CIM Bulletin, Vol. 4, No. 6, 2009
M. Sharifzadeh and A. Iranzadeh
With the significant advances in capacities of thrust and torque, as well as the development of large-diameter rolling cutters, tunnel boring machines (TBMs) are now extensively utilized in tunnelling. The prediction of TBM performance in different rock masses was considered an important part of project schedulling and the selection of tunnelling methods. In the past, various models have been proposed based on field observations and laboratory tests. These models are generally divided into two categories: empirical and theoretical-empirical.
The performance of TBMs depends on the efficiency of the cutters, which can be influenced by geological conditions and rock mass properties. In this study, a three-dimensional numerical model, based on the discrete element method, was used to evaluate the effects of a TBM cutter on joint orientation and spacing of the fragmentation process. Several models were constructed to study the effects of variable orientation. These models included different spacings, variable spacings, different orientations of the fragmentation process and different sections of the block model.
In accordance with previous studies and pressure core theory, the fragmentation and indentation process is complicated. Tensile cracks are the main cause of crack initiation and chip formation. To evaluate crack initiation and propagation in rock by a TBM disc cutter, three parameters (induced tensile stress, induced shear stress and maximum displacement) were calculated. Based on pressure core theory, induced tensile stress played an important role in forming tensile cracks. Tensile stress values were recorded in periodic steps, in the plane section that the cutter force enters and in parallel planes at distances of +0.15, -0.15, +0.3 and -0.3 metres. Using the innovate algorithm, the stress level with the probability of creating chips was established as critical stress in each section and the values of these critical stresses were compared in different joint orientations, spacings and sections to determine critical joint orientation and spacing.
Results show that initiation and propagation of cracks is generally located along the tensile break zone. Two modes of crack initiation and propagation are possible. In the first mode, cracks initiate from the joint surface and develop up to the free surface; in the second mode, cracks initiate from the broken section and develop up to the cracked surface.
Naturally, some differences between the results of the numerical model and the observations were witnessed. The main reason for these differences is that, with numerical modelling, the filling and aperture of joints are not taken into account, nor is the continuous boring process considered. According to the provided results, the following conclusions can be drawn:
The critical angle of joint orientation is in the range of 45 to 60º, where the possibilities of the highest penetration rate can be reached.
The critical spacing of joints, which has important effects on the value of critical stress, is 200 mm.
Joint orientation generally has a greater effect than does spacing, as joint orientation affects the induced stress field.