Cohesion of compacted unsaturated sandy soils and an equation for predicting cohesion with respect to initial degree of saturation

Embankments which carry highway or railway need firm foundations, for that reason they are generally constructed with compacted sandy soils for which shear strength plays a very important role in stability. However, there is limited available information in existing literature on shear strength behavior of unsaturated sandy soils. Generally the saturation conditions of soils of embankment during raining season are nearer 70%. However, during heavy rains, the soils become more saturated thereby reducing shear strength and consequently failure occurs.

The purposes of this study are to observe the shear strength variation of unsaturated sandy soils with degree of saturation more than 70% and to develop a constitutive equation for predicting shear strength with respect to initial degree of saturation. A series of triaxial shear strength tests conducted on 3 types of sandy soils (taken from Okayama and Hiroshima areas of Japan) with the same void ratio but varying degree of saturation. In all, 9 specimens of each type of soils were considered for undrained triaxial tests with pore-water pressure measurement. The specimens were prepared by static compaction with different initial degree of saturation ranging from 70% to 100% but with the same void ratio for each soil type (0.61, 0.65 and 0.70). The chosen void ratio gives around 90% proctor compaction. Experimental results show that, the shear strength decreases linearly with increase in degree of saturation. Moreover, experimental studies to determine the shear strength parameters of unsaturated soils are generally costly, time-consuming and difficult to conduct; therefore, many equations have been proposed to predict the shear strength of unsaturated soil with respect to matric suction in the last two decades. However, the measurement of matric suction of unsaturated soil is not easy. This paper also proposes an equation to predict cohesion of unsaturated sandy soils with respect to initial degree of saturation based on experimental results.