Study On Construction Disturbance Settlement Control Of Long-distance Four-line Tunnel Closely Underneath Crossing The Existing Station

With the upsurge of large-scale development and utilization of underground space in major cities in China,the phenomenon of crossing between new and old metro lines has become more and more common.The distance of four-line tunnel of Beijing metro line 19 underneath crossing the existing Xin Gong station is relative long,and the underneath crossing section is close to the station(the net distance is only 0.22m),which is a typical long-distance and large-area closely underneath crossing project.In addition,as the newly-built tunnel is in gravel stratum,it is very easy to be disturbed during the construction process,resulting in large settlement of the existing station structure.In order to ensure the safety of the existing station,it is very necessary to select proper construction parameters to control the deformation of the existing station.Based on the research background of this project,this paper first elaborates the engineering conditions and the position relationship between the newly-built tunnel and the existing station,and analyzes the main factors that induce the deformation,thus puts forward the corresponding deformation control indexes.Secondly,combining the on-siting monitoring and numerical simulation,the whole process of underneath crossing is simulated by using the finite element software,and the settlement and deformation characteristics of the existing station structure during the underneath crossing construction are analyzed.Finally,the paper analyzes the additional deformation and stress characteristics of the existing station under different construction parameters,grouting reinforcement range and grouting strength conditions,and puts forward the appropriate construction scheme and optimization of grouting parameters.Based on the above research,the main conclusions are as follows:(1)The number of pilot tunnels in the underpass section is large and the distance is relative close,and there is time-sharing and block excavation,and there is “multi-tunnel effect” in the excavation process of pilot tunnels.The settlement of the existing station structure can be effectively controlled by measures such as optimizing the construction sequence,selecting a reasonable excavation scheme and strengthening the stratum with full section deep hole grouting.(2)The settlement simulation and actual measurement results of the existing station structure and emergency evacuation exits show that the settlement does not exceed the limit value of 3mm in the construction process;the settlement of the existing station structure occurs mainly in the underneath crossing construction process,the settlement decreases gradually in the process of soil excavation,and the settlement increases gradually in the process of secondary lining construction.(3)According to the excavation sequence,excavation method and construction parameters of the tunnel with flat roof and straight wall,the numerical simulation is carried out.The existing station structure and the vault settlement of newly-built tunnel are selected as the evaluation indexes.Considering the construction security and period comprehensively,and the construction scheme is optimized: the settlement of the existing structure increases with the increase of the excavation footage,and the appropriate excavation footage is 2m;the settlement of the existing structure decreases with the increase of the working face distance and tends to be stable,and the appropriate working face distance is 6m;the settlement of the existing structure increases with the increase of the single demolition length of temporary support,and the appropriate single demolition length is 6m;the excavation sequence of the pilot tunnel is“first up and then down,first edge and then middle ”,which leads to the minimum settlement deformation of the existing structure;the settlement and deformation of the existing structure caused by the excavation of the underneath crossing section tunnel with CRD method is smallest.(4)According to the range and strength of grouting reinforcement,a verity of working conditions are set for numerical simulation.The existing station structure and vault settlement of newly-built tunnel are selected as evaluation indexes.Considering the construction security and cost comprehensively,it is considered that the settlement of the existing structure decreases with the increase of grouting reinforcement range and finally tends to be stable,and the appropriate grouting reinforcement range is 2m outside the tunnel contour;the settlement of the existing structure decreases with the increase of the grouting reinforcement strength and tends to be stable;in the aspect of controlling the settlement of the existing structure,when other conditions are certain,it is more effective to increase the grouting reinforcement range than to increase the grouting reinforcement strength.The above conclusions have certain reference significance for the design and scheme optimization of similar projects.