Mechanism of crack development of large cross section loess tunnels under unbalanced load

In order to study the cracking situation of the lining of large-section tunnel under bias load and clarify the cracking mechanism of large-section loess tunnel under bias load, this paper takes the Shaojiatang highway tunnel project of the G30 Lianhua Expressway expansion project in Qingshuiwei to Zhonghe section as the research object, and based on the extended finite element method under the actual working conditions of the crown and right shoulder with a 30° bias load range, the lining cracking situation in the tunnel mouth section under bias load is simulated by controlling the size of the applied bias load to be equal for each group and taking the angle of the bias load position$\alpha$as the variable. The axial force and bending moment of the lining cut sections after cracking are extracted, and the cracking mechanism of the bias load tunnel is analyzed by the angle of the internal force. The rationality of the model is verified by calculating the internal force distribution of the lining and comparing it with the numerical model internal force distribution based on the in-situ monitoring data of the lining. The research results show that: there are tensile and compressive regions on both sides of the lining in the bias load range and a tensile main crack is formed on the inner surface of the lining in the shape of $\mathrm{\Lambda }$ in the bias load range; as the angle of the bias load position$\alpha$increases from 10° to 30°, the length of the lining crack decreases from 4 m to 2.8 m, the depth of the lining crack increases from 9 cm to 22 cm, and the trend of the lining crack changing from extending in the length direction to expanding in the depth direction is presented as the angle of the bias load position increases; the width of the surface crack increases overall as the angle of the bias load position $\alpha$increases; the lining cracks at 0.256 MPa and the maximum surface crack width of the lining at 30° angle of the bias load position$\alpha$is 3.3 mm, and the surface crack width at 10° angle of the bias load position is 2.4 mm; the internal force extraction shows that the crown and right shoulder are most affected by the bias load, and there is the maximum axial tensile force and negative bending moment; it is calculated that the eccentricity of the crack section decreases as the bias load increases, and the eccentricity of the crack section at the angle of bias load$\alpha$= 30° is the largest under the same bias load, while the smallest at the angle of 10°. The research results can provide theoretical reference for the operation and maintenance of in-service bias tunnels.