## 1 Introduction  
A large number of defects, such as joints, holes and precracks, commonly exist in rock mass, which has an important effect on the propagation of a blast wave and the fracture behavior of blast-induced cracks [1]. In general, the blast wave is often obstructed and attenuated after it encountered the defect and induces severe rock failure around the defect. Thus, predicting the failure pattern of the rock mass with a joint is particularly significant in the safety evaluation of underground excavation of roadways.  
Numerous on-site surveys showed that there commonly exist two kinds of joints in natural rock mass, namely the filled joint and the unfilled joint. The filled joint is often filled with weak materials, which is generally treated as a composite rock mass with different mechanical properties. Considering different kinds of filled material, many researchers have investigated the propagation characteristic of the blast waves, as well as the induced cracks using analytical analyses [2], experimental methods [3, 4] and numerical simulations [5, 6]. The unfilled joint, on the other hand, is filled with no material. Due to the absence of the material in the joint gap, a remarkable reflection of blast waves could occur at the unfilled joint, induce significant stress concentration around the joint, and severely affect the fracturing of the blast-induced crack. In addition, the local stress field around the tip of the dynamic crack could also result in a strong stress concentration and induce wing crack initiation at the end of the unfilled joint. Therefore, it is very important to study the fracture mechanism of both the blast-induced crack and the wing crack for fully understanding the failure pattern of the rock mass with unfilled joint.  
Due to the complicacy of the rock failure under blasting, experimental investigation has become a main technique for studying the dynamic fracture of rock blasting. Among the optical experimental methods, caustic technology is proved to be an efficient way to study the fracture characteristic of the blast-induced crack, especially for investigating the local stress concentration of crack tip [7; 8]. [PERSON] [9] carried out the caustic method to study the influence of a joint on the propagation behavior of a blast-induced crack. [PERSON] [10] studied the dynamic crack propagation behavior of material containing pre-flaws. [PERSON] [11] studied the influence of joint spacing on crack propagation between two adjacent boreholes. Apart from the experimental techniques, some researchers also used a numerical method, such as discontinuous deformation analysis (DDA) [12], the distinct lattice spring model (DLSM) [13], and the discrete element method (DEM) [14], to analyze the stress wave propagation in the material with unfilled joint. In addition, the influence of the joint number [15] and joint orientation [16] on the rock fragmentation by blasting is also studied. From these studies, it can be concluded that the unfilled joint considerably hindered the propagation of blast waves and induced serious damage between the joint and the blast hole. However, due to the complexity of the blast waves, the influence of the different type of blast waves on the crack initiation and propagation has not been clearly clarified, which is quite important for optimizing the blast parameters to control the damage of a jointed rock under blasting.  
The main objective of this study is to investigate the influence of the unfilled joint on the propagation of the blast waves, as well as the crack initiation and propagation behavior in jointed rock mass. Section 1 reviews the previous studies on the crack propagation in the jointed rock and analyzes the limitations in these studies. Section 2 introduces the formation of caustic curves. Section 3 describes the caustic experiment and the results in detail. Section 4 further analyzes the blast wave propagation in the medium containing an unfilled joint through numerical simulation. And the conclusions are summarized in Section 5.