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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.

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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.

Since the load exerted on USH-5 to USH-8 sample was greater than the Ci threshold, and the load borne by some samples was even greater than the Cd threshold. The rock samples had undergone three typical creep stages. With the increase of load, the slope of time-strain curve increased; that is, with the increase of load, creep of the samples increased in per unit time. At the same time, with the increase of load, the failure time decreased and the final deformation increased. Failure modes of the samples were mainly vertical and oblique cracks which cut through the stratified structural plane, and the angle between the failure structural plane and the horizontal plane was generally greater than 45. Among them, the USH-7 rock sample showed a lateral bulging failure pattern parallel to the horizontal structural plane. It was considered that the failure was caused by transverse extrusion of weak material between the horizontal structural planes under high load. Except for the semipenetrating failure surface of the USH-4 rock sample, the failure surfaces of the other samples all presented top-to-bottom penetrating failure. Therefore, the failure of USH samples usually require enough load to penetrate rock bedding. In general, the interbedding weak material does not play a decisive role in the failure mechanism of the USH samples, which is also the reason why the USH samples require greater load than the USO samples and the USV samples.

Strain-time curves of stratified rock samples have a nonlinear trend during stress increasing stage (Figure 10). The samples begin to deform when load is applied, and as the load gradually increases, the evolution of new cracks and the development of native cracks begin inside the samples. The effect of this process on the rheological parameters is significant. In this stage, deformation rates of the samples are an important index to measure the change state of their internal structure. The strain generated by the USH samples is the smallest while that of the USV samples is the largest in the process of load increase. This is caused by the induced tensile stress of horizontal bedding plane compression and the minimum strength in the tensile stress environment. Because the bedding planes in the USH samples are arranged along the direction of the minimum tensile stress and shear stress, the instantaneous strain is the smallest. The maximum instantaneous strain of the USO samples is caused by the bedding plane orientation close to the natural shear plane. The USV samples show intermediate instantaneous strain, which is caused by the decrease of the contact area between the samples and the press plate due to the crack sliding along the bedding plane. On the whole, strain is positively correlated with load in the process of load increase, and its evolution can be expressed as an exponential function relationship with base e, which is ε in=aebσ . [Please download the PDF to view the inline formulas].where ε in is the strain generated during the load increase stage, σ is the load, and a and b are the coefficients. H, O, and V refer to three kinds of structurally anisotropic sandstones with horizontal, oblique, and vertical layered structures, respectively. The coefficient a ranges from 0.0081 to 0.0132, and the coefficient b ranges from 0.0439 to 0.0504. The coefficient a of the USV samples is the minimum value of 0.0081, the coefficient a of the USH samples is the middle value of 0.0096, and the coefficient a of the USO samples is the maximum value of 0.0132. The coefficient b of the USV samples is the largest (0.0504), the coefficient b of the USO samples is in the middle (0.045), and the coefficient b of the USH samples is the lowest (0.0439).

The strain and duration of steady-state creep stage are important indexes to evaluate the mechanical properties of samples, so it is necessary to analyze and evaluate these two parameters for each sample. Under the influence of the native cracks parallel to the bedding, the USH samples have a small response to tensile stress. So the creep strain in the steady-state creep stage is the minimum. Under tensile stress perpendicular to the bedding plane, cracks, pore spaces, and openings exist along the bedding plane, which are the reasons for the large creep strain of the USV samples in the steady creep stage. The main reason for the medium creep strain of the USO samples in the steady creep stage is that the contact area between the samples and the machine tool pressure plate decreases due to the sliding and spalling of the samples layer by layer. There is a nonlinear negative correlation between the duration of steady-state creep stage and the load. It can be seen from the curve that, compared with the USV samples and the USO samples, the USH samples have the longest duration in the steady-state creep stage, followed by the USO samples, and the USV samples have the shortest duration. On the whole, the duration of steady-state creep stage decreases with the increase of load; it can be expressed as a logarithmic relationship, which is ts=alnσ +b (Figure 11). [Please download the PDF to view the inline formulas].where ts is the duration of the steady-state creep stage and σ is the load. The duration of the steady-state creep stage decreases with the increase of the load, indicating that the influence of structural anisotropy on the duration of the steady-state creep stage decreases with the increase of the load. When the load reaches 0.8σ c of the rock sample, the steady-state creep stage lasts for a short time, and the accelerated creep stage has a great influence on the deformation of the sample.

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