Monitoring Of Horizontal Displacement And Optimization Of A Foundation Pit’s Supporting Structure In Beijing

Subway engineering is a typical linear engineering, so the foundation pit of subway station is long and narrow, and the ratio of length to width ratio is often close to 10, or even greater. Therefore, compared with foundation pits of other structures, the foundation pit of subway has its own characteristics in geometric shape and structure, supporting form and deformation law in excavation process. Taking foundation pit engineering of Dahongmen Station of Line 8 of Beijing Subway as the background, this paper summarizes the future construction demand and its unique shape and structure of the foundation pit of subway stations. And it also concludes the deformation and stress characteristics of underground supporting diaphragm wall through selecting reasonable monitoring data, and analyzing the monitoring data which are obtained by monitoring horizontal displacement of wall body on site. Meanwhile, based on analysis of monitoring data, this paper proposes an optimized scheme according to the existing supporting scheme; and finally establishes numerical models of two schemes and corresponding measurement systems for the quantities, to verify advantages of the optimized scheme in safety and project cost. Specific conclusions are as follows:1) Horizontal displacement variation of underground diaphragm wall body can be divided into three stages: Stage I, at initial stage of foundation pit excavation, the wall body displacement shows an overall toppling phenomenon; and the displacement direction has a variation law to incline towards outside of the foundation pit first and then towards inside of it. Stage II, when the excavation depth reaches 1/3, the stress displacement deformation shows a funnel shape, and the displacement direction is towards inside of the foundation pit. Stage III, when the excavation depth reaches 1/2 of excavation depth, the stress displacement deformation shows a parabolic shape; and the displacement direction is towards inside of the foundation pit.2) Upon the completion of the foundation pit excavation, the horizontal displacement curve of the wall body shows a parabolic shape, and all horizontal displacement curves of the wall body are smooth; and the wall body displacement has the deformation law to incline towards inside of the foundation pit first, and then towards outside of it and finally towards inside of it again. Therefore, it can be inferred that, all three of the wall body, earth mass and supporting system are inter-coordinated in the excavation process.3) In the original supporting scheme, cross-sectional horizontal displacement cloud chart for foundation pit excavation process is a funnel shape at the beginning, and turns into a shell shape after the excavation depth reaches 10 meters, which can coincide with the curves of three stages of development of the horizontal displacement monitoring curves of the wall body. In the optimized supporting scheme, cross-sectional displacement cloud chart in the foundation pit excavation process is a funnel shape at the beginning, and turns into a butterfly shape after the excavation depth reaches 10 meters, which can coincide with the curves of first two stages of the horizontal displacement monitoring curves of the wall body. Comparing the two, the optimized scheme has the characteristics that the impact on surrounding earth mass is slight, the maximum horizontal displacement generated is small, and the center of foundation pit deformation is on the surface in the excavation process; while the existing scheme has the characteristics that the impact on surrounding earth mass is great and maximum horizontal displacement occurred is large, and the center of foundation pit deformation is developing unceasingly towards the depth of the foundation pi in the excavation process. Therefore, it has been verified that the optimized scheme is more safety, and its stress characteristic is more reasonable.4) The two schemes are compared by establishing a measurement scheme to measure structural materials in terms of the volume and measure the quantities of the duration in terms of the work required for the excavation. The optimized scheme can save 19% of materials used for the structure, and 17% of consumables used in the construction(the construction duration).