Research on the Mechanical Properties of Surrounding Rock of Deep Shield Tunnel in Structural Loess

The structural properties of loess will change due to the shield construction. In order to explore the influences of variate structural properties on the mechanical properties of surrounding rock of shield tunnels, the structural parameters of stress ratio were introduced to modify the Mohr-Coulomb strength criterion and Fenner formula. By establishing the continuous conditions of force and displacement between loess strata and segments, the radius of plastic zone of surrounding rock was solved, and the analytical solutions were compared and verified. Finally, the influences of key parameters on the mechanical form of surrounding rock were analyzed. The results show that: (1) The calculation results of the Terzaghi surrounding rock pressure formula are between the upper limit and the lower limit of the analytical solution; (2) With the increase of the structure parameter of stress ratio, the corresponding plastic zone radius and surrounding rock pressure value are continuously decreasing; (3) With the increase of thickness span ratio, the radius of plastic zone decreases and the value of surrounding rock pressure increases. When the structural properties of loess are relatively strong, the analytical formula of surrounding rock pressure is applicable after the cover depth reaches a certain value; (4) The surrounding rock pressure and the radius of the plastic zone are approximately linearly positively correlated with the excavation radius.