STUDY ON THE THEORETICAL MODEL OF DEFORMABLE PRIMARY SUPPORT FOLLOWING THE PRINCIPLE OF DEFORMATION COORDINATION

The large deformation of surrounding rock in soft rock tunnels subjected to high ground stress is a pain point for the safe construction of deep engineering. The deformable primary lining consists of yielding steel arches and deformable shotcrete lining incorporating yielding elements, which has a certain deforming capacity and can release a certain amount of rock strain energy, thus relieving the support pressure. The deformation coordination between steel arches and shotcrete lining is the core issue in the research of the deformable primary support. This study uses the theoretical analysis method and firstly constructs the mechanical models of the yielding steel arches and deformable shotcrete lining incorporating yielding elements, and provides corresponding theoretical expressions for their support pressure - radial displacement relationships. For the yielding steel arch, its mechanical model includes three deforming stages: elastic - yielding - elastic stages. According to the deforming characteristic of yielding element, the mechanical model of deformable shotcrete lining also includes three stages, but its second stage is further divided into two sub-stages: yielding and compaction stages. Secondly, based on the principle of deformation coordination, a theoretical model for the deformable primary support is established. According to the principle of maximizing the utilization of support bearing capacity, a theoretical expression for the support parameters that should be satisfied is provided. Furthermore, by comparing with the numerical simulation results in previous reference, it is found that the theoretical model established in this study has good consistency with the numerical simulation results in characterizing the relationships between the displacement and support force for the yielding steel arch, the deformable shotcrete lining, and the deformable primary support. This well indicates the reliability and feasibility of the theoretical model established in this study. Finally, a comprehensive parametric investigation is conducted based on the established theoretical model. It is found that reducing the longitudinal spacing of the steel arch or increasing the lining thickness can improve the support stiffness and bearing capacity; Increasing the sliding joint height of the yielding steel arch and the yielding element length can improve the deformation capacity of the primary support, making the surrounding rock and support become stable; When the surrounding rock and support reach an equilibrium, a higher support pressure and a smaller radial displacement can be seen with considering the compression behaviour of the yielding element. The results of this study can provide relevant theoretical support for solving related engineering problems.