Dynamic characteristics of saturated subgrade silty clay under traffic induced cyclic vertical compression and horizontal shear stresses

Subgrade soils are subjected to multidirectional traffic-induced cyclic loading, under which excess pore water pressure and permanent deformation govern long-term performance. To clarify the distinct and coupled effects of vertical cyclic stress and horizontal cyclic shear stress, cyclic single-shear tests were conducted on saturated Harbin silty clay under fully laterally confined conditions for vertical stress amplitudes of 15 ∼ 45 kPa, horizontal shear amplitudes of 3 ∼ 9 kPa, and loading frequencies of 0.5 ∼ 3.0 Hz. The dynamic resilient modulus, dynamic shear modulus, and accumulative pore-water pressure were measured under closed undrained conditions, while the accumulated vertical plastic strain was determined under top drain valve close and bottom drain valve open conditions. The results show that vertical cyclic compression primarily induces skeleton hardening and modulus increase, whereas horizontal cyclic shear causes initial softening and generates pore water pressure and plastic strain more efficiently per unit stress, with both responses increasing approximately linearly with shear stress amplitude. When vertical and horizontal components act simultaneously, principal stress rotation yields higher dynamic stiffness and significantly amplified responses: at medium stress levels, the final plastic strain under complex cyclic loading reaches about 1.10 ∼ 1.74 times that under pure vertical loading and 1.69 ∼ 6.04 times that under pure horizontal cyclic shear stress. A loading frequency of approximately 1.5 Hz, representative of highway traffic, produces the largest pore pressures and plastic strains, in some cases up to about three times those at 0.5 or 3.0 Hz. Overall, these results indicate that design and assessment methods based solely on vertical cyclic stress are likely to underestimate the risks of pore pressure accumulation and permanent deformation in saturated subgrade silty clays; explicit consideration of horizontal cyclic shear stress and compression–shear coupling is necessary for a more realistic evaluation of subgrade performance under traffic loading.