Discontinuities play a key role in the failure process of rock and rock mass. Small fractures can decrease the strength of rock while large-scaled joints can have a significant effect on the overall stability of rock mass (Eberhardt et al. 2004). Discontinuities are often separated by intact rocks, which are known as “rock bridges”. These rock bridges contribute to the stability of jointed rock mass by providing a strength reserve (Gehle and Kutter 2003) that needs to be broken first before failure can take place. In recent years, a number of experimental studies have been performed to investigate the mechanism of crack propagation in rock-like material under uniaxial (Shen et al. 1995; Wong and Chau 1998; Sagong and Bobet 2002; Xu et al. 2013; Yin et al. 2014; Cao et al. 2015), biaxial (Bobet and Einstein 1998), and shear (Gehle and Kutter 2003) stress conditions. It was revealed that under load conditions, wing cracks first appear at the tips of pre-existing cracks (flaw) while shear cracks typically lead to coalescence and failure. The aforementioned studies also indicated that the process of coalescence was rather complex and depended on the rock material and geometry of pre-existing cracks.

While the mechanism of crack propagation is relatively well understood, the extent to which pre-existing cracks can affect the strength characteristics of rocks still remains unclear. Ramamurthy and Arora (1994), Yang et al. (1998) and Park and Bobet (2009) noted that the characteristics of joints such as their number and orientation can affect the strength of rock. Shen (1995) and Park and Bobet (2009) reported that the joint roughness and the friction of the filled material (Shen 1995; Park and Bobet 2009) can also influence the strength of rock and rock mass. These joint characteristics are considered in the joint strength parameter, which is commonly used in engineering practice (Ramamurthy and Arora 1994; Zhang 2010). Unfortunately, much less attention has been given to the length and width of joints, which are parameters that can also affect the strength of rock and rock mass. This study seeks to address this issue by investigating the effects of pre-existing cracks of different width and length on the unconfined compressive strength (UCS) of rock-like material of various strengths. This technical note presents and discusses the obtained results.