The strength of jointed rock masses is usually anisotropic and depends on the behavior of discontinuities and intact rocks. In the present study, Particle Flow Code in 3 Dimensions (PFC3D) is used to simulate the compression tests of jointed rock so as to find out the anisotropic strength characteristics of jointed rock masses. The numerical models having a single joint or one and two sets of joints are used. And the testing conditions include the uniaxial and triaxial compression accompanying the different joint persistence. Many valuable conclusions obtained are as follows. Duo to presence of joints, the strength of the jointed rock is reduced. The strength of the jointed rock is dependent on the inclination and persistence as well as number of the joints. The degree of strength anisotropy of the jointed rock is the highest at uniaxial compression condition. The degree of strength anisotropy decreases with increasing confining pressure and persistence of the joints. The superimposition principle is not suitable to estimate the strength of rock containing several sets of joints because interaction between joints is not considered. The unaxial strength of rock having one set of joint is related to the inclination and number of the joints. As the number of joints increases, the rock strength gradually decrease and close to a constant.
Rock masses are often intersected by discontinuities, such as regular bedding planes, fissures, fractures, joints or faults, producing many types of structure. One of the most important features of the joints in rock is that they introduce anisotropy in strength and deformability. If the mass is not highly fractured and the joint system has only few sets (say five or less), then the mass usually behaves anisotropically provided that no set has a dominant effect (Singh et al. 2002). Joints are invariably present in all rock masses and mainly govern the mechanical behavior of the mass, especially under uniaxial or low confining pressure conditions. The mass under such stress conditions may fail because of splitting and shearing of intact material, rotation of blocks or sliding along the critical joints (Singh et al., 1997). The influence of the intrinsic anisotropy on rock mass failure strength is one of the basic data required for predicting rock mass performance. Although many attempts have been made in the past to describe the strength anisotropy of jointed rock mass, no general methodology has emerged yet.
The properties of the intact rock between the discontinuities and the properties of the joints themselves can be determined in the laboratory where as the direct physical measurements of the properties of the rock mass are very expensive. For determining the rock mass properties indirectly, a theory needs to be established and tested in some independent way. A number of experimental studies have been conducted both in field and in the laboratory to understand the behavior of natural as well as artificial joints. Laboratory studies show that many different failure modes are possible in jointed rock.
