Effect of a filter cake on shear behavior of sand-concrete pile interface

A filter cake is often formed between soil and concrete during casting concrete in the ground, such as constructions of diaphragm walls and bored piles. The present study aims to investigate the effect of the filter cake on the shear behavior of the sand-concrete pile interface. A series of sand-concrete interface direct shear tests were performed with a large-direct shear apparatus while considering different roughness ( I =0, 10, 20 and 30 mm) and filter cake thickness (∆ h =0, 5 and 10 mm). For a smooth interface without a filter cake, the shear stress-horizontal displacement curves showed a “softening” response. The peak shear strength and friction angle decreased exponentially with increasing the ∆ h . Whereas, for a rough interface with ∆ h =5 or 10 mm, the shear stress-horizontal displacement curves presented a “hardening” response. The peak strength, as well as friction angle, decreased linearly with increasing the ∆ h . Moreover, a critical roughness I cr of 10 mm was observed in the tests without a filter cake. The interface shear strength initially increased with increasing I but gradually decreased when the I exceeded I cr . In addition, the filter cake could reduce the roughness sensitivity on shear strength.