Compressive Stress Concentration Research Articles

Effects of carbon dioxide blasting on hot dry rock reservoirs considering thermal damage

A cutting-edge technique for igniting hot dry rock (HDR) reservoirs is carbon dioxide blasting. The cooling effect of the drilling fluid was taken into consideration during a numerical simulation of the action range of carbon dioxide blasting-induced cracking. A temperature difference was used to determine the reservoir's material properties. Additionally, temperature distribution functions were used to create the temperature field in the reservoir. The blasting load is calculated using the pertinent theories and formulas of explosive blasting, and the process of blasting carbon dioxide to excite the HDR reservoir is modeled using COMSOL. The findings show that several stress concentrations take place during the blasting process. The fracture zone is created by the tensile stress concentration outside of the crushing zone, whereas the compressive stress concentration close to the blast hole creates the crushing zone. Furthermore, the effectiveness of carbon dioxide blasting fracturing would be affected by the beginning temperature and pressure plate thickness. Although the scope of the fracture zone is mostly unchanged, the initial temperature has a significant impact on the blasting crushing zone. The size of the crushing zone, which determines how the blasting fracture zone is distributed, is unaffected by the pressure plate's thickness.

Precise identification of delamination defect based on optimized deep learning method to understand mechanical property reduction

Ceramic matrix composites are advanced thermal structural materials used in aerospace and other fields due to their excellent performance. However, the common delamination defects in the interior can easily lead to material failure, posing significant threats to component reliability. Addressing this issue requires the development of precise and intelligent methodologies for the rapid identification of delamination defects and the evaluation of their impact on mechanical properties. In this study, delamination defects were intentionally introduced into SiCf/SiC composites to facilitate intelligent identification and quantitative analysis. We propose a novel network combining Mamba and Convolutional Neural Network, which surpasses existing models in defect segmentation by precisely capturing both global and local information, while optimizing computational efficiency and memory usage. Mechanical tests reveal that variations in delamination defects lead to an initial reduction followed by an increase in both tensile and compressive strengths. The minimum tensile and compressive strengths are 69.30% and 74.30% of those in non-defective control samples, respectively. Tensile strength is more sensitive to defect volume, as the reduced crack propagation path facilitates premature tensile fracture of the fibers, whereas compressive strength is more affected by defect depth due to compressive instability and stress concentration. This strategy provides a novel avenue for the intelligent identification and risk assessment of delamination defects in ceramic matrix composites.

Crack coalescence and failure behaviors in slate specimens containing a circular cavity and a pre-existing flaw pair under uniaxial compression

The flaws (fractures) widely existing in rock mass pose a threat on the stability of many underground cavities. This study aims at investigating the failure process of a cavity affected by adjacent flaws. A number of slate specimens containing a circular cavity and a pre-existing flaw pair were tested under uniaxial compression. Varied flaw configurations were obtained by changing the flaw inclination angle with respect to horizontal and the spacing between flaw pair and cavity. Three main types of cracks emanated from the pre-existing flaw tips, and played a dominant role in the failure process of cavity, comprising primary wing cracks (tensile cracks) and two types of secondary cracks (quasi-coplanar shear cracks and oblique shear cracks). The initiation and propagation of these cracks were highly dependent on the flaw pair configurations. When the pre-existing flaw pairs were non-parallel to the loading direction, (1) mostly, coalescence occurred both in the tensile and compressive stress concentration regions around cavity; (2) in most cases, the quasi-coplanar and oblique shear cracks were the predominant cracks leading to the coalescence between the compressive stress concentration regions of cavity and the pre-existing flaw pair tips. When the pre-existing flaws were parallel to the loading direction, the combination of inclined shear cracks and tensile cracks or inclined shear cracks only dominated the failure of cavity. The initiation angles of wing cracks, quasi-coplanar shear cracks and oblique shear cracks agreed well with the theoretical predictions by the maximum tangential strain criterion (MTSN), Mohr-Coulomb criterion (M−C) and maximum shear stress criterion (MSS), respectively.

Compressive Strength Analysis of Additively Manufactured Zirconia Honeycomb Sandwich Ceramic Parts with Different Cellular Structures

In this study, ZrO2 honeycomb sandwich structures with different cellular geometry were manufactured by SLA 3D-printing technology to analyze the compressive strength behaviour. After the printing procedure, the samples were sintered at 1450 °C for 2h. Among the samples with different cellular geometry, ZrO2 parts with circular cells were superior to that of square and triangular honeycomb structures and 1867±320 MPa compressive strength was obtained for this structure. The stress distributions in honeycomb structures were investigated using the COMSOL Multiphysics® for exposing the effect of cellular geometry on compressive strength. While more uniform stress distributions were seen on the inner wall of the circular honeycomb sample, the cellular structure of the square and triangle honeycomb samples mostly displayed compressive stress concentration on the joints of the honeycomb structure. Also, according to Rankine failure criterion, the parts with square cellular geometries were found to be more prone to failure. The highest specific compressive strength was obtained for the ZrO2 parts with circular cellular geometry. These findings demonstrated that the ZrO2 honeycomb sandwich structures with circular cellular geometry produced using SLA ceramic 3D-printing technology may be a suitable material to utilize in lightweight structural designs.

True triaxial modeling test of high-sidewall underground caverns subjected to dynamic disturbances

Seismicity resulting from the near- or in-field fault activation significantly affects the stability of large-scale underground caverns that are operating under high-stress conditions. A comprehensive scientific assessment of the operational safety of such caverns requires an in-depth understanding of the response characteristics of the rock mass subjected to dynamic disturbances. To address this issue, we conducted true triaxial modeling tests and dynamic numerical simulations on large underground caverns to investigate the impact of static stress levels, dynamic load parameters, and input directions on the response characteristics of the surrounding rock mass. The findings reveal that: (1) When subjected to identical incident stress waves and static loads, the surrounding rock mass exhibits the greatest stress response during horizontal incidence. When the incident direction is fixed, the mechanical response is more pronounced at the cavern wall parallel to the direction of dynamic loading. (2) A high initial static stress level specifically enhances the impact of dynamic loading. (3) The response of the surrounding rock mass is directly linked to the amplitude of the incident stress wave. High amplitude results in tensile damage in regions experiencing tensile stress concentration under static loading and shear damage in regions experiencing compressive stress concentration. These results have significant implications for the evaluation and prevention of dynamic disasters in the surrounding rock of underground caverns experiencing dynamic disturbances.

Influence of Incident Orientation on the Dynamic Response of Deep U-Shaped Cavern Subjected to Transient Loading

In deep rock engineering, caverns are often disturbed by engineering loads from different directions. To investigate the dynamic response of deep U-shaped caverns under different incident orientations, a theoretical solution of the dynamic stress concentration factor along the cavern boundary was derived based on the wave function expansion and conformal mapping method, and the failure characteristics around the cavern were further investigated by PFC2D (Particle Flow Code in two dimensions). As the incident orientation increases from 0° to 90°, the dynamic compressive stress concentration area transforms from both the roof and the floor to the sidewalls, and the peak dynamic stress concentration factor of the roof decreases from 2.98 to −0.20. The failure of the floor converts from dynamic compression shear failure to dynamic tensile failure. Compared to a stress wave incident from the curved boundary, a stress wave incident from the flat boundary causes severer damage. When the stress wave is incident from the sidewall, the cavern with a larger height-to-width (h/w) ratio exhibits severer damage. Conversely, the cavern with a smaller h/w ratio tends to fail as the stress wave is incident from the floor. This paper provides a basic understanding of dynamic responses of the deep U-shaped cavern.

Intra-plate Jal Az-Zor strike-slip faulting deciphering the enigma of local earthquakes in Kuwait

Abstract Kuwait National Seismic Network (KNSN) data show the clustering of earthquakes in northern and southern clusters. Spatial correlation between these clusters and oil fields led previous studies to declare that oil production/injection triggered earthquakes, but some suggested the possibility of tectonic causes. This study addresses the genuine objective of uncovering the origin of Kuwait earthquakes by analysing relationships between earthquake spatial and temporal patterns, oil production/injection, structural and tectonic setting, and subsurface fluid pressures. A kinematic model of the Jal Az-Zor dextral-slip fault was presented as a decipherer to the earthquake's origin. The northern and southern clusters represent leading quadrants, where increased mean stress causes earthquakes. Dibdibah Trough and Kuwait Bay represent trailing quadrants with decreased mean stress and a lack of earthquakes. The small percentage of earthquakes falling outside clusters are caused by the concentration of regional compressive stresses related to Arabian Plate motion on pre-existing faults. Triggering earthquakes by oil field operations requires 10% pressure increase above the original pressures, which never occurs in Kuwait oil fields. The results of this study emphasize the significance of understanding fault kinematics to assess earthquake hazards and the need to focus on engineering requirements for developments in the leading quadrants areas.

Test, modelling and design of a demountable stainless steel bar connection system for precast concrete pavements

To promote the reuse of precast concrete pavement units, an innovative demountable stainless steel bar connection system is proposed in this paper. Monotonic load tests were conducted to investigate the effects of the width of the stainless steel bar connection, the width and the end distance of the stainless steel plate on the mechanical behaviour of the proposed connection system. As localised concrete crushing was observed after tests, the stainless steel bar connection system was updated with an expanded contact area between concrete and steel. Test results revealed that specimens with the updated connection system exhibited high initial stiffness with low concrete compressive stress concentration. Finite element analysis (FEA) was then carried out to further investigate the parameters that affected the structural performance of the updated connection system. According to the deformations of the stainless steel bar connection and high-strength bolts, an analytical approach was developed to predict the design load of the proposed connection system. Design recommendations were finally summarised based on the obtained test and parametric analysis results to guide the demountable connection design.

Investigation of the Application of Complex Function Theory in Underground Mine Design: A Case Study

This study, with the engineering background of the design of a stope involving a sublevel mining method in a certain underground metal mine, explored the application of stress-solving methods based on the complex variable function theory in actual engineering. Three mathematical calculation models based on the functions of a complex variable were established. Through triangle interpolation, mapping functions of a plane with a roadway section and a plane with the stope section were determined. An improved Schwarz alternating method was adopted to study the stability of the roadway and the influence of an adjacent roadway from the perspective of the stress field. In addition, in light of the distribution characteristics of a gangue in the stope, the design parameters of a pillar were optimized, with the pillar’s optimal dimensions determined. The results showed that when the magnitudes of two far-field principal stresses in the rock mass are relatively close, the distribution of the surrounding rock stress around the roadway is more uniform, and tensile stress is less likely to occur. The excavation of a neighboring roadway exacerbates to some extent the side stress of the other roadway, especially the compressive stress concentration on the side closer to the neighboring roadway. However, when the distance between the two roadways is significantly larger than the roadway size, this effect is not pronounced. In the engineering case studied in this research, the thickness of the pillar is approximately linearly positively correlated with the safety factor of the pillar approximately linearly negatively correlated with the recovery rate. Overall, this research explored the application of the complex variable function theory in an underground mine design, demonstrating its accuracy and practicality.

A Quasi-2D Exploration of Mixed-Mode Fracture Propagation in Concrete Semi-Circular Chevron-Notched Disks

Most semi-circular bend (SCB) tests on concrete have been conducted with a pre-crack with a straight-through tip, thereby undermining the determination of the tensile fracture toughness (KIc). Therefore, the present study involved mixed-mode (tensile–shearing) fracture propagation in concrete semi-circular chevron-notched disks (i.e., with a sharp notch tip) using SCB tests and the FRANC2D numerical simulation software. The inclined notch angle (β) was varied from 0° to 70° while the other settings remained fixed, and the crack mouth opening displacement (CMOD) of the notch was measured constantly. The stress distribution was analyzed using finite-element simulations, and the experimental results showed that this testing method was robust. The maximum failure load and the fracture propagation angle increased with β, and wing fracture was observed. With FRANC2D simulating these SCB tests successfully, it was found that the tensile stress concentration around the notch tip moved toward the upper face of the notch, and the compressive stress concentration formed on the notch tip. The tensile mode was generated as the CMOD kept increasing for β = 0–30°, whereas the mixed mode became more evident as the CMOD kept decreasing for β = 45–70°. The fracture process zone was found for β = 0–30° but not for β = 45–70°. This mixed-mode fracture is predicted better by the criterion of extended maximum tangential strain than by other criteria, and there is a linear relationship between CMOD and KIc, as examined previously for pavement and concrete materials.

Theoretical analysis of dynamic stress distribution around a circular damaged roadway under transient disturbance

AbstractExcavation damaged zone (EDZ) widely exists in the surrounding rock of underground roadway due to blasting excavation, which can further change the surrounding rock property and deteriorate the stability of the roadway. In the presented research, a damaged roadway model was developed to investigate the dynamic stress concentration factor (DSCF) distribution around the damaged roadway under a transient plane P wave. The influencing factors on DSCF distribution were discussed, including the shear modulus (μe/μd) of the intact surrounding rock to that of the damaged surrounding rock, the thickness of EDZ (Rd/R0), and the wavelength of the incident stress wave. Results indicated that the damaged surrounding rock around the roadway belongs to stress‐relaxed zones, and the existence of EDZ can reduce the overall DSCF of surrounding rock. As the ratio of μe/μd is increasing, both the maximum compressive and tensile stress concentrations in the damaged zones decrease gradually, which is opposite to that in the intact surrounding rock. A larger EDZ can induce a higher DSCF in the damaged surrounding rock and a smaller DSCF in the intact surrounding rock. As the incident stress wave's wavelength is increasing, the peak value of DSCF grows first before declining, finally followed by a constant value. Furthermore, the maximum compressive and tensile stress concentrations are always found near the surrounding rock that is perpendicular to and along the incoming direction of stress waves, respectively. The stress concentration in both intact and damaged surrounding rock near the incoming side of stress waves is higher than that at the transmitted side.

Excavation analysis of large-scale slope considering effects of folded structure and in-situ stress

A cut slope in Rajamandala, Indonesia exhibited unexpected large-scale deformation during the excavation process which was revealed by the clinometers attached to the preventive piles. A geological survey indicated that the mudstone layer composing the slope had been folded by tectonic movement and was highly weathered and eroded at the surface. The above internal factors could have resulted in the significant deformation of the slope, although few studies have focused on the influence of folded structures and horizontal in-situ stress on the deformation behavior of cut slopes. Therefore, the objective of this paper is to clarify the influence of the folded structure and anisotropic in-situ stress state induced by tectonic movement on the deformation behavior of the aforementioned cut slope. To achieve this clarification, numerical analyses were performed based on the explicit finite difference method included in the FLAC2D software using different geological structures and in-situ stress states. Consequently, it was revealed that such folded structures can lead to larger displacement in cut slopes when compressive stress concentration occurs at the bottom of the fold. Moreover, an anisotropic in-situ stress state can result in shear failure at the foot of the mudstone and reproduce the displacement of piles that possess shapes as close as possible to those observed at the site. These analytical results confirmed that the folded structure and anisotropic in-situ stress state were the inevitable factors in the large deformation of the cut slope in Rajamandala, Indonesia.

Effects of hole shape on mechanical behavior and fracturing mechanism of rock: Implications for instability of underground openings

Rock failure commonly initiates near the wall of openings with various geometries in underground excavation. In this study, to elucidate the hole-shape influences on the mechanical response and fracturing behavior on rock sample subjected to uniaxial compression, a thorough experimental–numerical-theoretical investigation is conducted on the intact and pre-holed rock samples with different hole shapes, including circular, elliptical, rectangular, and inverted-U shapes. Digital image correlation technology is employed to monitor the crack initiation and development around the hole during the loading process. The results show that the presence of different hole shapes leads to varying degrees of mechanical property deterioration in rock samples. The load capacity and stiffness of the tested samples follow the same order: intact sample > circular-holed sample > elliptical-holed sample > inverted-U holed sample > square-holed sample. Irrespective of the hole shape, the primary tensile crack initiates at the top and bottom of the hole, but its development is trapped. The growth and coalescence of shear cracks near the sidewalls eventually give rise to the sample failure. Stress analysis around the holes reveals a positive dependence of the reduction in load capacity on the maximum compressive stress concentration which is controlled by the shape of the hole. This indicates that the amplification of compressive stress induced by the hole governs the rock collapse. These findings offer valuable insights for the design and construction of underground engineering projects.

Experimental and theoretical analysis of spalling in deep hard rock tunnels with different arch structures

To study the spalling characteristics and induction mechanism of tunnels with different arch structures, a series of true triaxial tests were conducted on three types of cubic granite specimens with a through-going arched hole and analytical stress solutions were obtained for the three arch sections. Test results show that the failure process of specimens includes four periods: a quiet period, a buckling deformation period, a gradual buckling and exfoliation period of rock fragments, and a V-shaped notch formation period. Specimens with different arch structures are listed in a descending order as the three-centered arch with f/B (arch rise/tunnel width) = 1/3 (II), three-centered arch with f/B = 1/4 (III), and semi-circular arch with f/B = 1/2 (I) according to the size of rock fragments. Before the spalling of arched tunnels, the acoustic emission (AE) count is low, while the AE count and cumulative AE count are high during spalling. Stress analysis results show that as the vertical stress rises, the arch feet of tunnels undergo initial failure at first due to stress concentration; spalling cracks appear in both side walls due to compressive stress concentration, which induce spalling failure; tensile cracks initiate in the vault and invert due to tensile stress concentration. The sequence of spalling in tunnels with different arch structures varies due to a change in section structure. Therefore, the quality of section structures can be evaluated according to the stress distribution characteristics of surrounding rocks. The vertical stress inducing the initial failure of tunnel surrounding rocks increases gradually in different arch structures from the II, III, to I; the failure severity of surrounding rocks decreases in different arch structures, also from II, III, to I. In addition, the stress analysis results also verify the accuracy of the test results. In practical engineering applications, to consider both the section utilization ratio and control of spalling in surrounding rocks, selecting tunnel sections with f/B = 1/4 is more economic and safer than that of f/B = 1/3.

Dynamic characteristics of rockbolt anchorage structure under radial cylindrical P wave

The anchorage structure formed of rockbolt, anchoring agent and surrounding rock plays a significant role in supporting underground engineering, however, there are still damage and failure cases of rockbolt supporting systems under blasting load. Therefore, it is important to clarify the mechanical behavior of rockbolt anchorage structure subjected to dynamic load. In this paper, we derive the analytical solution of dynamic response around anchorage structure subjected to radial cylindrical P wave based on the wave function expansion method, and the transient response is obtained by using Fourier transform. Two cases with and without anchoring agent are taken into consideration, and the elastic-slip model is introduced into describing the interaction between the surrounding rock and rockbolt without anchoring agent. The influence of multiple parameters such as thickness, stiffness and distance from the disturbance source on dynamic response is investigated in detail. The steady-state results demonstrates that the alternating tensile and compressive stress concentration is generated with the increase of wavenumber. The dynamic stress concentration can be reduced by appropriately decreasing the thickness and stiffness of anchoring agent, and the optimal values are obtained. The dynamic stress distribution in rock is opposite to that in anchoring agent under transient incidence and the tensile damage in rock along the incident direction and that in anchoring agent at the vertical direction of propagation must be alert. The novelty of the manuscript consists in studying the dynamic mechanical behaviors of anchorage structure subjected to radial dynamic load. The implications of these investigations for practical engineering will serve as a reference to design supporting methods and improve anchoring agent parameters.

Temporary stress regime reversal immediately after the complete release of the localized compressive stress by the 2021 Luxian Ms 6.0 earthquake, Sichuan Basin

Aftershocks can provide fundamental information of the seismogenic structure and the stress condition of the source region to reveal the seismogenic mechanism of earthquakes without clear surface rupture. On 16 September 2021, a destructive Ms 6.0 earthquake broke the seismic quiescence of the Luxian county in the inner Sichuan Basin, China. Here, we explore the seismogenic structure and the postseismic stress condition of the Luxian earthquake through characterizing sedimentary faults in the spatiotemporal distribution of aftershocks, through the focal mechanism solutions and further stress inversion, and through the coseismic rupture model based on synthetic aperture radar interferometry (InSAR) data. Our results show that aftershocks are concentrated at two clusters in the broad Yujiasi syncline. Seismicity of the northern cluster reveals that the main shock ruptured an east-southeast striking fault with a dip angle of ∼ 45° to the southwest (F0), and activated a gently dipping (∼ 18°) fault (F1) below F0 and a nearly vertical shallow fault (F2) on the northern side of F0. These three faults outline the edges of a thrust wedge in the sedimentary cover. The northern cluster occurred on a conjugate fault system within which steep faults with different dipping direction interlaced downward forming ‘V’ shapes. The main reverse rupture on the east-southeast striking F0 fault is inconsistent with the background northwest-southeast tectonic stress field. Combined with the morphology of anticlines, we suggest that the main shock was resulted from the localized NE-SW compressive stress concentration due to the inhomogeneous deformation of the sedimentary cover. Aftershocks with magnitude ≥ 2.0 are characterized by the dominant normal slip. Coulomb stress change (ΔCFS) results show that normal slip is enhanced by the main rupture. However, the amplitude of ΔCFS (a few bars) is not large enough to reverse the dominant slip direction of faults. Postseismic stress inversion results show a temporary stress regime reversal from the nearly north-south subhorizontal compressive stress regime before the main shock to the normal slip favored stress regime with a subvertical maximum principal stress (σ1). We suggest that the Luxian Ms 6.0 earthquake as an unusual event completely released the localized horizontal compressive stress and hence caused a temporary reversal of the localized stress regime.

Substantiation of the optimal mining sequence of an ore deposit under conditions of high stress and low rock mass strength

Introduction. When choosing the mining sequences of an ore deposit, technological and organizational factors are mostly taken into account. However, when using systems with caving which involve considerable mined-out spaces, the geomechanical factor, which determines stability of mine workings, plays a determining role. The research objective is to reveal the dependence between the stability of the development and face-entry headings within the mined-out space influence zone and the excavation sequence of the isolated ore block under conditions of high stress and low ore and rock mass strength. Methods of research. Numerical modeling of the enclosing ore and rock mass secondary stressstrain state formation was carried out by the finite element method in the Rocscience RS3 program together with the original FEM software package. Modeling was performed for two variants of block excavation – from the center to the flanks and from one flank to the other along the strike. For the analysis, we proposed a coefficient representing the product of compressive stress concentration zones length by their existence duration. Results. Based on the analysis of the maximum stress concentration areas distribution in the marginal rock mass, as well as the time of their influence, the optimal sequence of actual mining was substantiated according to the factor of influence on bottom workings stress-strain state – from flank to flank along the strike. Conclusions. The obtained result confirms the model representations about the mechanism of rock mass stressed redistribution in the process of lens-shaped deposit excavation. The representations suggest that during mined-out space development along the strike, compressive stresses concentrate at its ends. However, the intensity of end sections expansion in the flank to flank variant is two times as high as in the center to flank variant. The duration of the stress concentration zones impact on the bottom workings is therefore less, which is indicated by the proposed coefficient.

Stability Analysis of Gentle Dip Thick Ore Body Mining Based on the Integration of SURPAC-FLAC3D

Issues of large exposed roof area, poor stability, and significant roof support workload are prevalent in the mining of metal ores with gently inclined thick bodies. In response to these challenges, a downward cemented fill mining method has been proposed to enhance the stability of the surrounding rock and filling body. By integrating SURPAC and FLAC3D software, a three-dimensional numerical model that conforms to the actual geological morphology of the mining area was established. Numerical calculation results indicate that after the first stage of ore body excavation, the surrounding rock settlement mainly occurs in the roof and the hanging wall of the -65m level, with the hanging wall settlement primarily concentrated between the 3rd and 4th vertical exploration lines. The filling body demonstrates a weak compressive stress capacity, leading to a gradual transition of compressive stress to the surrounding rock of the mine stope. As the excavation level increases, the compressive stress on the pillars also increases, forming a compressive stress concentration area at the -65m level. A mixed plastic zone of shear and tension is generated in the roof and hanging wall, while a shear plastic zone is present in the inter-pillar area. The findings of this study offer valuable insights to ensure the safety of mining in gently inclined thick ore bodies.

Mechanical behavior and failure characteristics of rock with double holes

Holes are common defects in underground engineering structures, which will reduce the strength and stability of surrounding rocks. Understanding the mechanical behavior and damage characteristics of specimens with holes under static loading is important to the safety of underground engineering construction. In this paper, PFC2D is used to study the mechanical properties and failure behavior of the specimens with double holes under uniaxial compression. First, the influence of rock bridge angle on the strength characteristics of the specimens was analyzed. The results show that with an increase in rock bridge angle, the peak stress and elastic modulus decrease first and then increase, and the curves approximately exhibit a ‘V’ shape. When the rock bridge angle is 45°, the peak stress and elastic modulus reach the minimum. Subsequently, the crack propagation, stress field evolution and crack closure mechanism of the double-hole specimens for different stress levels were investigated in detail. The results show that tensile stress concentration exist on the top and bottom of the hole, and compressive stress concentration exist on both sides of the hole in the initial loading stage. The crack initiation position is located near the hole and mainly in the tensile stress concentration area. 7According to the evolution of crack propagation, contact force chain and displacement field around the hole, three typical crack closure forms are proposed, which are mixed tensile and shear (type I), shear (type II) and shear (type III).

Mechanical behaviour and fracture evolution of coal specimens containing an over‐excavated hole: Experimental study and numerical modelling

AbstractUtilising a series of mechanically over‐excavated cavities along borehole is a novel technique for enhancing the permeability of soft coal seams and, consequently, gas drainage. The evolution of cracks induced by a wide range of pressure‐relief around an over‐excavated hole is intrinsically complex. In this study, the mechanical behaviour and crack evolution of the specimens containing an over‐excavated hole under uniaxial compression loading were studied by experimental and 3D numerical simulation. The results indicated that the peak strength and elastic modulus of the specimens gradually decrease with increasing cavity diameter and length, which is also verified by the numerical simulation. The inclusion of cylindrical cavities in over‐excavated holes results in reduced crack initiation stress and a greater degradation of peak stress and elastic modulus, despite having an equivalent volume to the ellipsoidal cavity. This is likely attributed to the difference in stress concentration between the cylindrical and ellipsoidal cavities. The crack propagation process can be classified into four stages based on the acoustic emission (AE) event counts, initial crack compaction, stable crack propagation, unstable crack propagation and post‐peak failure stage. The two AE indices, rise angle and average frequency value, demonstrated that the failure is dominated by tensile crack and gradually transformed to shear crack. Stress redistribution is essential in the initiation and propagation of cracks. Tensile stress concentration leads to cracks forming at the top and bottom of the hole, which propagate in the direction of loading. Compressive stress concentration results in shear cracks forming at the left and right sides of the hole, which propagate diagonally. The failure pattern of the specimen is ultimately determined by a combination of tensile and mixed crack propagation. The experimental and numerical results contribute to a deeper understanding of the crack evolution mechanism of coal seams with over‐excavated holes.