Interaction study on surrounding rock and bolting-shotcrete U-shaped steel support structure in deep soft rock roadway

Convergence-constraint method is a design approach for underground engineering structures that combines theoretical analysis, field measurements, and engineering experience. It is a commonly used method for analyzing the interaction between surrounding rock and support system, as well as for conducting optimized support design. Based on the basic principle of convergence-constraint method, the classical supporting characteristics and supporting structure deformation equations were summarized. The supporting characteristic curves of supporting structures such as shotcrete, bolt (cable) and U-shaped steel were obtained by theoretical calculation. The effects of the geometric size (diameter, length), row spacing and material strength of supporting structure on the support pressure were analyzed. As the thickness and strength grade of shotcrete increase, the support stiffness and support pressure provided by shotcrete gradually escalate. Simultaneously, the support pressure of bolts experiences a significant increase with the augmentation of bolt (cable) diameter, length, material strength, and reduction in row-spacing between them. Furthermore, the support pressure delivered by U-shaped supports intensifies as the row-spacing decreases and the material strength increases. By using the Mohr-Coulomb strain softening constitutive model embedded in FLAC^3D, the numerical analysis model of deep soft rock roadway considering the post-peak strain softening and dilatancy characteristics of rock was established. The deformation curves of roadway longitudinal section and the characteristic curves of surrounding rock under different stress states were calculated. The applicability of three kinds of combined support technologies such as bolting shotcrete, bolt (cable) shotcrete and bolting-shotcrete U-shaped steel to the large deformation control of surrounding rock in deep roadway were analyzed. The feasibility of applying the combined support technique of bolting-shotcrete U-shaped steel in deep roadway support projects has been verified. The numerical simulation results of the strain-softening constitutive model considering the post-peak strain-softening and dilatancy characteristics of rocks are quite different from those based on the classical Mohr-Coulomb constitutive model. The numerical simulation results of the Mohr-Coulomb constitutive model are conservative, and the supporting pressure provided by the supporting structure cannot meet the stability control requirements of the deep roadway. After roadway support, the surrounding rock exhibits pronounced deformation, which may give rise to safety incidents such as roof collapse and wall instability.