Theoretical Analysis Of The Arching Effect For Large Tunnels With Shallow Overburden Depth And The Resulted Rock Load

Large tunnels with shallow overburden depth have been experiencing rapid growth in recent years.While the previous tunneling projects provide a wealth of empirical experience,the theoretical principles and mechanical behaviors behind the supporting effect of the ground-arch on such tunnels are still limited,and in need of future practices.As researchers and engineers have well recognized the support effect of the ground-arch on shallow tunnels since its first presence,some implicit assumptions behind these arching theories are unappropriated and oversimple in practical use.Also,the arching effect of large underground constructions with shallow overburden depth is more complex and requires a detailed theoretical study.Considering that theoretical vacuum,the author carefully studied the supporting effect of the ground-arch on large tunnels with shallow overburden depth through laboratory tests,in-situ measurements,theoretical derivations,and numerical calculations.Research studies in this thesis cover four main topics which are: the principal-stress-rotation based arching effect of shallow tunnels,the progressive arching effect with its resulted ground pressure on tunnels,the combinations of the ground-arch in three-dimensional space due to sequential excavation and the support of tunnel face,and discussions on appropriate approaches for estimating the ground load for large tunnels with shallow depth.Major innovations include:(1)A modified model was proposed to improve the original Terzaghi’s model based on the major principal stress rotation resulted from tunnel excavations.(2)According to the rotation of the shear surface and the correlation between the direction of the shear surface and that of the major principle stress,a new model was established to study the progressive development of the ground arching effect.(3)Considering the different ground disturbance caused by sequential excavations on the plane and in the space,a new model was proposed to study the ground-arch combinations.The specific work and the main results are concluded as follows:(1)The stress redistributions after the excavation of shallow tunnels are studied.The shear forces which prevent rock masses from moving into the tunnel periphery concentrate on the margin of the ground-arch.The trace of the major principal stress holds an upward arch shape,which forms a so-called tangential arch above the tunnels.The angle between the direction of the major principal stress and the horizontal line is zero on the tunnel axis and is p/ 4(10)j/ 2 on the shear surface.On the horizontal line,from the tunnel axis to the shear surface,the rotation angle varies linearly with the distance to the tunnel axis.(2)Based on the rotations of the principal stress,the original Terzaghi’s arch theory was modified,and the resulted loosening pressure loaded on shallow tunnels was calculated.The application background of Terzaghi’s arching theory and its extensions,including the modified approach,was analyzed through finite element limit analysis.Results indicated that the modified method is more suitable for calculating the loosening pressure on shallow tunnels.Terzaghi’s arching theory and its extensions provide confidential results of the loosening pressure only for tunnels under the limit state.Under such a state,the shear band extends from the bottom of the tunnel to the roof and then directly to the ground surface,and the mobilize friction angle on the shear band is the friction angle of the rock masses.(3)Through connections between the rotations of the principal stress and the shear face,the author established a new mechanical model that provides a continuous solution for the progressively developed ground-arch from the initial state to the final one.Results shew that accompanied by the excavation induced disturbance,the development of the ground arch mainly includes four stages,which are the elastic stage,the initial stage,the developed stage,and the final stage.The corresponding loosening pressure experiences the decreasing phase,the minimum load phase,the increasing phase,and the final stabilizing phase.The previously modified static ground-arch could calculate the final loosening pressure on the tunnel.(4)For large tunnels with shallow overburden depth,effects of the sequential excavation and the sub-tunnel face on the ground-arch combinations were analyzed,and a solution taking into account such impacts were proposed.The combinations occur both in the two-dimensional plane and the three-dimensional space.In the two-dimensional plane,the size of sub tunnels,the additional support from the mid-bench and the rock mass strength are of critical importance.In the three dimensional space,the lagged distance among sub-tunnel faces dominates.(5)Based on an ongoing project named the Xiabeishan tunnel and through the theoretical results,the numerical back analyses and the in-situ measurements,applicabilities of various ground-arch theories,roc-mass-classification based empirical approaches and the methods suggested by the standard on calculating the loosening pressure for large span tunnels with shallow depth are discussed.Results indicated that the modified approach provides similar results like that from in-situ measurements,while Terzaghi’s original arch theory and the Protodyakonov’s Pressure Arch Theory overestimate the pressure.Among Terzaghi’s rock load system,the rock mass rating system(RMR)and the rock quality system(Q),the last provides the most confidential results while the others give overestimated results.Methods suggested by the standards overestimate the loosening pressure too.