Experimental And Theoretical Study On Earth Pressures And Pore Pressures Of Underground Engineering Of Urban Rail Transit System In Coastal Area

With the acceleration of urbanization, peoples in coastal cities are busy with the constraction of urban rail transit system in order to solve the traffic congestion problems. The passive earth pressure for the soils between the retaining wall in long foundation pit of subway stations and the limit support pressure of the face stability subjected to water level fluctuation in the river-crossing shield tunnels are new problems accompanied with the constraction of urban rail transit system in coastal cities. Model tests, FEM analysis and analytical solution were used to solve the problems above. The main work and conclusions are as follows:1. A series of laboratory model tests with the different width of backfill was conducted for the passive case of a rigid retaining wall subjected to horizontal translation. The change in lateral earth pressure distribution from the at-rest condition to the passive condition was monitored by a set of pressure cells. The particle image velocimetry technique was employed to observe the development of a failure zone in the soils. The experiment results show that there is a good agreement between the measured earth pressures with the Coulomb’s solution in case of infinite soils. However, the passive earth pressures on the moving retaining wall for finite soils are much more than the Coulomb’s solution. As the decrease of soil width, the limited displacement of the wall under passive state seems to be increase, and the passive earth pressures also increase significantly accompanied with the heights of the application points of the resultant earth pressure moves down gradually. Moreover, with the decrease of soil width, the heave of the backfill surface arguments gradually, the inclination angles of the slip surface increase slightly, and the lateral earth pressures on the fixed boundary also increase gradually.2. FEM analysis models through COMSOL were presented to solve the pore pressure response problems affected by water level fluctuation during the excavation of shield tunnel. The FEM analysis results show that there is no phase delay in the process of transmission of water level fluctuation if the shied tunnel is excavated in a sand layer (the permeability coefficient is quite big), while, if the shied tunnel is excavated in a silt layer (the permeability coefficient is much smaller than a sand layer), the amplitude attenuation and phase delay in soil layer is significant. In the axis of the shield tunnel, the distribution of pressure head along the horizontal distance is consistent with negative exponential function. In the vertical plane of symmetry of the tunnel, the distribution of pressure head and total head along the depth is nonlinear if close to the face. Moreover, the influence on the horizontal seepage force caused by the thickness of cover layer, the thickness of under layer, the depth of water, the diameter of tunnel, the permeability coefficient, the amplitude and the cycle on the boundary is discussed.3. Based on the FEM calculation of seepage affected by water level fluctuation and classical "wedge-prism" limit equilibrium model, the formula of the limit support pressure on the tunnel face affected by water level fluctuation was obrained. And, the method proposed in this paper was applied in a case study of the Qingchun Road Tunnel in Hangzhou. The semi-analytical solution results show that the limit support pressure on the tunnel face in a silt layer fluctuates in sine fluctuation, and phase delay is also significant. The horizontal seepage force is the main part of the limit support pressure, and become the key facter of the face stability. Moreover, the influence on the limit support pressure caused by the effective cohesion, the effective angle of internal friction, the pressure head in the chamber, the thickness of cover layer and the diameter of tunnel is discussed.4. A simplified seepage analysis model affected by water level fluctuating during the excavation of shield tunnel was proposed, and then, a approximately analytical solution of pore pressure response ws deduced employing the Laplace transformation and series expansions. The comparisons between the approximately analytical solution and the semi-analytical solution have shown the approximately analytical solution is somewhat conservative.