It is very important for an engineer to understand the evolution characteristics of pressure-arch of a highway tunnel under different stress conditions. After the numerical model of the highway tunnel being built, considering the lateral pressure coefficients varied from small to large, the forming process, distributional pattern and evolution characteristics of the pressure-arch of the highway tunnel under different stress states were analyzed. The numerical simulation of the step-bystep excavation was carried out along the tunnel axis, the distribution morphology and variation characteristics of the pressure-arches located in the front of working face and roof and floor of the tunnel during excavation were revealed. The results showed that the pressure-arch shapes of the tunnel mainly varied from spire, flat, convex, to sag top with the lateral pressure coefficient from small to large. And the stability of the surrounding rock near the tunnel working face became worse and worse with the principal stress difference increasing. The results are of great significance for the construction and reinforcing design of the similar engineering practice.

It is of importance to analyze the morphological characterization, the evolution process and the skewed effect of pressure-arch of a double-arch tunnel in the water-rich strata. Taking a buried depth 80 m double-arch tunnel as an example, a computational model of the double-arch tunnel was built by using FLAC3D technique. Then considering some aspects including groundwater conditions, tunnel depth, construction sequences and permeability coefficients, the coupling effect of stress field and seepage field in the pressure-arch of the double-arch tunnel was analyzed. The results show that the thickness of the pressure-arch induced by step-by-step excavation and display a step-descent skewed distribution from the left to the right of the double-arch tunnel. The permeability coefficient has a significant influence on the shape and the skewed effect of the pressure arch. The excavation of the bench method has a better arching condition than that of the expanding method. The abtained results provide a basic reference for the rock reinforcement design and safety construction of double-arch tunnels in the water-rich strata.