## 1 Introduction  
In underground coal mines, coal pillars are often used for the purpose of ground control [1; 2]. During the process of mining, coal pillar instability is often encountered [3]. Many researchers have reported that coal pillar instability is the product of destruction of coal-rock masses under high stress [4; 5]. Practices showed that the coal pillar instability is affected not only by their internal cracks and joints, but also by the strength of the roof and floor [6]. It is necessary to study the mechanical properties and failure behavior of the system composed of roof/floor rocks and coal. Coal and rock mass failure is a long-standing issue, and its mechanical properties and failure characteristics have been essential parameters for underground coal engineering, such as mining design, gateroad support design, etc. [7; 8; 9; 10]. Numerous studies [11; 12; 13; 14; 15; 16; 17] have been carried out to study the mechanism of coal and rock mass failure by testing the pure coal/rock specimens. It has been shown that common types of coal and rock mass failure mechanism include tension failure, shear failure, and tension-shear failure. The above results provide a basis for understanding the instability of coal pillars. In fact, coal pillars are often left to support the overlying strata in underground engineering, and then the coal, roof and floor bear loads together [18]. Thus, it can be expected that the mechanical properties and failure behavior of underground coal and rock mass may different from that of pure coal/rock specimens. In other words, it is hard to understand the deformation and failure characteristics of underground construction in accordance with the experimental results of coal/rock specimens. Therefore, a kind of coal-rock combined (CRC) specimen was tested and its mechanical properties, and failure behavior were found to possibly be useful for forecasting the deformation and failure of the coal pillar. Generally, extensive studies have been conducted to understand the mechanical properties and failure behavior through the application of different methods, including numerical methods, laboratory experiments, and theoretical analysis [19; 20; 21]. [PERSON] et al. [22] found that when the CRC specimen was loaded, AE (Acoustic Emission) events are more likely to be generated in the coal section, if damage first occurred in the coal section. [PERSON] et al. [23] also suggested that the mechanical properties and deformation failure characteristics of the coal-rock combined body were governed mainly by the coal section. These studies indicated that the strength of a CRC specimen is not only affected by its mechanical properties, but also by other parameters such as the properties of coal-rock interfaces and the interface dipping angle. A literature review showed that the effects of the coal/rock strength on the mechanical behavior and failure mode of the CRC specimen is still unclear, since the mechanical behavior of the CRC specimen is more prominent than that of pure coal/rock specimens. Since [PERSON] [24] first pointed out the link between excess strain energy and the damage due to strainbursts, an increasing number of studies have been performed to understand the energy evolution during coal and rock mass failure. [PERSON] et al. [25] and [PERSON] et al. [26] point that from the perspective of energy in the energy transformation, coal and rock mass failure is energy-driven instability that includes energy absorption, evolution, and dissipation. [PERSON] and [PERSON] [27] studied rock energy characteristics under dynamic cyclic loading, and found that higher rock strength leads to greater energy absorption. Conducting compression tests on bedded-sandstone with bedding layers in different orientations, [PERSON] et al. [28] found that for rock with bedding layers inclined in shallow angles failures, energy release could be more violent than failures of rock with steeply oriented bedding layers.