Experimental Study On Stability Of Tunnel Face In Sand Layer Under The Influence Of Multiple Factors

With the gradual development of urban rail transit construction technology and the increasing problem of "labor shortage" in construction,the construction of interval tunnels in urban rail transit tends to be mechanized large-section construction,because the surrounding rock conditions are often encountered during construction.Since the new idea method was accepted by more scholars,the stability of the tunnel face has attracted more and more attention.There are many factors involved in the stability of the tunnel face,the tunnel construction factor is the main disturbing factor,the stratum factors are intricate,and the tunnel face reinforcement measures are numerous.This paper is funded by the National Key Research and Development Program(2018YFC0808701).It mainly selects the sand tunnel face as the research object,and conducts research on its stability,using model tests,numerical analysis and other methods.The main work content and results are as follows:(1)Through carrying out model tests,a total of ten working conditions under different construction methods,different support methods and different formation treatment conditions were set up to study,and the ultimate extrusion deformation before failure of the tunnel face was used as the evaluation method for the stability of the tunnel face.Index to obtain the ultimate extrusion deformation under the test environment.The instability failure of the tunnel face is divided into middle and lower extrusion failure and dump failure.Starting from the destruction characteristics of the surrounding rock in front of the tunnel,the development trend of the soil damage in front and above the tunnel under the sand geological conditions is determined as: the damage first develops toward the front of the tunnel,and then the damage zone toward the tunnel in front of the tunnel.The upper part of the soil body develops to form a wedge-shaped cavity.As the height of the damage zone increases,the damage zone gradually expands and extends to the top of the support.The top of the damage zone shows an arch effect.(2)Using Midas GTS NX to simulate the influence of many factors on the stability of the tunnel face,the deformation laws of the tunnel face and the front surrounding rock under different excavation methods and different supporting conditions are obtained.The results show that there is a coupling between the influencing factors.The amplification effect or the coupling reduction effect,and the coupling amplification effect is more harmful to the stability of the tunnel face.Under the combined action of various factors,the influence on the stability of the tunnel face is not a simple linear increase or decrease relationship.In the range above the extension line of the vault in front of the tunnel,the vertical stress gradually decreases and the reduction increases,reaching a minimum when the height of the tunnel vault is reached,and then the vertical stress increases as the depth increases.Gradually increase.The reduction in horizontal stress far exceeds the reduction in vertical stress.(3)The face grouting and advanced support have a significant effect on improving the stability of the face.By grouting,changing the parameters of the leading core soil,increasing the cohesion and internal friction angle can enhance the stability of the face.Under the condition of interbedded sand and clay,the relative position of the clay layer and the tunnel has a great impact on the stability of the tunnel face.The clay layer at the bottom of the tunnel has a significant effect on the settlement of the vault and the deformation of the tunnel face,and the effect is the best.Located on the upper part of the vault can only significantly reduce the settlement of the vault,but cannot reduce the bulging displacement of the tunnel face.(4)Taking the risk event of tunnel face collapse as the research object,the coupling mechanism of multiple risk factors and the coupling amplification effect are analyzed,and the calculation method of the coupling coefficient between the risk factors is given,which can be calculated according to the formula.Under certain circumstances,different risk factors lead to the coupling magnification factor of the tunnel face collapse,and the risk of tunnel face collapse is evaluated.