References 182


Residual soils are formed by themechanical and chemical weathering of parent rocks at the original location. Poor drainage conditions favour formation of montmorilloniterich, expansive soils (vertisols) in semi-arid zones, while good internal drainage favours the development of kaolinite-dominant ferruginous soils in the sub-tropical zones (FAO, 1989; Fookes, 1990). Occurrence of low groundwater table coupledwith evapotranspiration effects often render the residual soils unsaturated (degree of saturation, Sr <1). A special feature of these unsaturated residual soils is their volumetric instability to changes in water content at constant net applied stress (σ −ua). Moisture induced volumetric instability in residual soils manifest as swell (increase in void ratio upon water absorption), shrinkage (decrease in void ratio from reduction in water content) and collapse (decrease in void ratio upon water absorption) as illustrated in Figure 5.1. The rates of swell and shrink of unsaturated soils are much slower than rate of collapse. The swell process is essentially governed by moisture migration into the microstructure, while shrinkage depends onmoisture evaporation from themicrostructure. Microstructure-dependent moisture exchange makes the swelling and shrinkage a slow process. Collapse is a more rapid process as it essentially depends on the rate of moisture migration into the porous macrostructure. The magnitudes of volumetric deformations in response to changes in moisture content at given net applied stress

are influenced by the mineralogy of clay fraction, soil density and moisture content, surcharge pressure and soil microstructure.