Variability of soil-water characteristic curve
Date of Issue2016-05-05
School of Civil and Environmental Engineering
Environmental Engineering Research Centre
In recent years, unsaturated soil mechanics as related to soil above ground water table and compacted soil, has received increased attention in geotechnical engineering. The soil-water characteristic curve (SWCC) defines the relationship between the water amount in the soil (i.e. gravimetric water content, w, volumetric water content, w, or the degree of saturation, S) and the suction in the soil. The SWCC contains the fundamental information to solve problems related to unsaturated soil mechanics. Other unsaturated soil properties such as permeability and shear strength can be predicted from SWCC. SWCC can be determined from experimental measurement or predicted from basic soil properties. Experimental measurement provides more reliable SWCC rather than the method of prediction from the basic soil properties. However, there is variability in SWCC regardless of which method is used. This variability can affect the accuracy of predicting unsaturated soil properties. Therefore, it is necessary to understand the variability in SWCC before using it to predict other unsaturated soil properties. In this research variability of SWCC during experimental measurement and data interpretation was investigated and discussed. Equations for quantification of variability in SWCC, determination of SWCC variables such as air-entry value, slope at the inflection point, residual suction and residual water content, hydraulic conductivity from fitting parameters in Fredlund and Xing’s (1994) equation were derived. The equation for estimation of hydraulic conductivity from SWCC using capillary model was proposed. The equations for estimation of wetting SWCC from drying SWCC using capillary model was also proposed. The variation in measured water content for compacted mixture of sand and kaolin and residual soil prepared in this research during drying and wetting process was presented. The variability in SWCC during data interpretation with respect to soil volume change, number of data points adopted and suction range covered for the best fit procedure was presented and discussed. The variation in SWCC variables using both conventional graphical method and equations proposed in this research was also presented. The variability in SWCC and hydraulic conductivity for residual soils in Singapore was quantified using confidence limits proposed in this research. A new frame which incorporated with the confidence limits was proposed for prediction of SWCC for residual soils in Singapore from saturated hydraulic conductivity. From the study in this research, both water content and soil volume change is suggested to be monitored during SWCC measurement, and more data points to be collected within the estimated transition zone. SWCC variable such as air-entry value instead of SWCC fitting parameters is suggested to be correlated with soil properties.