Discontinuities are responsible for mechanical anisotropy causing directional variation of rock strength. Therefore the effect of discontinuity dip and dip direction on anisotropic response of rocks has been the focus of researches. Although numerous studies have been concentrated on the effect of dip variation, but no attention has been paid to the dip-direction variation on the strength response of the rocks. Hence, the effects of dip-direction on the rock mass strength were studied through testing of artificial material. Specimens were fabricated using plaster. A specific mold was used for this purpose. The samples were tested under triaxial tests subjected to different confining pressures. A parameter was defined known as. Anisotropy Effect. in order to highlight the changes in strength of rocks due to the discontinuities dip-direction variation. The details of the investigations, tables of the evaluated test results and the empirical equation deduced are presented in the paper.


Discontinuities are one of the most important phenomena that cause the mechanical anisotropy in rocks. This mechanical anisotropy would, in turn, impose reduction of strength. Therefore, the discontinuities and degree of their influence on the rocks must be studied carefully in order to define the rocks behavior.

There have been efforts to investigate the effects of discontinuities on rock strength in recent years, but dip-direction of joints and their effects on rock strength were ignored.

An extensive investigation was planned and undertaken in order to study the strength response of rocks when the dip-directions, of two induced discontinuities, are changing with respect to each other. The investigations were conducted using artificial specimens made out of plaster. The result of these studies is presented in this paper.


Many investigators have carried out the measurement of the strength anisotropy for various rock types e.g. Chenevert and Gatline (1965), McLamore and Gray (1967), Hoek (1968) Attewell and Sandford (1974) and Brown et al. (1977) concentrated their studies on Shales and slates, Deklotz et al. (1966), Akai et al. (1970) McCabe and Koerner (1975) Nasseri et al. (1996,1997) and Singh (2000) researched on response of gneisses and schists, Ramamurthy et al. (1988), on Phyllites Horino and Ellicksone (1970), Rao et al. (1986) and Al-Harthi (1998) considered sandstones, Pomeroy et al. (1971) on coal, Allirote and Boehler on diatomite (1970) and Tien and Tsao (2000) undertaken their investigation through artificial materials. An overall analysis and review of their works exhibit that maximum failure strength is either at β= 0° or 90° (β= angle of discontinuity with respect to σ1) and the minimum strength is usually attained around β=30°, more precisely at (45-φ/2), where φ= friction angle along the plane of weakness.

3.1 Inherent Anisotropy

In this type of anisotropy, the weak surfaces are related to the rock formation processes, such as the foliation and schistose planes which are developed during the formation of rocks, Ramamurthy (1993).

This content is only available via PDF.
You can access this article if you purchase or spend a download.