Rock Bridge Research Articles

Frequency properties of acoustic emissions from the dry and saturated rock

Different frequency bands (FBs) of acoustic emission (AE) signals from rock fractures contain diverse information about the physical and mechanical properties of a rock. This study proposes a frequency division processing method for the standard description expressions of discrete signals and for defining the integrated characteristics of AE signals. The frequency evolution of rock fracture-related AEs was investigated under uniaxial loading conditions, and rocks in dry and saturated states were examined and compared using AE information in FBs. The correlation between the water state and FBs of AE signal was addressed. The FB features (e.g., energy concentration) were analyzed, and their changes in water conditions were also considered by focusing on the FBs with the greatest energy concentrations. During rock fracturing, dry and saturated samples developed various shear-tensile damage patterns or “rock bridge” modes. The moisture property of rock affected the peak strength, plastic deformation, and intensity of the rock destruction, and the statistical relationship observed between rise time/amplitude and average frequency showed that the crack characteristics depended on the presence or absence of moisture. The most suitable FB to distinguish the water state was 31.25–62.5 kHz. The parameter ζ (i.e., variable energy coefficient) exhibited a sudden increase in the FB of 0–31.25 kHz. This rapid change was more remarkable than that in the other FBs. This FB should be considered useful for monitoring to help predict the final rupture. The method developed here provides a new way for examining the rock damage characteristics and discovering the fracturing behavior patterns.

Cracking Behaviors and Mechanical Properties of Rock‐Like Specimens with Two Unparallel Flaws under Conventional Triaxial Compression

Flaws existing in rock masses are generally unparallel and under three‐dimensional stress; however, the mechanical and cracking behaviors of the specimens with two unparallel flaws under triaxial compression have been rarely studied. Therefore, this study conducted comprehensive research on the cracking and coalescence behavior and mechanical properties of specimens with two unparallel flaws under triaxial compression. Triaxial compressive tests were conducted under different confining pressures on rock‐like specimens with two preexisting flaws but varying flaw geometries (with respect to the inclination angle of the two unparallel flaws, rock bridge length, and rock bridge inclination angle). Six crack types and eleven coalescence types in the bridge region were observed, and three types of failure modes (tensile failure, shear failure, and tensile‐shear failure) were observed in experiments. Test results show that bridge length and bridge inclination angle have an effect on the coalescence pattern, but the influence of bridge inclination angle is larger than that of the bridge length. When the confining pressure is low, coalescence patterns and failure modes of the specimens are greatly affected by flaw geometry, but when confining pressure rose to a certain level, the influence of confining pressure is larger than the effect of flaw geometry. The peak strength of the specimens is affected by flaw geometry and confining pressure. There is a critical value for the bridge length. If the bridge length is larger than the critical value, peak strengths of the samples almost keep constant as the bridge length increases. In addition, as the bridge inclination angle increases, there is an increase in the probability of tensile cracks occurring, and with an increase in the confining pressure, the probability of the occurrence of shear cracks increases.

Stability analysis method for rock blocks formed by curved fractures

Analyzing the stability of rock blocks is important in rock engineering. Most block analysis methods assume the blocks are formed by planar fractures whereas the stability of blocks formed by curved fractures are rarely investigated. However, large fractures observed in field are often curved. This paper presents a method for analyzing the stability of blocks formed by both planar and curved fractures. The blocks are represented as assemblages of element-blocks with possible cracked rock bridges. First, the triangle meshes representing the curved boundaries of blocks are simplified using an edge-collapsing-based method. Then, possible cracking rock bridges are determined by extending fracture facets and comparing the boundary surfaces in the new block system with those in the original one. The connection relation of the element-blocks are used to determine possible moving blocks, and the resisting force on rock bridges are considered when calculating the safety factor of each possible moving block. Finally, the possible moving block with the minimum safety factor is regarded as the actual moving block and the associated minimum safety factor is defined as the actual safety factor of the block. Several analytical examples are given to validate the proposed method and the results show that blocks formed by curved fractures may be unstable if the cracking of rock bridges is considered.

Experimental study on the strength and fracture mechanism of sandstone containing elliptical holes and fissures under uniaxial compression

To investigate the strength and fracture mechanism of flawed rock, a series of uniaxial compression tests on sandstone specimens containing elliptical holes and fissures were carried out using acoustic emission (AE) monitoring and digital image correlation (DIC) system. Based on the experimental results, the stress-strain curve, mechanical parameters, AE counts and strain field evolution were analysed, and they indicated that the ligament angle is a significant factor influencing the strength and deformation of pre-flawed sandstone specimens. As the ligament angle increases, the peak strength, peak strain and elastic modulus first decrease and subsequently increase. Observations show that a stress fluctuation corresponds to an inflexion point in the accumulated AE count curve and AE counts reach the maximum value at or after peak strength. Additionally, the highlighted strain localization zones are indications of crack initiation, and crack propagation behaviour is a progressive evolution of the highlighted strain localization zones. Based on an analysis of the ultimate failure patterns, four types of rock bridge coalescence modes are identified, i.e., no coalescence, shear coalescence, double-shear coalescence and double-tensile coalescence.

New understandings of the June 24th 2017 Xinmo Landslide, Maoxian, Sichuan, China

On June 24, 2017 (21:39 UTC, June 23rd), a catastrophic landslide occurred at Xinmo village of Mao County, Sichuan Province, China. Soon after the event, some research teams carried out field investigations in order to both support the emergency operations and to understand the failure mechanism and possible evolutionary scenarios. Based on further in-depth interpretation of high-resolution remote-sensing images and detailed field surveys, it is newly found that there are at least six old rockfall deposits in the source area that prove the historic activity of the landslide scarp. Seismic data of the event and morphological evidences along the slope indicate that the landslide was preceded by a significant rockfall. Mechanical calculations show that the surface force due to pore water was far less than the impact force due to the rockfall. It means that the subsequent major rock avalanche was more likely due to the impact of the rockfall on the rock slope below, which broke the rock bridges and caused drop of shear resistance along the fractures. According to these new understandings, a different triggering mechanism for the landslide is proposed.

Numerical simulation of shear mechanism of concrete specimens containing two coplanar flaws under biaxial loading

In this paper, the effect of non-persistent joints was determined on the behavior of concrete specimens subjected to biaxial loading through numerical modeling using particle flow code in two dimensions (PFC2D). Firstly, a numerical model was calibrated by uniaxial, Brazilian and triaxial experimental results to ensure the conformity of the simulated numerical model\'s response. Secondly, sixteen rectangular models with dimension of 100 mm by 100 mm were developed. Each model contains two non-persistent joints with lengths of 40 mm and 20 mm, respectively. The angularity of the larger joint changes from 30 to 90. In each configuration, the small joint angularity changes from 0 to 90 in 30 increments. All of the models were under confining stress of 1 MPa. By using of the biaxial test configuration, the failure process was visually observed. Discrete element simulations demonstrated that macro shear fractures in models are because of microscopic tensile breakage of a large number of bonded discs. The failure pattern in Rock Bridge is mostly affected by joint overlapping whereas the biaxial strength is closely related to the failure pattern.

Fracturing Behavior Study of Three-Flawed Specimens by Uniaxial Compression and 3D Digital Image Correlation: Sensitivity to Brittleness

Brittleness has significant effects on the fracturing behaviors of preflawed rock-like materials. In this study, uniaxial compressive experiments are conducted on rock-like specimens containing three flaws, and a three-dimensional digital image correlation technique is applied to capture the real-time displacement and strain fields on the rock-like specimens’ surface. The present experimental study focuses on the effects of the brittleness of rock-like materials on crack initiation, propagation, and coalescence behaviors in three flaw-contained specimens. The experimental results show that the crack initiation mode transforms from the tensile mode to the shear mode with a decrease in the brittleness index of rock-like materials. Meanwhile, crack coalescence modes transform from the tensile crack coalescence mode to the mixed tension–shear coalescence mode, then to the shear coalescence mode as the brittleness index of rock-like materials decreases. Moreover, the ratio of the crack initiation stress to the peak stress increases, while the ratio of the crack coalescence stress to the peak stress decreases as the brittleness index decreases. This paper also addresses the influences of the rock bridge angle on the mechanical and cracking behaviors of three flaw-contained specimens. For specimens with the lower brittleness index, the ratio of the crack initiation stress to the peak stress decreases, while the ratio of the crack coalescence stress to the peak stress increases as the rock bridge angle increases. However, for specimens with the higher brittleness index, the ratio of the crack initiation stress to the peak stress and the ratio of the crack coalescence stress to the peak stress decrease with increasing the rock bridge angle. The experimental results provide a better understanding of the sensitivity of mechanical and cracking behaviors of three flaw-contained specimens to the brittleness of rock-like materials under uniaxial compression.

Failure Mechanism of the Qianjiangping Slope in Three Gorges Reservoir Area, China

The Qianjiangping landslide is the first large-scale rock slide in Three Gorges Reservoir (TGR) Area, China, after the impoundment of the TGR. Previous studies on the slope showed that most researchers agreed that reservoir impoundment and rainfall were the two main triggering factors of the slope failure. However, there were different views about the influence degrees of the two factors on the slope failure. In order to clarify the influence degrees of each of three conditions (reservoir impoundment, rainfall, and combined effect of reservoir impoundment and rainfall) on the failure of the Qianjiangping slope and reveal the failure mechanism of the slope, underground water tables and stresses in the slope were calculated under the three conditions, respectively, based on fluid-solid coupling theory using the Abaqus software in this paper; then, the failure approach index (FAI) was adopted to analyze the failure characteristics of the slope under each of the three conditions. Research results show that the influence degree of rainfall is greater than that of reservoir impoundment on the slope failure, and the influence degree of the combined effect of reservoir impoundment and rainfall is greater than that of rainfall; the sliding surface runs through only in the condition of the combined effect of reservoir impoundment and rainfall. Study results suggest that with the reservoir water level rising, the toe of the slope was gradually submerged in reservoir water and the strength of rock mass submerged by reservoir water decreased due to water-rock interaction; furthermore, the heavy rainfall was rapidly injected into the slope through the interlayer staggered zone and slope surface, the groundwater table in the middle part of the slope rose rapidly, the sliding force of the slope increased, and the stress concentration appeared at the lower part of the slope; finally, the rock bridges submerged by reservoir water in the front of the slope fractured, and the failure of the slope occurred.

Study on the Rock Mass Caving and Surface Subsidence Mechanism Based on an In Situ Geological Investigation and Numerical Analysis

To deeply understand the mechanism of the rock mass caving and associated surface subsidence during sublevel caving mining, the Xiaowanggou iron mine was selected as an engineering project case study. The study area was analyzed by means of an in situ geological investigation and numerical simulation. First, a borehole television (BHTV) system and a GPS monitoring system were used to monitor the caving process of the roof rock mass and the development of the surface subsidence; the monitoring time was thirteen months. Then, a numerical simulation was used to analyze the damage evolution of the rock mass. Research shows the following:(1)in situ geological monitoring results indicate that the caving process of the roof rock mass presents intermittent characteristics, where slow caving and sudden caving are conducted alternatively and the arched-caving trend is more pronounced with continuous mining. The surface subsidence, horizontal displacement, and horizontal deformation of the hanging wall are higher than that of the footwall, and the subsidence center gradually deflects to the hanging wall in the late stage of the +45m sublevel mining.(2)Numerical simulation results indicate that the extension and penetration of the shear and tensile cracks along the joints and intact rock bridges are the main factors causing the rock mass caving and the existence of the stress arch and its evolution process is the fundamental reason for the intermittent caving of the rock mass. The rapid development of damage to the hanging wall (the damage angle reduced) is the main cause of the deflection of the subsidence center affected by joints and the mining size.(3)In the future of mining, large-scale subsidence will occur on the surface of the hanging wall.

3D Particle-Based DEM Investigation into the Shear Behaviour of Incipient Rock Joints with Various Geometries of Rock Bridges

A 3D particle-based DEM model was established taking into account the geometries of rock bridges. The model was used to investigate the shear behaviour of incipient rock joints. Fifty-seven direct shear tests were conducted under constant normal load (CNL) boundary conditions using the established model, in which rock bridges with 19 different geometries and incipient joints with various areal persistence (between 0.2 and 0.96) were involved. Our results show that, for the cases having a single rock bridge, cracks often initiated around the edges of the rock bridges and coalesced first in the middle of the rock bridge areas. While for other cases containing multiple rock bridges, cracks initially appeared at the connection points (located in the middle of the joint planes) of the rock bridges and then propagated to the edges. High crack initiation stresses were measured, which were often more than 60% of the shear strength of the tested incipient rock joints. Sudden failures of the rock bridges subjected to shearing were observed, accompanying dramatic increases in the number of cracks. Another important conclusion derived from this research is that both joint areal persistence and rock bridge geometry played significant roles in the shear failure of the simulated Horton Formation Siltstone joints. The present study has shown that shear strength increased gradually when joint areal persistence was decreased. Interestingly, different shear strength values were measured for rock joints with the same areal persistence (e.g. K = 0.5). Shear velocity was also found to have a significant influence on the shear characteristics of the Horton Formation Siltstone joints. A higher shear strength was measured when the shearing velocity was increased from 0.01 to 1 m/s.

Challenges in the characterisation of intact rock bridges in rock slopes

Intact rock bridges have been recognised as of critical importance in the stability of rock slopes but still remain a poorly understood and challenging engineering problem both in respect to their measurement and their incorporation into design analyses. The objective of this paper is not to provide conclusive answers as to the definition and measurement of rock bridges, and the manner that they can be accounted for in rock slope stability analysis. Rather, we highlight important and as yet unanswered questions to allow review of fundamental rock bridge concepts and then proceed to provide new insight into how rock bridges may be incorporated into slope stability analysis. Early definitions of what constitute a rock bridge are somewhat limited, and the authors suggest that they could be improved by including important constraints such as “block forming potential” and “block kinematics”. In this context, this paper introduces an improved terminology (RBij) commonly used by the discrete fracture network (DFN) community to define rock bridge intensity relative to the sampling region. In rock engineering, the measurement of rock bridges is exacerbated by the fact that rock bridges are not visible unless the rock mass is exposed by human activities or by natural events such as rockfalls. This constitutes a major problem for engineering design scenarios, since it would not be possible to validate any assumption made with respect to extent of rock bridges without performing some form of field testing or back-analysis. The results of a of the Finite-Discrete numerical analysis are presented to support our conclusions with respect to the limitations current methods used to characterise rock bridge strength. The role of scale effects, block theory, kinematics and what we term “negative rock bridges” in stability analysis is discussed.

Numerical Study on Crack Propagation Characteristics Under Different Loading Conditions by FRACOD2D Approach

The fracture of rock mass is affected by different stress state in underground engineering. It is important to understand the crack propagation characteristics and stress distribution under different loading conditions. In this study, the FRACOD2D (the fracture propagation code) numerical modeling was conducted to analyze the propagation characteristics and stress distribution of rock-like material specimen under different loading conditions and crack geometric distribution. Crack propagation characteristics were distinct under different loading conditions, namely the tensile crack propagation under the uniaxial loading and shear crack propagation under the biaxial loading. In the uniaxial loading, the wing crack initiated at an angle of 90°, and then propagated along the maximum principal stress direction. The stress concentration occurred at the pre-existing crack tips. The coalescence of Model 1 rock bridge was caused by wing cracks and pre-existing cracks, while there was no coalescence occurred in the Model 2 or Model 3 rock bridge. Under the biaxial loading condition, shear cracks initiated at an angle of 150°, which developed from the pre-existing crack tips and propagated perpendicularly along the pre-existing cracks gradually. Moreover, secondary cracks propagated largely, whose stress concentration increased gradually with their propagation. Secondary shear cracks were interconnected, which led to the coalescence of Model 1 rock bridge The corrugated propagation of secondary cracks was observed instead of rock bridge coalescence in the Model 2 rock bridge. Pre-existing cracks of Model 3 propagated into two independent systems without cracks initiation in the rock bridge. Numerical modeling results were evidenced by laboratory tests.

Influence of two unparallel fissures on the mechanical behaviours of rock-like specimens subjected to uniaxial compression

Preexisting unparallel fissures in rock mass significantly affect the overall mechanical behaviours of rock engineering structures. Understanding the mechanical properties of rocks containing unparallel fissures is beneficial for evaluating the overall stability of rock structures. This study comprehensively investigates the geometrical influence of two unparallel fissures on the mechanical behaviours of rock-like specimens, including the fissure inclination angle (α2), the rock bridge lengths (d) and the bridging angles (β). Experimental results indicate that the strength, deformation characteristics and the crack coalescence modes of fissured specimens are highly dependent on the geometrical configurations of fissures. The elastic modulus of the tested specimen increases with the increase of fissure inclination angle (α2), rock bridge length and bridging angle. Fissured specimens with lower rock bridge length and higher bridging angle are characterised by higher strengths. Energy analysis reveals that, the specimens with lower fissure inclination angle, higher rock bridge length and lower bridging angle dissipate more energy under uniaxial compression. The specimens with lower fissure inclination angle (α2), lower rock bridge length and higher bridging angle release more elastic energy. Moreover, the crack evolution of the fissured specimens is numerically revealed by analysing the spatial development of cracks and the stress field in the fissured models.

The Mechanism Study of Cracks Propagation of Different Floor Strata Combinations Under Mining

To obtain the water resistance performance of floor strata under different rock layers and fracture combination, the two cracks’ propagation path under different situations was given by using minimum plastic zone theory according to the change of the fracture factor and the plastic strain of different strata combination cracks under the influence of fault, water pressure and mining. The shortest distance of the rock bridge between the two cracks after bursting in different strata combination was obtained. And the water resistance performance of every strata combination was analyzed. The conclusion was that the water resistance performance of the floor strata under the 3214 combination of soft–soft–hard–hard was the worst. While the 1243 combination of soft rock, hard rock, soft rock, hard rock was the best. The fracture factor combination and the plastic strain of the crack would reach maximum under mining condition. And the crack was mostly easy to destroy. The distance of the rock bridge between the two cracks after bursting was the shortest under the 3214 combination of soft–soft–hard–hard strata. The distance of the rock bridge between the two cracks after bursting was the longest under the 1243 combination of soft rock, hard rock, soft rock, hard rock strata. And the distance of the rock bridge between the two cracks after bursting would be shorten when considering the water pressure inside the crack and fault, which resulted in the decrease of rock water resistance performance.

A PHASE FIELD METHOD TO SIMULATE CRACK NUCLEATION AND CRACK PROPAGATION IN ROCK-LIKE MATERIALS

In the phase field method, by defining a continuous distribution function to approximate discontinuous cracks, a variational-based energy minimization framework was established to obtain the governing equations of the physical field. As an outstanding feature, it allows describing crack nucleation and branching without any prescription of the shape, size and orientation of the cracks, thus providing a very robust numerical framework for crack propagation simulation. In this paper, rock specimens containing two parallel cracks with different rock bridge angles are simulated under uniaxial compression condition to investigate crack nucleation and propagation configuration with the phase field method. The simulation results are compared with the experimental data, showing that the phase field method, as a new numerical technique in rock mechanics community, works quite satisfactorily in simulating the cracks nucleation and propagation of rock-like materials under the considered loading paths. Meanwhile, perspectives would be envisaged that the phase field method has important engineering significance in stability and durability analysis of rock structures.

Dynamic numerical investigation of a stepped-planar rockslide in the Central Andes, Chile

In this paper, static and dynamic analyses were performed considering the reconstituted geometry of a prehistoric 0.1 km2 rockslide with an exposed stepped-planar failure surface using a distinct element code (UDEC®). The rockslide occurred on the northern slope of the Cerro Catedral mount, located in the headers of the Maipo river drainage basin (Main range of the Andes Cordillera) at the latitude of the Santiago city, Chile, between the Late Pleistocene and the Holocene. The remaining failure surface consists of two distinct sliding planes dipping at an angle of ~37° and connected by a sub-vertical step. The original geometry of the rock slope was estimated from a terrestrial laser scan elevation model (LIDAR), the deposit that remained in the valley, and the hill surface topography. The laser scan of the sliding surfaces and Schmidt hammer field measurements allowed the estimation of the sliding surface mechanical properties, whereas laboratory test results were used to estimate the rock mass material properties. Results from 2D static numerical analyses indicate that the rock slope requires the presence of at least one rock bridge at the sliding surface connecting step in order to be statically stable. Furthermore, we studied the effect of different acceleration time histories recorded during shallow crustal earthquakes, compatible with the regional seismicity, in the rockslide response. The dynamic numerical analyses show that the input earthquake records induce a pronounce topographic amplification in the rock slope, that the rockslide displacement highly correlates with the Arias Intensity, the peak velocity, and the peak acceleration calculated in the rock slope center of gravity, and that the rockslide may have been triggered by horizontal accelerations larger than 0.6 g in the free field. Finally, the slope's response is evaluated under the coupled action of horizontal and vertical accelerations.

On the shear failure of incipient rock discontinuities under CNL and CNS boundary conditions: Insights from DEM modelling

This paper presents a numerical investigation of the effects of boundary conditions, i.e., constant normal load (CNL) and constant normal stiffness (CNS), on the failure mechanism of incipient rock discontinuities in direct shear. A series of numerical simulations were performed using the particle-based discrete element method (DEM), in which rock matrix and rock bridges (on incipient joint planes) were modeled as an assembly of rigid particles that were bonded together at their contacts. Smooth-joint model was assigned to particles of the persistent portions of incipient rock joints. Input micro-parameters of particles, bonds and smooth-joint were calibrated against a series of laboratory experiments. The study reveals that CNL and CNS boundary conditions significantly affect shear characteristics of incipient rock discontinuities. Peak shear stress increased significantly (up to three times) in the CNS direct shear in comparison with that measured in the CNL direct shear under the same initially applied normal stresses. The significant increase of shear stress in the CNS direct shear tests conducted in this study was related to the opening of newly created micro-cracks and creation of the rupture zones within the rock bridges, leading to a dramatic increase in the normal stresses. In the meanwhile, yield behavior was observed in the CNS direct shear while brittle failure was noticed in the CNL direct shear. It is also found that micro-cracks initiated at the vicinity of rock bridges in both CNL and CNS shear tests, while they propagated differently due to the gradual increase of normal stress under CNS boundary conditions.

Use of a remotely piloted aircraft system for hazard assessment in a rocky mining area (Lucca, Italy)

Abstract. The use of remote sensing techniques is now common practice in different working environments, including engineering geology. Moreover, in recent years the development of structure from motion (SfM) methods, together with rapid technological improvement, has allowed the widespread use of cost-effective remotely piloted aircraft systems (RPAS) for acquiring detailed and accurate geometrical information even in evolving environments, such as mining contexts. Indeed, the acquisition of remotely sensed data from hazardous areas provides accurate 3-D models and high-resolution orthophotos minimizing the risk for operators. The quality and quantity of the data obtainable from RPAS surveys can then be used for inspection of mining areas, audit of mining design, rock mass characterizations, stability analysis investigations and monitoring activities. Despite the widespread use of RPAS, its potential and limitations still have to be fully understood.In this paper a case study is shown where a RPAS was used for the engineering geological investigation of a closed marble mine area in Italy; direct ground-based techniques could not be applied for safety reasons. In view of the re-activation of mining operations, high-resolution images taken from different positions and heights were acquired and processed using SfM techniques to obtain an accurate and detailed 3-D model of the area. The geometrical and radiometrical information was subsequently used for a deterministic rock mass characterization, which led to the identification of two large marble blocks that pose a potential significant hazard issue for the future workforce. A preliminary stability analysis, with a focus on investigating the contribution of potential rock bridges, was then performed in order to demonstrate the potential use of RPAS information in engineering geological contexts for geohazard identification, awareness and reduction.

Experimental study on propagation behavior of three-dimensional cracks influenced by intermediate principal stress

Many laboratory experiments on crack propagation under uniaxial loading and biaxial loading have been conducted in the past using transparent materials such as resin, polymethyl methacrylate (PMMA), etc. However, propagation behaviors of three-dimensional (3D) cracks in rock or rock-like materials under tri-axial loading are often considerably different. In this study, a series of true tri-axial loading tests on the rock-like material with two semi-ellipse pre-existing cracks were performed in laboratory to investigate the acoustic emission (AE) characteristics and propagation characteristics of 3D crack groups influenced by intermediate principal stress. Compared with previous experiments under uniaxial loading and biaxial loading, the tests under true tri-axial loading showed that shear cracks, anti-wing cracks and secondary cracks were the main failure mechanisms, and the initiation and propagation of tensile cracks were limited. Shear cracks propagated in the direction parallel to pre-existing crack plane. With the increase of intermediate principal stress, the critical stress of crack initiation increased gradually, and secondary shear cracks may no longer coalesce in the rock bridge. Crack aperture decreased with the increase of intermediate principal stress, and the failure is dominated by shear fracturing. There are two stages of fracture development: stable propagation stage and unstable failure stage. The AE events occurred in a zone parallel to pre-existing crack plane, and the AE zone increased gradually with the increase of intermediate principal stress, eventually forming obvious shear rupture planes. This shows that shear cracks initiated and propagated in the pre-existing crack direction, forming a shear rupture plane inside the specimens. The paths of fracturing inside the specimens were observed using the Computerized Tomography (CT) scanning and reconstruction.

Influence of the rock bridge in the roof of the developed space on the loading and deformation of stooks

The main regularities of deformation and loading of the stook during creation of the developed space with the great area of a side job are considered in the article. The possibility of formation of the set of balance among the stook barriers is estimated. The assessment of influence of rock "bridge" on the stook loading is given.