| The red-bed soft rock with special properties is widely distributed in central Sichuan,which greatly affects the railway construction in this region.The upper arch of tunnel base is one of the obvious indigenous diseases.At present,the research on the foundation arch disease of soft rock tunnel at home and abroad is still in the initial stage of post-characteristic induction and reason derivation.There is a lack of strong experimental data and systematic analysis on the stress change,displacement deformation and their relationship of surrounding rock after tunnel excavation and the influencing factors of foundation arch.From the point of view of the relationship between normal stress and horizontal stress,based on the actual engineering geological conditions and field investigation,through the generalized physical model,the centrifugal test of the upper arch of the soft rock tunnel foundation,the original tunnel model and the single factor numerical simulation of the upper arch of the soft rock tunnel foundation are carried out to study the stress change and displacement deformation of the surrounding rock of the tunnel excavation.The data were processed by Get Data software to obtain the deformation characteristics of surrounding rock after tunnel excavation.The stress variation law and tunnel structural strain after tunnel excavation were obtained through the corresponding measurement elements.The influence of tunnel excavation on surrounding rock and the relationship between stress and displacement deformation were preliminarily studied.After the numerical simulation of the original model and the feasibility of the actual condition monitoring,the results of the single factor numerical model of the arch on the base of the soft rock tunnel are analyzed.The main conclusions are as follows:(1)By analyzing the variation characteristics of stress and deformation in centrifugal test and numerical simulation of the original model,it is shown that the stress redistribution caused by soft rock tunnel excavation will cause a large degree of attenuation of the normal stress of the arch foot at the bottom of the tunnel and the increase of the lateral pressure coefficient,which leads to the obvious deformation of the arch foot at the bottom of the tunnel.The influence of stress redistribution on underlying strata decreases with the increase of burial depth,while the attenuation of normal stress is still greater than that of horizontal stress.The increase of lateral pressure coefficient decreases,resulting in the decrease of arch deformation with the increase of burial depth.The strain data of tunnel structure are analyzed.The results show that the lateral deformation of tunnel structure is flat oval under earth pressure,the top and bottom of tunnel are compressive deformation,and the two sides are tensile deformation.The longitudinal compression deformation gradually decreases,and the tensile deformation gradually increases.(2)The stress distribution characteristics of the numerical simulation of the original model are as follows : when the burial depth increases,the normal stress decreases gradually,and the change trend of horizontal stress changes from decrease to slight increase.The overall deformation characteristics of the surrounding rock at the bottom of the tunnel are that the upper arch of the surrounding rock at the depth of 1 m is larger than that at the bottom of the tunnel,and the upper arch deformation of the underlying strata decreases with the increase of the buried depth.The cross-sectional deformation of the tunnel and underlying strata is folded.The upper arch amount of the middle line of the tunnel is the largest,and the convergence of the left and right side walls is small.The upper arch amount of the longitudinal section at the bottom of the tunnel at 3 m is the largest,while the upper arch amount of the longitudinal section of the underlying strata increases along the longitudinal direction of the tunnel.The contact surface is used to restore the soft rock surface,and it is found that the arch displacement direction of the upper layer is closer to the normal direction than that of the upper layer,which leads to the sudden increase of the arch amount on the soft rock surface and the surrounding rock,which is in line with the phenomenon of layer effect.(3)The single factor numerical simulation of soft rock tunnel arch shows that :(1)The change of soft rock depth and soft rock thickness has little effect on the initial stress of the model,and the increase of initial lateral pressure coefficient increases the initial horizontal stress.After excavation,the lateral pressure coefficient increases,indicating that the decrease of normal stress caused by excavation is greater than that of horizontal stress.(2)The upper arch characteristics of the transverse and longitudinal sections of the tunnel are less affected by single factor,and the maximum upper arch amount is significantly affected,which increases with the increase of the initial lateral pressure coefficient,and the maximum is29.32 mm,increasing by 71 %;it decreases with the increase of soft rock burial depth,with the minimum of 19.81 mm,decreasing by 13.6 %;the influence decreases with the increase of soft rock thickness,and the influence weakens rapidly when the thickness of soft rock is greater than 4.4 m.(3)The maximum arch depth and range of surrounding rock under tunnel decrease with the increase of initial lateral pressure coefficient,soft rock depth and soft rock thickness.(4)Combined with the results of centrifugal test and numerical simulation,the deformation of surrounding rock strata at the tunnel base and arch foot is serious.The corresponding normal stress decreases and the horizontal stress increases greatly,and the lateral pressure coefficient increases significantly.With the increase of buried depth,the variation of normal stress,horizontal stress and lateral pressure coefficient of underlying strata decreases,and the deformation amplitude of strata decreases.It can be seen that the relative size of normal stress and horizontal stress is the key factor to cause the upper arch of the tunnel and its underlying strata,indicating that the change of geostress after excavation is an important reason for the upper arch disease of the base.Therefore,it is necessary to drill and measure the stress in the underlying strata when the tunnel line selection and construction are carried out in the soft rock strata in the high geostress area.The line tunnel should try to select the areas with small lateral pressure coefficient,far upper layer of soft rock from the bottom of the tunnel and large thickness of soft rock interlayer. |
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