Construction Mechanical Behavior And Application Of Long-span Multi-arch Tunnel Without Middle Wall

After China’s reform and opening up,the traffic pressure is increasing with the sharp increase of population and the number of motor vehicles,especially in the 21 century,which promotes the rapid development of transportation infrastructure in our country.highway,high-speed railway,urban rail transit and so on are becoming more and more perfect.Among them,the number of tunnel construction is increasing,the scale is getting larger and larger,the technical requirements are getting higher and higher,the geological conditions encountered are becoming more and more complex,and the forms of supporting structures are becoming more and more diverse.In order to adapt to more topography,improve the space utilization rate and reduce the construction cost,the multi-arch tunnel is favored by tunnel builders because of its unique advantages.In order to explore the construction mechanical behavior of long-span multi-arch tunnel without middle wall,taking the Xiqu Tunnel of Dali-Nanjian Expressway as the research object.Based on experimental research,theoretical analysis,numerical simulation and practical engineering,the construction mechanical behavior and application of long-span multi-arch tunnel without middle wall are studied.The results show that:(1)In the model test of the multi-arch tunnel,with the advance of the tunnel face,the stress value in the vertical direction of each measuring point increases gradually,and the stress increase of the surrounding rock increases sharply near the collapse of the tunnel;at the same time,the vertical deformation of the surrounding rock in the middle of the model test box increases gradually during the simulated excavation.when the tunnel catastrophe occurs,the surrounding rock will have instantaneous large deformation,and its displacement will form a substantial increase.(2)In the process of numerical simulation of Mao cave excavation,the stress and displacement of surrounding rock in X,Y and Z direction of rock mass increase with the advance of tunnel face.However,when the advance hole and the back hole are excavated and connected,the stress decreases twice successively.The reason is that after all the rock mass in the tunnel area is excavated,the stress in the surrounding rock will be released,and the stress in the surrounding rock will decrease.(3)In the process of numerical simulation of excavation under actual conditions,with the advance of the tunnel face,the surrounding rock stress in X,Y and Z direction increases gradually and the growth trend is small before the back tunnel excavation,and when the back tunnel is excavated for 5 m,at this time,due to the sudden increase of the excavation section of the whole tunnel,the surrounding rock stress in X,Y and Z direction increases suddenly.When the back tunnel of the multi-arch tunnel is completed,the accumulated stress in the last excavation part of the surrounding rock is released,and the surrounding rock stress decreases greatly;at the same time,the surrounding rock displacement shows a uniform increase trend.The simulation results show that when the multi-arch tunnel is constructed by this method,the tunnel deformation is small and meets the safety requirements.(4)The numerical simulation results show that if the three-step reserved core soil method is used to excavate the leading hole,the improved CD method is used to dig the later tunnel,and the steel arch frame of the rear tunnel is mounted on the steel arch frame of the leading tunnel,the multi-arch tunnel needs to strengthen the design of the common side wall structure of the lower step of the multi-arch tunnel in the process of design and construction,and the base bearing capacity of the multi-arch tunnel needs to be increased at the same time.(5)Based on theoretical calculation,model test and numerical analysis,a complete set of construction method of long-span multi-arch tunnel without middle wall is formed and successfully applied,which greatly reduces the occupied area of the tunnel,is beneficial to environmental protection and saves land cost.The use of this method not only ensures the safety of construction,but also reduces the amount of steel,greatly saves the construction cost,shortens the construction procedure,and effectively controls the disturbance to the advance hole.