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Understanding salt rock creep properties is important for designing and operating salt caverns. Triaxial multistage creep test of salt rock and acoustic emission (AE) monitoring were conducted, revealing the influence of deviatoric stress on the creep and AE characteristics of salt rock. Based on the creep AE test of salt rock, the evolution characteristics of AE during the creep process of salt rock are revealed from a microscopic point of view. Previous constitutive models describing rock creep only considered the damage effect during the accelerated stage, and could not accurately depict the damage evolution during the creep process. Using AE characteristic parameters, a damage constitutive model was established, revealing an exponential increasing trend in damage evolution during salt rock creep process. Considering the damage effects during salt rock creep, a novel creep damage constitutive (CDC) model was constructed based on fractional derivatives, and expressions for one-dimensional and three-dimensional (3D) stress states were given respectively. The proposed model and the classical model were calibrated by the triaxial creep test data, indicating that the proposed model can better describe the nonlinear creep characteristics of salt rock. [ABSTRACT FROM AUTHOR]

The behavior of fluid flow through rock fractures in the form of radial flow is a critical issue in various subsurface geological engineering projects. Previous work has considered to couple the radial seepage flow and the normal stress on the fracture surface, while ignored the influence of the fracture surface morphology on the seepage. Therefore, to quantitatively assess the fracture morphology influences on radial permeability, we conducted research adopting seepage testing and fracture surface scanning experiments and the tests results were present in this paper. The difference about fracture surface morphology of each sample can be quantitatively described by fractal dimension. Compared with natural fractures, artificial fractures had larger fractal dimension, rougher surface, and lower radial permeability. And the average radial seepage flow of natural fractures was about 1.57 times that of artificial fractures. In the same sample, the single hole permeability of different outlet holes was also different, which was related to the fracture surface morphology and its fractal dimension of the section line in this direction. The magnitude of fracture radial seepage flow increased with decreasing fractal dimension, and the change law of both followed a negative exponential function. A similar change tendency appeared in both artificial fracture samples and natural fracture samples. [ABSTRACT FROM AUTHOR]

Understanding the long-term creep mechanical behavior of granite is critical to evaluating the safety of the nuclear waste repository. It is unreasonable to extrapolate the creep test results of hard rock samples at high stress to lower stress. In this work, the uniaxial creep test of granite lasting for 1117 days is carried out at lower axial stress levels (60 and 87 MPa) to reveal its long-term time-dependent deformation characteristics. The test results show that under 60 MPa and 87 MPa, the creep deformation of granite accounts for only 21.7% and 36% of the total deformation, while that of salt rock is about 80%, making the creep of hard rock difficult to be detected. The steady creep rate of granite is 9.14 × 10−12 s−1, which is 1–2 orders of magnitude slower than that of salt rock under low stress, and 3–6 orders of magnitude slower than that under short-term granite creep test. At the same stress ratio of about 0.45, it takes 47.6 days for granite samples to enter the steady creep stage, which is much shorter than 213 days for salt rock samples. In the short-term (several hours or days) creep test under low stress level, the hard rock sample does not actually enter the steady creep stage. Based on the fractional derivative theory and damage mechanics, a novel nonlinear creep-damage constitutive model that can well describe the long-term brittle creep characteristics of granite is proposed. This research reveals the very long-term time-dependent deformation behavior of hard rock, which is conducive to the evaluation of long-term stability of nuclear waste repositories. [ABSTRACT FROM AUTHOR]

At present, hydrofracture technology is mainly used for shale gas exploration and exploitation, and the effect of volume fracturing is tightly linked to the mechanical properties (including anisotropy and brittleness) of shale. Meanwhile, Sichuan Basin is also an important area for unconventional natural gas exploration and development in China. This work analyzed mineral composition and microstructure of outcrop shale matrix in Lower Silurian Longmaxi Formation through XRD test, slice observation, and scanning electron microscope (SEM). How anisotropy affects shale deformation, strength and failure characteristics at different bedding angles (0°, 30°, 45°, 60°, and 90°) under diverse confining pressures (0 MPa, 25 MPa, 50 MPa, 75 MPa, and 100 MPa) in conventional uniaxial/triaxial experiments were discussed. The acoustic emission (AE) evolution was introduced to reveal the cracks closure and propagation of Longmaxi Formation shale and qualitatively characterized the brittleness of the shale. What's more, based on the existing brittleness index, this study proposed an indicator BInew that considers the shape of the stress–strain curve before and after the peak stress to evaluate the brittleness of the Longmaxi Formation shale. The index BInew can better describe the changes of brittleness with confining pressure and bedding angle. Finally, this work elaborately explained the relationship between AE characteristics, brittleness, and failure modes of shale. AE characteristics can not only qualitatively characterize shale brittleness but also show the failure process and modes of shale samples. Change of failure modes rests with the difference in brittleness and also reflects the level of brittleness. The change of brittleness with failure modes can be expressed as shear along bedding plane (Sh-Al) > splitting through bedding plane (Sp-Th) > conjugate shear failure (Co-Sh) > shear along bedding plane (Sh-Al) > shear through bedding plane (Sh-Th). Highlights: Uniaxial/triaxial compression and acoustic emission (AE) monitoring of Longmaxi formation shale were carried out. Influence of anisotropy of shale on mechanical properties under different confining pressures was analyzed. Different brittleness indexes were used to evaluate the brittleness of shale, and a new brittleness index was established. Relationship between AE characteristics, brittleness, and failure modes of shale was discussed in detail. [ABSTRACT FROM AUTHOR]

In order to reveal the mechanism of surrounding rock deformation and instability of soft rock roadway in 12# working face of Puhe coal mine, and synthetical analysis of the surrounding rock, deformation and failure of the roadway shows that the original support scheme is difficult to maintain the stability of roadway. Thus the basic concept of roadway surrounding rock control: Single anchor bolt form a compression zone to control the failure and expansion of surrounding rock, and a comparable number of anchor bolts form a homogeneous compression zone to inhibit the development of surrounding rock plastic zone. The "anchor blot + anchor cable net + concrete shotcrete support plan (arrangement of anchor cables four to five)" is proposed. Furthermore, and FLAC3d numerical simulation software was used to verify the rationality and reliability of the supporting scheme of "bolt + anchor cable net + concrete shotcrete support scheme (four to five anchor cables)" support scheme. The on-site monitoring results show that the soft rock roadway in Puhe Coal Mine 12# working face return airway passes through the "bolt + anchor cable net + concrete shotcrete support scheme (anchor cable four to five arrangement)" to reduce the surrounding rock deformation and become stable. The sinking amount of the roof and floor in the roadway is 251.2 mm, the convergence amount of the two sides in the roadway is 231 mm, supporting system and surrounding rock form the stable compression zone, the soft rock roadway can be effectively controlled by the support scheme, which can provide reference value for other similar engineering. [ABSTRACT FROM AUTHOR]

The mechanical model of the basic roof fracture structure is established on the basis of key block theory to study the roof breaking mechanism of gob-side entry retaining under roof cutting and pressure relief, and the analytical formula of roof support resistance is derived when the key block of the basic roof is stable. The influence of roof cutting angle and cutting height on roof support resistance is also analyzed. Determining the cutting seam parameters of the retained roadway roof is necessary to identify the support resistance of the roadway roof due to the correlation between the roof cutting parameters and the support resistance. Taking the II 632 haulage drift of the Hengyuan coal mine as the engineering background, FLAC3D numerical simulation is used in this paper to analyze the influence of different roof cutting angles and cutting heights on the surrounding rock structure evolution of retained roadways. Results show that the roof cutting angle and cutting height respond to the support resistance of the retained roadway roof, and the support resistance required by the roof increases with the roof cutting angle and cutting height. This condition ensures that the side roof of the gob can be cut off smoothly, and the support resistance required by the roof of retained roadways is within a reasonable range. Through theoretical and numerical simulation analysis, the reasonable roof cutting height of II 632 haulage drift is 8 m and the roof cutting angle is 15°. The theoretical analysis and numerical simulation results reveal that the required support resistance to maintain the stability of the roadway roof is 0.38 MPa. The supporting scheme of the roof of the II 632 haulage drift in the Hengyuan coal mine is then designed. Finally, the field industrial test is used for verification. The borehole imaging results show that the overall line of the retained roadway roof is small based on the description of field monitoring results. The deformation of the surrounding rock surface of the retained roadway is less than 100 mm, and the roadway is 40 m from the lagging working face. The deformation rate of surrounding rock decreases with the increase in distance from the working face. The integrity of the retained roadway roof is good, and the deformation of the surrounding rock is effectively controlled. [ABSTRACT FROM AUTHOR]

As the mining depth increases, the deformation of the roadway becomes more difficult to control. As a main supporting structure for maintaining the stability of roadway, the fully anchored bolt is widely used to reinforce deep mine. At the same time, the analysis of the stress distribution law of fully anchored bolt is the basic work to optimize anchor design. Therefore, this paper establishes a fully anchored bolt-surrounding rock interaction model based on the law of surrounding rock deformation and derives the analytical expressions for the axial force and shear stress of the fully anchored bolt during normal support and critical failure. At the same time, the effects of surrounding rock properties, support resistance, and bolt length on the stress distribution of fully anchored bolt are analyzed. The results show that the stress distribution of fully anchored bolt is consistent with the "neutral point" theory and the most important is the fact that the conditions of surrounding rock, the supporting resistance, and the length of bolt affect the actual stress distribution of the fully anchored bolt. It provides a certain theoretical basis for the design and development of anchoring and supporting technology. [ABSTRACT FROM AUTHOR]

Hydraulic coupling often leads to progressive rock failure accidents. Mechanical tests were performed over a range of combined pore water pressure and confining pressure stress path conditions to study the progressive failure characteristics of sandstone under hydraulic-mechanical coupling and explore the crack initiation and pore water fracture mechanism. The closure stress and damage stress were determined by the axial deformation stiffness and volume deformation stiffness. The experimental results indicate that confining pressure is the main controlling factor in the crack propagation stage, and pore water pressure enhances crack evolution. With increasing effective confining pressure, the effective peak deviatoric stress strongly increases and the characteristic stress increases linearly. The initiation stress and damage stress decrease with increasing pore water pressure. The moduli in stages I, II, and III are similar to the law of the transverse and radial deformation ratio with notable differences in stage IV. The fracture trend angle was determined by the ratio of axial crack strain and radial crack strain. Compared with the experimental results, the internal cracks in the sandstone samples are mainly type-II cracks, and type-I cracks are also locally present. After stress damage, the cracks expand and extend at an angle close to the real fracture. [ABSTRACT FROM AUTHOR]