Shield Tunneling Is Used To Excavate The Upper Subway Influence Analysis Of Circuit Stability

With the continuous expansion of urban rail network,the situation of urban underground engineering projects is more and more complex,the new shield subway tunnel inevitably has to go through many existing underground and overground projects.It is becoming more and more common for subway tunnels to be underrun.For example,there are more than 30 underrun projects of guangzhou rail transit.These underpass projects undoubtedly bring new difficulties to the shield tunneling construction.In the design and construction,engineers need to consider not only the influence of the shield tunneling construction on the stability of the ground structures,but also the influence of the construction on the surrounding rock and the existing upward engineering stability.In this environment,it is necessary to explore the influence of shield tunneling on the stability of the existing uplink line and the factors affecting the structural stability of the existing uplink line.Firstly,the main factors affecting the stability of the existing upstream tunnel are summarized and analyzed,and the weight coefficients of the influencing factors are studied and sorted by ahp.It is concluded that the factors affecting the stability of the existing upstream tunnel are mainly divided into 3 categories and a total of 9 influencing factors.The results of analytic hierarchy process show that the weight distribution of the three kinds of influencing factors is geological factor > engineering factor > other factors.The order of 9factors is as follows: surrounding rock lithology > close construction distance >groundwater > geological activity > supporting mode and supporting time > time factor >tunnel excavation method > in-situ stress field > section form.The study shows that rock lithology is the key factor affecting the stability of existing tunnel.In view of this conclusion,the influence degree of surrounding rock lithology on underpass engineering is studied,which provides some theoretical basis for the design and construction of related engineering in the future.Secondly,combined with the actual engineering background,the site monitoring method is used to monitor the settlement of the surface,the vertical displacement of the existing upward tunnel and the convergence of the new tunnel in the case of shield tunneling under the construction,and the monitoring data are analyzed and obtained: In the process of shield construction,the maximum settlement of the surface is basically about 20 m within the excavation range of the tunnel.After a period of time after the completion of shield tunneling,the surface settlement tends to be stable and the location of settlement no longer shifts.The trend of vertical deformation and surface deformation is roughly the same.Within 0.5 times of the tunnel axis diameter,the difference of vertical displacement is obvious.In the range of shield tunneling,the vertical displacement increases with the increase of depth,and the loose zone extends to the surface.The monitoring results of tunnel convergence are as follows:with the progress of shield tunneling,the convergence deformation around the tunnel increases;however,after the completion of shield tunneling for a period of time,the development of tunnel convergence deformation tends to be gentle.At the same time,the geotechnical simulation software is used to simulate the constructio process according to the actual construction steps on site,and the results are as follows:When the shield tunneling is carried out on the right line of the new tunnel,the horizontal displacement of the existing upward tunnel structure increases first and then decreases with the construction progress of the new tunnel,and the bending moment of the existing upward tunnel structure decreases with the construction progress of the new tunnel shield tunneling.When the shield tunneling is carried out on the left line of the newly-built tunnel,the horizontal displacement and bending moment of the existing upward tunnel structure are approximately the same,but the position of the maximum deformation point is changed.Comparing the simulation data with the monitoring data,it can be seen that the numerical simulation results are more reliable,which can provide guidance for the subsequent construction and focus on monitoring the possible maximum displacement deformation point.Finally,the major risk sources are identified,including the following four major risk sources: construction in arrival section,gushing,segment floating,shield jamming and wear damage of cutter head and screw machine in full section hard rock section.And in view of the risk elaborated in the design stage,the construction stage and after the completion of underpass construction measures.The conclusions obtained in this paper can be used to guide the subsequent construction of shield tunneling and provide a reference for other similar projects.