Mechanical properties of two expansive clay soils from Sudan

Abstract

The mechanical responses of two highly expansive plastic alluvial soils from Sudan are reported. When wetted from their in-situ state, such soils frequently lead to damage of civil engineering structures. Disturbed samples of a black cotton clay and a more plastic red clay were collected from two locations in the vicinity of Khartoum. Intrinsic properties were established from reconstituted samples using oedometer and triaxial apparatus. Natural undisturbed block samples of the black cotton clay were obtained and tested in an unsaturated state, as found in situ. Basic characteristics of both soils are presented. Detailed information on the intrinsic properties of both clay soils was established and found to conform with Burland's earlier published framework. Shear planes formed during shearing in almost all triaxial samples and strengths reduced to post-rupture values. K0-consolidated reconstituted samples exhibited a more brittle response to undrained shearing compared with isotropically consolidated samples. Peak failure conditions can be represented by linear failure envelopes for both soils, regardless of consolidation path, although those from shearing in extension do not pass through the origin: reasons for this are discussed. Oedometer tests on the intact black clay soil indicate a high potential for swelling on wetting (volumetric strains of almost 50%), decreasing with increasing stress level (swelling pressure of about 1 MPa). Unconsolidated undrained triaxial tests on the intact unsaturated samples show increasing strength and decreasing brittleness with increasing confining pressure.