Study on the Mechanical Characteristics of Shallow Buried Four-Hole Tunnel with Small Distance through Expressway

Based on the Mohr-Coulomb failure criterion and combined with mathematical optimization method, considering the key indexes such as the stress of surrounding rock, the deformation of cavern and the settlement of subgrade, the comprehensive evaluation equation of safety degree and the quantitative optimization scheme are established, and the mechanical characteristics of tunnel construction considering the construction process and support effect are studied. The intercalated rock between the first excavated right vehicular tunnel and the pedestrian tunnel produces strain penetration, the soil covering settlement of the vehicular tunnel is funnel-shaped, and the inverted arch bulges, and the plastic strain of the first excavated vehicular tunnel is mostly concentrated at the arch waist, while the arch crown subsidence and lateral deformation of the later excavated left vehicular tunnel are large. It can be seen that the unloading and stress release effect of the first excavated tunnel can not be ignored. During construction, attention should be paid to grouting reinforcement of the intercalated rock in the first excavated tunnel, and reinforcement measures such as foot locking anchor bolt and arch grouting should be set at the left arch foot of the two vehicular tunnels. When the upper bench is excavated, the vault settlement of the tunnel and the settlement of the overlying highway subgrade increase sharply, and the vault deformation at this stage exceeds 50% of the total deformation value. If the surrounding rock deteriorates or the environmental control standard is more strict, the short bench reserved core soil method or double side wall heading method should be considered. The settlement of the overlying subgrade of the tunnel presents an asymmetric "W" distribution, and the left vehicle tunnel behind is larger. The calculation results are verified by field monitoring data. There are stress concentration areas at the foot of pipe roof and the contact surface with surrounding rock, which reveals the stress transfer effect of pipe roof with larger stiffness. The installation of pipe shed can effectively slow down the vault settlement and subgrade settlement rate, ensure that the total settlement value of overlying subgrade is not more than 20 mm, and meet the design requirements. The research work provides better technical support and engineering guidance for similar projects.