The existence or many joints brings about the problems of interaction between joints. In this regard, it is very important to explore the interaction between two joints under shear loading condition. In this study, the shear performance of double joints was studied using the laboratory experiments, where the smooth cleavage joints were generated by sawing off and the rough joints were generated by the Brazilian Split test. The experimental results of double joints of sandstone show that, under lower normal stress the interlayer rock between two joints does not fracture, and the peak shear strength of the specimen is determined by the weaker joint. In contrast, under higher normal stress, the peak shear strength is attained when the tensile fractures initiate in the interlayer rock, and it has also relevancy to the JRC of double joints and interlayer thickness. Additionally, numerical simulation of the double-joint shear tests show that the direction of the cracks trends to be parallel with that of the maximum principal stress, and the stress concentration in the joint surfaces causes penetration between different joints, which leads to a lower strength.
Shear strength of a single joint can be effectively estimated by an empirical formula , However, the structural planes or joints in the real rock mass are usually not exist alone, the interaction between different joints has important influence on the overall strength. Yang et al.  studied the strength and deformation of rock specimens cut with parallel structures under the uniaxial compression conditions. It indicated that the failure modes of specimens with multiple structures can be divided into three types. The stress-strain characteristics of rock mass with multiple joints based on the double axial compression tests have been conducted by Yoshinaka and Yamabe , and the related constitutive equation was established. Through the true triaxial compression tests on large size multi-jointed rock specimens, Reik and Zacas  have found that the deformation and failure mode of jointed rock mass are both related to the direction of the joints and the stress state. Kulatilake et al. deemed that the fracture tensor component can be used to establish a nonlinear relationship with the strength of rock mass with many groups of joints. Jaeger  and Bray  predicted the strength of rock mass containing one or two joints by the principle of stress superposition, and they considered that the weakest structure played a decisive role in the strength of rock mass. Hoek and Brown  have also set up a prediction formula of jointed rock mass strength based on the uniaxial compression test. In addition, the numerical simulation method is used to study the mechanical properties of the multi-jointed rock, and the results show that the direction, number and spacing of the structural planes have influence on the overall strength-. Generally speaking, the current understanding of the interaction between different joints is still not enough, especially on the interpenetration between two or more rough joints.