Collapsible soils

Partly saturated/dry soils are often relatively stiff and they suffer only small compressions under normal foundation loads. However, under constant normal loads such soils undergo a marked decrease in their volume upon wetting. The rate of volume change depends on the rate at which water diffuses into the soil mass. The volume changes are rapid when the water has an easy access and the subsequent phenomenon is referred to as “collapse”. The amount of collapse is function of several factors including degree of saturation, initial void ratio, stress history of the materials, thickness of the collapsible strata, and the amount of stress applied (Barden et al., 1973). Loess is a collapsing soil. It is predominantly a silt-sized (5 to 50μm) alkaline, amorphous sediment deposited through aeolian agencies in pen-glacial regions. Such collapsing soils of the loessial type are found in many parts of the earth, including India. The distribution of loess soils in the northern parts of India is given in Fig. 1.15. The figure indicates the prevailing manifestation of aeolian processes both in cold arid and hot arid regions. The collapse mechanism in these soils is found to be due to the local shear failure between soil grains upon wetting. Several land subsidence problems have been noticed in such soils. Land settlement can damage man-made structures such as foundations, pavements, and irrigation works. The collapse process has been studied experimentally in terms of two components of effective stresses, viz. the applied stress and the suction (Barden et al., 1969). It was shown that the following three conditions favor the collapse (Barden et al., 1973):

(i) An open and potentially unstable partly saturated structure

(ii) A high applied stress to develop a metastable condition

(iii) A high value of soil suction to stabilize inter-granular contacts

The reduction in soil suction on wetting leads to collapse. Thus, the stability of these soils depends on the matric suction, which is the unsaturated soil state variable. Thus it stresses the importance of mechanics of unsaturated soils in understanding the stability.