ABSTRACT:

The relevant tunneling methods of various excavation sequences and support systems are reviewed in detail. In addition, a numerical model was established to evaluate the performances of these methods. In the numerical analysis, the strain softening constitutive law of squeezing rocks was considered to simulate the behavior of the media surrounding a tunnel. The factors of three-dimensional effects near the excavation face, excavation sequence and support installation time during tunneling were also taken into account. The support systems analyzed include not only traditional bolts, steel ribs and shotcrete, but also some newly developed support methods such as yielding bolts, bearing type bolts, adjustable ribs and slot shotcrete. The results of the numerical analysis indicate that step excavation, among which the inner pilot tunnel method might be a better way for tunnel excavation in squeezing ground, can minimize the plastic deformation of the surrounding rock. The support stress can be reduced by the passive support approaches and the performances of various support systems are also presented. Finally, based on the results of this study, some effective strategies for the excavation and support of tunneling in squeezing ground are recommended.

INTRODUCTION

Squeezing has become one of the major obstacles for tunnel construction with high overburden or under adverse geological conditions. Large tunnel deformation in squeezing ground may cause the support failure, the insufficient tunnel clearance, or the raveling or collapse of the tunnel. In the past, many approaches including modifications of tunnel cross section and excavation sequence, and some newly developed support methods, have been applied to minimize the deformation and thus stabilize the tunnel. However, these approaches lack systematic analysis and evaluation on their effect against squeezing ground. The performances of these tunneling methods still need to be investigated in detail.

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