The Research On Land Subsidence Caused By Engineering Dewatering

With its rapid urban development, Beijing has increased the investment in subway construction. Underground excavation in the subway construction requires that dewatering measures be taken to ensure that the earthwork is conducted in dry conditions. Dewatering inevitably results in land subsidence. And land subsidence endangers social development, urban construction, environment protection and public life. Therefore, in-depth research needs to be done in this field. Land subsidence results from changes in the pressure borne by pore water in the soil and the effective stress borne by soil skeleton. When underground water is drawn out, soil pore water pressure diminishes, the original soil-water balance is broken, the effective stress changes, and the soil becomes deformed. Due to the non-linearity, non-elasticity and porosity of soil, the mechanics, water reserving capacity and permeability of the soil change consequently. Land subsidence, as the exterior manifestation of the earth-water interaction and the interior stress change, is closely related to soil deformability and water seepage. In this paper land subsidence is introduced and mathematical models simulating underground water withdrawal in the past decades are analyzed. The mechanism of land subsidence and the major factors affecting land subsidence are elaborated. The microstructure of soil is analyzed from the perspectives of soil pores, soil pore water, interaction between soil grains and water and interaction between soil grains to give a microcosmic explanation of seepage consolidation and to probe into the rules of seepage consolidation. The study is based on the Beijing Line 5 Subway dewatering project, with Yutingqiao area as the focus. A mathematical model of underground water is built. The software of Feflow is used to estimate the dewatering depths. The model of land subsidence caused by dewatering is established to calculate the ground settlement volume in the research area. With the results being close to the actual situation, this approach is found to be useful in the assessment of land subsidence caused by engineering dewatering and in the elicitation of relations between dewatering depths and land subsidence. The dominating factor in seepage consolidation directly appears to be the involvement of confined water in the seepage. And seepage consolidation ultimately depends on the clay content and chemical ingredients of soil, the micro-pore and microscopic properties of soil, and the interaction mechanism between clay and water. In the process of consolidation, both free water and confined water are involved in pore water seepage. From the viewpoint of micro-mechanism, seepage consolidation depends on which ionic layer dominates between grains and in the center of pores with changes in pore water volume. The impact of confined water's involvement in the seepage should be considered theoretically to correspond with practices of drain consolidation. And the rules of change of permeation coefficient, compression coefficient and consolidation coefficient should be found out to modify the expression of effective stress.