Undrained behaviour of intact soft clay under cyclic paths that match vehicle loading conditions

Vehicle traffic loading appears to contribute significantly to long-term settlement beneath highways, airport runways, and metro lines in China. Wheel loading imposes cycles in both the magnitude and direction of the principal stresses acting on the soils beneath pavement or rail-track structures. Conventional cyclic triaxial (CT) testing, which is not capable of imposing such stress paths, may underestimate how heavy traffic loading affects any underlying soft clay layers. A hollow cylinder apparatus (HCA) can simulate such traffic loading stress paths more accurately, including rotation of the principal stress directions. This paper presents a systematic experimental study of cyclic HCA (CHCA) tests on K-0-consolidated saturated soft clay involving cyclic variations in both vertical and torsional shear stresses, along with a parallel programme of CT tests, considering the undrained response of saturated samples of intact soft clay. It is shown that when applied above certain critical cyclic stress ratios, principal stress rotation accelerates excess pore-water pressure and permanent strain development. Corresponding changes are also seen in the resilient modulus and damping ratio trends. The discrepancies between the behaviour of CHCA and equivalent CT tests grow as the cyclic stress ratios increase.