Experimental studies carried out in the past on multiple cracks propagation show discrepancies between literatures in terms of crack coalescence behavior. Apart from crack geometries including inclined angles and tip spacing, the filling material and surface features of crack affect the interventions between the tip fields and hence the crack growth pattern. An investigation on propagation behavior of filled cracks is carried out through fracture tests of rock-like specimen under uniaxial compression. The digital photographical technique is used to quantify the information of propagation paths. Detailed analyses are given by classifying the propagation paths into four types, namely original, secondary, wing and anti-wing cracks. It was observed that the crack types differ from each other in terms of the initiation time, initiation location and propagation speed. It was also found the filling thickness has an influence on the crack path, failure modes and the initiation of anti-wing crack.
Cracks can be classified into closed crack, open crack and filled crack according to the mechanical features of the joint surface. Current research on the propagation behavior of cracks is mostly focused on open cracks and closed cracks, while in engineering practice, cracks change state from open to close due to varying stress states. Cracks found in the nature are often filled with sands, ooze and etc. Similarly, during the construction process such as grouting and shotcrete, the cracks can be filled with mortar and concrete etc. Compared with the unfilled cracks, it was concluded that the filling will decrease the concentration of stress level around the cracks and the propagation behavior of cracks will be different (Zhang et al. 2012). Mughieda (2004) concluded that the rock bridge failure between two cracks is caused by the linkage of wing cracks which is due to the tensile failure as shown in Figure 1. Mughieda's conclusion is based on the uniaxial compression test where unfilled a crack is formed by the preset thin iron sheet in the mortar specimen and then take the sheet out during the curing of specimen. Zhuang (2012) found similar conclusions by model this problem using the meshless method as shown in Figure 2. While Zhu (2002) concluded that the rock bridge failure between two cracks is cause by secondary crack which is believed to be shear failure as shown in Figure 3. Zhu's conclusion is based on the uniaxial compression test using rock similar material with cracks filled with mica sheet. Though the specimen materials are different in the above two experiments, it is found that the existence of filling material changes the crack propagation and failure modes.