Construction Technology Of Open Excavation Foundation Pit In Subway Protection Area

With the increasingly crowded ground space,the development and utilization of underground space is gradually popularized,especially in recent years,the development of underground engineering ushered in a climax.Underground works such as subway,comprehensive corridor,underground parking lot and underground commercial street are common in all parts of the country.It has brought great convenience to people's life and work.It has greatly helped the management of the government and relieved the pressure of the ground space.However,because of the rapid development of underground engineering,various problems have followed,such as the influence of Underground Space Excavation on the ground buildings,the great uncertainty of the hydrogeological conditions in the underground space,the problem of the urban environment caused by the underground engineering,and so on,among which the excavation of the foundation pit will cause the unloading of the soil.The soil uplift at the bottom of the foundation pit,the lateral displacement of the soil after the wall and the ground settlement outside the pit will affect and even change the stress and strain state of the subway tunnel under its underneath,which causes the vertical and horizontal displacement of the tunnel,and the cross section will also produce the convergent deformation.When the tunnel is serious,the safety hidden danger of the tunnel pipe fracturing and seaming seepage will appear.In addition,the soil disturbance caused by soil reinforcement,diaphragm wall or pile foundation construction and foundation pit dewatering will also have an impact on tunnel safety.Based on the single underground space project of Shenzhen listening to Sea Avenue(Guiwan),this paper studies the influence of different excavation methods on the adjacent subway tunnel.The following work is as follows:(1)read relevant literature,understand the current development situation at home and abroad,and sum up experience.(2)analyze the similarities and differences,advantages and disadvantages of two kinds of construction methods of open-excavation foundation pit(which is operating subway tunnel)by "triple-digging and one-by-one" sequential excavation and excavation of "small shaft anti-floating plate" foundation pit;(3)the feasibility analysis of the scheme of "Digging directly from three to one" and "Floating plate + uplift pile" has been carried out,and the construction parameters and schemes of "Floating plate + uplift pile" have been summarized by numerical simulation.(4)Through reasonable monitoring scheme under different construction schemes,the deformation of adjacent metro tunnels has been observed.And the deformation law of tunnel under foundation pit with the excavation of foundation pit has been summarized by analyzing the data.(5)By comparing the results of numerical simulation and monitoring data,the control effect of the structure system of "anti-floating slab + anti-uplift pile" on the floating capacity of existing metro has been clarified.And the influence factors of foundation pit excavation on the deformation of adjacent metro tunnels have been obtained.