Small- and large-strain behaviour of a cement-treated soil during various loading histories and testing conditions

Abstract

The effects of loading histories on small-strain and large-strain stiffness of a compacted cement-mixed well-graded gravelly soil were evaluated by consolidated drained triaxial compression (TC) tests. Various cyclic loading histories were applied during otherwise continuous monotonic loading at different confining pressures, σhs, and multiple-step loading with stepwise increase or decrease of σh. To evaluate small-strain stiffness, minute unload/reload cycles were applied during TC loading. The value of peak-to-peak secant modulus from a minute unload/reload cycle was defined as the equivalent Young’s modulus, Eeq. The average of Eeq values measured during continuous monotonic loading at low deviator stresses was rather close to the elastic modulus, Ee. As the Ee increased with an increase in the axial stress, the Eeq value increased first and then decreased due to an increase in creep strains. The stress–strain behaviour after the start of a large-scale yielding and the peak shear strength is not noticeably affected by the previous cyclic loading with relatively large stress amplitude. However, the large-strain stiffness during reloading is significantly affected by the following factors controlled by pre-cyclic loading history; (a) strain hardening; (b) strain nonlinearity; (c) hysteresis effect; (d) viscous effect; and (e) damage to bonding by immediately preceding large unloading. The tangent stiffness, Etan, at a given stress level during reloading becoming larger with an increase in the effects of factor (a) and smaller by the effects of factors (b), (c), (d) and (e). In addition, increase of σh has a negative effect on stiffness of primary loading curves and a positive effect on stiffness of reloading curves. The effect of loading history on the small-strain stiffness in terms of equivalent Young’s modulus, Eeq, was found to be significant. This effect depends on various factors. Four factors—axial stress level, strain-hardening effect, creep effect by the viscous properties and damage due to a loading history—were identified to be responsible for equivalent Young’s modulus, Eeq variations. Axial stress increase and strain-hardening effect both increase Eeq, making Eeq closer to the Ee. However, creep strains by the viscous properties and damage due to a loading history both decrease Eeq.