ABSTRACT

The influence of joint spacing and orientation on penetration rate of disc cutter and chip formation has simultaneously been studied using the discrete element method (DEM). To do this, three ranges of joint spacing together with seven values of joint orientation were included in the model and hence 21 models were conducted. The analysis results of this study showed that with increase of joint spacing in given joint orientation, the TBM penetration decreases. Increasing penetration trend for orientation 45 to 75° had a higher gradient value than that for 0 to 30°. The analysis results showed that the optimum joint orientation from the viewpoint of TBM penetration is about 60 to 75°.

1 INTRODUCTION

Rock mass parameters including weak surfaces are the most important parameters which should be taken into account while analyzing the rock penetration by disc cutters. Up to now, many experimental, theoretical and numerical simulation based researches have been performed regarding the interaction of TBM disc cutter performance and these two joint specifications. However, in most of these works, the influence of joint spacing and orientation on penetration rate of disc cutter and chip formation have individually been studied. Among them the researches by Sanio (1985) on studying the effect of rock mass anisotropy including bedding and shistosity on cutter load, Howarth (1981) on studying experimentally the impact of joints spacing on TBM performance, Bruland (1998) on introducing a fracture factor in NTNU TBM performance prediction model and Gong et al (2005, 2006) on investigating joint orientation and spacing effects on rock fragmentation by TBM cutters using numerical modeling.

In this study, the simultaneous effects of joint spacing and orientation on rock indentation and fragmentation process by TBM disc cutters were investigated by using the discrete element method (DEM). To do this, three intervals of joint spacing together with seven values of joint orientation (i.e. totally 21 models) were considered in the study. The required input data for simulation were obtained from the sandstone along the Alborz service tunnel which is currently excavated by a 5.2 m diameter Open TBM in Iran.

2 CONFIGURATION OF THE NUMERICAL MODEL

Universal distinct element code (UDEC) is used for simulating the rock fragmentation mechanism by disc cutters. The key concepts of the DEM are that the domain of interest is treated as an assemblage of rigid or deformable blocks/particles and that the contacts among them need to be identified and continuously updated during the entire deformation/motion process, and be represented by appropriate constitutive models (Itasca, 1996).

Three ranges of joint spacing with representative values of 150, 200 and 300 mm together with seven joint orientations were considered for modeling. The dip direction of the joint set is assumed to be in the same direction as the cutting load, and the joint dip angle (α in Figure 1) varies from 0, 15, 30, 45, 60 and 75 to 90 (all in degrees). The model configuration is schematically shown in Figure 1.

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