A Critical Review of the Methodologies Employed for Suction Measurement for Developing the SWCC

Transcription

1 The 12 th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 08 Goa, India A Critical Review of the Methodologies Employed for Suction Measurement for Developing the SWCC S. Sreedeep Dept. of Civil Engineering, Indian Institute of Technology Guwahati, Assam, India D. N. Singh Dept. of Civil Engineering, Indian Institute of Technology Bombay, Mumbai, India Keywords: suction, soil-water characteristic curve, tensiometer, pressure membrane extractor, dew point potentiameter ABSTRACT: Several methodologies have been developed and used by researchers for measuring either total suction ψ (sum of matric suction ψ m and osmotic suction ψ o ), or only ψ m. While employing different methodologies for establishing the soil-water characteristic curve, SWCC, there is a possibility that the factors such as the type of the soil suction measured, measurement range, equilibration time, presence of salts or contaminants in the soil etc., may influence it. Thus there is a need to understand the uniqueness of SWCC determined by using some of the commonly adopted methodologies such as insertion tensiometer, a pressure membrane extractor for measuring ψ m, and a dew point potentiameter for measuring ψ. For this purpose, two locally available uncontaminated fine grained soils, of entirely different characteristics were considered in this study. In addition, to understand the influence of salt content on the SWCC, another chloride rich soil of marine origin was also considered. The study indicates that the SWCC established by using different methodologies may not be unique and hence due care should be taken while reporting the SWCC of a soil. 1 Introduction The role of soil suction in unsaturated soil mechanics is very well recognized and many models relating it to the basic soil characteristics have been developed in the past few decades (van Genuchten, 1980; Fredlund and Rahardjo, 1993; Knight and Mitchell, 1996; Sreedeep and Singh, 05). Suction measurement finds its wide application in transportation engineering projects, environmentally sensitive projects such as waste containment, nuclear storage installations etc. (Fredlund et al., 1994; Oberg, 1995; Rahardjo et al., 1995; Sudhakar and Revanasiddappa, 00). From these studies, it is quite explicit that properties such as hydraulic conductivity, shear strength, compressibility and swelling potential of unsaturated soil are function of suction, and hence its measurement is very important. In the recent past, different methodologies have been developed and reported by the researchers for measuring matric suction, ψ m, or total suction ψ, either directly or indirectly. For measuring ψ m, researchers have employed tensiometers, pressure-plate apparatus or a pressure membrane extractor, thermal conductivity sensor and centrifuges (Lee and Wray, 1995), while, total suction ψ has been measured by employing instruments such as transistor or thermocouple psychrometers, filter paper, and a dew point potentiometer (Leong et al., 03). The obtained suction values are further employed for developing the soil-water characteristic curve, SWCC, which is a relationship between the soil suction and its water content, and represents the unsaturated soil behavior. Since there are different methodologies available for establishing SWCC, there is a need to understand the uniqueness of unsaturated soil characterization employing these methods. The factors such as type of the soil suction, measurement range, equilibration time, presence of salts or contaminants in soil etc., may influence the SWCC obtained by using different methods. To investigate this presumption, some of the commonly adopted methodologies for determining SWCC such as insertion tensiometer (IT), a pressure membrane extractor (PME) and a dew point potentiameter (WP4) were carefully evaluated and compared. For this purpose, two locally available fine-grained soils, a silty soil (designated as SS), and a white clay (designated as WC) were chosen. In addition, to understand the influence of salt content on the SWCC, another chloride rich soil of marine origin, designated as clayey silt (CS), was also considered in this study. The study brings out the fact that the SWCC established by using different methodologies may not be unique. Further, it has 1988

2 been demonstrated that the SWCC obtained by employing WP4 is significantly influenced by the presence of salts or contaminants in the soil. 2 Experimental Investigations Three fine-grained soils were used in this study, which include locally available silty soil (denoted as SS), clayey silt (CS) and commercially available white clay (WC). CS is a chloride rich soil of marine origin whereas SS and WC are uncontaminated. These soils were characterized for their specific gravity G, particle size characteristics, liquid limit w L, plastic limit w P, plasticity index PI, and the specific surface area, SSA, by following the guidelines provided in the literature (ASTM ). The mineralogical composition of these soils was obtained using the X- Ray diffraction technique. The cation exchange capacity, CEC, of these soils was measured following the guidelines presented in IS 27. The obtained results are summarized in Table 1 along with the soil classification as per ASTM D Table 1. Properties of the soils used in the study Soil Property SS WC CS G % Sand size ( mm) % Silt size ( mm) % Clay size (<0.002 mm) w L (%) w P (%) PI (%) Soil Classification ML CH MH SSA (m 2 /g) CEC (meq/0 g) Minerals present Albite, Anorthite, Montmorillonite Kaolinite, Illite Quartz, Anorthite, Montmorillonite The background concentration of the soil was obtained on an extract of 2:1 water to soil ratio (by weight) and by analyzing it for chloride and sulphite contents with the help of Indion Easy test kit (Ion Exchange, India Ltd.). In addition, ph and electrical conductivity, EC, of the extract is measured. The details of these chemical analyses are presented in Table 2. Table 2. Properties of the pore-solution of different soils Soil ph EC Chloride Sulphite (ms/cm) ppm ppm SS < 5 < 5 WC < 5 < 5 CS It can be noted from the table that, the EC of soils SS and WC are negligibly small as compared to that of the soil CS. This clearly indicates that the soil CS is contaminated where as soils SS and WC are uncontaminated. The chloride concentration of the soil CS is high, which can be attributed to its marine origin. It can be noted that ph of these soils is comparable and is close to neutral range. 2.1 Measurement of soil suction Three different suction measuring methodologies were employed in this study: (a) Jib-P insertion tensiometer (IT), make TENSIO-TECHNIK, Geisenheim, Germany, to measure ψ m directly in the range of 0 to 0 kpa. The measurement is based on the direct equilibration of negative pore water pressure of the soil with that of the water pressure in the tensiometer reservoir, (b) the pressure membrane extractor (PME), make Eijkelkamp, Germany, to measure ψ m in the range of 0 to 10 kpa. The working principle of the PME is based on the well established axis-translation technique (Fredlund and Rahardjo, 1993), and (c) the dew point potentiameter (WP4), make Decagon devices inc. USA, measures total suction ψ in the range of 0 to kpa (Thakur et al., 06). WP4 employs chilled mirror technique (Leong et al., 03) and relative humidity principle for measuring wide range of ψ. It must be noted that these techniques are entirely different in nature and hence would yield differet suction results. 1989

3 2.2 Establishment of soil-water characteristics curve Two commonly used soil-water characteristic curve, SWCC, equations proposed by van Genuchten (1980) and Fredlund and Xing (1994), represented by Eqs. 1 and 2, respectively, were used in this study. ψ ln1 + m θ ( ψ) = θ s 1 ln 1 + h r 6 h r ln exp(1) + n [ vg ] m ( ) = + ( ) vg θ + ( a ) ψ θ r θs θr 1 vg ψ (2) where θ(ψ) is the volumetric water content at any suction, ψ orψ m, θ r is the residual water content, which is the water content below which there is negligible change in water content corresponding to an increase in soil suction; θ s is the volumetirc water content at saturation; a f, and a vg are fitting parameters primarily dependent on the air entry value, AEV; AEV is the suction at which air starts entering into the largest pores in the soil; n f and n vg are fitting parameters that are dependent on the rate of extraction of water from the soil beyond the AEV; m f and m vg are fitting parameters which depend on θ r and h r is the suction (in kpa) corresponding to θ r. ψ a f n f f (1) 3 Results and discussion The suction values obtained by using IT, PME and WP4 for soils SS and WC are plotted as a function of θ, as depicted in Figure 1. In the figure, ψ refers to either matric or total suction depending upon the measuring methodology. Sufficient care was taken to maintain identical compaction state of the soil for its suction measurements by resorting to these methodologies. For this purpose, soil mixed with desired water content was compacted in a perspex mold in which tensiometer test was performed (Sreedeep and Singh, 07). Further, the soil samples for WP4 and PME measurements were extruded from the same compacted soil (Shah et al., 06) IT PME WP4 SS WC θ (%) ψ (kpa) Figure 1. Comparison of the measured soil suction by employing different methodologies 1990

4 It can be clearly noted from the figure that suction measured by using the three methodologies does not match well with each other. The ψ m measurement obtained using IT deviates appreciably with those obtained using PME and WP4 and the deviation increases as θ decreases. This may be mainly due to insufficient time of equilibration towards the upper limit of tensiometer measurement (i.e., about 0 kpa), which needs to be investigated in detail. The WP4 results indicate mismatch with PME and IT towards its lower limit (i.e., close to 0 kpa). This is mainly due to the low sensitivity of WP4 in measuring lower suction values (<00 kpa), as reported by Shah et al., (06). The lower suction values obtained by employing PME match reasonably well with IT and the higher values closely match with the WP4 results. Hence, it can be stated that PME results prefectly bridge the results obtained from the IT and WP4 for both the soils. It should be noted that different methodologies adopted for suction measurement do not yield a unique value. However, it is worth appreciating that each of these methodologies complement well with each other for a better characterization of the unsaturated state of the soil. The study highlights that it is essential to clearly mention the methodology employed for measuring soil suction so that its results could be used for studying the unsaturated soil behavior, precisely. Table 3 lists the SWCC fit parameters of Eqs. 1 and 2 for the suction values obtained by the three methodologies. The equations are fitted by following the procedure reported in the literature (Sreedeep and Singh, 06a). It can be noted that in most of the cases, the SWCC parameters of the same soil obtained by three different methodologies do not match well. Such a difference is mainly due to the different range of suction values obtained by employing different methodologies to which SWCC equation is fitted to. This clearly indicates that the measured data points and the range of measurement of the instrument do influence the SWCC determined by using different equations. This observation is similar to those reported in the literature (Shah et al., 06). Table 3. Details of SWCC fit parameters for two soils SWCC Parameter SS WC IT PME WP4 IT PME WP4 Fredlund a f And n f Xing m f (1994) h r Van a vg Genuchten n vg (1980) m vg To understand the influence of contamination, the SWCC has been obtained for chloride rich soil CS by employing WP4 and PME. The soil was mixed with deionised water to achieve a dilution ratio DR of 2. The slurry was stirred intermittently and stored for 24 h. Later, it was centrifuged and the solution was decanted using Whatman s No. 42 ashless filter paper. The electrical conductivity EC of the filtrate was measured to check for its contamination due to the presence of dissolved salts. The process was repeated until the electrical conductivity EC of the filtrate reduced to a negligible value. Details of this study are listed in Table 4. The negligible value of EC corresponding to 7 th washing indicates that CS is free of the dissolved salts. Table 4. Details of CS washing with deionised water Washing Cumulative dilution ratio DR EC (ms/cm) The washed soil CS was further subjected to suction measurement by employing WP4 and PME and compared with those obtained earlier for chloride rich soil CS, as depicted in Figure 2. It can be noted that suction values obtained by using WP4 differs significantly for original and washed states of the soil CS. For a given θ, original soil CS exhibits very high suction as compared to when it was washed. This difference is due to the additional osmotic suction component (Sreedeep and Singh, 06b), which is due to the presence of the chlorides in this soil, which becomes negligible due to repeated washing. As expected, the suction values obtained by using PME for original and washed states of the soil CS match very well. This indicates that presence of contaminants does not influence the matric suction values measured by employing the PME. These observations call for an initial chemical analysis of the soil to ascertain the presence of salts or contaminants before selecting a suitable methodology for establishing the SWCC. 1991

5 θ (%) Original Washed 1 1 WP4 PME Ψ (kpa) Figure 2. Influence of contamination on suction values of the soil CS WP4 PME θ (%) ψ (kpa) Figure 3. Comparison of suction obtained by employing the WP4 and PME for the washed soil CS For completeness, the suction values obtained by employing the WP4 and PME for washed soil CS have been compared as depicted in Figure 3. It can be noted that the suction values match reasonably well in the lower suction ranges. However, as suction increases the PME exhibits a higher value as compared to the value obtained from the WP4. 4 Concluding Remarks This study deals with an investigation on the influence of methodologies used for suction measurement on the soil-water characteristic curve, SWCC. Emphasis has been laid on factors such as type of soil suction measured, 1992

6 measurement range, and presence of salts or contaminants in the soil. The uniqueness of the SWCC determined by using insertion tensiometer (IT), a pressure membrane extractor (PME), and a dew point potentiameter (WP4) has been investigated. The study indicates that the SWCC established by using different methodologies may not yield a unique trend. It has also been observed that the suction of the soil, measured by employing the WP4 gets significantly influenced by the presence of salts or contaminants in it. Based on this observation, it can be recommended that initial chemical analysis of the soil is essential before measuring its suction. 5 References ASTM ASTM Book of Standards Volume 4.08: Construction: Soil and Rock (I), Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA, USA. ASTM D Standard Classification of Soils for Engineering Purposes (Unified Soil Classification System), Annual Book of ASTM Standards, 04.08, ASTM International, West Conshohocken, PA, USA. Fredlund D.G., Rahardjo H Soil mechanics for unsaturated soils, John Willey and Sons, Inc., New York. Fredlund D.G., Xing A Equations for the soil-water characteristic curve, Canadian Geotechnical Journal, 31(3), Fredlund D.G., Xing A., Shangyan H Predicting the Permeability function for unsaturated soils using the soil-water characteristic curve, Canadian Geotechnical Journal, 31, IS 27, Part Methods of test for soils: determination of cation exchange capacity, Indian Standards Institute, New Delhi, India. Knight M.A., Mitchell R.J Modelling of light nonaqueous phase liquid (LNAPL) releases into unsaturated sand, Canadian Geotechnical Journal, 33, Lee H.C., Wray W.K Techniques to evaluate soil suction-a vital unsaturated soil variable, Proceedings of the first international conference on unsaturated soils, Paris, 2, Leong E.C., Tripathy S., Rahardjo H. 03. Total suction measurement of unsaturated soils with a device using the chilledmirror dew-point technique, Geotechnique, 53(2), Oberg A.L Negative pore pressures-seasonal variation and importance in slope stability analysis, Proceedings of the first international conference on unsaturated soils, Paris, 2, Rahardjo H., Chang M.F., Lim T.T Shear strength and in situ matric suction of a residual soil, Proceedings of the first international conference on unsaturated soils, Paris, 2, Shah P.H., Sreedeep S., Singh D.N. 06. Evaluation of methodologies used for establishing soil-water characteristic curve, Journal of ASTM International, 3(6), Page Count: 11, Published online: April 06. Sreedeep S., and Singh D.N. 05. A study to investigate influence of soil properties on its suction, Journal of Testing and Evaluation, ASTM, 33(1), Sreedeep S., Singh D.N. 05. Estimating unsaturated hydraulic conductivity of fine-grained soils using electrical resistivity measurements, Journal of ASTM International, 2(1), Page Count: 11, Published online: January 05. Sreedeep S., Singh D.N. 06b. Methodology for determination of osmotic suction of soils, Journal of Geotechnical and Geological Engineering, Springer, 24(5), Sreedeep S., Singh D.N. 06a. Nonlinear curve-fitting procedures for developing soil-water characteristic curves, Geotechnical Testing Journal, ASTM, 29(5), 1-. Sudhakar M.R., Revanasiddappa K. 00. Role of matric suction in collapse of compacted clay soil, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 126(1), Thakur V.K.S., Sreedeep S., Singh D.N. 06. Laboratory investigations on extremely high suction measurements for finegrained soils, Journal of Geotechnical and Geological Engineering, Springer, 24(3), Van Genuchten M.T A closed form equation for predicting the hydraulic conductivity of unsaturated soils, Soil Science Society of America Journal, 44,