Simplified prediction of the stratigraphy of the active zone in expansive soils in Kibaha, Tanzania

Abstract

Considerable distress to lightly loaded engineered structures in various parts of the coast of Tanzania is due to development of heave and swelling pressure in the active zone of expansive clay soils. The active zone is the region of soil near the surface in which the water content varies due to precipitation and evapo-transpiration. Even though the soil may have the potential to shrink and swell below the depth of active zone, volume changes will not take place because the water content of the soil is constant. Because the water content distribution does not change with time below that zone, the soil should be either stabilized or removed down to that depth or the foundation must extend to a depth that exceeds that of the active zone. A logical soil investigation needs to be carried out by an appropriate professional to determine the depth of active zone based upon the site specific soil conditions that are dependent on the water content in the soil at the specific site in question.

To monitor the water content changes with depth, samples from different open pits at earmarked intervals and depths within a 100-hectare section in Kibaha were analysed. Based on the moisture content variation with depth, the depth of the 'active zone' ranged between 1.5 m and 2.0 m.

Keywords: Depth of active zone, soil water content, soil profile and heave/ shrinkage