Field and Laboratory Suction- Soil Moisture Relationship of Unsaturated Residual Soils

Transcription

1 Americn Journl of Environmentl Sciences 1 (1): 34-4, 25 ISSN X Science Publictions, 25 Field nd Lbortory Suction- Soil Moisture Reltionship of Unsturted Residul Soils 1 Bujng B.K. Hut, 2 Fisl Hj. Ali nd 2 A. Abdullh 1 Deprtment of Civil Engineering, University Putr Mlysi 434 UPM Serdng, Selngor, Mlysi 2 Deprtment of Civil Engineering, University of Mly, 563 Kul Lumpur, Mlysi Abstrct: Soils locted bove the groundwter tble such s residul soils re generlly unsturted nd possess negtive pore-wter pressures. A soil-wter (moisture) chrcteristic curve (SWCC) tht reltes the wter content of soil to mtric suction is n importnt reltionship for the unsturted soil mechnics. The SWCC essentilly shows the bility of n unsturted soil to retin wter under vrious mtric suctions. It hs similr role s the consolidtion curve of sturted soil tht reltes void rtio or wter content to effective stress. This study describes study tht hs been crried in the field nd in the lbortory to exmine the suction soil moisture reltionship of unsturted residul soils of grnite nd sedimentry rocks origin. The field mesurement shows decresing trend of suction with depth for both soils. The suction soil moisture reltionship shows two distinct curves, wetting (sorption) curve nd drying (desorption) curve. While from the lbortory study, it is observed tht there is significnt decrese in the soil moisture with incresing suction in the lower suction rnges, until de-sturtion or ir entry point for both soils. Beyond this point, the mgnitude of the decrese in soil moisture for the equl increment of pplied suction is less. The de-sturtion point of prticulr soil ppers to be dependent on the mount of cly content. Higher mount of fines in the soil constitute more compct prticle rrngement nd smller pore size. Soils with smller pore sizes de-sturte t higher mtric suction. Key words: Soil Moisture Reltionship, Mtric Suction, Unsturted Soil Mechnics, SWCC INTRODUCTION The microclimtic conditions in n re re the min fctors cusing soil deposit to be unsturted. Therefore, unsturted soils or soils with negtive porewter pressures cn occur in essentilly ny geologicl deposit. An unsturted soil could be residul soil, lcustrine deposit, nd soils in rid nd semi rid res with deep ground wter tble. Tropicl residul soils hve some unique chrcteristics relted to their composition nd the environment under which they develop. Most distinctive is the microstructure, which chnges in grdtionl mnner with depth. Their strength nd permebility re likely to be greter thn those of temperte zone soils with comprble liquid limits. Most clssicl concepts relted to soil properties nd soil behvior hve been developed for temperte zone soils, nd there hs been difficulty in ccurtely modeling procedures nd conditions to which residul soils will be subjected. Engineers pper to be slowly recognizing tht residul soils re generlly soils with negtive in situ pore-wter pressures, nd tht much of the unusul behvior exhibited during lbortory testing is relted to mtric suction chnge in the soil [1, 2]. There is the need for relible engineering design ssocited with residul soils [3]. When the degree of sturtion of soil is greter thn bout 85%, sturted soil mechnics principles cn be pplied. However, when the degree of sturtion is less thn 85%, it becomes necessry to pply unsturted soil mechnics principles [4]. The trnsfer of theory from sturted soil mechnics to unsturted soil mechnics nd vice vers is possible through the use of stress stte vribles. Stress stte vribles define the stress condition in soil nd llow the trnsfer of theory between sturted nd unsturted soil mechnics. The stress stte vribles for unsturted soils re net norml stress (σ u ) nd mtric suction (u u w ), where σ is the totl stress, u is the pore-ir pressure nd u w is the pore-wter pressure. The stress stte in n unsturted soil cn be represented by two independent stress tensors s suggested by Fredlund nd Morgenstern [5] s follows: ( σ u ) x yx zx ( σ u ) y xy zy xz ( ) yz σ z u (1) 34

2 ( u u ) w ( u u ) w ( ) u uw (2) Where, σ x, σ y, σ z in eqution 1 re the totl norml stresses in the x-, y-, nd z-directions, respectively; nd xy, yx, xz, zx, zy, yz re the sher stresses. A soil-wter (moisture) chrcteristic curve (SWCC), which reltes the wter content of soil to mtric suction, is n importnt reltionship for the unsturted soil mechnics [3]. The SWCC essentilly shows the bility of n unsturted soil to retin wter under vrious mtric suctions. It hs similr role s the consolidtion curve of sturted soil tht reltes void rtio or wter content to effective stress. The SWCC of soil dicttes the mnner by which the permebility, sher strength nd volume chnge of the soil will behve t different mtric suctions upon drying nd wetting [2]. This study describes study tht hs been crried in the field nd in the lbortory to exmine the suction soil moisture reltionship of unsturted residul soils of grnite nd sedimentry rock origin. FIELD TEST APPARATUS LABORATORY TEST APPARATUS In this study, the suction - soil moisture reltionship, which is lso known s the soil-wter (moisture) chrcteristic curve (SWCC), is studied in the lbortory by using the modified Rowe cell nd modified (double wll) trixil cell. Smples used in the tests were tken t depth t.6m by men of block smpling in the field of unsturted residul soil of grnite nd sndstone (sedimentry) rocks origin. Modified Rowe Cell: The conventionl Rowe Cell ws modified nd used together with the GDS pressure controllers, using the principles of the pressure plte or xis trnsltion technique [6] for the ppliction of suction. The modifiction involved removl of the rubber membrne from the cell top, detchment of the side dringe porous lyer, blocking of dringe outlet nd the fbriction of completely new bse to include the seting for high ir-entry cermic disc nd spirl grooved comprtment for flushing the diffused ir from below the disc. Figure 2 shows the detils of the modified Rowe cell. For mesurement of the field suction, quick drw tensiometer probe ws used. This probe ws design to mesure soil suction in the field. It is bsiclly modified jet filled tensiometer, portble, nd for rugged field use. Figure 1 shows detil of the quick drw tensiometer. Note tht the tensiometer cn only mesure suction up to 1 br (1 kp). Fig.1: Detils of Quick Drw Tensiometer Probe (Soil Moisture Equipment Corp., USA) Fig. 2: Modified Rowe Cell The schemtic of the test rrngement is shown in Fig. 3. Air pressure ws pplied through the top of the cell vi vlve E. The pore wter ws pplied using the GDS pressure controller with vlve 1A open, nd with vlve 1B nd vlve 2 closed, to the desired vlue of suction. The GDS pressure controller records the volume of wter flowing in or out of the smple. The flushing of the diffused ir from below the cermic disc ws done using ir-wter bldder, with vlve 1A closed, vlve 1B nd vlve 2 open. 35

3 Fig. 3: Schemtic Arrngement of Test Setup Using Modified Rowe Cell The flow of wter contining diffused ir into the diffused ir volume indictor (DAVI) ws mde possible by creting pressure grdient between the bldder nd the DAVI. The soil smple, 1mm dimeter, 25 mm thick ws obtined from block smple using split body smpler (Fig. 4) by trimming the height. specilly fbricted double wll cell. One of the two GDS 2 MP pressure controllers ws used for pplying nd mesuring the inner cell pressure, while the other ws used to pply nd mesure the pressure in the nnulr volume between the outer nd inner cell. The design ws bsed on the principle of equl pressure being pplied to both sides of the inner nd outer wll, thus producing no volume chnge to the cell. Fig. 4: Split Body Smpler Modified (Double Wll) Trixil Cell: A specilly modified trixil cell ws lso used to study the suction soil moisture reltionship of unsturted residul soil in the lbortory, in prticulr the effect of confining pressure. The typicl setup for this test is s shown in Fig. 5. The conventionl Bishop-Wesley cell ws modified to Fig. 5: Schemtic Lyout of Specilly Modified Trixil Test Figure 6 shows detil of the double wll cell. The cell comprises of two Perspex cells sndwiched between top nd bottom metl pltes nd seled using pproprite O -rings. The sel prevent ny movement of liquid from the inner to the outer cell or vice vers. The top nd the bottom pltes were held in plce by four rods screwed onto the bottom plte t one end nd set of nut tightened t the top plte. The whole double 36

4 At both sites field test were done t done of t depth of 3 cm, 45 cm nd 6 cm within soil of wethering grde VI, following the generl clssifiction of rock wethering of Little [8]. For the lbortory tests, block smples of similr soils were tken t depth of bout.6 m. Some physicl nd index properties of the soil smples from both sites re shown in Tble 1. RESULTS AND DISCUSSION Fig. 6: Detils of the Double Wll Cell wll cell ssembly is then fixed to the bse of the Bishop Wesley cell by nother set of four rods. FIELD SITES AND SOIL SAMPLES Two field sites, nmely site A nd site B were chosen for this study. Site A ws cut slope t km 31 long the Kul Lumpur Krk highwy, ner Kul Lumpur, Mlysi. The cut slope bsiclly composed of residul soil tht hd developed over the more commonly outcropping Permo-Trissic Mesozonl grnite rock of Peninsulr Mlysi [7]. Site B ws locted within the university (University of Mly, Kul Lumpur) cmpus. The soil ws tht of wethered sedimentry rock of the middle nd upper Trissic sequences of sndstone, shle nd volcnic rocks. Tble 1: Physicl nd Index Properties of the Soil Smples Site A B Wethering VI / Grnite VI / Sndstone Grde/Prent Rock Description Yellowish brown silty cly Reddish brown sndy cly Nturl moisture % - Content Coefficient of x 1-8 m/s - permebility, k Liquid Limit 95% 66% Plstic Limit 45% 32% Specific Grvity Prticle Size Distribution: Grvel Snd Silt Cly 1.7% 11.3% 47.% 4.%.5% 36.8% 27.7% 35.% Cly Minerl Kolinite Kolinite nd Illite Field Test Results: In the field, the reltionship of suction nd soil moisture of unsturted residul soil is studied using the quick drw tensiometer. Dt ws collected t depths of 3 cm, 45 cm nd 6 cm for both the sites A nd B. The vlues of suction recorded for the site A rnges from 5 kp to 72 kp, while for the site B, suction vlues recorded rnges from 5 kp to 81 kp, the lrger vlue of suction recorded t the shllower depth (3 cm). A similr observtion of suction vlue decresing depth ws mde by Sweeney [9] for Hong Kong soils. Figure 7 shows the response of suction build up with time for both site A nd B. As shown there is generl trend for both the sites, whereby the time required for the higher suction vlues to stbilize is longer compred to time tken for lower suction vlues. Dt ws collected to study the time tken for the suction mesured to come to stte of equilibrium. This study would help in judging the pproprite time to stop the mesurement, s time tken to rech 1% equilibrium cn be quite long. () Site A (b) Site B Fig. 7: Suction Build up with Time 37

5 The fster response for lower suction could be ttributed to the cpillry conductivity of the soil. The suction cpillry vlue of soil decreses with incresing suction. A low cpillry conductivity induces lower trnsfer rte of wter from the probe to the soil, thus incresing the time required for stbiliztion of the suction reding. The grph cn ssist in estimting time required for mesurement in wet nd dry condition. Figure 8 shows the field suction moisture curves for both site A nd B. The dt obtined for both sites showed wide sctter with mrked hysteresis between the wetting nd the drying curves. () Site A showing rpid decrese in suction for higher suction nd leveling off t suction below 2 kp. The drying curve exhibits plteu t high suction vlues nd steeper curve below 6 kp suction vlue nd possibly levels off below suction vlue of pproximtely 5 kp. The drying (desorption) curve nd the wetting (sorption) curve pper to converge t both ends (high nd low) suction vlues. It is lso of interest to note tht the suction - moisture curve for site A shows number of intermedite curves before joining either the drying or wetting curve. These curves cn be identified s the initil intermedite curves or scnning curves. The curve following the drying curve initilly cn be devited from the upper bound curve due to the effect of moisture content chnges, which my explin the considerble sctter in the dt. When the moisture content increses the drying curve will no longer be followed but new wetting curve pth would be followed. Lbortory Test Result: Figure 9 shows plot of suction versus soil moisture obtined using the modified Rowe cell. Fig. 9: Lbortory Suction Soil Moisture using Modified Rowe Cell (b) Site B (c) Combined Plot for Site A nd B Fig. 8: Field Suction Moisture Curve The upper bound represents the drying curve while the lower bound represents the wetting curve. The wetting curves for both the sites re chrcteristiclly concve, A significnt decrese in the soil moisture is observed with incresing suction in the lower suction rnges, until de-sturtion or ir entry point. After this point, the mgnitude of the decrese in soil moisture for the equl increment of pplied suction is less. At this stge, the drining of wter out of the soil pores becomes more nd more difficult. This phenomenon is minly due to the incresing surfce tension force t the contrctile lyer s the suction increses. The de-sturtion point for the site A smple is pproximtely t 25 kp, wheres for the site B smple the de-sturtion point ws slightly lower, t bout 2 kp. The lower vlue of site B smple is probbly minly due to soil grding. Site A smple hs more cly content thn the site B smple (Tble 1). Higher mount of fines in the smple constitute more compct prticle rrngement nd smller pore size. Soils with smller pore sizes will de-sturte t higher mtric suction [1]. 38

6 In order to study the effect of confining pressure on the suction - soil moisture reltionship of unsturted residul soil, double wll trixil cell hs been specificlly prefbricted for this purpose. Only soil smples of site A were used in this t study. A confining pressure of 4 kp ws pplied to the soil smple. The mtric suction ws first rised to 2 kp nd then reduced to 5 kp, i.e. the wetting prt of the chrcteristics curve. The drying curve ws obtined by incresing bck the suction until 25 kp. The plot of suction nd moisture obtined is shown in Fig. 1. In generl the shpe of the curve obtined is similr to tht of the field study (Fig. 8). prticulr moisture content re of orders of mgnitude different. In ny cse the mximum vlue of suction mesured in the field is not greter thn bout 1 kp. Both the field nd lbortory curves however ppers to converge t low suction vlues. () Site A Fig. 1: Lbortory Moisture Curve (Drying nd Wetting) under Confining Pressure of 4kP (Site A Smple) The effect of confining pressure on the soil moisture chrcteristics curve is shown in Fig. 11. It ppers tht the confining pressure shifts the curve considerbly below the unconfined curve t lower suction level. Both curves however pper to converge t higher suction level, i.e. t pproximtely the ir entry (de-sturtion) point for the soil. Fig.11: Effect of Confining Pressure on Suction Moisture Curve (Site A Smple) Comprison of Field nd Lbortory Dt: It is lwys desirble to obtin comprison between the field nd lbortory mesurements. Figure 12 shows the combined plot of the field (tensiometer), lbortory (modified Rowe cell) suction nd soil moisture. While the generl shpe of the field (wetting) curve is similr to tht of the lbortory, but the vlues of suction t ny (b) Site B Fig.12: Combined Field nd Lbortory Suction Soil Moisture It is of interest to note tht such comprisons between this field nd lbortory mesurements inevitbly involve different degree of disturbnces tht cn hve significnt influence on the soil behvior. In the field, the soil hs lwys been under the lod from the soil bove nd confined by its surrounding soil. It hs been shown erlier tht the confining pressure hs n effect in pushing down the suction soil moisture curve. For exmple s shown in Fig. 11, t moisture content of 26%, the corresponding vlue of suction t confining pressure of 4 kp is hlf tht of suction t zero confining pressure. However the desturtion or ir entry points of the confined nd unconfined curve re bout the sme. CONCLUSION The field mesurement shows decresing trend of suction with depth. The suction soil moisture reltionship shows two distinct curves, wetting (sorption) curve nd drying (desorption) curve. The wetting curve is chrcteristiclly concve, showing 39

7 rpid decrese in suction for higher suction nd leveling off t low suction. While the drying curve exhibits plteu t high suction vlues nd levels off t low suction. The two curves pprently converge t both ends (high nd low) suction vlues. From the lbortory study, it is observed tht there is significnt decrese in the soil moisture with incresing suction in the lower suction rnges, until de-sturtion or ir entry point. Beyond this point, the mgnitude of the decrese in soil moisture for the equl increment of pplied suction is less. At this stge the drining of wter out of the soil pores becomes more nd more difficult, which is believed to be due to the incresing surfce tension force t the contrctile lyer s the suction increses. The de-sturtion point of prticulr soil ppers to be dependent on the mount of cly content. Higher mount of fines in the soil constitute more compct prticle rrngement nd smller pore size. Soils with smller pore sizes de-sturte t higher mtric suction. On the effect of confining pressure, it ppers tht the confining pressure shifts the suction-soil moisture curve considerbly below the unconfined curve t lower suction level. Both curves however pper to converge t high suction level, i.e. t pproximtely the ir entry (de-sturtion) point for the soil. When compring the field nd lbortory suction-soil moisture reltionship, it ppers tht the generl shpe of both wetting curves re similr, but not the vlue of suction t ny prticulr moisture content. REFERENCES 1. Fredlund, D.G. nd H. Rhrdjo, Theoreticl context for understnding unsturted residul soils behviour. Proceedings 1 st Intl. Conf. Geomechnics in Tropicl Lterite nd Sprolitic Soils, So Polo Brzil, pp: Fredlund, D.G. nd H. Rhrdjo, Soil Mechnics for Unsturted Soils. New York: John Wiley nd Sons Inc. 3. Ali, F.H. nd H. Rhrdjo, 24. Unsturted Residul Soil. Chpter 4 in Tropicl Residul Soils Engineering. Hut et l., (Eds.). Leiden. Blkem, pp: Fredlund, D.G. nd H. Rhrdjo, Soil mechnics principles for highwy engineering in rid regions. In soil mechnics considertions: Arid nd semirid res. Trnsporttion Reserch Record, 1137: Fredlund, D.G. nd N.R. Morgenstern, Stress stte vribles for unsturted soils. J. Geotechnicl Engineering Division, ASCE, 13: Hilf, J.W., An investigtion of pore pressure in cohesive soils. US Bureu of Reclmtion. Technicl Memorndum. No Rj, J.K., Chrcteriztion of the wethering profile developed over porphyritic biotite grnite in Peninsulr Mlysi. Bull. Intl. Assoc. Eng. Geol., Pris, 32: Little, A.L., The engineering clssifiction of residul tropicl soils. Proceedings 7 th Intl. Conf. Soil Mechnics Foundtion Engineering, Mexico, 1: Sweeney, D.J., Some in situ soil suction mesurements in Hong Kong s residul soil slopes. Proceedings of 7 th S.E.A.G.S. Conf., Hong Kong, 1: Croney, D. nd J.D. Colemn, Soil structure in reltion soil (pf). Soil Sci., 5: