Evolution Of Deformation Mechanism In Layered Soft Rock Tunnels With Interlayers

With the rapid development of Chinese economy,the ground resources and space available for development is becoming less and less,and its utilization rate has gradually tended to saturate.In order to solve the increasingly serious traffic congestion problem,more and more tunnels are constructed on the foundation soil rich in soft rocks.Some of the large tunnels are constructed in weak rock strata and often suffer from engineering disasters such as the instability of the tunnel face and even surface collapse,which not only affects the engineering of the tunnel,but also the safety of the tunnel.construction schedule,making it more difficult,and posing a serious threat to staff safety,as well as to the national economic development.significant impact.Therefore,in order to understand the causes of the above-mentioned problems,to reveal the deformation and evolution of the weak rock body,and to propose effective protective The paper takes the Pengjiazhai Tunnel in Xuanen County,Hubei Province as the engineering background,and through theoretical research,numerical simulation and onsite monitoring analysis,a series of studies on the stability and deformation evolution mechanism of the excavation of layered soft rock tunnels containing interlayers were carried out.The main tasks are as follows.(1)By reviewing a large number of literature,the current research status of soft rocks,laminated rocks,and interlayers is summarized;the classification of structural surfaces,apparent morphology and filler characteristics,mechanical properties and main features,and deformation mechanisms of laminated surrounding rocks are discussed to provide the theoretical basis for this study.(2)The excavation simulation of K1+010～K1+030,a typical excavation section of Pengjiazhai Tunnel,was carried out,and the results were compared with the measured data at field.It was found that the simulation results were roughly in line with the change rule of the measured data,which verified the accuracy of the model.(3)Numerical simulations were used to analyse the excavation of downstream soft rock bodies,and the dip angle of the surrounding rock levels,the spacing between levels,the location of the interlayer and the effect of the thickness of the interlayer on the stability of the excavation.The settlement of the entire tunnel circumference can be divided into three stages: a fast growing stage,a slow growing stage and a stabilizing stage,though the influence of different factors varies,which makes the inflection point towards smoothness different.A comprehensive analysis of several conditions at different levels of inclination,different levels of spacing,different locations of the interlayer,and different thicknesses of the interlayer revealed that excavation in down-grade soft rock formations is mainly characterized by bending and fracture damage in the vertical direction of the layer and shear slip damage in the parallel direction of the layer.Under the joint action of the two modes of destruction,the main deformation law for the left side wall part of the displacement settlement greater than the right side wall.After the intervention of the interlayer,the displacement deformation of both further increases,and in order to effectively control the settlement and avoid the collapse accident,it is recommended that the left side walls should be designed with advance pre-grouting and support reinforcement.(4)Analysis of the causes of the phenomenon of tunnel face falling blocks at K1+50 of Pengjiazhai Tunnel,combined with the laminated soft rock tunnel.According to the conclusion on the study of the deformation evolution mechanism,comprehensive reinforcement treatment is made for this tunnel section,namely,adding temporary steel support and spraying slurry to seal the tunnel face.As for the vault and the left side wall part,they are reinforced using advance pre-grouting.After treatment mentioned above,continuous monitoring is carried out on this section,and the results show that the deformation of the vault and the peripheral convergence of this section are in line with the design specifications,which verifies that the reinforcement measures are reasonable and can be a guide to the practical project.