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In situ rock mass usually contains discontinuities in various forms, including fissures, joints and fractures, etc., and it becomes more heterogeneous when the discontinuities are backfilled with artificial grouting. It is incredibly challenging to obtain an accurate wave velocity model for those rocks with random inclusions, making the AE sources more difficult to locate. This study first proposed a projective reconstruction method (PRM) to obtain the wave velocity model for heterogeneous rock. An improved AE source localization algorithm was proposed using the arrival time-based governing equations. Flawed sandstone samples with various inclination angles were then prepared and subjected to unconfined compression until failure. The fragments were restored to generate post-grouted samples with random inclusions. The feasibility and validation of the PRM-based wave velocity model and the proposed AE source localization algorithm were verified by pencil lead break tests. The AE source localization results were compared using three wave velocity models, i.e., averaged wave velocity model, 1D approximated wave velocity model and PRM-based wave velocity model. The results indicate that the PRM-based AE source localization for the post-grouted samples is much more accurate than the other two models. Finally, the fracturing mechanisms of the post-grouted samples with random inclusions under uniaxial compression conditions were illuminated based on the results of the AE source localization and moment tensor inversion. It is confirmed that the PRM-based wave velocity model was promising for the AE source localization in heterogeneous rocks with random inclusions. Highlights: A projective reconstruction method for obtaining the wave velocity field of heterogeneous rocks. More accurate AE source localization was achieved for post-grouted sandstones with random inclusions. The proposed PRM-based wave velocity model are promising for microseismic localization application in the practical field. [ABSTRACT FROM AUTHOR]

A large amount of slates is distributed along Kangding No. 2 tunnel of Sichuan–Tibet Railway (China). The bedding of the Kangding slate is rich, showing apparent anisotropy of strength. For underground tunnels excavated in the anisotropic rock mass, the rock material is vulnerable to damage in tension because of its low tensile strength. Therefore, the tensile strength and fracture modes are important for the support design of layered surrounding-rock tunnel. In this study, first, a series of direct tensile tests were carried out on slate samples with five kinds of bedding dip angles. Second, the effect of bedding dip angle on the slate's tensile strength, failure modes, and acoustic emission characteristics was analyzed. Third, based on the Nova–Zaninetti criterion and the experiment results, a new phenomenological anisotropic criterion was proposed by introducing anisotropic coefficient, which could describe the nonlinear variation trend of tensile strength with foliation angles. Compared with four typical tensile strength criteria, the accuracy and universality of the new criterion were verified. Finally, the new criterion was implemented into the combined finite-discrete element method (FDEM) to conduct direct tensile simulation of slate with different foliation angles. The results indicated that the direct tensile strength followed an S-shaped increasing trend with the variation of foliation angles. The direct tensile failure modes of slate samples with different foliation angles could be divided into layer activation failure, mixed failure, and rock matrix failure. The acoustic emission process of slate with different bedding inclination under direct tension could be roughly divided into the following three stages: quiet period—stepped saltatory increase—drastic increase. The fitting effect of the new criterion was the highest among Kangding slate and other six typical layered rocks, which showed that the new criterion was suitable for predicting its Brazilian strength and direct tensile strength. At the same time, the numerical simulation results were in good agreement with the experimental results, which further verified the accuracy of the new criterion. Highlights: The acoustic emission characteristic parameters helped to reveal the direct tensile failure characteristics of layered rocks. A new phenomenological anisotropic tensile failure criterion was proposed. The direct tensile test data and the Brazilian splitting test data were used to study the accuracy and universality of the new anisotropic tensile failure criterion. FDEM was used to study the accuracy of the new anisotropic tensile failure criterion. [ABSTRACT FROM AUTHOR]

The accuracy of locating acoustic emission (AE) source location is closely related to the reasonability of the adopted wave velocity model. However, it is literally hard to obtain an accurate wave velocity model for a heterogeneous rock material due to its inner invisible inclusions and local defects, making the AE source localization very challenging. To obtain a more appropriate wave velocity model for the heterogeneous rock material having unknown inclusions, in this study the projective reconstruction method (PRM) was utilized to determine the wave velocity model. Because the PRM is irrelevant to the continuous assumption, the proposed wave velocity model can well reflect the discontinuous characteristics of the real wave velocity field in rock materials. Then, the proposed wave velocity model was incorporated into an arrival time-based governing equation to locate the AE source. To demonstrate the effectiveness of the proposed method in improving the AE source location accuracy and monitoring the rock failure process, lead-break tests and three-point bending tests were conducted on the composite rock and rock-like specimens, respectively. Results show that the improved wave velocity model obtained by the PRM can significantly improve the accuracy of AE source location in the heterogeneous rock material with unknown inclusions. [ABSTRACT FROM AUTHOR]

This study experimentally investigates the influence of heating/cooling cycles on the mechanical properties and cracking features of Rucheng granite. The specimens, which were previously subjected to various numbers of heating (350 °C) and cooling (in 21 °C tap water) cycles, were tested under unconfined compression. Integrated acoustic emission (AE) and digital image correlation (DIC) techniques were used to study the micro/macrocracking characteristics of the specimens upon different heating/cooling cycles. The changes in the amount of surface cracks and microcracks, P-wave velocity, unconfined compressive strength (UCS), elastic modulus, and Poisson's ratio of the specimens prior to and after various heating/cooling cycles were comprehensively analyzed. The results showed that as the number of heating/cooling cycles increased, more surface cracks and internal microcracks (including intergranular and intragranular) were observed, leading to more remarkable decreases in P-wave velocity and substantial loss of the strength and stiffness. The change in the P-wave velocity is more sensitive than the UCS upon the cyclic heating/cooling treatments. An increase in the cycle number yielded an increase in the brittleness and a decrease in the ductility with respect to the stress-strain response. Furthermore, after exposure to a greater number of heating/cooling treatment cycles, the onset of unstable crack propagation will initiate at a lower stress threshold, while the postpeak softening response becomes more prominent. In addition, the failure mode changes from a typical shear failure to an axial splitting fashion as the number of heating/cooling cycles increases. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study in which molded gypsum pre-cracked specimens with two types of flaw (single and X-shaped cross flaws) were tested under biaxial compression. Results show that acoustic emission (AE) energy rate curves can be divided into three different periods during uniaxial compression, which correspond to crack closure, linear elastic and unstable failure stages of specimens. However, only two periods are observed during biaxial compression, which correspond to linear elastic deformation and unstable failure. Furthermore, two proposed AE parameters, namely the AE energy ratio and AE energy release ratio, and a classical AE parameter b -value were used to analyze the influence of biaxial compression loading on AE energy, respectively. With increasing confining pressure, the AE energy release ratio, as well as AE b -value decreases gradually while AE energy ratio increases. Moreover, maximum and average AE energy rate values decrease as confining pressure increases. Data presented herein is useful to study the AE characteristics of pre-cracked specimens under biaxial compression. [ABSTRACT FROM AUTHOR]

Since rock mass in many fields of rock engineering usually undergoes a cyclic heating and cooling process, it is very meaningful to investigate the variations of rock micro/macro physical and mechanical properties subjected to cyclic heating and cooling. However, due to the complex and invisible characteristics of rock microstructure, the effect of cyclic heating and cooling on rock macro/micro physical and mechanical properties still requires further investigation. In this study, to explore the microscopic mechanism underlying the variations of rock macroscopic properties during cyclic heating and cooling, uniaxial compression tests and P-wave velocity tests were conducted to obtain the macromechanical properties of red sandstone specimens subjected to varying numbers of heating and cooling cycles. Acoustic emission (AE) tests were also carried out to capture the variations in the microscopic damage process during each heating and cooling cycle. A scanning electron microscope and differential thermal analysis–thermal gravimetric analyzer were used to analyze the development of the microcracks and variations in the micrograin mass after each cyclic heating and cooling. The test results demonstrate that after being heated to 500 °C, some of the minerals (microcline, albite and calcite) in the sandstone decomposed and microcrocks developed due to the uneven thermal stress. When the sandstone was cooled in water, more microdefects were induced to the microstructure due to mineral transformations and uneven contractions, causing further deterioration to the integrity and compactness of the rock matrix. This phenomenon was also reflected in the uniaxial compression tests and AE tests. Due to the damage caused by each heating and cooling cycle, the AE hit rate decreased and the macrophysical and mechanical properties of rock deteriorated as the number of cycles increases. Comparison of the test results obtained from samples subjected to different heating and cooling cycles reveals that although each cycle could cause damage to the rock sample, the first and fifth cycles induced more severe damage, as indicated by the sharp decrease in the physical and mechanical properties of the rock after the first and fifth cycles. [ABSTRACT FROM AUTHOR]

In this study, a two-dimensional Voronoi element based – explicit numerical manifold method (VE-ENMM) is developed as a new approach to investigate the acoustic emission characteristics during the failure process of intact rocks. The micro-granular structures of the intact rocks are approximated by the random Voronoi polygons and the interactions among rock grains are interpreted using a modified interface contact model. A new acoustic emission (AE) monitoring technique considering the clustering effect of micro-cracks is developed based on the explicit time integration scheme of numerical manifold method. With the newly developed approach, a series of numerical simulations are carried out. Firstly, numerical uniaxial compressive and Brazilian tests are conducted to validate the new approach on simulating micro-/macro-mechanical behaviors of granite samples. After that, the acoustic event properties, such as time evolution, spatial clustering and frequency-magnitude distribution of AE events during the failure process under uniaxial compression are studied in details. Simulation results show that the development of acoustic emissions is in good consistence with the damage evolution process of the numerical model, which may help to reveal the micro-fracturing mechanism of intact rocks in terms of seismic energy dissipation. To further illustrate the capability of the developed approach in monitoring the failure process as well as failure characteristics during TBM tunnel excavation, an additional numerical simulation is conducted and the fracturing behaviors as well as the related AE properties of the surrounding rock mass during TBM tunnelling are investigated. [ABSTRACT FROM AUTHOR]

Experiments for the rectangular rock-like samples (made of high-strength gypsum and water-cement ratio is 1) with two parallel pre-existing flaws subjected to uniaxial compression were carried out to further investigate the influence of varying flaw geometries on mechanical properties and crack coalescence behaviors. According to the tests results, eight crack types were characterized on a basis of the mechanisms of crack nucleation, formation and propagation, and seven coalescence modes occurred through the ligament, including S-mode, M1-mode, M2-mode, M3-mode, T1-mode, T2-mode and T3-mode. The AE and photographic monitoring techniques were adopted to further clarify the procedure of the crack coalescence and failure during uniaxial compression tests and in consequence the whole process of crack emergence, growth, coalescence and failure was recorded in real time. The results of AE technique revealed that the characteristics of acoustic emission energy associate with crack coalescence modes, and AE location method can emphasize the moments of crack occurrences and follow the crack growth until final failure. This study put forward better understanding of the fracture and failure mechanism of underground rock engineering, like rock burst. [ABSTRACT FROM AUTHOR]

Crack initiation stress (CI) is an important index of rock and the index of thermally damaged rock under uniaxial compression is examined in this study. Methods for determining CI are first reviewed, and the CI values of two thermally damaged rocks are then identified using several methods including crack volumetric strain (CVS) method, lateral strain response (LSR) method, and cumulative acoustic emission hit (CAEH) method. The results show that CI estimated from the CVS method and the CAEH method is comparable with each other. On the other hand, it is found that CI determined using the LSR method is generally lower than these determined by the other two methods. Because the lateral strain of a thermally damaged rock is small (sometimes positive in the initial stage), the LSR method is not applicable for determining CI of thermally damaged rocks. A modified LSR (MLSR) method, which excludes the initial collapse deformation stage, is proposed, and the new method truly reflects new crack initiation. By comparing the determined CI with other methods, it is found that the proposed MLSR method is capable of determining CI of thermally damaged rock. Variation of CI with the degree of thermal damage is also studied. It is found that CI generally decreases as the number in the cyclic treatment increases, while the difference between CI to uniaxial compressive strength (UCS) ratio and crack damage stress (CD) to UCS ratio decreases with increasing thermal damage in the rock. The results indicate that unstable cracking propagation is easy to occur as the thermal damage in the rock increases. • Previous methods for determining crack initiation are reviewed and compared. • A MLSR method is proposed to determine crack initiation of thermally damaged rock. • Difference between CI/UCS and CD/UCS decreases as thermal damage in rock increases. • Unstable crack propagation is easy to occur when the rock is thermally damaged. [ABSTRACT FROM AUTHOR]

In this paper, to study the influence of grouting on meso-failure mechanism, acoustic emission (AE) localization technique was used to monitor the micro-cracking during the uniaxial compression tests on single flawed sandstone specimens with or without grouting. Due to the inhomogeneous geometrical structure of the flawed specimens, the mesoscopic damage evolution is also inhomogeneous during the loading process. As a result, the wave velocity of the specimens varied temporally and spatially, which greatly increased the difficulty in accurate AE localization. To solve this problem, the wave velocities versus the axial load for the studied sandstone were first measured, and the stress concentration effect near the flaw was also theoretically analyzed. Accordingly, the localization arithmetic was also improved to match the complex wave velocity field. Results show that compared to the specimens without grouting, more microcracks (particularly the shear microcracks) are induced in the grout-infilled specimens prior to the final failure of the specimens. Moreover, the effect of grouting on microcrack initiation also varies with the oblique angle of the flaw. For θ (flaw inclination) ≤30°, grouting significantly increases the shear microcrack at macrocrack propagation stage which states that the axial tensile cracking was limited by the supporting effect of the grout material and made the failure mode trend to shear mode. For θ = 45° and 60°, grouting improved the ratio of shear microcrack during the whole failure process, indicating that the direction of maximum shear stress is close to the flaw direction. Hence, failure develops in stable shear mode. For θ = 75° and 90°, the primary microcrack generated during the first macrocrack initiation changed to shear mode, but the grouting has little influence on the subsequent crack propagation. So that, the reinforcement effect is poor in this set of specimens. • The AE localization technique was improved by adopting a temporally and spatially varied wave velocity model. • Grouting decreases the stress concentration near the prefabricated flaw, which changes the mechanism of crack evolution. • The development of a macro crack in the flawed specimen follows a tensile-shear-alternating meso damage evolution mode. [ABSTRACT FROM AUTHOR]

Understanding the micro-mechanism of brittle creep in saturated rocks and studying their mechanical behaviors after creep damage are of great significance to the design of rock engineering and the prediction of the long-term evolution of the Earth's crust. In this study, we first performed a series of uniaxial creep tests with different creep stage cut-offs on saturated intact sandstones. The brittle creep mechanism of the specimens was explored microscopically by analyzing P-wave velocity, acoustic emission (AE), and nuclear magnetic resonance (NMR) measurements. Then, specimens with different degrees of creep damage were subjected to a secondary short-term loading test or creep test, and their short- and long-term mechanical behaviors, together with AE monitoring, were systematically investigated. The experimental results show that (1) the anisotropy of the specimens increases significantly during creep, with the axial and lateral P-wave velocities exhibiting a difference of 30% at the onset of tertiary creep. (2) The tensile cracks dominate the transient creep stage and the early part of the steady creep stage, while the shear cracks dominate thereafter. (3) The porosities of the specimens increase with the loading stages, and the porosity increments corresponding to small pores are greater than those corresponding to large pores, for all creep stages. (4) Specimens with different degrees of creep damage show greater deformability, and their short-term and creep mechanical parameters, except for uniaxial strength, change dramatically. (5) The AE characteristics of creep-damaged specimens differ significantly from those of intact specimens, and the Kaiser stress memory effect was observed during reloading, with the memory stress being the previous creep stress. [ABSTRACT FROM AUTHOR]