## 1 Introduction  
In recent years, with the depletion of fossil energy worldwide and the rapid development of guidance technology, more and more underground mining projects have shifted from shallow excavation to deeper excavation. The research on the mechanical properties of layered rock has been gradually paid attention to and intensified [1]. As early as 1967, [PERSON] [2] put forward the concept of layered rock mass in the study of igneous rock, which had an important influence on geological research. Subsequently, many scholars [3; 4; 5; 6; 7; 8] have conducted a lot of studies on the properties of single-layered rock mass such as shale and sandstone. However, in deep engineering, more and more problems of layered rock masses, which are different from general rock mechanics, have been found. Additionally, these rock masses often have various shapes and complex mechanical properties. It brings challenges to underground engineering construction such as hydraulic fracturing, oil and gas exploitation, underground excavation, and so on [9; 10; 11; 12]. Therefore, it is of great significance to explore the mechanical properties of layered rock for underground engineering construction. In 1981, [PERSON] [13] introduced the term of sandwiched rock mass when classifying rock mass. According to the research progress, the layered rock mass can be divided into soft sandwiched rock mass, hard sandwiched rock mass, interbedded rock mass and so on. The current studies into layered rock mainly focuses on mechanical properties and deformation mechanism. In terms of mechanical properties, [PERSON] et al. [14] used coal and oil shale with high strength and low elastic modulus to prepare the sandwich type and superposition type combination rock samples. Through uniaxial compression test, they found that the difference in the mechanical properties of coal and oil shale affected the stress state near the interface, resulting in the increase in coal strength and the decrease in oil shale strength. [PERSON] et al. [15] found that, based on mineral composition, the uniaxial compressive strength and triaxial compression tests of shale reservoir, it was found that the mechanical properties of the shale reservoir in parallel bedding direction are less affected by lithology under uniaxial conditions. There is an increase in the compressive strength of rock, with a higher confinement in triaxial tests. The change in stress state is an important factor leading to the change in rock strength in the sample. Later, many scholars proved that the mechanical properties of combined rock mass are closely related to the content and strength of components. [PERSON] et al. [16] studied the uniaxial compressive strength of the sandwich rock mass with different strength ratios of original materials. The relationship between the strength difference of lithological units, the percentage of soft rock and uniaxial compressive strength in soft rock and hard rock with alternating layered rock mass is revealed. [PERSON] et al. [17] studied the influence of water pressure on the mechanical properties and permeability of layered rock mass through triaxial compression tests under different water pressures and low confining pressures. The results show that the increase in water pressure reduces the peak strength of layered rock mass, but accelerates the expansion of cracks and significantly enhances the peak permeability of layered rock mass. [PERSON] et al. [18] conducted uniaxial compression tests on combination rock samples to reveal their mechanical properties. The conclusion is that combination rocks with thick interlayers are more likely to fail than combination rocks with thin interlayers. [PERSON] et al. [19] prepared superimposed and sandwiched rock masses using coal and rock coal bi-material samples. Uniaxial compression tests showed that the strength of coal determines the overall strength of the bi-material sample. [PERSON] et al. [20] studied the influence of the difference in rock strength between adjacent rock strata on cracks.