Investigations On Seepage Field And Face Stability Of Shield Tunnel Subjected To Water Level Fluctuation

In the coastal areas of China,tunnel construction may often encounter the situation in which the tunnels need to cross river and sea.Subriver tunnels can effectively alleviate urban and intercity traffic congestion,improving the travel efficiency.Subsea tunnels are one of the important cross-sea transport infrastructures,which can greatly improve the traffic and develop the social economy in coastal areas.Shield tunneling is the main method for underwater tunnel construction in soft soil or soft rock.During the construction of underwater shield tunnels,the dynamic water conditions acting on the tunnel face can extremely complicate the distribution of stress and pore water pressure in the surrounding soil,which may easily lead to water flush accidents and instability of the tunnel face.Combined with the Qingchun Road Tunnel in Hangzhou,the thesis studies the stability of shield tunnels under fluctuating water level,through the methods of seepage field numerical calculation,semi-analytical analysis with limit equilibrium model,semi-analytical analysis with upper bound model and fluid-solid coupling numerical calculation.The main work and conclusions are as follows:(1)By using the finite element software COMSOL Multyphysics,the seepage field near the shield tunnel face under fluctuating water level is calculated and analyzed.The influence of soil parameters on the transient seepage field ahead of the tunnel face is studied.The shortcomings of the existing limit equilibrium analysis are improved,and a new method of calculating the seepage force is proposed.Numerical results show that the fluctuation and propagation of pore water pressure are determined by the ratio of the hydraulic conductivity to the volumetric specific storage of soil.With the decrease of the permeability of soil and the increase of the volumetric specific storage,the amplitude and maximum value of horizontal seepage force in the wedge(Fx)gradually decrease,and the phase lag to the boundary water level fluctuation becomes more significant.When calculating the support pressure,the influence of the phase difference between the seepage forces in different directions and different blocks of the model should be taken into account.(2)Based on the numerically calculated two-dimensional seepage field,the upper bound analysis of the stability of shield tunnel face subjected to water level fluctuation is carried out.The difference between two-dimensional and three-dimensional seepage field calculation is studied.The upper bound solution of the support pressure on the tunnel face is solved,and compared with the existing limit equilibrium solution.Results show that the two-dimensional seepage field calculation is consistent with the three-dimensional one in the propagation law of pore pressure fluctuation,but the absolute value of pore pressure response and the pore pressure distribution are different at each moment.The two-dimensional calculation underestimates the horizontal seepage ahead of the tunnel face and overestimates the vertical seepage in shallow soil.The influence of transient seepage on the limit support pressure in two-dimensional upper bound model is similar to that in three-dimensional limit equilibrium model.(3)In the finite element software COMSOL Multyphysics,the seepage field ahead of the tunnel face subjected to water level fluctuation is numerically calculated by using the fluid-solid coupling seepage control equation.The results are compared with those obtained by the uncoupling method,and the differences between the two methods in hydraulic elements,propagation law and seepage force calculation are studied.Results show that the hydraulic head in soil follows the same rule in the process of wave propagation in both methods.For the horizontal seepage force in the wedge,the fluctuation amplitude is larger in the fluid-solid coupling calculation,and the phase lag to the boundary fluctuation water level is larger in the uncoupling calculation.For the vertical seepage force in the wedge and the prism,the fluctuation amplitude is larger in the uncoupling calculation,and there is no notable phase difference between the coupling and uncoupling results.The Biot coefficient a has limited influence on the horizontal seepage force in the wedge(Fx)calculated by fluid-solid coupling method.