## 1 Introduction  
Underground rock engineering structures encounter commonly complex loading paths. The stability of the engineering structure is not only related to its mechanical properties but also depends on the loading path and properties of external loads. When the direction of the external load is oblique to the axis of the engineering structure, the oblique loading path appears. In this situation, the engineering structure is subjected to both compressive and shear loads, for instance, inclined layered rock mass in the process of excavation [1], soil sandbags used in foundation reinforcement [2], concrete piles reinforced by stratum [3], flexible piles in rock anchorage [4], reinforced concrete walls [5] and many constructions with building masonry [6], which would be subjected to combined compression-shear loading caused by the dip of layered rock mass, induced stress, and earthquakes. Therefore, it can be said that engineering structures are subjected to combined compression-shear loads everywhere. In the process of underground mining, the inclined pillar is a typical support structure under compression and shear loads, the function of which is to support the overburden pressure and maintain the stability of the surrounding rock [7; 8; 9]. Due to the influence of the ore-body dip angle, the problem of mining-induced pressure is prominent in the mining process, and the stress concentration degree, stress peak value, and failure mechanism of the surrounding rock are quite different from those of horizontal ore-body mining [10]. The reason is that the inclined pillar is subjected to both compressive and shear loads, which occur in pairs and change with the change of the ore-body dip angle [11; 12]. Under the combined compression-shear load, the core confinement of the pillar is reduced, and the shear failure occurs along the discontinuous surface [13], as shown in Figure 1. Once a pillar is unstable, the domino instability of other pillars may be triggered [14; 15], even inducing several pillar bursts in the same area [16]. However, the root of pillar burst is not that the load exceeds the compressive strength, but the result of the transformation of compressive stress into shear stress [17]. The consequences of pillar instability will lead to disastrous accidents [18; 19]. It can be seen that without stable pillars, mining and ground control is impossible [20]. Therefore, the stability evaluation of pillars becomes the key to safe mining. The mechanical behavior and strength characteristics of the pillar are the basis of pillar design and stability evaluation. At present, laboratory tests and numerical simulation methods are mainly used to study the mechanical behavior and strength characteristics of inclined pillars. According to the compression-shear loading characteristics of the inclined pillar, the combined compression-shear loading test is proposed to study the mechanical behavior of the inclined pillar. [PERSON] et al. [21] conducted compression-shear tests on gypsum and sandstone pillar samples with diameters of 50 mm and different width-to-height ratios in the laboratory and studied the deformation and strength characteristics of inclined pillar samples. The results showed that pillar strength decreased with the increase of dip angle. [PERSON] et al. [22; 23] conducted combined compression-shear tests of basalt and granite; the results displayed that both strength and elastic moduli of rock decrease as the sample inclination increases, which is closely related to the strength and modulus under uniaxial compression. With the increase in inclination angle, the failure modes of the rock samples are inevitably converted to shear failure from axial splitting with increasing specimen inclination. [PERSON] et al. [24] carried out combined compression-shear tests on granite samples. The results showed that the failure strength of granite has an obvious compression-shear coupling effect, and the larger the dip angle, the smaller the peak strength of rock failure.