Rock masses contain discontinuities inherently, such as faults, fractures, bedding planes and other planes of weakness [2; 5; 11; 15; 23]. For example, the Baihetan Hydropower Station, a very large construction project, indicated by field geological investigations that many discontinuities developed in the rock masses [4]. In the open-pit mine located to the west of Fushun City, China, the Cretaceous rock mass between reverse faults F1 and F1A was imposed a substantial impact by strong shear stresses, resulting in a large amount of fractured rock mass [16]. The shear strength of rock joints was lower compared with the intact rock materials, and shear failure of rock masses along pre-existing fractures was one of the most common failure modes in rock slopes, rock dams and tunnels [8]. Thus, it is important to understand and predict the shear behaviours of rock masses containing joints for the design and stability analysis of rock structures.
In recent years, great effort has been paid to investigate the mechanical features, energy evolution and morphology characteristics of rock fractures from various perspectives, e.g. complete shear stress-shear displacement curves, energy dissipation and absorption, and change regularity of surface roughness [1; 24; 25; 27]. Similar to intact coal sample easily influenced by confining pressure and external stress level, it is general accepted that influencing factors, such as shear load, applied shear stress level and cyclic shear stress may stronger affect the mechanics and failure properties of rock fractures (Meng et al., [7]. Due to the difficulty to obtaining natural rock fractures, the cement mortar fractures or rock fractures obtained by artificial splitting was mainly used to carry out relevant tests [26].
In terms of direct shear tests, Zhou et al. [26] prepared the irregular serrated structural planes with three different asperity heights made of cement mortar to perform direct shear tests to study the effect of structural plane to the slip burst, and the mechanism of in-situ slip burst was analyzed preliminarily. Afterwards, Meng et al. [6] carried out shear tests under constant normal loading (CNL) on tensile joints in three different types of rock, the test results indicated that the maximum AE hit rate of marble joints was attained before the peak shear stress when the normal stress was 1 MPa, and the AE hit rate became quiet during the ultimate sliding stage. Moreover, Meng et al. [10] carried out direct shear tests on split granite joints at various shear rates (0.001-0.1 mm/s) under different normal stresses, and the influences of the shear rate on the shear strength, post-peak shear behaviour and acoustic emission characteristics are analysed and discussed. Recently, Meng et al. [9] conducted direct shear tests under constant normal stress condition on artificial splitting cement mortar fractures and monitored the AEs during the shear failure process. Furthermore, rock fractures was usually subjected to tiered cyclic stress, such as s periodical change of the reservoir level and reciprocating motion of the train [19]. Thus, it is of considerable significance to study the impacts of tiered cyclic stress on the mechanical behaviours of rock fractures. Zhai et al. [20] conducted experiments on jointed marble specimens using servo-controlled testing machine to investigate the energy and deformation characteristics of rock joints under multi-stage shear loading-creep-unloading conditions. Additionally, to study the evolution characteristics of interface slip field under cyclic loading, a double shear model test study of granite specimen under constant confining pressure was designed and carried out by Yang et al. [17]. More recently, Zhai et al. [19] experimentally investigated rock samples with artificially prefabricated fractures by RDS-200 servocontrolled rock direct shear experimental apparatus, measured displacement characteristics and energy evolution under pre-peak tiered cyclic shear loading at a constant normal loading. And like intact coal and rocks, creep of discontinuities is also a part of the fundamental properties for rock fractures, which was prone to contribute to the large deformation of slope and surrounding rock of underground tunnel, or even landslide hazard [22], and delay failure may took place several days or years. Many scholars in this field have conducted large quantity of research work, and they have gained rich achievements. For example, Wang et al. [12] induced two types of fractures by means of modified shear and indirect tension and assembled the fractured rock samples following their initial formation and direct shear tests were performed under a constant normal force and stepwise increased shear creep loadings. Zhang et al. [21] conducted restrictive shear creep experiments to investigate the creep characteristics of key unit rocks in the slope potential slip surface and established creep models reflecting the creep-failure time and progressive-failure characteristics of rock. More recently, Wang et al. [13] investigated the three creep stages of shear creep tests in detail and proposed a method for predicting the accelerating creep stage, established an empirical creep model, and so on.
As illustrated by the above analysis, extensive research has been conducted on mechanical and failure features of the rock fractures by different authors. Nevertheless, minimal investigations exist which simultaneously deal with the shear stress-shear displacement, creep, and cyclic shear failure characteristics. Therefore, in this study, direct shear tests, pre-peak tiered cyclic shear tests and multistage loading shear creep tests are conducted on the rock fractures obtained by artificial prefabrication method to study the shear failure characteristics and instability behaviour under various loading paths. The results of this study will contribute to the understanding of failure instability mechanism of rock fractures under different shear loading paths, and are of great significance for the prevention and control of slip instability hazards of rock mass with fractures in geotechnical engineering.