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In view of a previous study of the intermediate principal stress effect at a limited σ2 range, a series of true triaxial tests, covering a full range of intermediate principal stresses that vary from the generalized triaxial compression stress state (σ2 = σ3) to the generalized triaxial tensile stress state (σ1 = σ2), was carried out on sandstone and granite samples. The experimental results revealed that the deformation, failure strength and failure mode have a significant dependence on the stress state. As an effect of the intermediate principal stress on crack evolution, the deformation difference known as stress-induced deformation anisotropy occurred and should be considered when developing the mechanical model. Moreover, a post-peak deformation with a step-shaped stress drop is observed and illustrates that there will be a multi-stage bearing capacity after the rock failure. The peak strength is non-symmetrical with the increasing σ2 and is closely related to the Lode angle. Based on the final fracture surface and SEM analysis under true triaxial compression, three failure modes and failure zones, including tension failure, shear failure and mixed failure, are delineated and discussed. Combining the failure mode and the strength under true triaxial compression, it is found that the strength variation exhibited a close relationship to the failure mechanism.

To study the difference in the failures of laboratories #7 and #8 of the China Jinping Underground Laboratory Phase II (CJPL-II), different loading and unloading stress path tests were carried out under a true triaxial compression condition based on the measured in situ stress of CJPL-II. The strength, deformation and failure mechanisms of Jinping marble under different stress paths were thoroughly investigated. The results showed that under the same stress state as that at sample failure, the bearing capacity and deformation of the unloading stress path were greater than those of the loading stress path. The bearing capacity and deformation in the σ2 and σ3 directions of stress path II and stress path III were similar, and the others decreased from stress path IV to stress path V under unloading conditions. The failure mode results reflect the violence of the unloading failure. The failure features and mechanisms of different stress paths were analysed though the acoustic emission (AE) ring, cumulative energy and strain energy trend. This research may provide a reference for the excavation and support of deep underground works.

Neuropathic pain (NP) is caused by damage to the nervous system, resulting in aberrant pain, which is associated with gene expression changes in the sensory pathway. However, the molecular mechanisms are not fully understood. A non-coding Ribose Nucleic Acid (ncRNA) is an RNA molecule that is not translated into a protein. NcRNAs are involved in many cellular processes, and mutations or imbalances of the repertoire within the body can cause a variety of diseases. Although ncRNAs have recently been shown to play a role in NP pathogenesis, the specific effects of ncRNAs in NP remain largely unknown. In this study, sequencing analysis was performed to investigated the expression patterns of ncRNAs in the spinal cord following spared nerve injury-induced NP. A total of 134 long non-coding RNAs (lncRNAs), 12 microRNAs (miRNAs), 188 circular RNAs (circRNAs) and 1066 mRNAs were significantly regulated at 14 days after spared nerve injury (SNI) surgery. Next, quantitative real-time polymerase chain reaction (PCR) was performed to validate the expression of selected lncRNAs, miRNAs, circRNAs, and mRNAs. Bioinformatics tools and databases were employed to explore the potential ncRNA functions and relationships. Our data showed that the most significantly involved pathways in SNI pathogenesis were ribosome, PI3K-Akt signaling pathway, focal adhesion, ECM-receptor interaction, amoebiasis and protein digestion and absorption. In addition, the lncRNA-miRNA-mRNA and circRNA-miRNA-mRNA network of NP was constructed. This is the first study to comprehensively identify regulated ncRNAs of the spinal cord and to demonstrate the involvement of different ncRNA expression patterns in the spinal cord of NP pathogenesis by sequence analysis. This information will enable further research on the pathogenesis of NP and facilitate the development of novel NP therapeutics targeting ncRNAs.

Energy principles, which can favorably explain the complete rock failure process, are one of the most reliable analysis approaches in rock mechanics and engineering. In this study, a strain energy approach under true triaxial compression (TTC) is proposed. On this basis, the energy evolution characteristics and variations of different failure behavior types (Class I, Class II and ductile failure) under TTC are analyzed. The variations of the strain energy characteristics of Beishan granite with σ2 and σ3 under TTC are studied. The results indicate that the total strain energy U and the elastic strain energy \\[U^{e}\\] of Beishan granite increase with the increasing σ2 or σ3. The dissipated strain energy \\[U^{d}\\] rapidly increases when the value of ε1/ε1peak is approximately 0.6–0.8. The influence of σ3 on the rock failure mode and energy evolution characteristics is greater than that of σ2. In highly brittle rocks, the tensile cracking of the rock microstructure is dominant, and the rock has a high strain energy storage capacity and a low strain energy dissipation capacity. The cumulative acoustic emission (AE) count rate curve shows the same trend as the total dissipated strain energy \\[U^{d}\\] curve. The research results show that the proposed strain energy analysis method for TTC can explain the macroscopic failure behaviors, microscopic failure mechanism and AE characteristics of Beishan granite under TTC, thereby providing new ideas and methods for investigating the behaviors of deep underground hard rock.

Deep rock masses are in true triaxial stress (TTS) fields, and few studies have examined the brittleness evaluation of rock under TTS thus far. In particular, research has not been conducted on the brittleness evaluation index and the effect of TTS. Therefore, based on a summary and analysis of pre-existing brittleness indexes, this paper proposes a brittleness index of rock under TTS that considers both stress-induced anisotropy and energy conversion characteristics during Class I behavior and Class II behavior. Meanwhile, the studied brittleness index is analyzed and verified. The brittleness of rock was found to increase with increasing σ2 while σ3 is constant; when σ2 is constant, the rock brittleness decreases with the increase in σ3. When σ3 is high, the increase in the rock brittleness amplitude with increasing σ2 is large; conversely, when σ3 is low, the amplitude is small. For the same σ3 (σ3 = 5 MPa), the order of brittleness of the three rocks is basalt > granite > marble. Finally, this paper shows that the proposed brittleness index can be applied to conventional triaxial and uniaxial stresses. In addition, under different TTS, a brittleness threshold of σ2/σ3 = 5 was found for the rocks, and when σ2/σ3 < 5, the rock brittleness enters a stress sensitive zone. The research results in this study can provide a relevant basis and guidance for the study of the brittle fracture process and the brittleness of rock under TTS.

During underground excavation, rocks are generally subjected to weak dynamic disturbances caused by blasting, machine vibration, rockburst. To investigate the failure characteristics of hard rock subjected to weak dynamic disturbance, two types of tests, dynamic and static true triaxial tests, with different minimum principal stresses σ3 (0.5, 5, 10, 15, 20, 25, and 30 MPa) were conducted on granite specimens. In the former type of true triaxial test, a weak dynamic disturbance was included; while in the latter type of true triaxial test, only a static stress state was considered. Then, the strength and failure mode of the tested specimens were systemically investigated. The experimental results indicate that the rock strength from the dynamic true triaxial tests is lower than that from the static true triaxial tests, indicating a reduction effect of the weak dynamic disturbance on the rock strength. This reduction effect will gradually increase with σ3. In addition, the failure of the tested specimens from the dynamic true triaxial tests is much more serious than that of the tested specimens from the static true triaxial tests, which may be due to the promotion effect of the weak dynamic disturbance on the crack development. Specifically, when σ3 is low, the dynamic disturbance primarily promotes the development of subdominant cracks (vertical tensile cracks and short oblique tensile-shear cracks) within the tested specimens; when σ3 is high, the dynamic disturbance primarily promotes the development of predominant cracks (through oblique tensile-shear cracks).

True triaxial short-term and long-term tests were conducted on three different types of hard rocks, namely, marble, basalt and granite. During true triaxial long-term creep tests, the crack evolution information was collected by using an acoustic emission system. The micro-structures before and after creep tests were analysed by scanning electron microscopy and polarising microscopy. The time-dependent propagation of cracks in hard rocks is affected by applied stress and the mineral composition. The high axial stress makes the cracks propagate with time development during creep stage. When the axial stress exceeds the damage stress, the AE signals intensity have increased significantly. The damage stress values determined through short-term and long-term tests are different, but the difference is small so that the former can be used efficiently to test the damage stress in terms of long-term strength analysis. Accumulation of extensile cracks induced by high applied axial stress eventually can cause creep failure of hard rocks; however, the time-effect seems to only increase the micro-cracking inside the rock. The ultimate macroscopic failure surface of the rock sample appears to only be affected by the difference between σ2 and σ3. The time-dependent dilatancy of Jinping marble and Baihetan basalt exceeded that of granite specimens. Under creep loading, calcite and pyroxene, which have lower hardness values, are more prone to crack time-dependent expansion than feldspar and quartz, which have higher hardness values.

Hard rocks exhibit three-dimensional (3D) stress-dependent failure under true triaxial compression. The deformability and strength of hard rocks under true triaxial compression differ from those under traditional loading schemes of conventional triaxial compression tests. For the purpose of characterizing these distinctive features, including 3D stress-dependent brittleness and the failure process and 3D stress-induced anisotropy, a new model suited for hard rocks was proposed in this paper. In the new model, the 3D stress-dependent brittleness under true triaxial compression tests was reflected by a determination method of the internal variable considering the influences of both σ2 and σ3. For the failure process, different evolutions in cohesion and the friction angle were applied to realize different failure processes under different 3D stresses, and their 3D stress dependency was identified by the 3D brittleness index defined in this paper. The 3D stress-induced anisotropy involved in the deformation and failure of hard rocks under true triaxial compression was described via the deformation modulus evolution. The formulation for the stress-induced stiffness matrix and the model framework is fully thermodynamically consistent. The model was implemented in the 3D elasto-plastic cellular automaton system, and good agreement was achieved between the numerical simulation and experimental results, indicating that the new model can be applied to describe the failure behaviours of hard rocks under 3D stress.

Because true triaxial compression testing is generally more complicated, time-consuming, and expensive than conventional triaxial compression testing, further research is needed to reduce the cost and time of true triaxial compression tests. In this study, the true triaxial apparatus developed by Northeastern University is used, which can change the principal stress in one direction independently. Multi-stage true triaxial compression tests are carried out on Jinping marble, and the effectiveness of the new test method is evaluated. The strength parameters obtained from the multi-stage tests and single-stage tests are compared. It is suggested that constant slope unloading should be carried out before the peak strength to ensure reliable and effective experimental results. The multi-stage true triaxial compression test exhibits good repeatability and can obtain reliable strength parameters in a short time for reference in engineering practice. Under different multi-stage loading conditions, Jinping marble presents different strength, deformation and failure characteristics, and the macrofailure mechanical properties of a specimen are affected by the propagation of microcracks within that specimen.