Small strain shear modulus of saturated and unsaturated cohesive soils under anisotropic consolidation

Cohesive soils in nature are created under anisotropic stress and have various stress histories. Embankments generate greater vertical loads underground. Moreover, associated excavation activities can exacerbate the extensional stress state. This study investigated the effects of induced anisotropy on the shear modulus in saturated and unsaturated cohesive soils. A triaxial testing apparatus, equipped with local small strain (LSS) measurement devices and bender elements (BEs), was used to measure the small strain shear modulus. Two series of tests were conducted: (1) LSS and BE tests used specimens normally consolidated under a constant mean effective stress of p’ = 300 kPa or net mean stress p_net = 300 kPa with different stress ratios to investigate the effects of anisotropic consolidation. The values of the applied stress ratios, represented as K = σ'_h/ σ'_v for the saturated soil and K_net = (σ_h–u_a)/(σ_v–u_a) for the unsaturated soil, were 0.35, 0.43, 0.6, 0.8, 1.0, 1.5, 2.0, 3.0, and 3.5. (2) BE tests used specimens consolidated under various mean effective stresses in the order of p’ = 50, 100, 200, 300, 400, 500, and 600 kPa, and swollen in reverse order under K of 0.35, 0.43, 0.6, and 1.0, to elucidate p’ and the effects of the overconsolidation ratio (OCR). The results demonstrated that K-consolidation under constant p’ produces large differences in initial shear modulus G₀ in saturated cohesive soil, but K_net produces only slight differences in unsaturated cohesive soil because of the influence of strong matric suction. Finally, G₀ was normalized successfully considering the effects of void ratio e, K, and OCR.