Expansive soils are residual soils which are the result of weathering of the parent rock. The depths of these soils in some regions may be up to 6 m or more. Normally the water table is met at great depths in these regions. As such the soils become wet only during rainy seasons and are dry or partially saturated during the dry seasons. In regions which have well-defined, alternately wet and dry seasons, these soils swell and shrink in regular cycles. Since moisture change in the soils bring about severe movements of the mass, any structure built on such soils experiences recurring cracking and progressive damage. If one measures the water content of the expansive soils with respect to depth during dry and wet seasons, the variation is similar to the one shown in Fig. 18.9.
During dry seasons, the natural water content is practically zero on the surface and the volume of the soil reaches the shrinkage limit. The water content increases with depth and reaches a value wn at a depth Dus , beyond which it remains almost constant. During the wet season the water content increases and reaches a maximum at the surface. The water content decreases with depth from a maximum of wn at the surface to a constant value of wn at almost the same depth Dus. This indicates that the intake of water by the expansive soil into its lattice structure is a maximum at the surface and nil at depth Dus. This means that the soil lying within this depth Dus is subjected to drying and wetting and hence cause considerable movements in the soil.
The movements are considerable close to the ground surface and decrease with depth. The cracks that are developed in the dry seasons close due to lateral movements during the wet seasons.
The zone which lies within the depth Dus may be called the unstable zone (or active zone) and the one below this the stable zone. Structures built within this unstable zone are likely to move up and down according to seasons and hence suffer damage if differential movements are considerable.
If a structure is built during the dry season with the foundation lying within the unstable zone, the base of the foundation experiences a swelling pressure as the partially saturated soil starts taking in water during the wet season. This swelling pressure is due to constraints offered by the foundation for free swelling. The maximum swelling pressure may be as high as 2 MPa (20 tsf). If the imposed bearing pressure on the foundation by the structure is less than the swelling pressure, the structure is likely to be lifted up at least locally which would lead to cracks in the structure. If the imposed bearing pressure is greater than the swelling pressure, there will not be any problem for the structure. If on the other hand, the structure is built during the wet season, it will definitely experience settlement as the dry season approaches, whether the imposed bearing pressure is high
or low. However, the imposed bearing pressure during the wet season should be within the allowable bearing pressure of the soil. The better practice is to construct a structure during the dry
season and complete it before the wet season.
In covered areas below a building there will be very little change in the moisture content except due to lateral migration of water from uncovered areas. The moisture profile is depicted by curve 1 in Fig. 18.9.
Figure 18.9 Moisture content variation with depth below ground surface