Study On Excavation Method And Support Optimization Of Large Section Tunnel In Grade Ⅳ Surrounding Rock

In recent years,the development of high-speed railway in China has become the world’s highest,when the train speed in the tunnel,not only can increase the tunnel cross-section area makes the comfort of the train get promoted,also can reduce the hole by blocking ratio of micro pressure wave,and thus reduce the influence of the air pressure of surrounding structures,therefore,the large cross section tunnel is gradually widely used in the high-speed railway tunnel.However,due to a variety of landforms,complex and changeable geological conditions,the increase of the tunnel excavation section is no different from increasing the difficulty of construction,resulting in tunnel collapse,water gushing and other accidents.At this time,it is particularly important to determine the appropriate excavation method,excavation footage and support parameters.Based on the engineering background of a railway tunnel,the finite element software MIDAS/GTS simulated,using the actual data of the bid for a place,different excavation method and the variation characteristics of the tunnel under different excavation,determine reasonable excavation method and excavation,and the rationality of the model is validated with the measured data,and then optimize the supporting parameters.The main research results are as follows:(1)Finite element simulation was carried out for the common excavation methods of three kinds of large-section tunnels,and the characteristics of displacement,stress and plastic zone around the tunnel after excavation were analyzed.The results show that compared with the other two methods,the construction method of three steps and seven steps is suitable for the large section tunnel of grade IV surrounding rock.(2)Based on the three-step and seven-step excavation method,the simulation analysis is conducted for different excavation footage.The results show that reducing the excavation footage can effectively reduce the settlement of the vault and the compressive stress on the surrounding rock,but the plastic zone after excavation is roughly the same.When the excavation footage is 1.5m,the settlement of the vault decreases by 7.32% compared with the excavation footage of 2m,and by 29.80%compared with the excavation footage of 2.5m.(3)By comparing the simulation results with the actual field results,the optimized simulation value is slightly smaller than the actual monitoring measurement result,but the overall trend is roughly the same,which proves the rationality of the model.At the same time,the field measured monitoring data of the tunnel are analyzed and the results show that the surrounding rock velocity is stable at about 0.1mm/d when the tunnel is excavated for about 23 days,and the surrounding rock of the tunnel basically reaches a stable state.At this time,the maximum settlement value of the vault is 14.65 mm.(4)Optimize the initial support of the tunnel.Orthogonal tests are designed mainly for the length,diameter,spacing,thickness and grade of shotcrete.Three evaluation indexes were established and analyzed by range and variance.The conclusion shows that the optimal initial supporting parameters are: the anchor rod with 3m length Φ nm and the shotcrete with C25 strength and 150 mm thickness.