Studies On The Effects Of Rock Bolting And Rock Parameters On The Stress Distribution Around Tunnel

This thesis presents studies on the effects of rock bolting and rock parameters on the stress distribution around tunnel by using an elasto-plastic model. The main objectives of this research are: (1) To simulate stress distribution around tunnels and interaction between tunnels at different overburden depths and to compare the results with that of elastic model; (2) To study the effects of material parameters on the stress distribution around tunnels and interaction between tunnels at different overburden depths; (3) To study the effects of multiple rock bolts on the stress distribution around tunnel, including the position and orientation of bolts, overburden depths, the lengths of bolts etc. In (1) four models of one circular tunnel 2m in diameter with different overburden depths of 150m, 200m, 300m and 350m has been used; As in (2) six models that have two circular tunnels 2m in diameter with different overburden depths of 150m, 200m, 300m and 350m and the different distance of separation between two ones that are 2D, 3D, 4D, 5D, 6D and 7D are used. In (3) the effects of the positions and orientations of multiple bolts of 2m and 1m lengths in the roof and lateral sides of tunnel, different overburden depths of 350m, 300m, 200m, 150m respectively and spacing between bolts on the stress distribution around tunnel are studied. The finite element method for elasto-plastic problems has been utilized in this research. The stress distributions around tunnels at different depths of overburden rock are calculated based on the rock behaviour as non-linear materials. The numerical analyses results shown that stress distribution and stress concentration around the tunnels vary with the overburden depths. It is found that the coefficients of stress concentration for elasto-plastic medium are lower than those for elastic one. Furthermore, the interaction between the two tunnels decreases quickly with the increase in separation distance between them. For quantitatively describing the interaction between two tunnels, we introduce a critical separation distance that is defined as the distance between two tunnels when the stress concentration values in both of the two tunnels are equal to that for one tunnel case at the same depth. Through comparing according to the above given definition of critical distance, the critical distances between two tunnels at different overburden depths are given. By considering the variations of the cohesion and the angle of internal friction for rock, the parameter studies on the stress distribution around one and two tunnels were carried out. For the constant value of friction angle, the values of cohesion are selected as respectively. While for the constant value of cohesion c = 0.75MPa, the selected values of friction angles ? are 180, 220, 260, 300, 340, 360, 420 ,460 , 500 and 540 respectively. The two sets of parameters has been used for modeling of one and two tunnels at different overburden depths of 150m, 200m, 300m and 350m with different distances of separation of 2D, 3D, 4D, 5D, 6D and 7D. The numerical analyses results show that stress concentrations around one and two tunnels increases as cohesion and angle of internal friction increase. The laws of variations of maximum stresses on the boundary of tunnels with increasing values of c and ? are given respectively, which are quite different. Finally, the effects of rock bolting on the stress distribution around tunnel, including position and orientations of bolt, overburden depths, the lengths of bolts etc., are simulated by using an elasto-plastic model. A very important finding is obtained, that is the tensile stress region that is very dangerous for rock in the bottom of tunnel quickly grows with the increasing overburden depths when rock bolts are installed only in the roof or lateral sides of tunnel, which indicates that the whole stress distribution around tunnels with rock bolts should be considered in addition to the regions of stress concentration. In addition, the rock bolting around tunnel can obviously reduce the coefficients and the region size of stress concentration, especially the rock bolts are installed in high stress areas. These above results of research are very important and essential for the design of tunnels to ensure engineering safety and economy in tunnel engineering.