter chrcteristic curves of sndy soils. Soils nd Foundtions, 5(1), [7] Yng, H., Rhrdjo, H., Leong, E. C., & Fredlund, D. G. (24). Fctors ffecting drying nd wetting soil-wter chrcteristic curves of sndy soils. Cndin Geotechnicl Journl, 41(5), [8] Chin, K. B., Leong, E. C., & Rhrdjo, H. (21). A simplified method to estimte the soil-wter chrcteristic curve. Cndin Geotechnicl Journl, 47(12), [9] Herkelrth, W. N., & Delin, G. N. (21). Long-term monitoring of soil-moisture in hrsh climte using reflectometer nd TDR proes. US Geol. Surv., Menlo Prk, CA. [1] Kizito, F., Cmpell, C., Cmpell, G., Coos, D., Tere, B., Crter, B., & Hopmns, J. (28). Frequency, electricl conductivity nd temperture nlysis of low-cost cpcitnce soil moisture sensor. Journl of Hydrology, 352(3), [11] Fredlund, D. G., & Xing, A. (1994). Equtions for the soil-wter chrcteristic curve. Cndin geotechnicl journl, 31(4), Copyright Int. J. of GEOMATE. All rights reserved, including the mking of copies unless permission is otined from the copyright proprietors. 16