## 1 Introduction  
There are a lot of natural joints in the rock mass around strategic protection engineering, which are often filled with sand and clay. Such granular materials are usually weak and have a large difference in wave impedance from natural rocks [1]. Reflection and attenuation occurred in rock joints during stress wave propagation. At the same time, the infill squeezed each other and broke by friction. This process consumes a large amount of incident energy and effectively weakens the stress wave intensity [2]. The weakening effect was significantly different with different filling materials. Therefore, by discussing the seismic response of rock joints filled with different granular materials under a seismic wave, especially under high strain rate conditions, recommendations are provided for existing absorbing layer materials and further weaken the damage effect of ground penetrating weapons and nuclear blast effects on protective works [3]. Under high strain rate conditions, inertial force would be generated when the loading rate exceeded the quasi-stationary state region. At the macroscopic scale, this is manifested in the form of increased stiffness and strength; at the particle scale, changes in the forces between particles may lead to fragmentation of the particles, thus affecting the energy dissipation of wave propagation. The split Hopkinson pressure bar (SHPB) equipment was widely used in the high strain rate laboratory test. By using the modified Kolsky method [4], the sand or clay was confined in the rigid sleeve to limit its radial strain. This method has a good effect in an impact test. By using the shell theory, [PERSON] et al. [5] proposed an alternative theoretical method to estimate the stress and deformation state of granular materials in passive constrained SHPB tests. Then, high strain rate impact tests were carried out on calcareous sand with water content of 25% and 30% to verify the feasibility of the theoretical method. By using the modified Kolsky method, [PERSON] et al. [6] carried out high strain rate impact tests on Stockton beach sand with different initial densities and saturations. [PERSON] et al. [7] explored the effect of mass density on the compressive properties of dry sand, and pointed out that the dynamic stress-strain relationship of dry sand under constraint conditions was highly sensitive to the initial mass density. Meanwhile, [PERSON] et al. [7] estimated the energy absorption characteristic of sand according to the area surrounded by the stress-strain curve, and found that the specific energy absorption range of dry sand was 5-25 J/g. [PERSON] et al. [8] carried out high strain rate impact tests on sand with five water contents (mass ratios were 0%, 4.2%, 8.2%, 12.4% and 16.5%, respectively). The paper pointed out that the stress-strain behavior of partially saturated sand was strongly dependent on confining pressure, initial mass density and saturation degree. With the increase of saturation, the mechanical behavior of Eglin sand with mass density of 1.52 g/cm\({}^{3}\) was softening first and then becoming stiffer; but the lock-up phenomenon did not happen when the mass density of sand was 1.75 g/cm\({}^{3}\). [PERSON] et al. [9] studied the effect of water content on the compressibility and particle crushing of loose sand. The results were similar to those of [PERSON] et al. [8]. When the moisture content reached 15.0%, the specimen was completely saturated and the particles were rarely crushed. [PERSON] et al. [10] pointed out that under high strain rate impact, spherical particles were more difficult to be crushed and had a higher stress-strain relationship with the same compactness and lateral constraint. When the incident wave propagates through the rock joints, part of the wave will be reflected, part of the wave will be dissipated due to the friction and fragmentation of the particles inside the joints, and finally, the remaining part of the wave will continue to propagate. A lot of energy will be consumed in this process [11].