In this chapter, the three-dimensional finite difference method was used to investigate the tunnel face deformation behavior both with and without slurry pressure and to estimate the relationship between maximum tunnel face deformation and various ground temperature (T), geotechnical and geometrical parameters, including cohesion
(c) and friction angle (φ) of frozen soil, tunnel diameter (D) and cover depth (C). Then, four additional groups of additional simulations were conducted with the effective face support pressure of 0.5, 1.0, 1.5 and 2.0 times of horizontal stress at tunnel centerline.
Based on the numerical results, a new calculation model was presented, through which the minimum allowable slurry pressure can be figured out. Meanwhile, in order to consider the thawing effect caused by concrete pipe and injected lubricant, the comparative analysis was carried out between with and without thawing effect. Finally, the slurry pressure calculated from the numerical results was compared with the classical theoretical solutions, which proves the validity of the prediction of slurry pressure by using the proposed calculation model. The following points are concluded upon above results:
(1) The maximum face deformation decreases with the reduction in ground temperature. In cold region, the tunnel face becomes more stable than that in the unfrozen ground, because freezing temperature immensely increases the bonding strength between soil particles and mechanical property of frozen ground. In addition, the face deformation profiles are nearly independent of friction angle and cohesion of frozen soil owing to the dominant ascendancy of deformation modulus of frozen ground for face deformation.
(2) The deformation mechanism of tunneling face is different with that of unfrozen ground and hardly no blow-out damage happens as conducting pipe jacking within frozen ground. What is more, the gradient temperature distribution causes the strength gradient of frozen ground along vertical direction from low to high temperature, which may cause uneven cutting of tunnel face and results in jacking misalignment or even get stuck of drivage machine.
(3) The geometric parameters, tunnel diameter and cover depth, have obvious impact on the deformation of tunnel face. When the geotechnical parameters stay constant, the tunnel face deformation increases with increasing in tunnel diameter or cover depth.
(4) A new calculation model is presented to estimate the minimum allowable slurry pressure applied on the tunnel face based on the numerical results, which can be a well reference value in the design and construction of pipe jacking tunnel in cold regions. The results show that the larger slurry pressure is needed with the condition of relative higher temperature (-5℃), larger tunnel diameter and cover depth, lower cohesion and friction angle of frozen ground.
(5) The thawing effect induced by the excavation and injected lubricant causes larger face deformation and results in larger minimum allowable slurry pressure as well.
(6) The Leca and Dormieux (L&D) lower bound solution is found to have relative approximate evaluation of the slurry pressure with numerical prediction. That is to say the L&D lower bound solution is more suitable to predict the slurry pressure when application of pipe jacking in frozen ground as a reference value during design stage.
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