Model tests on disturbance characteristics of surrounding rock of loess shield tunnels during excavation

In order to explore the disturbance characteristics of the surrounding rock by excavation of loess shield tunnels under different water contents and buried depths, the similarity model tests are carried out. Firstly, the physical parameters and strength indexes of the undisturbed loess under different water contents are measured by the triaxial tests. Through a large number of tests, the test soil with similar strength indexes of the undisturbed loess with different water contents is obtained. Then, considering the characteristics of shield tail gap of shield tunnels, the simulated tests on the shield excavation under different water contents and buried depths are carried out. The variation laws of stratum deformation, surface settlement and rock stress under different working conditions are analyzed by means of the monitoring components such as micro-earth pressure sensors, dial indicator and digital image technology. It is found that the self-stabilizing capability of the surrounding rock is strong when the water content is low. The higher the water content, the greater the buried depth, the greater the disturbance of excavation to the stratum. The maximum ground settlement is positively correlated with the water content and negatively correlated with the buried depth. The residual stress of the surrounding rock increases with the increase of the water content and buried depth, but when the water content increases to a certain value (26.6 %), the residual stress of the surrounding rock decreases sharply. Based on the development law of the stress and deformation of the surrounding rock under different water contents, the water content of loess can be divided into self-stabilization, deformation and loosening water contents. Based on the loosening of the surrounding rock caused by the water content and buried depth, the pressure action mode of the surrounding rock of loess shield tunnel under different working conditions can be calculated according to the deformation pressure and loosening pressure, so as to better adapt to the characteristics of loess strata.