Study On Mechanical Effect Of Undercut Construction Of Small Clear Distance Metro Station Under Dominant Joint Control

Since the 21st century,China’s highway transportation industry has witnessed rapid developmen.The construction of subways in the prosperous areas of the city is subject to the boundary of the building on the ground.The construction environment is harsh and the tunnel exhibition is very difficult.The jointed tunnels with small spacing usually exist in the construction of Metro stations.Because of the unique structure and geology of jointed tunnels with small spacing,the stress situation is very complex.In this paper,the numerical simulation combined with the similar model test method is used to study the influence of joint parameters on the construction stability of small spacing tunnel and the mechanical effect of small clear distance metro station construction under the dominant of joint control.The main work and conclusions of the thesis are as follows:1.Based on the Hoek-Brown criterion,FLAC^3D is used to analyze the construction stability of small spacing tunnels with different dip angle joints,different location joints and unjointed conditions,and summarize the influence law of joint parameters on the construction stability of small spacing tunnels.The research shows that the existence of joints leads to the serious bias of the tunnel group,the joint with the dip angle of 30° and the tunnel passing through the left and right tunnels has the greatest stability to the construction of small spacing tunnels.2.According to the influence of joint parameters,the joint is selected as the dominant joints when the dip angle is 30°,and then the influence of the clear distance and construction method on the construction stability of the small spacing tunnel is studied.The research shows that the reasonable minimum clearance of the Class IV surrounding rock tunnel is between 0.5B and 0.75B.Choosing a suitable construction method is the key to ensuring tunnel construction safety.3.Based on the similarity theory,the joint passing through the right tunnel with an dip of 35 degrees is selected as the dominant joint in the model test.Considering that the upper layer of the joint is sandstone and the lower layer is sandy mudstone,the similar proportion of surrounding rock materials is studied.Through the analysis of the stress,displacement and axial force variation of the monitoring point during the model test excavation process,combined with the simulation results under the same working condition of FLAC^3D,it is concluded that the influence of tunnel excavation on the stress around the right hole is larger than that of the left hole;The stress redistribution in the middle rock pillar is complex,and the support should be applied in time;the anchoring effect of the anchor is good,and the closer to the observation point of the anchor end is,the greater the axial force is.4.Based on the model test,a three-dimensional numerical analysis model of Tushan Station on Chongqing Circle Line is established by FLAC^3D.According to the design document,the excavation of the double-side-drift method is selected.By simulating the excavation and support process,the variation law of displacement,stress and plastic zone of each observation point is analyzed.It is concluded that the dominant joints of stratum lead to significant bias of tunnel group,the vertical displacement and horizontal displacement of the right tunnel are larger than that of the left tunnel,and the surface settlement deformation is asymmetrical distribution;After the excavation is completed,the left and right tunnels have tensile stress at the bottom of the tunnel;the right arch foot of the right tunnel has a small plastic zone due to the excavation of the left following tunnel.5.By comparing the numerical simulation results with the field monitoring data,the correctness of the FLAC^3D finite difference element software in the tunnel construction calculation is verified,which ensures the safety of the construction of the Tushan station using the double-side-drift method.