Three-dimensional theoretical analysis of seepage field in front of shield tunnel face

To evaluate hydraulic head distribution in front of a shield tunnel in a saturated soil layer,theoretical analysis and numerical simulations are carried out in this study.Based on the partial differential equilibrium equation of seepage flow,a three-dimensional(3D)theoretical analytical model of the shield tunnel face and the seepage field in front of it is established using the eigenfunction and the Fourier series expansion methods,and the hydraulic head calculation formula is derived.Combined with engineering cases,the theoretical analysis results and the 3D numerical simulation results are compared and analyzed.The effect of the water pressure of the tunnel face on the hydraulic head distribution is also analyzed.The results of the proposed analytical solution are in agreement with those of the numerical simulation solutions;moreover,the proposed analytical solution requires less time to calculate the seepage hydraulic head than the numerical simulation.The ratio of the initial water table to the diameter(D)of tunnel face has a more significant impact on the hydraulic head distribution at a position 0.5D above the tunnel vault.When the water pressure on the tunnel face is not considered,the values of the hydraulic head are significantly underestimated.

Experimental investigation of face instability for tunnels in sandy cobble strata

In order to investigate the influence of face instability for tunnels with different burial depths in sandy cobble strata on earth pressure and the instability region,geomechanical model tests and numerical simulations were performed.The continuous excavation method was adopted to reduce the pressure of the soil bin and restore the real engineering situation.Earth pressure in three directions of the obser-vation section in front of the tunnel face was monitored during the tunneling of the shield.Evolutions of the lateral stress ratios at dif-ferent stages were also investigated.The instability area of the shield tunnel face in sandy cobble strata with different burial depth ratios during the instability stage was obtained based on the change ratio of earth pressure and compared with existing researches.The earth pressure began to change when the excavation was one shield diameter away from the observation section,and when the excavation reached the observation section,the earth pressure decreased significantly.The burial depth of shield tunnel in the sandy cobble strata has a significant impact on the evolution of soil arch and the size of the failure area.The numerical simulation of the continuum medium cannot reflect the stress redistribution characteristics of the granular body like sandy cobble strata,and the failure area or stress distur-bance area obtained by the model test is larger than the numerical simulation result.Existing methods have deviations in analyzing the failure area of shield tunnel face in sandy cobble strata.It provides not only guidance for shield tunnel excavation engineering in sandy cobble strata,but also a reference for the theoretical research on failure areas.

Influence of permeability anisotropy of seepage flow on the tunnel face stability

This paper focuses on the influence of permeability anisotropy of seepage flow on the face stability for a shied tunnel.An analytical model has been proposed to present the hydraulic head distribution around the tunnel face in the anisotropic ground,considering the difference of permeability coefficient in the horizontal direction and the vertical direction.The rationality of the proposed model is ver-ified by a series of numerical simulations.Then,an analytical model of face stability for a tunnel under the anisotropic seepage has been established based on the limit analysis upper bound method.Comparisons of the analytical solutions and the numerical simulations are conducted,and the limit support pressure of the two methods is consistent.The effect of permeability anisotropy and water pressure on the stability of the tunnel face is analyzed through the three-dimensional analytical solution.Anisotropy of permeability has a significant impact on the stability of the tunnel face,and its impact gradually decreases.It can also be found that the water pressure coefficient of the tunnel face has a significant effect on the limit support pressure and the failure area when the ratio of the horizontal permeability to the vertical permeability is large.