Geotechnical aspects of mechanized tunneling in rock

In the construction of large tunnels in rock where the length of the construction period dominates the overall price of a tunnel, the tunnel boring machines (TBM) provide an attractive alternative to drilling and blasting with the added advantage of reducing disturbance to the rock around the excavation.;Among the TBMs, the double-shield tunnel boring machines are modern tunneling systems which utilize the action of grippers to achieve very high performance rates. However, in addition to the stress changes caused by the ground excavation, the TBM grippers bring along another significant stress change with unfavorable stress concentration particularly along the edges of the grippers. In weak rock, the overstressing caused by grippers can result in loosening part of the tunnel wall which may lead to an increase in permeability, potential of overbreak, and consequently to an increase in lining load. In the extreme case, the grippers can cause a bearing capacity failure of the tunnel wall signifying inapplicability of double-shield TBM to these ground conditions.;In the current practice, the design of TBM grippers does not address the relevant geotechnical concerns. As a result, the TBMs are designed with a number of features which are not fully compatible with the ground, thus requiring changes and modifications to be carried out on the system during the early stages of the drive which causes delays in construction and increases the cost.;The thesis deals with a geotechnical evaluation of the high performance tunneling systems utilizing double-shield TBMs operating in rock. The objective of the thesis is to obtain better understanding of the mechanics of the ground response to the double-shield TBMs. The research involves analyzing the stress changes and possible associated overstressing resulting from both the tunnel excavation and gripper action. A three-dimensional finite element stress analysis is carried out to investigate the shape and extent of plastic zones induced by the excavation, and grippers in discontinuous rock mass to define optimal position of grippers on the TBM.;In order to assess the applicability of a double-shield TBM, a rock mass bearing capacity formula was derived as a combination of the empirical Hoek-Brown failure criterion and a lower bound solution. Using the bearing capacity formula and considering the required TBM driving forces, a series of calculations resulted in the development of a number of design charts which define the allowable gripper pressures for various rock types. The analysis is superimposed on the distribution of radial stresses and the extent of plastic zones induced by the excavation.