ABSTRACT

A series of two dimension numerical modeling were performed using the discrete element method (OEM) to explore the effect of joint orientation on rock fragmentation process by a tunnel boring machine (TBM) rolling cutter. Numerical results show that the joint orientation can significantly influence the crack initiation and propagation, as well as the fragmentation pattern, and hence affect the penetration rate of the TBM. Two crack initiation and propagation modes are found to break the rock mass due to the variation of joint orientation. One is that the cracks initiate form the joint plane and propagate upwards to the free surface. The other is that the cracks initiate form the crushed zone and propagate downwards to the joint plane. The results also show that the stress field is deflected in different extent due to the occurrence of joint and the variation of the joint orientation. It makes the chipping angle increase with the increase of the angle between the tunnel axis and joint plane.

INTRODUCTION

The tunnel boring technology has been improved over the years. Hence, TBM is extensively utilized in underground construction. The effects of the joint orientation on the TBM penetration rate were observed by many researchers. In situ measurements by Aeberli and Wanner (1978) in a homogeneous zone of schistose phyllite showed that the advance rate of the TBM increased with the increase of the angle between the tunnel axis and the planes of schistosity. Similar phenomena were also observed by Thuro and Plinninger (2003) in phyllite and phyllite-carbonate-schist inter-stratification. Bruland (1998) summarized the in situ measurement results over 100 km tunnel by TBM tunneling. In term of the classes of joints, the effects of joint orientation were respectively obtained. The principal rule is the same. But with the increase of joint spacing, the effect of joint orientation on TBM penetration shows a little bit difference. A theoretical analysis of the interaction between disc cutter and rock by Sanio (J 985) showed a similar trend. Although the above mentioned phenomenon was noticed, little research Work such as numerical investigation has been undertaken to explain the mechanism of rock fragmentation by TBM cutters at different joint orientation. This may be probably owing to the theoretical difficulty In simulating crack initiation and propagation.

During the past few years, finite element method (FEM) has been used to simulate the rock material fragmentation using an indenter. Cook et al. (1984) employed a linear axisymmetric elastic finite element model to numerically investigate the fracture process in a strong, brittle rock by a circular, flat-bottomed punch. The results showed a pretty good agreement with the laboratory experiment. Chiaia (2001) used a lattice model implemented in the FEM program to simulate the penetration process in heterogeneous material by a hard cutting indenter. He found that the indentation process characterizes various interaction mechanisms, amongst them the dominant modes would be plastic crushing and brittle chipping.

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