Specimens of size 63.5 cm × 27.9 cm × 20.3 cm, made of 72 % silica sand, 16 % cement and 12 % water by weight were tested. The joint inclination angle kept constant and equal to 45°, while offset angle i. e. angle between the plane of the joint and the line connects the two inner tips of the joints, changed from 0° to 120° with an increment of 15°. The failure mechanism monitored by visual inspection and magnifier to detect cracks initiation. In all of the tested samples curvilinear cracks called wing cracks initiated at the joints tips due to high tensile stresses concentration this wing cracks directed along the direction of the uniaxial load. The coalescence mechanism of two cracks was investigated. Results showed that open cracks can coalesce by shear failure or tensile failure. The coalescence path manly depends on the inclination of the rock bridge between cracks.
Rock masses are usually discontinuous in nature as a result of various geological processes they have subjected to. Consequently, joints and rock brides formed in the rock mass. The initiation, propagation and coalescence of rock cracks are important factors in controlling the mechanical behaviour of brittle rocks. Crack propagation and coalescence processes primarily cause rock failure in slopes, foundations and tunnels. Many studies have been performed on the initiation, propagation and coalescence of cracks since Griffith (1921) have studied the growth of preexisting two-dimensional crack. The studies that perform on jointed rock can help to explain the joint (crack) propagation mechanism and serve as models for the behaviour of jointed rock masses. Joint propagation and coalescence can reduce the stiffness of jointed rock masses causing the shear failure of rock slopes (Einstein et. al 1983). Also, it can induce earthquakes by forming shear faults (Deng and Zhang, 1989). A number of studies have been performed on crack propagation in different materials under uniaxial compression. Lajtai (1969) performed direct shear test on natural rock specimens with two parallel slots, Segall and Pollard (1980) studied analytically the stress field In rock bridges between two stepped cracks, Nernat-Naser and Horri (1982) investigated the coalescence behaviour of multi-cracks in polymer specimens, Reyes and Einstein (1991) performed uniaxial tests on gypsum specimens with two inclined flaws and Shen (1995) conducted uniaxial tests on gypsum specimen with two cracks. However, all experimental studies have been conducted on a small size sample and on limited test material. In this current study we conducted a series of uniaxial loading tests on rock-like material specimens of size 63.5 cm × 27.9 cm × 20.3 cm, made of 72 % silica sand, 16 % cement (Type I) and 12 % water by weight. Each specimen contains two pre-existing open cracks arranged in different geometries.
In the present study a model material was used. This model material was made of mixing silicasand, ordinary Portland cement type I and water in 72%, 16%, and 12% proportion by weight respectively.