Experimental Study On The Water-earth Pressure Of Deep & Large Shaft Construction In Strong Permeable Strata

Construction-induced water and earth pressure of the shaft and foundation pit is an eternal problem in geotechnical field. Nowadays the classical theories are widely used because of their simplicity and convenience. However, there is a big difference between calculation pressure and in-situ value. Especially for the pervious strata affected by groundwater seepage. Based on the previous research of the water-earth pressure on foundation pit and data of deep and large shaft of Lanzhou Subway Station locating aside Huanghe River, this paper present an analyses of distribution of water and earth pressure on shaft construction in cobble strata. The presented analyses are mainly based on literature research, theoretical analysis, indoor model test and numerical simulation. The main contributions of this thesis are as follows:(1) Design the indoor model test. First, operating permeability test, direct shear test and other laboratory soil tests to prepare the model soil and obtain its physical and mechanical parameters. Then, producing a model diaphragm wall in gypsum, diatomaceous earths and water based on the principle of stiffness equivalence. Finally, producing a piezometric tube with acrylic tube based on the principle of communicating vessels, then measuring the water pressure behind the diaphragm wall.(2) Experimental and numerical results show that when the shaft in Cobble Stratum excavated with the support forms of diaphragm wall and lateral bracing, the active soil pressure outside the wall is less than Rankin’s active soil pressure. Therefore, the soil pressure should be reduced appropriately in design and calculation work. The distribution of soil pressure outside the wall is roughly "R" type after the completion of the vertical shaft excavation. While in water-rich formations, the distribution of soil pressure outside the wall is roughly linear. According to the experimental results, the distribution of soil stress is more related to the displacement of the wall in anhydrous formations.(3) Hydrostatic pressure can be almost fully transferred in pebble and water-rich formation without considering reduction of water pressure. Compared with static conditions, total pressure outside the wall decreases while active soil pressure increases and water pressure declines in Cobble Stratum, which worth attention during engineering design.(4) In Cobble Stratum under different intensity of seepage, the distribution of water pressure is studied after the completion of the shaft in model test. Comparing with several common theoretical calculation methods, test results are different from the empirical algorithms which indicates that the empirical method may not applicable. Bent Hanson method is available while the scope is very limited. Lian-sheng Tang schemata theory is reasonable in theory, but still holds large deviation between calculation results and measured values. Therefore, the available theories need to be further revised.