| Due to the constraints of various factors such as terrain,geology,and tunnel site selection during the construction of mountain tunnels,as well as the need to take into account factors such as construction difficulty and cost,small spacing tunnels have been widely used.However,compared to traditional small spacing tunnels,small spacing tunnels under complex conditions such as shallow burial,eccentric pressure,and staggered platforms have more complex deformation and structural stress of the surrounding rock,making them prone to accidents such as lining cracking and even collapse during construction.Therefore,studying the surrounding rock pressure and construction mechanical effects of shallow buried offset pressure staggered small spacing tunnels is of great significance for controlling the stability of surrounding rock and ensuring safe tunnel entry.This thesis takes the entrance section of the Yiliang Tunnel on the Chongqing Kunming High Speed Railway as the engineering background,and adopts a combination of theoretical analysis,numerical simulation,and on-site monitoring to study the calculation of surrounding rock pressure,the variation law of surrounding rock pressure under various parameters,construction mechanical effects,advanced support schemes,and design parameter optimization of shallow buried and unsymmetrical staggered small spacing tunnels.The research results are expected to provide reference and guidance for similar tunnel engineering construction in the future.The specific research content and results are as follows:（1）Conduct research on relevant engineering cases and analyze the failure mode of shallow buried unsymmetrical pressure staggered small clear distance tunnels.When the tunnel fracture surface intersects below the ground surface,establish a tunnel load calculation model,based on the limit equilibrium method,derive the formula for surrounding rock pressure,and determine the applicable conditions of the formula.Using simple numerical examples,study the changes in surrounding rock pressure caused by bias angle,tunnel burial depth,step offset,and tunnel clearance.（2）Using FLAC3Dsoftware,establish a three-dimensional numerical model for this type of tunnel under different bias angles,tunnel burial depth,step offset,and tunnel net distance to calculate the working conditions.The displacement,stress,maximum shear strain increment,and plastic zone of the surrounding rock are used as stability analysis indicators to study the changes in the surrounding rock under different influencing parameters,in order to verify the reliability of the derived formula.（3）Based on the Yiliang Tunnel portal section project,the geological overview is introduced,and the tunnel type is determined based on the tunnel definition standards.Using the AHP method and numerical simulation,it was determined to choose the three step temporary inverted arch method as the excavation method for the tunnel entrance section.At the same time,numerical simulations were conducted on the construction sequence of the left and right tunnels,the construction offset of the tunnel face,and the length of the steps to determine the construction sequence of excavating the right tunnel first and then the left tunnel.The construction offset of the tunnel face should be 50 meters,and the length of the steps should be 4 meters.（4）Due to the poor stability of the surrounding rock at the entrance section of the Yiliang Tunnel under initial support,advanced support is required.Based on the comparison and selection of advanced support measures,three advanced support schemes are proposed:pipe shed support,pipe shed and small conduit combined support,and small conduit support.Establish a three-dimensional numerical model,compare and analyze the stability analysis indicators of surrounding rock,and conclude that the support effect is the best for pipe shed support,followed by joint support of pipe shed and small conduit,and the worst for small conduit support.At the same time,while ensuring the stability of the surrounding rock,combined with factors such as the length of the pipe shed support effect and construction difficulty,the optimal advanced support scheme is determined to be the combination of pipe shed and small conduit support.（5）Based on the selection of advanced support schemes,different design parameters of the pipe shed are optimized and compared.Establish a relevant three-dimensional numerical model to calculate the working conditions,compare and analyze the vertical and horizontal displacements of surrounding rock under different design parameters,and determine the design parameters of the pipe shed based on controlling the deformation of surrounding rock and considering factors such as engineering cost and construction technology:the thickness of the grouting reinforcement area is 3 m;the length of pipe shed reinforcement should be controlled at about 40 m;the angle should be about 2°;the reasonable value of the spacing ring0.3 m;the optimal diameter of the pipe shed is 100 mm.Simultaneously combining monitoring measurement and numerical simulation,evaluate the effectiveness of advanced support,and verify the safety,economy,and rationality of the selection of pipe shed design parameters. |
