Search

Water injection, as a widely used technique to prevent coal burst, can restrain the fractured coal seam and released the energy storage. In this study, laboratory tests were firstly carried out on standard coal specimens with five different water contents (i.e., 0%, 0.6%, 1.08%, 1.5%, 2.0%, and 2.3%). The failure mode, fragment size, and energy distribution characteristics of coal specimens were investigated. Experimental results show that strength, elastic strain energy, dissipated energy, brittleness index, as well as impact energy index decrease with increasing water content. Besides, the failure mode transitions gradually from splitting ejection to tensile-shear mixed failure mode as water content increases, and average fragment size shows positively related to water content. Moreover, scanning electron microscope tests results indicate that water in coal sample mainly causes the mineral softening and defects increase. Furthermore, a numerical model containing roadway excavation was established considering the water on coal burst prevention. Modelling results revealed that water injection can reduce degree of coal burst and ejection velocity of coal blocks, while it will raise up the depth of crack zone and surface displacement of roadway. Combined with laboratory tests and numerical results, the micro mechanism, energy mechanism, and engineering significance of water injection on coal burst prevention were finally analyzed. [ABSTRACT FROM AUTHOR]

To investigate the effects of the oval hole on the strength and the failure mechanisms of rock materials, a series of uniaxial and biaxial compression numerical simulations were carried out using the combined finite-discrete element method (FDEM). Based on the numerical simulation results, the effects of dip angle (β), major–minor ratio (ω) and position of the oval hole on rock strength characteristics crack initiation and development process and final failure modes were analysed in detail. The numerical simulation results indicate that: (1) Under uniaxial compression, the ultimate failure modes of rock containing oval holes can be mainly classified into single inclined plane shear failure, X-shaped tensile-shear composite failure and Y-shaped tensile-shear composite failure. Under biaxial compression, the ultimate failure mode is single-inclined plane shear failure dominated by shear cracks. Whether it is uniaxial compression or biaxial compression, the crack initiation position of the main fracture surface is near the end of the major axis of the oval hole. (2) Under uniaxial compression, the axial stress–strain curve of rock containing the oval hole is smooth before and after the peak, but under biaxial compression, there is obvious fluctuation in the post-peak stage due to crack initiation and propagation. In both uniaxial and biaxial compression, the pre-peak elastic stage of the axial stress–strain curve of the rock containing the oval hole is significantly shortened and the peak stress is significantly reduced compared with the intact rock, which indicates that the bearing capacity of the rock is seriously reduced by the existence of oval hole. (3) When the position and major–minor axis ratio of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength increase with the increase of the dip angle of the oval hole. When the position and dip angle of the oval hole remain unchanged, the uniaxial compressive strength and biaxial compressive strength decrease with the increase of the major–minor axis ratio of the oval hole. (4) At different dip angles, the brittleness index Bi of rock containing oval holes decreases with the increase of confining pressure, and the brittleness degree shows a gradual weakening trend. At the same time, when the dip angle increases, the decreasing amplitude of brittleness index Bi decreases with the increase of confining pressure. Three types of failure modes of rock with elliptical hole are systematically summarised, and the failure mechanisms of elliptical hole rock are revealed. The influence of different positions, major–minor axis ratio, and dip angle of rock containing oval holes on uniaxial compression and biaxial compression are studied. The relationship between brittleness index and failure mode of rock containing the oval hole is investigated. [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 design of rock engineering projects requires an estimation of the strength of intact rock blocks. These blocks can be significantly larger than the typical laboratory test specimen; thus, understanding the specimen size effect on mechanical properties is essential. In this paper, a series of uniaxial compression tests and Brazilian disc tests were conducted on cylindrical samples with diameters varying from 20 to 150 mm, and direct shear tests were conducted on cubic samples with side lengths varying from 50 to 200 mm. The results showed that the uniaxial compressive strength (UCS), cohesion, and Young's modulus decrease with increasing specimen size, whereas the tensile strength initially increases and then decreases. Moreover, the applicability of different size effect models to experimental data was assessed. It was concluded that the fitting equations of UCS and cohesion are almost the same, whereas that of tensile strength is significantly different. The size effect law of rock mechanics parameters may be affected by the failure mode. [ABSTRACT FROM AUTHOR]

To study the toughening effect and the anchoring mechanism of rock bolts, fractured rock specimens with different crack angles and different crack lengths are made of cement mortar. The prefabricated specimens are applied with different anchoring modes for reinforcement, containing no-anchoring, full-length bonded anchoring and pre-stressed anchoring. The uniaxial compression tests of fractured rock specimens under different anchoring modes are carried out, and then the mechanical properties and the failure modes of fractured rock specimens before and after anchoring are analysed. The 'Cavity Damage Effect' is defined and the test results show that the peak strength of the fractured specimens are roughly proportional to the crack inclination angle due to the 'Cavity Damage Effect', the crack is equivalent to a 'cavity damage' inside the prefabricated specimen if the opening of the crack is larger. The crack length can significantly affect the strength and deformation characteristics of no-anchoring specimens, while has little influence on the anchoring specimens. The full-length bonded anchoring and the pre-stressed anchoring can both significantly increase the mechanical parameters (the peak strength, residual strength and elastic modulus) of the specimens, enhance the resistance to deformation of fractured rock mass, and inhibit the expansion and coalescence of cracks. [ABSTRACT FROM AUTHOR]

The large deformation mechanisms and predictions of soft rock tunnels have always been important but difficult to solve problems in the field of geotechnical engineering. A combined finite–discrete element numerical simulation method (FDEM) was used to study large deformation mechanism, classification and prediction. The deformations or failure mechanisms of tunnel surrounding rock were revealed, and the failure modes and displacement value prediction of surrounding rock with different strength-stress ratios were also investigated. The following conclusions were obtained: (1) Critical hysteresis damping can be adopted to simulate the progressive large deformation process of soft rock tunnels, which can obtain the final deformation of unsupported tunnels and avoid a dynamic response. (2) Under concentrated tangential stress, the surrounding rock undergoes X-shaped conjugate shear fracture, and this type of fracture network continues to propagate toward the depth of the surrounding rock until the model reaches a stable state; the reduction in tunnel cross-section is mainly caused by the macroscopic movement and volume expansion of rock fragments, and the latter is due to the generation of a large number of macroscopic voids. (3) As the strength-stress ratio decreases, the deformation or failure modes of the surrounding rock can be divided into four categories: elastic–plastic deformation, closed fracturing, shear dilation and broken expansion. (4) Finally, a prediction equation for isotropic and homogeneous soft rock unsupported tunnel deformation with general size driven by hydrostatic in situ stress is obtained, which indicates that with an increasing strength-stress ratio, the surrounding rock displacement decreases as an exponential function, with a correlation coefficient of R 2 = 0.997. In addition, the robustness and reliability of the prediction equation is verified. [ABSTRACT FROM AUTHOR]

• A new phenomenological tensile failure criterion with an anisotropic coefficient was proposed. • Direct tensile test of the Kangding slate was carried out to verify the accuracy of the new criterion. • The tensile strength data of eight typical layered rocks were collected and compared with the other six typical tensile strength criteria to verify the universality and superiority of the new criterion. • Based on the sensitivity analysis of tensile rate, the loading rate of 0.01 m/s was recommended for the samples with various foliation angles. • Layer thickness has little effect on tensile strength, but a significant effect on tensile failure modes. The tensile strength is a key parameter for the design of many geotechnical engineerings such as tunnel support, slope support, and petroleum drilling. The tensile strength in layered rocks, however, is anisotropic due to the existence of bedding planes. In this paper, to study the anisotropic tensile behaviors, a series of direct tensile tests were conducted using Kangding slate with five different foliation angles (β = 0°, 30°, 45°, 60°, and 90°). Based on the N-Z criterion, a new tensile failure criterion with the anisotropic coefficient was proposed. The tensile strength data of nine typical layered rocks were collected to evaluate the prediction ability of seven anisotropic tensile failure criteria including the new criterion. Embedding the new criterion into FDEM, the loading rate-dependent and layer thickness-dependent tensile mechanism of layered rock under direct tensile test were simulated. The experimental and numerical simulation results showed that the foliation angle significantly affected the tensile strength and failure modes of the slate. With the increase of foliation angle (β), the tensile strength of nine typical layered rocks, including Kangding slate, presented a non-linear increase trend. Compared with the other six typical anisotropic tensile failure criteria, the new criterion has the best prediction capability on the tensile strength of rocks with four different anisotropic degrees, and therefore the new criterion was recommended. The new criterion was embedded in FDEM to carry out direct tensile simulation of slate with different β. The numerical simulation results were in good agreement with the experimental results, further verifying the accuracy of the new criterion. The loading rate sensitivity analysis indicated that a tensile rate of 0.01 m/s was recommended to assure quasi-static loading of the specimen. Through the sensitivity analysis of layer thickness, it is concluded that layer thickness has very little influence on tensile strength, but has a very significant influence on the final failure modes of the specimens. [ABSTRACT FROM AUTHOR]

The failure mechanism and failure mode of anisotropic rock masses are very different from those of isotropic rock masses. For unsupported tunnels, the failure modes of anisotropic rock masses can be summarized as composite failure, V-shaped notch failure and spalling of bedding planes. Different failure modes are controlled by the mechanical parameters of the rock masses, the environmental occurrences and the excavation conditions. The combined finite-discrete element method (FDEM) was employed to investigate the failure mechanism and failure process of layered rock masses and the influence of different factors on the failure mode. The study results indicate that (1) composite failure is the basic failure mode of the layered rock masses and that it is caused by the combined action of shear-slip fractures F1 parallel to the bedding plane, tensile fractures F2 perpendicular to the bedding plane, and conjugate shear fractures F3; the rotation movement of slab-like rock fragments induced by fractures F1 and F3 into the tunnel causes significant deformation of the surrounding rock masses; (2) on the basis of composite failure, when fractures F1 disappear and fractures F2 obliquely intersect with the bedding plane, V-shaped notch failure occurs; furthermore, spalling of the bedding plane occurs when the propagation of fractures F3 is blocked at the bedding plane; and (3) with increasing rock mass strength, lateral pressure coefficient of in situ stress and layer thickness, or decreasing tunnel span, the surrounding rock masses change from composite failure to V-shaped notch failure and then to spalling of the bedding plane. However, the effect of the elastic modulus on the failure mode of anisotropic rock masses is weak, and the influence of the dip angle of the rock layer on the failure mode is complicated. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach (Y-slope) to simulate entire slope failure processes, from initiation, transport to deposition. The algorithm is implemented in a combined finite-discrete element method code. Absorbing boundary conditions are implemented to improve computational efficiency for the initial stress state equilibrium. Strength reduction methods, considering both tensile and shear failure modes, are implemented to evaluate the slope stability, where the safety factor and critical failure surface are automatically obtained. The energy dissipation mechanism, due to blocks' friction and collision, is incorporated to accurately simulate the block kinematics during the post-failure stage. The accuracy and robustness of Y-slope are validated by numerical tests, and the failure mechanism and failure progress of a homogeneous and jointed rock slope are presented. Results indicate that Y-slope can not only evaluate the slope stability state (e.g., safety factor and critical failure surface), but also simulate the entire failure process (e.g., slope deformation, failure surface evolution, block transport and deposition). In addition, the critical role of existing discontinuities on the slope stability and failure mechanism are also highlighted. This work proposes a promising tool in understanding the failure mechanism and assessing the potential risk by predicting the entire failure process of rock slopes. [ABSTRACT FROM AUTHOR]

Rockburst has become a main factor threatening the safe construction of deep-buried tunnels. To monitor microseismic (MS) event and comprehend evolution process of rockbursts, a MS monitoring system was established in a deep-buried tunnel excavated by drilling and blasting. The mechanisms of three typical rockburst cases were investigated through the analysis of the MS sequence and development process. The results showed that the MS sequence of first rockburst case was a main-aftershock type, corresponding to a strain rockburst that occurred in intact surrounding rocks. Most of the MS events were failure in tensile, with few MS events presenting mixed failure mode. For the second rockburst case, the MS sequence was a fore-main-aftershock type, corresponding to a strain-structural slip rockburst that occurred in surrounding rocks with a considerable number of small-scale structural planes. The MS events in tensile failure were predominated, and a few MS events were failure in mixed and shear. The MS sequence of the third rockburst case was a new type observed in the present study, i.e. main shock type, corresponding to a strain-structural slip rockburst. It occurred in high strength surrounding rocks with rigid structural planes, which easily induced collapse in the rockburst process. The MS events in tensile failure were predominated. The precursor phenomena of former two cases of the rockburst can be identified through the analysis of MS sequence and source parameters. While before the third rockburst case occurred, no or a few MS events were obtained. It is difficult to predict this type of rockburst based on source parameters. For this case, destress blasting method was adopted. The number of high-energy MS events decreased abruptly, and no intense rockbursts occurred. This method can effectively release the concentrated stress of the surrounding rocks and reduce the risk of rockbursts. The results of this study could provide guidance and reference for the early warning and prevention of rockbursts in deep-buried tunnels. • The temporal and spatial evolution process of microseismicity of three typical rockbursts were studied. • The evolution mechanism of three typical rockbursts was revealed. • Mitigation measures were adopted to reduce the risk of rockbursts. [ABSTRACT FROM AUTHOR]

Abstract A Voronoi-element based explicit numerical manifold method is adopted to investigate the evolution process and failure characteristics of the excavation damaged zone around a deeply buried TBM tunnel. An explicit time integration scheme of numerical manifold method is adopted as the numerical platform, based on which a Voronoi polygon assemblage is generated to approximate the blocky structure of the surrounding rock mass resulting from discrete fracture networks. A new mathematical mesh generation method is developed to better adapt to the Voronoi polygons and an excavation algorithm including both in-situ stress equilibrium and TBM excavation modelling techniques is proposed to capture the mechanical response of the TBM tunnelling process. With the newly developed VE-ENMM approach, the progressive failure process of the surrounding rock mass in a headrace tunnel of Yindajihuang Project is simulated and the formation mechanism of the excavation damaged zone is explored. Simulation results show that a large extent of excavation damaged zone is easily formed in the highly fractured rock mass due to the high in-situ stress in deep grounds. Besides, relatively large convergence displacements are predicted around the tunnel surface, leading to a high risk of TBM shield jamming as in the practical situation. However, the development of damaged zone is effectively restrained and the convergence displacements are well controlled by an early installation of the tunnel lining segments, thus preventing the occurance of TBM shield jamming. Additional simulations are conducted to study the effects of the lateral in-situ stress ratio on the excavation damaged zone. It is inferred that the increase of lateral stress ratio facilitates the fracturing of the surrounding rock mass, which results in a larger extent of the damaged zone and more severe squeezing deformation compared with those under lower lateral stresses. [ABSTRACT FROM AUTHOR]

Grouting is one of the primary methods used to reinforce the strength of fractured rock masses. In this study, a three-dimensional discrete element method (3D Co-DEM) grouted joint shear model, which consists of upper and lower intact rock blocks, 3D real joint surfaces and a grout-filled layer, was developed with the cohesive zone model. The 3D real joint surface was imported into the model using 3D scanning point cloud data, and the grout-filled layer was described by a cohesive zone model. Then, a numerical simulation algorithm was developed to implement the numerical direct shear test and grout-filled layer cracking simulation. Also, the parameters of the proposed method were calibrated, and the effectiveness of the cohesive zone model and direct shear numerical test were verified by comparison with analytical and experimental results. Finally, grouted jointed specimens with regular sawtooth were used to analyse the influencing factors of shear characteristics. Numerical tests showed that shear peak strength and residual strength decrease with increasing grout water-cement ratio but increase with increasing JRC. Specifically, with increasing sawtooth inclination angle, the shear peak strength first decreases and then increases, while the peak shear strength increases as the sawtooth height increases. [ABSTRACT FROM AUTHOR]