| The western region of China belongs to the plateau loess area,and the topography and geology are complex,which brings great difficulties to the design and construction of highspeed railways.In order to meet the requirements of smoothness and safety of high-speed railways,tunnels are increasingly applied to the construction of high-speed railways in the Loess Plateau.The loess tunnel has the characteristics of poor stability and large collapsibility,and it is prone to instability of surrounding rock and failure of supporting structure during tunnel construction.The construction of shallow-buried small-distance loess tunnels has always been a key concern in tunnel engineering.In particular,the selection of excavation methods and presupport methods is an important aspect of controlling the deformation of surrounding rock and ensuring the safe and smooth construction.Based on the engineering background of Baishi No.1 Tunnel,which is the supporting project of Lanzhou Hub of China-Lanzhou Passenger Dedicated Line,combined with engineering practice and numerical simulation,this paper studies the construction scheme of shallow buried small spacing loess tunnel,compares and analyzes the deformation law and mechanical characteristics of different schemes in the construction process,selects the reasonable excavation method of the tunnel,and optimizes the construction parameters of the optimal construction method.The numerical simulation of the inverted arch cracking disease in the actual construction is carried out,and the corresponding treatment method is put forward.The stability of the slope of the tunnel under different construction sequences and excavation methods was analyzed by the strength reduction method,and the influence of the properties of the middle rock column on the stability of the surrounding rock was studied.According to the numerical simulation results,the optimal advance support method and layout range of the tunnel are selected.The main work and research results are as follows :(1)Aiming at the optimization of excavation method of shallow buried small spacing loess tunnel,the deformation and stress characteristics of tunnel surrounding rock,middle rock column and supporting structure,as well as the size of surface settlement and the range of plastic zone of surrounding rock were studied by numerical simulation analysis method during excavation of two-bench method,three-bench method and three-bench temporary bracing method.According to the calculation results,the excavation method of three-bench temporary bracing method was determined,and the optimization analysis of different step lengths and different cyclic footages was carried out.Finally,the 0.6m cyclic footage and 3m step length were determined as the optimal construction parameters.(2)In view of the cracking disease of the inverted arch of the first hole in the construction process of the Baishi No.1 tunnel,the finite element simulation method is used to study the stress and deformation characteristics of the inverted arch of the first hole in the tunnel excavation process.It is determined that the tensile stress concentration of the inverted arch structure of the first hole caused by the excavation of the second hole is the main reason for the cracking of the inverted arch,and the protective measures for the grouting reinforcement of the lower soil and the middle rock column area of the inverted arch of the first hole are proposed.Finally,the effectiveness of the measures is verified by numerical simulation analysis.(3)In view of the construction of the bias section of shallow buried and small spacing loess tunnel,the stability of the soil around the tunnel and the adjacent slope is studied by using the finite element local strength reduction method when the ’ left tunnel is excavated first ’ and the ’ right tunnel is excavated first ’.Based on the analysis results,the construction sequence of’ left tunnel is excavated first ’ is determined,and the two-bench method,three-bench method,ring excavation reserved core soil method and three-bench temporary cross bracing method are compared and analyzed by using the same method.The results show that the three-bench temporary cross bracing method is more suitable for the construction of the bias section of shallow buried and small spacing loess tunnel.Finally,the influence of the properties of the middle rock pillar on the stability of the tunnel surrounding rock is analyzed.The results show that the rock pillar area in the small spacing tunnel can be strengthened in the actual construction to improve the stability of the surrounding rock.(4)A finite element model is constructed for the advance support method of shallow buried small spacing loess tunnel.The influence of tunnel excavation on surrounding rock,middle rock pillar soil and surface settlement under advanced small pipe support and advanced pipe shed support was studied.Based on the numerical simulation analysis,the advanced pipe shed support scheme was determined,and the reinforcement effect of advanced pipe shed support under 90°,135° and 180° support range was compared and analyzed.The results show that the pipe shed pre-support effect is the best according to the layout range of 180°. |
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