The hazards induced by stratified rock mass creep are still one of the major problems that threaten the safety of underground engineering. This paper takes safe construction of underground roadway in Urumqi mining area as the research background. In this study, we mainly adopted rock mechanics experiments to accomplish the research on creep behavior and crack evolution of stratified structural sandstone. Creep deformation characteristics of stratified structural sandstone under different load were revealed; also, we analyzed the reason why a part of rock samples failed but others were not under the same load. Creep behavior and crack evolution of rock samples without stratified structure have significant randomness. The crack evolution and failure characteristics of stratified structural rock samples were mainly manifested as failure along and cutting through structural plane and their combined forms. Creep strain, creep duration, and creep rate of rock samples with stratified structure had a nonlinear relationship with applied load, such as exponential function or logarithmic function. Understanding the evolutionary relationship between the above parameters and load provides a basis for obtaining the creep behavior of stratified rock mass under different load conditions.
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Creep of rock is common in the underground engineering, which leads to time-sensitive characteristics of crack evolution in rock deformation [1-3]. In terms of deep rock engineering construction, the service time of large underground tunnel increases obviously, and the general expected life is from several decades to more than a hundred years. Therefore, it is necessary to consider time-dependent properties of rock mass in the design, construction, and routine maintenance [4-6]. Particularly when in situ stress is less than short-term strength of rock mass and original layered structure exists in the rock mass, it is easy to generate creep and crack propagation [7, 8]. Therefore, it is important for disaster control to study the development of creep deformation and crack evolution of deep rock mass.
An experimental study is an effective method to obtain the creep behavior and typical parameters of rock mass, which is widely favored by researchers due to its advantages of rapidity and intuitive. Especially in recent years, with improvement and upgrading of experimental equipment, application scope of experimental research has been promoted and expanded to a large extent. Researchers optimized and improved experimental equipment according to their requirements, which accelerated the process of rock mass creep research. Liu et al. [9] studied creep behavior and characteristic of saturated rock under high stress in uniaxial single-stage load and graded incremental cyclic load mode, providing a basis for deformation control and disaster relief of deep saturated rock mass. Dubey and Gairola [10] used experimental means to study the influence of internal anisotropy of rock salt on its creep behavior and control effect. They believed that structural anisotropy had a strong control effect on the development of instantaneous strain, transient strain, steady strain, and accelerated strain, and the influence of structural anisotropy on rock salt deformation had a negative correlation with the stress level. Zivaljevic and Tomanovic [11] adopted a uniaxial creep experimental method to analyze the creep characteristics and behavior of marl, focusing on the influence of compressive stress preconsolidation level and load time on the creep parameters of marl. Pellet and Fabre [12] carried out static, quasistatic, and cyclic creep experiments on sedimentary rock, and the results showed that the content of clay particles had a significant impact on the creep behavior, and the creep of particles had an adverse effect on the creep behavior. Rahimi and Hosseini [13] carried out triaxial creep experiments on thick-walled hollow columnar rock salt samples to study the effects of confining pressure, eccentricity stress, and strain rate on the creep behavior of rock salt. The results showed that the strain rate increased with the increase of eccentricity stress and confining pressure, and the lateral pressure was more important than the eccentricity stress in the change of tangential strain rate. Grgic and Amitrano [14] studied the influence of water saturation on rock creep by multistep uniaxial creep experiments of polycrystalline porous rocks under partially saturated conditions and explained the important role of microcrack in the creep process by analyzing strain and acoustic emission monitoring data.
Fabre and Pellet [20] carried out creep experiments on argillaceous rocks under a variety of stress environments and found that the overall mechanical properties of argillaceous rocks deteriorated rapidly when the cracks propagated unsteadily, and the creep of clay particles caused viscoplastic strain. Brantut et al. [21] proposed a micromechanical model that could describe the brittle creep of saturated rock under triaxial stress with time and studied the micromechanics of brittle creep. Davis et al. [22] carried out triaxial compression experiments on dolomites with different particle sizes under variable temperature conditions and revealed the differences of creep mechanism between coarse-grained dolomites and fine-grained dolomites with different grain sizes. Smit et al. [23] studied the structure and microstructure of garnet polycrystals in eclogites and analyzed the creep mechanism of garnet in eclogites by using optical microscopy, element mapping, and electron backscatter diffraction. Rybacki and Dresen [24] carried out creep experiments on plagioclase samples under dry and wet conditions and determined two different creep mechanisms of dry and wet plagioclase. Heap et al. [25] studied the creep mechanism of pore water in sandstone by using microstructure analysis, acoustic emission source location, and macroscopic creep law. Brückl and Parotidis [26] analyzed the deep creep mechanism of slope rock mass with simulation study and pointed out that the main factor controlling the deep creep mechanism was the expansion of subcritical cracks. Bresser [27] obtained the pressure sensitivity and strain rate sensitivity of flow stress through experiments and revealed the creep mechanism of calcite dislocation at high temperature based on the experimental data of microphysical model. Gratier et al. [28] carried out indentation experiments on quartz crystals, which provided characteristic time scales for the transient creep and sealing processes of quartz-rich rocks after earthquakes.
Researches have carried out experiments on rock without primary structures and obtained instructive results [29-31]. However, stratified structural rock mass widely exists in deep engineering, and it is characterized by structural anisotropy. Related studies have found that structural anisotropy has a controlling effect on the creep behavior and crack evolution of rock mass. Therefore, the study on creep behavior and law of stratified structural rock is of guiding significance to discover the failure mechanism of such rock mass. Also, it is an important supplement to the study of rock mechanics. The rock samples used in this study were taken from the surrounding rock of underground roadway in Urumqi mining area. Through systematic creep experiments under different loads, the control effect of structural anisotropy on creep of stratified structural rock samples was studied. And the degree of difference in deformation rate caused by structural anisotropy in rock samples was analyzed to obtain the creep behavior and crack evolution of stratified structural rock mass.
Figure 2(a) shows the state of USN-1 rock sample under load of 50% σ c. The sample had the first and second creep stages, in which the sample exhibited such changes as end slag shedding, axial shrinkage, and radial expansion. However, during the experiment lasting for 240.0h, the creep process did not enter the third creep stage but gradually had a stable state. Statistical results of AE monitoring data showed that there was basically no AE signal between 15.0h and 240.0h, indicating that the evolution of crack inside the sample was gradually weakening. The failure characteristics of USN-2 rock sample were obviously random, and the crack evolution was irregular (Figure 2(b)). Failure of this sample was dominated by oblique crack, and the angle between the oblique crack and the horizontal plane was different in size, which was not statistical. At the intersection of anisotropic cracks, the shallow part of the sample collapsed in layers or small blocks, which accelerated the overall failure of the sample. Due to the energy absorption effect, there was no large crack throughout the whole sample, so the damaged sample still had a certain residual strength.
Due to the sudden instability of the USN-3 rock sample, the transition characteristics of creep from the second stage to the third stage were not clear (Figure 2(c)). To some extent, this might lead to the overlap of creep stages, which made it hard to clearly distinguish the characteristic of each creep stage. Failure of the sample was dominated by the horizontal and vertical cracks, and the horizontal and vertical penetrating cracks were generated simultaneously. The overall failure was relatively complete, and the residual strength was almost equal to zero. When USN-4 rock sample began to break, multidirection cracks were generated on the surface of the sample and continued to extend, accompanied by small and irregular rock fragments spalling, until the cracks were fully developed to penetrate the sample and resulted in complete failure. Failure of the sample was dominated by the transverse crack, and the failure part fell off from the sample along the transverse crack in a block shape. The damaged part of the sample was pulverized and had minimal residual strength.
To sum up, there was almost no anisotropic structure in USN rock samples, and the cracks did not generate or develop along a specific direction. The evolution path of crack generation, development, and final failure were random. 2ff7e9595c
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