Analysis of the Interaction between Radial Passive and Active Grouted Rockbolts with Rock Mass in Tunneling Design Based on Convergence-Confinement Method

Grouted rock bolts are one of the most popular systems of support in underground mining and tunneling operations. These can be installed as passive or active pretensioned elements. Passive grouted rock bolts develop load as the rock mass deforms. However, the active types are preloaded before rock mass deformation. In order to improve bolting design, it is necessary to have a good understanding of interaction and behavior of rock bolts in deformed rock mass. This can be acquired through field monitoring, laboratory tests, numerical modeling and analytical studies. Application of the rock-support interaction concepts to the support system such as concrete linings, steel sets, and ungrouted rock bolts, can not be applied to the reinforcing elements such as grouted rock bolts or cables. This is because, they don’t act independently of rock mass and hence the deformations, which occur in both the rock mass and support system, cannot be separated. In other words, the reinforcing elements confine rock mass deformation and tunnel convergence via improving or conserving the overall it's properties from within the rock mass. Consequently, the use of rock mass reinforcing techniques such as fully-grouted bolts or cable has encountered many theoretical problems. In this paper after reviewing of the past studies conducted by Smart and Bischoff (1975), Grasso (1989), Indraratna and Kaiser (1990), Peila and Oreste (1995), Still (1989) in the field of grouted rock bolts, with assuming that grouted rockbolts increase internal pressure within a broken rock mass, and with consideration of Hoek-Brown strength criterion for rock mass and on the basis of strain softening behavior model, rock-support interaction concept and convergence confinement approach, analytical models are presented to simulate the interaction between radial passive and active grouted rockbolts with rock mass in tunneling design. In these methods, the equations of ground response curve which has been reinforced with passive and active grouted rock bolts are also derived for a circular underground excavation under hydrostatic stress field. The proposed models allow one to take into account the following specific conditions: the effect of the increasing spacing between bolts, the effect of the increasing pretension load in response curve. At the end, some examples solved with new analytical approaches and the results were found to be close agreement with the FLAC results in a quantitative manner.The example results show that decreasing spacing between bolts is more effective than preloading of them in confining tunnel convergence to allowable value. It is also seen that, the preloading will be more effective if the spacing between bolts decrease. It should be noted that, due to algebraic complexity, it is not possible to obtain closed-form solution; therefore a computer program was prepared for solving the problem on the basis of numerical methods.