The determinations of the peak strength of rocks have been comprehensively investigated and applied to the simulation of tunneling. However, only few achievements have been made to estimate the post-peak strength of rocks, which is becoming more and more important due to the rapidly increase of tunneling depth. Understanding both the peak and the post-peak strengths of brittle rock are necessary for the design of deep tunneling. After reviews of various estimation methods, the post-peak form of the Hoek-Brown failure criterion by introducing a strength loss parameter (ß) was adopted in the paper. The relationship between the strength loss parameter (ß), the post-peak strength and the confining stress were established on the basis of the triaxial compressive tests of marble. A numerical simulation of the tri-axial compressive test was conducted by the axisymmetric modeling in FLAC to verify the accuracy of the method suggested. The post-peak strength estimation method, so-called strength loss experiment method, may estimate the post-peak strength reasonably well.
By excavating an underground space, the state of stress and displacement in the surrounding medium are changed in comparison to the inital state. As time passes, the variation of the displacement mainly depends on the creep behaviour of the hosting rock mass. In this paper, an elasto-viscoplastic creep model is proposed. In the proposed model, the main purpose is to consider plastic deformations increasing with time. The viscoplastic behaviour of rocks plays a key role in the tunnelling works, especially for deep tunnels subjected to large in situ stresses. Using non-linear Hoek-Brown yield criterion in a creep model is the other important aim of this paper, which eliminates estimating specific equivalent Mohr-Coulomb strength parameters from the Hoek-Brown parameters. The equations related to the proposed model are derived and then, to reach a numerical solution of the equations, the finite difference software (FLAC2D-FISH Editor) is used. The application of the proposed model is illustrated through an example analyzed numerically using the finite difference software FLAC2D.
Since the establishment of the International Society for Rock Mechanics (ISRM) in the 1960s, there have been important scientific developments and technological advances both in rock mechanics and rock engineering. Particularly, modeling of rock behaviour, design methodologies for rock structures and rock testing methods are the main issues in these developments and advances. The models developed depend considerably on the input parameters such as boundary conditions and material and rock mass properties. For this reason, establishing how to obtain these input parameters for a particular site, rock mass and project is important. Accordingly, since 1974, the ISRM Commission on Testing Methods has spent considerable effort in developing a succession of Suggested Methods (SMs) for different aspects of rock mechanics with the contribution of a number of working groups. The SMs are intended as guidance, explaining the recommended procedures to follow in the works associated with the various aspects of rock mechanics, such as rock characterisation, testing and monitoring. In this paper; the past, present and future of laboratory and in-situ rock testing and monitoring techniques and then the general principles followed in developing the ISRM SMs, stages in their evaluation and the recent developments related to the SMs are briefly given.
Leelasukseree, Cheowchan (Chiang Mai University) | Pipatpongsa, Thirapong (Tokyo Institute of Technology) | Khosravi, Mohammad Hossein (Tokyo Institute of Technology) | Mavong, Narongsak (Tokyo Institute of Technology)
ABSTRACT The object of this study is to investigate the unloading failure mechanism of hard rocks in the unloading process. A commercial finite element program LS-DYNA was employed to simulate the rock unloading process. The implicit and explicit methods were performed in sequence to simulate the static initialization-dynamic unloading process of rocks. The numerical results indicated that the rock failure can be induced by releasing of the initial stress, and the previous result of the equivalent initial stress release rate (EISRR) theory based on the 1D stress state is not suitable for 3D stress state. In 3D stress state, a new definition of equivalent strain energy release rate (ESERR) was introduced. The further study indicated that the ESERR can characterize the effect of different confining stresses and different unloading path on rock unloading. A significant finding is that the ESERR can quantitatively describe the characteristics of the unloading process under 3D stress state. This finding indicated that in practical underground excavation engineering, dynamically controlling the ESERR can be used to increase excavation potential of rocks and minimize the needed external excavation energy by using the initial energy.’