## 1 Introduction  
Layered composite rock masses are commonly found in nature [1]. The varying mechanical properties across adjacent rock layers initiate the crack propagation patterns within layered composite rock masses, contrasting with the homogeneous rocks. The strength of these rock masses is not solely determined by layering but is also impacted by the natural rock defects. Weathering and erosion processes contribute to the formation of numerous defects, such as joints and voids, within the rock, thereby causing a decrement in its overall strength [2]. For jointed rock masses, failure is primarily governed by the evolution of cracks developing from these joint tips. Therefore, a thorough investigation into the mechanical properties and failure behaviors of jointed layered composite rock masses holds significance for assessing the rock instability. The strength and deformation parameters of jointed rock masses are dominated by the configuration of joints, encompassing factors like joint density [3, 4], joint length [5, 6], joint angle [7], and the relative positioning of joints [8, 9]. [PERSON] et al. [10] analyzed the failure characteristics of rocks containing parallel double joints, categorizing them into tensile and shear cracks. [PERSON] et al. [11] utilized a high-speed camera to monitor the loading process, examining the effects of parallel joint inclination, joint length, bridge angle, and bridge length on crack coalescence patterns. [PERSON] [12], [PERSON] [13, 14], [PERSON] [15, 16], and [PERSON] [17] employed digital image correlation (DIC) methods to document crack propagation patterns in jointed rock analogues and rocks. However, as unparallel joints are prevalent in nature, the interaction between parallel joints cannot describe the authentic failure characteristics of rocks. As a result, research endeavors have focused on the behavior of unparallel joints. [PERSON] et al. (2018) conducted uniaxial compression tests on non-coplanar joint sandstone, finding that peak strength is influenced by joint angle, whereas elastic modulus remains unaffected. [PERSON] et al. (2019) performed dynamic loading tests on rocks with non-coplanar fractures, suggesting that a larger joint inclination leads to a higher elastic modulus. [PERSON] et al. (2020), in their investigation of unparallel jointed rocks, identified 90\({}^{\circ}\) as a critical angle influencing peak strength, elastic modulus, and peak strain. Compared to rocks with parallel joints, the evolution of principal strain in the bridge zone of unparallel joints is more complex, influencing crack coalescence patterns. Notably, the aforementioned studies on unparallel jointed rocks focused solely on single-lithology rocks, necessitating research into the mechanical properties and failure modes of jointed composite rock layers. In the past few years, scholars have conducted laboratory experiments and numerical simulations on jointed soft-hard rock strata ([PERSON] et al., 2019; [PERSON] et al., 2020; [PERSON] et al., 2020). [PERSON] et al. (2020) analyzed the crack propagation patterns in composite rock masses with inclined bedding planes under tensile and compressive loading. [PERSON] et al. (2020) combined particle flow simulation and uniaxial compression tests to analyze the effects of crack length and angle on the strength and failure modes of composite rock masses. [PERSON] (2018) and [PERSON] (2016) explored how joint geometry and layer strength ratios impact the coalescence patterns and failure mechanisms in bi-jointed composite rocks. The aforementioned studies concentrated on jointed soft-hard composite rock with parallel joints. In addition, there are often differences in joint orientation within different layers. Therefore, further investigation on the failure mechanism of soft-hard composite rock with unparallel joints is still needed. In this study, a series of soft-hard composite rock models containing unparallel double joints are fabricated to investigate the effect of joint angle on mechanical and failure responses.