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.

Combined with the first case of MJS construction method pile strengthening soft foundation in railway related project in Fuzhou coastal area, the construction process parameters of MJS construction method pile soft foundation reinforcement were summarized and put forward. Based on the cylindrical cavity expansion model, an axisymmetric finite element numerical model was established. The detailed analysis considered the deformation law of the soil on the pile side during MJS shotcrete pile forming. The results showed that the lateral displacement of the soil on the pile side generally presents a Z-shaped distribution. With the release of pressure during the gradual lifting of shotcrete pile, the positive displacement amplitude gradually decreases and the negative displacement amplitude gradually increases, And the maximum displacement position gradually moves upward. The analysis of the influence of shotcrete pressure showed that when the shotcrete pressure is 1.3p ~ 1.6p, the lateral displacement of soil is small and the deformation behavior is good; The surface settlement is small and consistent, and the ground pressure coefficient is reasonably controlled at 1.3 ~ 1.6. The measured value is close to the calculated value. The maximum lateral displacement and surface settlement account for 13.4% and 50.4% of the early warning value respectively. They belong to micro disturbance construction and fully meet the engineering requirements. In the process of shotcrete pile forming in the actual project, it is the key to control the internal shotcrete pressure when close to the surface, and the surface settlement caused by the interaction between shotcrete pressure and ground pressure should be paid attention to.

To explore the characteristics of support bearing and deformation control, a mechanical model for the coupling effect of support and surrounding rock was constructed based on comprehensive consideration of support bearing characteristics such as steel arch frame, sprayed concrete, and advanced reinforcement. A total of 35 working conditions were constructed for 5 calculation types, and the influence of different support forms, support stiffness, and support timing on the deformation of surrounding rock was detailed analyzed based on a certain engineering example. The results showed tunnel support has a good control effect on the deformation of the surrounding rock in front of and behind the palm, and the combined support effect of advanced support and initial support is better, and this control has a spatial effect. A single support or a coordinated combination of multiple supports can achieve the same deformation control effect, depending on the stiffness matching of each support element. Steel arch frame, as a rigid support structure that can immediately and actively bear loads, has more advantages than sprayed concrete in bearing loads and controlling deformation. The combined support efficiency of advanced support and steel arch frame is higher. The earlier the support is applied, the earlier the deformation of the surrounding rock enters a controllable state, and the better the effect. With advanced support, the sensitivity of surrounding rock deformation to support timing is relatively low. Therefore, for complex tunnel projects, early implementation of advanced support is an important measure to compensate for inadequate initial support and reduce construction risks. This study theoretically clarified that reasonable stiffness matching between supporting structures is the key to achieve stress coordination and tunnel structure stability.