Determination of Shear Strength Parameters of Unsaturated Silts and Sands Based on the Water Retention Curve

Abstract

Great efforts have been made to understand and determine the stress parameters of unsaturated soils. However, this requires elaborate laboratory tests, which are both difficult and time consuming to perform. A hypothesis for determination of the shear strength of unsaturated soils is suggested. The hypothesis suggests a simple way of determining the shear strength based on the water retention curve and the angle of internal friction. Values of shear strengths have been taken from the literature and compared with shear strengths calculated according to this hypothesis. The shear strengths obtained from the hypothesis are in surprisingly good agreement with those found in the literature. The suggested hypothesis is critically examined and its limitations discussed. Finally, a reasonable way of applying the hypothesis in engineering practice is proposed.

Capillary water retention curve and shear strength of unsaturated soils

“…(15) was widely adopted, such as in Vanapalli et al (1996), Oberg and Sallfors (1997), Xu (2004), and Lu and Likos (2006). The normalized area of water (A w /A) is assumed to be in direct proportion to the normalized volumetric water content (i.e., degree of saturation, S r ) in the soil by applying Greens theorem (Fung 1977;Vanapalli et al 1996).…”

“…A number of shear strength criteria for unsaturated soils have been proposed in the literature during the past three decades or so. Some of them were based on regression analyses of experimental data from either direct shear or triaxial tests, such as Fredlund et al (1978); Gan et al (1988); Fredlund et al (1996); Oberg and Sallfors (1997); Khalili and Khabbaz (1998); Rassam and Williams (1999); Rassam and Cook (2002); Toll and Ong (2003); Tekinsoy et al (2004); Xu (2004). Other strength criteria were embedded within various constitutive models for unsaturated soils, such as Alonso et al (1990); Sun et al (2000); Sheng et al (2008); Alonso et al (2010).…”

“…It is often recognised that the shear strength of an unsaturated soil is closely related to the suction or degree of saturation in the soil. Therefore, the degree of saturation that can be obtained from the water retention curve (WRC) for a given suction has been widely embedded into strength criteria for unsaturated soils, such as Fredlund et al (1996); Vanapalli et al (1996); Oberg and Sallfors (1997); Toll and Ong (2003). Although some shear strength criteria for unsaturated soils do not contain the degree of saturation explicitly, such as the equations proposed by Khalili and Khabbaz (1998) and Sheng et al (2008), the key parameters involved in these criteria, such as the air-entry value (AEV), are embedded in the WRC.…”

“…No specific surface area is explicitly considered in these equations and hence the existence of adsorbed water is ignored. When the degree of saturation is embedded in the shear strength criteria (Oberg and Sallfors 1997;Lu and Likos 2006), it leads to significant overestimation of shear strength of unsaturated soils, especially for fine-grained soils and high suction values . Using the effective degree of saturation to replace the degree of saturation in shear strength criteria (Vanapalli et al 1996;Lu et al 2010;Konrad and Lebeau 2015) can somewhat improve the prediction, but still overestimates the shear strength for a wide range of suction values .…”

Model for the prediction of shear strength with respect to soil suction

“…For this reason, several semiempirical procedures have been proposed to predict shear strength of unsaturated soils using the relationship between the water content and soil suction expressed by the soil-water characteristic curve (SWCC) Vanapalli et al 1996;Oberg and Sallfours 1997;Bao et al 1998;Khalili and Khabbaz 1998;Toll and Ong 2003;Sheng et al 2008).…”