Large deformation mechanism and control technology of soft rock tunnel under tectonic stress

Large deformation of surrounding rock is a great challenging in soft rock underground engineering. We analyzed the microscopic and pore characteristics of typical surrounding rock using scanning electron microscopy (SEM) and established a mechanical model for high pre-stressed long and short anchor cables in a collaborative support system. By integrating test results of surrounding rock loose zones and calculations of surrounding rock pressure, the support parameters for long and short anchor cables are determined and verified. A suitable support scheme for large deformation sections using high pre-stressed long and short anchor cables is proposed. Additionally, numerical simulation techniques are employed to systematically analyze the large deformation of surrounding rock and the supporting mechanism of long and short anchor cables under tectonic stress. The results reveal that the surrounding rock of the DLII11 + 080 section of the branch tunnel of Haidong Tunnel is primarily composed of calcareous shale, characterized by developed fractures, high porosity, and a loose structure. The microscopic layered structure contributes to the rock's fragility, making it susceptible to external forces. Single-length short anchor cable support is ineffective in providing adequate reinforcement to the surrounding rock. However, the use of both long and short anchor cables enhances support by leveraging their synergistic effects. The short anchor cable primarily mitigates early plastic deformation, while the long anchor cable provides overall constraints to prevent the expansion of deep displacement. Through numerical simulations, the plastic zone and convergence deformation of surrounding rock are analyzed under varying lateral pressure coefficients and cohesion levels. The study also elucidates the influence of tectonic stress and surrounding rock strength on tunnel deformation. The results indicate that the depth of the vault plastic zone under the optimized long and short anchor cable support is 48.63% lower than without any support and 44.38% lower compared to support using 6-meter short anchor cables alone. The combined support of long and short anchor cables significantly enhances the uniformity of axial force distribution, improves the overall support effect, and stabilizes the surrounding rock. This research provides a scientific foundation and practical guidance for controlling large deformations under tectonic stress in soft rock underground engineering.