Experimental study of effects of suction history and wetting-drying on small strain stiffness of an unsaturated soil

The deformation characteristics of various earth structures such as retaining walls, foundations and tunnels are governed by soil stiffness at small strains (0.001% to 1%). The shear modulus at very small strains (0.001% or less), G0, is an important parameter for predicting ground deformation and dynamic responses of many earth structures. Natural soils are usually unsaturated in the field. However, the small strain stiffness of unsaturated soil has not been well studied. Due to changing of environmental conditions, soils in the field always undergo various wetting-drying cycles and suction histories. The principal objectives of this research are to investigate the effects of suction history, wetting-drying and compression and extension on small strain stiffness of an unsaturated soil. A double-surface conceptual framework is proposed for interpreting small strain behaviour of unsaturated soil, based on the concept of two-surface elasto-plastic kinematic hardening model for saturated soil and the use of two stress state variables for unsaturated soil (i.e., net normal stress and matric suction). Five series of constant net mean stress shear triaxial tests with different suction histories are carried out on compacted samples of completely decomposed tuff (CDT). Each sample is equipped with a suction probe, three pairs of bender elements and a set of Hall-effect local strain transducers. The effects of suction history, which refers to current suction ratio (CSR) and recent suction history, effects of wetting-drying and compression and extension on both G0 and shear modulus reduction curve are investigated. The effects of suction magnitude and net mean stress are also studied. The experimental test results are interpreted by the proposed double-surface conceptual framework. The measured magnitude of G0(vh), G0(hv) and G0(hh) are quite similar. The stiffness anisotropy of compacted CDT under isotropic stress condition is negligible. G0, small strain secant shear modulus (Gsec), elastic threshold strain (εe) and the rate of shear modulus reduction all increase with suction magnitude and CSR significantly, due to the decrease of void ratio and degree of saturation caused by expansion of inner yield surface of the double-surface conceptual framework. The effect of direction of recent suction path (θ) on shear modulus reduction curve is not prominent when the magnitude of recent suction path l=30 kPa. The effect of l on shear modulus reduction curve is significant when θ=-90° but not prominent when θ=90°. The effects of suction history on G0, Gsec, εe and the rate of shear modulus reduction may also be explained by the change of inner yield surface of the double-surface conceptual framework. G0, Gsec and εe after first wetting are significantly higher than those after first and second drying. This observation is probably because the inner yield surface after wetting is larger and void ratio and degree of saturation are smaller. Gsec along compression is consistently higher than that along extension. This is probably because the extension path is more close to the inner yield surface of the conceptual framework. However, the effect of compression and extension becomes negligible after a wetting-drying cycle. In other words, further wetting-drying paths can reduce the effects of compression and extension on small strain shear modulus