Shear stress-strain-time behaviour of clays

Shear stress-strain-time models for soils were examined in terms of undrained triaxial compression tests, with pore water pressure measurement, using reconstituted specimens of kaolinite and Cucaracha shale. Multiple-step constant load and constant rate of deformation tests were performed. The testing variables included final equal all-round consolidation pressure (100-800 lb/in²), overconsolidation ratio (1-8) and the magnitude of the first load increment (20-82% in terms of shear stress level). Additional constant load test data reported in the literature were analysed. The parameters of an exponential stress-strain model were expressed and interpreted in terms of the parameters of the hyperbolic stress-strain model. The parameters of both models can be expressed in terms of the undrained modulus to undrained shear strength ratio E_u/s_u and the axial strain at failure ε_f. The creep parameter λ which controls time or strain rate effects was correlated with E_u/s_u. It appears that any soil has a potential total strain at any shear stress level, and that λ is directly related to the magnitude of the remaining strain. A large value of E_u/s_u means a small strain at any small time. Therefore as E_u/s_u increases, the remaining strain and thus λ increase. Alternative combinations of the hyperbolic stress-strain and either power strain-time or power strain-strain rate relationships are suggested as possible creep models for the analysis of field problems and interpretation of laboratory tests.