Interface shear mechanisms in concrete–cement soil composite piles revealed by ring shear testing

Abstract Composite piles consisting of an inner precast concrete core and an outer cement soil column have been increasingly used in soft ground, whereas their load-transfer mechanism is governed by two interfaces, namely the concrete-cement soil interface and the cement soil-surrounding soil interface. In this study, ring shear tests were conducted on the two interfaces to investigate their shear behavior, failure characteristics, and strength evolution, and corresponding constitutive models and practical strength estimation methods were established. The results show that the concrete-cement soil interface exhibits a three-stage response of initial loading, post-peak softening, and residual stabilization, and its shear strength increases with both cement soil strength and normal stress. In contrast, the cement soil-soil interface shows a strain-hardening response followed by a stable stage, and its shear strength is primarily controlled by the normal stress and the properties of the surrounding soil. Based on the test results, a three-segment linear constitutive model was proposed for the concrete-cement soil interface, and its strength can be estimated from the intrinsic strength parameters of the cement soil. Moreover, a nonlinear hyperbolic constitutive model was proposed for the cement soil-soil interface, and its strength can be approximately evaluated from the internal friction angle of the surrounding soil. This study clarifies the interface mechanical behavior of composite pile foundations, providing experimental support and a theoretical basis for bearing-capacity analysis and design calculations of composite pile foundations.