Role of normal stiffness in 3D cyclic behavior of gravel–steel interface from large-scale direct shear tests

Abstract

The shaft capacity of piles is essentially the shear strength response of soil–structure interfaces. The design of piles requires sound knowledge of the three-dimensional (3D) interface behavior under constant normal stiffness (CNS) condition, which has seldom been addressed. In this research, a series of 3D tests of a gravel–steel interface were conducted in two-way circular, cross and beeline cyclic shear paths at different normal stiffness using a large-scale direct shear apparatus. The 3D behavior and constitutive rules of the interface under CNS condition and the influence of normal stiffness were explored in detail. Test results indicate that the normal stiffness remarkably affects the magnitude of normal displacement, normal stress, shear stress, initial shear modulus and degradation of cyclic shear strength. Increased normal stiffness leads to accelerated reduction in the normal stress and therefore accelerated degradation of cyclic shear strength. The variation of normal stress results in “induced aeolotropy” in the shear strength, while it has no impact on the inherent isotropy of strength coefficient. The shaft capacity of the piles degrades slowly and might be lost incompletely if the surrounding soil is relatively hard and the normal stiffness is sufficiently small. By contrast, the normal stiffness is of poor significance in the relationship pattern between performance parameters, in addition to the friction angle of the interface. The resultant stress ratio versus resultant tangential displacement response presents perfect consistency characteristics and can be described using a hyperbola model, independent of normal stiffness and shear path, thereby considerably simplifying the 3D constitutive description of the interface under CNS condition.