Effects of displacement-softening behavior of concrete-mudstone interface on load transfer of belled bored pile

Mudstone is a typical weak rock that fails to provide sufficient base resistance in the bored pile. The belled bored pile can replace the super-long bored pile in the super high-rise building due to the higher base resistance and lower settlement. However, ignoring the displacement-softening behavior of the concrete-mudstone interface probably overestimates the vertical bearing capacity of the belled bored pile in the mudstone. Thus, this study investigates its load transfer considering the interfacial softening behavior. The interfacial softening constitutive model is derived based on the in-door direct shear test. Full-scale simulation results show that the interfacial softening weakens the bearing capacity of the belled bored pile. The belled section’s side resistance is mobilized earlier than the lower half of the uniform cross-section. Its interfacial tensile failure accelerates the side resistance at the lower half to the peak and softening stages rapidly. This adjustment effectively resists most settlements over 30 MPa compared to the bored pile. Moreover, parametric sensitivity analysis reveals the significance of interfacial cohesion. This study is valuable for researching the analytical solution of side resistance by providing verification or comparison. It also indicates the belled bored pile can be gradually popularized into the super high-rise building in the mudstone stratum, saving cost and ensuring structural safety. Highlights The displacement-softening model for concrete-mudstone interface is derived based on in-door shear test. The side resistance of belled section is mobilized earlier than the lower half of uniform cross-section. The interfacial tensile failure rapidly mobilizes the side resistance at 15–20 m to peak and residual stages. The belled bored pile effectively resists the failure settlement over 30 MPa caused by interfacial softening.