Fracture initiation and propagation, being one of the most intensive subjects in rock mechanics; contribute significantly to the deformation of jointed rock masses. An experimental study on fractured granitic rock was carried out to determine the peak strength of specimen under triaxial loading conditions. The commonly observed failure mode for rock specimens in this study was found to be the shearing of the plane of weakness. The minimum peak strength of fractured rock is observed when the joint orientation is approximately 70o to the horizontal axis. Based on experimental test data, an empirical expression was developed to accommodate the effect of joint orientation and joint trace length in estimating the peak strength of fractured rocks. The predicted values of peak strength using the proposed equation well agree with the experimental results carried out on singly fractured specimens under triaxial loading conditions. The threshold stress values of crack initiation and propagation depend on joint geometrical parameters, their degree of interconnectivity, as well as surrounding stresses on the fracture plane.


In Singapore, due to the scarcity of land, underground space has been utilized for the development of storage facilities and deep sewerage system in rocks. Thus, a comprehensive understanding in the mechanism and the behaviour of rock structure is required as the ultimate objective is to control rock displacement into and around the underground excavations. In addition, reliable estimates of the shear strength and deformation characteristics of a rock mass are required for analysis of slopes and foundation.

This research program presents the effects of stresses on the deformation characteristics of a single rock fracture subjected to triaxial loading states. Furthermore, the study investigates the effects of joint orientation and joint length on the stress-strain behaviour of jointed rocks as well as the threshold stress values of crack initiation and propagation of fractured specimens.

Effects of joint orientation on the compressive strength of rocks

The overall strength and permeability of rock a mass and the stability of engineering structures are greatly influenced by joint orientations (Ranjith, 2000). Higher the interconnectivity of fractures which in turn lower the shear strength, the greater will be the risk of failure of a rock mass. Therefore, it is fundamentally important to study the influence of joint orientations on stress-strain characteristics of rocks under different loading conditions.

The peak strength developed by transversely isotropic rocks in triaxial compression vary with the orientation of the plane of anisotropy, foliation plane or plane of weakness, with respect to the principal stress direction (Donath 1972, McLamore Gray, 1967). Figure 1 shows measured variations in axial stress with the angle of inclination (α) of the major principal stress to the plane of weakness.

Brady and Brown (1994) introduced an instructive analysis of a case in which the rock contained a well-defined, parallel plane of weakness whose normal was inclined at an angle β (Figure 2) to the major principal stress direction.

(Figure in full paper)

Fracture initiation

According to Eberhardt et al. (1997), the point where Majority.

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