Coupling effect of roughness and normal pressure on mechanical characteristics and failure mode of deep-sea sediment interface

Revealing the failure mechanism at the soil–structure interface is an important prerequisite for foundations bearing capacity analysis. In deep-sea structures, due to the special sedimentary conditions and extremely low permeability coefficient of marine soil, obviously, the undrained behavior at the interface is becoming more complex and needs due attention. The objective of this study is to investigate the coupling effects of roughness and normal pressure on the interface mechanical characteristics and failure mode from the perspective of the pore pressure index through improved interface triaxial apparatus. The obtained results revealed that increased surface roughness makes it difficult for the soil particles in the grooves to climb along the structure surface and consequently the failure surface shifts from the interface to the soil. The confining pressure strongly influences the failure mode and critical roughness; Based on the response of two factors to the interface, a new piecewise Mohr-Coulomb failure criterion is established to reveal the real interface mechanical parameters, accordingly two new concepts are proposed and analyzed: transition roughness and critical normal stress; Furthermore, the relationship between the critical roughness and confining pressure is determined by the interface shear resistance equation considering the coupling effect; Finally, the interface failure mechanism and failure modes related to the confining pressure and surface roughness are revealed and distinguished. The dominant effect of effective interface friction parameters on the failure modes is evaluated for various surface roughness ranges.