Transition from Brittle Failure to Ductile Flow in a Sandy Soil

A series of experiments were carried out using a triaxial system modified for unsaturated soil testing to investigate brittle transitional and ductile failure behaviour of a sandy clay loam soil under varying soil water and effective confining stress levels. Mechanical properties and deviatoric stress-strain relationships of the soil were also established. The specimens were prepared under laboratory conditions where the inter-particle cementation bonds were allowed to form to their natural state. An unsaturated soil mechanics approach was used to define critical state relations for the soil. All specimens attained a critical state under continuous shearing. The results demonstrate that brittle/ductile behaviour of unsaturated soils and their tendency to dilate/compact at failure are both controlled by soil water and confining stress levels. An exponential model used to fit the deviatoric stress-axial strain test data accurately predicted the trends. Soil water significantly influenced the shear strength and internal angle of friction ( \') and hence the mechanical behaviour of the sandy soil. The regression equations developed showed that \' have quadratic relationships with soil water with an asymptotic surface (transitional stage). Hence, in civil works or soil tillage the transitional range (w.c. 5 - 9% d.b.) could be taken as a useful guide towards understanding soil behaviour upon loading Keywords : Triaxial testing, critical state, brittle failure, transitional, ductile flow, effective confining stress, soil water Discovery and Innovation Vol. 19 (2) 2007: pp. 140-146