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Keywords: intermediate principal stress

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Proceedings Papers

Paper presented at the ISRM International Symposium, August 30–September 2, 1989

Paper Number: ISRM-IS-1989-003

... ABSTRACT: To clarify the effect of the

**intermediate****principal****stress**on the strength and deformational behavior of rocks, three sandstones, marble, and shale were deformed under true triaxial stress state in which the**intermediate****principal****stress**is not equal to the minimum nor to the maximum...
Abstract

ABSTRACT: To clarify the effect of the intermediate principal stress on the strength and deformational behavior of rocks, three sandstones, marble, and shale were deformed under true triaxial stress state in which the intermediate principal stress is not equal to the minimum nor to the maximum principal stress. Failure strength increased with the relative increase of the intermediate principal stress except for the stress state where the intermediate principal stress approaches to the maximum stress. The relative increase of intermediate principal stress induced also changes the deformation of rock from the ductile mode to the more brittle mode. The difference between the intermediate principal strain and minimum principal strain increased markedly with increment of the intermediate principal stress. The fracture stability of rock specimen was suggested by the concept of the effective shear strain energy stored around the closed microcracks. The deformational anisotropy was indicated by the preferred orientation of stress-induced open microcracks. 1 INTRODUCTION: The Physical behavior of rock under triaxial stress state is essentially important for the underground mining and for the utilization of underground spaces for example, tunnels, electric powerhouse, storages of gas and oil, open pit mine, and high level waste repository and so on), because the act of excavations changes the stress fields and other physical and mechanical environments of rocks around the underground open space. The underground rock body is subjected to the initial stress prior to excavation. In many cases, it can be adequately assumed that the vertical normal stress is equal to the weight of the overlying rock mass, usually, 25–27MPa/Km on the average. The magnitude of the horizontal stresses lie widely between 0.5 and 3.5 times as large as the vertical normal stress at the depth shallower than 1000m(Brown and Hoek,1978). These observations suggest that the three principal stresses are different each others for many cases of initial. In order to investigate the mechanical characteristics of rocks around the underground openings, we have to adopt the testing equipment with which three different principal stresses can be reproduced precisely. The so-called triaxial compression test has been proved to be the most adequate and convenient testing method for the study of the mechanical characteristics of rocks for wide variety of confining pressures. In the conventional triaxial compression test, the longitudinal axial load is applied parallel to the axis of a cylindrical rock specimen through steel end pieces and the other stresses(confining pressure) are the cylinder by a fluid medium. The relatively homogeneous stress distribution can be produced in the specimen with the conventional triaxial test(confining pressure test). However, the intermediate principal stress is fixed equal to the minimum principal stress(compression test under confining pressure), in fewer cases, to the maximum principal stress(extension test under confining pressure), because the stress state in the confining pressure test is essentially axially symmetrical. According to the confining pressure experiments, failure strength increased remarkably with increase of the confining pressure, and relative transition from brittle to ductile behaviors is observed by the increase of the confining pressure.

Proceedings Papers

Paper presented at the ISRM International Symposium, September 12–16, 1988

Paper Number: ISRM-IS-1988-011

... that rock strength is independent of the

**intermediate****principal****stress**. This assumption implies that the yield surface is generated by lines parallel to the principal stress axes. Following experi- ments on hollowcylinders with external pressure and axial load, Obert and Stephenson(I965) sug- gested...
Abstract

ABSTRACT: Although it appears that the Hoek-Brown empirical failure criterion is still the most suitable and realistic criterion for predicting the strength of rocks or rock masses, difficulties have occured in applying the criterion to both laboratory and numerical modelling. It is shown that in three dimensional (3-D) principal stress space, the yield surface of the Hoek-Brown criterion is a combination of the components of six two dimensional parabolic surfaces. After a thorough investigation ?f the nature of the Hoek-Brown and some other yield surfaces, a modified 3-D Hoek-Brown criterion is suggested and tested in association with the finite element method. The proposed criterion has particular advantages in numerical analysis and also in predicting the strength of weak rock masses. 1 INTRODUCTION An important phenomenon manifested by rock strata in the vicinity of underground openings is their non-linear response to induced stress. In situ observations and experimental results show that this is caused by a combination of pre-peak non-linear elasticity and post peak behaviour. After failure, a rock mass may exhibit strain softening, perfect plastic or strain hardening behaviour depending on confining pressure, rate of loading and rheologic characteristics of the material (Hudson et al. 1972a,b; Farmer 1983). For the mechanical description of non-linear post failure behaviour of a rock mass, a yield criterion based on the theory of plasticity is usually used. To determine the conditions which govern the failure or yielding of a rock mass considered as an equivalent continuum, a 3-D strength criterion is generally required. Two of the classical strength criteria in incremental plasticity theory are the Mohr-Coulomb and Drucker-Prager criteria which define the simple yield surfaces of Fig. 1 in 3-D principal stress space. A suitable shape of yield surfaces for rock materials has been considered to be a curved, pointed 'bullet' with curved cross sections in the octahedral planes, similar to those of Mohr-Coulomb but without sharp intersection points or corners (Serata et al. 1967; Akai and Mori 1970; Franklin 1971; Kim and Lade 1984). Recently, Michelis (1987) interpreted his truly triaxial test data in the meridian and deviatoric planes of yield surfaces and verified most of the above characteristics. Taking into account the practical application of a strength criterion in excavation design, not only the intact rock, but also the rock mass behaviour has to be considered in the development of a criterion(Hoek and Brown 1980). On the other hand, a rock strength criterion is at best only an approximate fit to the observed data, and thus it should be expressed in a simple way and involving the minimum number of parameters for practical excavation design. 2 REVIEW OF ROCK STRENGTH CRITERIA Of the classical strength criteria, the Mohr- Coulomb criterion is generally considered the most acceptable for describing rock behaviour. However, through considering the strength criterion in 3-D principal stress space, the main discrepancies between the experimental results and those predicted by the Mohr-Coulomb criterion are the curvature of its yield· surface.