`

Proceedings of Indian Geotechnical Conference December 22-24,2013, Roorkee

A STUDY ON HYGROSCOPIC WATER CONTENT AND RESIDUAL WATER CONTENT OF SOIL

C. Chinumani, Research Scholar, Indian Institute of Technology, Guwahati, chinumani@iitg.ernet.in C. Malaya, Assistant Professor, Assam Engineering College, gogoi_chetia@yahoo.co.in ABSTRACT: Hygroscopic water (HW) is the water adsorbed by the soil from the environment due to electromolecular surface forces and is greatly influenced by the relative humidity of the environment. On the other hand, the residual water content (RWC) is a key parameter of soil-water characteristic curve (SWCC) and is defined as the minimum water content below which there is no appreciable change in water content with suction. The SWCC is a graphical relationship between soil suction and water content (gravimetric or volumetric) and is inevitable in the study of unsaturated soil-mechanics. The determination of SWCC is a costly and time consuming process. There is a possibility of having a close relationship between HW content and RWC of soils as both the water contents are dependent on the specific surface are of soils. With this in view, HW content and RWC have been determined for ten soil samples. It must be noted that there are very few studies available in the literature on the correlation between hygroscopic water content and residual water content of soil. Hence, an attempt has been made to correlate hygroscopic water content of the soil with the residual water content. Such a correlation would be of great help in predicting the RWC of soil by knowing its HW content. INTRODUCTION Hygroscopic water (HW) is the water adsorbed by the soil from the environment due to electromolecular surface forces, and is greatly affected by the relative humidity [1]. On the other hand, the residual water content (RWC) is a key parameter of soil-water characteristic curve (SWCC) and is defined as the minimum water content below which there is no appreciable change in water content with suction. SWCC is a fundamental constitutive relationship in unsaturated soil mechanics [2]. In general terms, the SWCC describes the relationship between soil suction and soil water content. More specifically, the SWCC describes the thermodynamic potential of the soil pore water relative to that of free water as a function of the amount of water adsorbed by the soil system. At relatively high value of water content and correspondingly low values of suction, the dominant pore water mechanism becomes capillarity, governed primarily by the particle and pore structure and pore size distribution. On the other hand, at relatively low water content and correspondingly high values of suction, where pore water primarily in the form of thin films on the particle surfaces, the mechanism contributing to suction are the relatively short-range adsorption

effects governed by the surface properties of soil. Since both the water content is dependent on surface property of soil grains, it may be possible that some relation may also exist between these two parameters of soil. Therefore, the present study investigates the relationship between HW content and RWC of soil. The study indicates that there exists a close relationship between these two water contents of soil. Theoretical Background The study is carried out using Van Genuchten (VG) (1980) [3] SWCC equation to analyze the SWCC for the soil.

( ) s rr mn

......................(1)

1a

θ −θθ ψ = θ +

ψ +

where θ(ψ), θs and θr are the volumetric water content at any suction ψ, saturated volumetric water content and residual water content respectively and a, n and m are the fitting parameters. The key parameters that are relevant for SWCC are:

Page 1 of 3

Chinumani, C. & Malaya, C.

1. The volumetric water content at saturation, θs, describes the water content at which the soil is completely saturated and typically depicts the initial state for the evaluation of the drying path.

2. The air-entry value (AEV), ψa, is the suction at which air enters the largest pore present in the soil sample during a drying process [4]. This is the point where the desaturation process starts.

3. Residual water content (θr) is the minimum water content below which there is no appreciable change in θ. Suction corresponding to θr is called residual soil suction, ψr [5].

RETC RETC [6] is a computer code for analyzing the soil-water characteristic of unsaturated soils. The program uses VG SWCC equation to represent the SWCC. The SWCC estimation is based on the VG SWCC equation (Eq. 1) and grain size distribution of soil. EXPERIMENTAL INVESTIGATIONS Ten locally available soil samples were used in this study. The physical properties of soils are presented in Table 1. Grain size distribution is obtained using guidelines provided by Indian standard (IS: 2720). Hygroscopic water content (wh) was obtained following the methodology presented in the literature [7]. Determination of Hygroscopic Water Content [7] A sufficient amount of the soils was oven dried and with the help of a wooden rammer, the clumps were broken up. Fifty grams of each of these soils was passed through a 980 μ sieve and spread uniformly in a tray. Later, the tray with the soil was placed in a humidity chamber, which maintains a specified relative humidity, RH. The moisture content of the sample was determined, following the methodology presented by ASTM [8], after different time t (=1, 3, 5, and 7 days) of storage, at different levels of RH (=45%, 52%, 58%, 78% and 90%) and at a constant temperature of 22±0.5°C. Estimation of SWCC Ten numbers of SWCC were estimated using the data available presented in the Table 1. The

estimated SWCCs are presented in the Fig. 1. The parameters of VG SWCC equation are estimated using RETC by inputing the grain size data which are shown in the Table 2.

Table 1 Physical properties and HW content of soils used in this study

Sample Sand Silt Clay wh (%) (%) (%) (%)

P1 14 65 21 28.28 P2 20 62 18 39.72 P3 9 69 22 50.55 P4 21 46 33 79.43 P5 19 49 32 93.88 P6 7 69 24 83.04 P7 6 68 26 72.21 P8 7 64 29 75.82 P9 8 64 28 68.60 P10 13 61 26 90.26

10-2 10-1 100 101 102 103 104 105 1060

5

10

15

20

25

30

35

40

45

θ (%

)

ψ (kPa)

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10

Fig. 1 Estimated SWCC of the soils used in the study

Page 2 of 3

A study on hygroscopic water content and residual water content of soil

Table 2 Parameters obtained from estimated SWCC

Sample AEV ψr θr θAEV kPa kPa % %

P1 62 3700 37.45 0.363 P2 60 3900 37.36 0.38 P3 61 3800 38.23 0.37 P4 50 18000 33.49 3.6 P5 30 20000 28.36 3.12 P6 39 8000 31.47 1.25 P7 30 15000 29.35 1.58 P8 45 7000 33.93 1.18 P9 42 8500 31.98 1.37 P10 29 14000 30.46 1.65

Results and Discussion The variations of residual water content with respect to hygroscpic water content are presented in the Fig. 2. It can be observed from the trend depicted in the figure that r increases with increase in HW content for the range of HW content considered in this study. r shows almost a linear relation with the wh. The regression coefficient for the relationship is found to be 0.83. Due to the lack of such studies in the literature, the obtained trends could not be compared and validated.

0 2 4 6 8 10 12 14 160.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5 θr vs. whθr=0.28wh-0.86 R2=0.83

θ r (%

)

wh (%) Fig. 2 Correlation between r and wh

CONCLUSIONS The study investigates the relationship between the hygroscopic water content and residual water content of SWCC for the soils under consideration. The SWCC parameters have been estimated for ten numbers of soils using computer code RETC. The study indicates that HW content is closely related to the residual water content of soil. It has been observed that HW content, wh shows a good correlation with residual water content, θr. Therefore, it can be said that HW can be used to calculate the r of soil. REFERENCES 1. Saarenketo, T. (1998), Electrical properties of

water in clay and silty soils, Jl. of Applied Geophysics, 40, 73–88.

2. Fredlund, D.G. and Rahardjo, H. (1993), Soil Mechanics for Unsaturated Soils, John Wiley and Sons, Inc., New York.

3. Van Genuchten, M.T. (1980), A closed form equation for predicting the hydraulic conductivity of unsaturated soils, Soil Sci. Soc. Am. Jl., 43, 892-898.

4. Brooks, R.H. and Corey, A.T. (1964), Hydraulic properties of porous media, Hydrology paper no. 3, Dept. of Civil Engineering, Colorado State Univ., Fort Collins, Colorado.

5. Yang, H., Rahardjo, H. and Fredlund, D.G. (2004), Factors affecting drying and wetting soil-water characteristic curves of sandy soils, Canadian Geotech. Jl., 41(5), 908-920.

6. Van Genuchten, F.J. Leij and Yates, S.R. (1991), The RETC code for quantifying the hydraulic functions of unsaturated soils, U. S. Salinity Laboratory, Agricultural Research Service, Riverside, California.

7. Paresh H. Shah and D. N. Singh. (2006), Methodology for determination of hygroscopic moisture content of soils, Jl. of ASTM International, 3(2).

8. ASTM D 2216-92 (1994), Laboratory determination of water _moisture_ content of soil and Rock, Annual Book of ASTM Standards, Vol. 04.08, ASTM International, West Conshohocken, PA, PA, 1994, 177–180.

Page 3 of 3