Damage evolution of bi-body model composed of weakly cemented soft rock and coal considering different interface effect

Considering the structure effect of tunnel stability in western mining of China, three typical kinds of numerical model were respectively built as follows based on the strain softening constitutive model and linear elastic–perfectly plastic model for soft rock and interface: R–M, R–Cs–M and R–Cw–M. Calculation results revealed that the stress–strain relation and failure characteristics of the three models vary between each other. The combination model without interface or with a strong interface presented continuous failure, while weak interface exhibited ‘cut off’ effect. Thus, conceptual models of bi-material model and bi-body model were established. Then numerical experiments of tri-axial compression were carried out for the two models. The relationships between stress evolution, failure zone and deformation rate fluctuations as well as the displacement of interface were detailed analyzed. Results show that two breakaway points of deformation rate actually demonstrate the starting and penetration of the main rupture, respectively. It is distinguishable due to the large fluctuation. The bi-material model shows general continuous failure while bi-body model shows ‘V’ type shear zone in weak body and failure in strong body near the interface due to the interface effect. With the increasing of confining pressure, the ‘cut off’ effect of weak interface is not obvious. These conclusions lay the theoretical foundation for further development of constitutive model for soft rock–coal combination body.