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In designing suitable support systems and ensuring safe excavation of a tunnel, deformation and block failure assessment around the opening is a crucial aspect of tunneling. In this study, a distinct element modeling approach was employed to evaluate the distribution of failed blocks, failure modes, and displacements of the tunnels to gain insight into support recommendations for the Wabe Hydropower Project in central Ethiopia. For this purpose, three representative numerical models were developed considering different rock mass along the tunnel alignment. Subsequently, the influence region classification technique was introduced, and the models were systematically classified into three distinct regions. This technique enabled the consideration of blocky rock mass as discontinuum through the direct inclusion of field-measured joints with average spacings of 0.2, 0.56, and 1.2 m into a region surrounding the tunnel opening. The simulation results indicated that tunnels in closely jointed rock mass behave anisotropic, with failed blocks following the joint inclinations of N253/72 and N035/79 and exhibiting a tensile failure mode. Tunneling in the fault zone induced a shear failure mode, with a significant distribution of failed blocks aligned in the maximum principal stress direction. However, under low horizontal in situ stress, both shear and tensile failure could exist, with tensile failure affecting the roof and floor. Furthermore, tunnels in closely jointed rock mass are primarily influenced by horizontal displacement, whereas tunneling in fault zones led to both greater horizontal and vertical convergences, with horizontal displacement being more significant. Finally, the obtained results were used to propose support recommendations. [ABSTRACT FROM AUTHOR]

The excavation of the tunnel on the expressway in northwest Yunnan province induced landslide and a series problem such as ground surface cracks of the slope, sliding of the slope, and cracks in the tunnel lining. This research aims to reveal the interaction relationship between the tunnel and the landslide from the prospective of field monitoring and numerical simulation. Firstly, the engineering geological conditions of the slope where the tunnel was located were obtained by field investigation. The "landslide traction segment-tunnel longitudinal tensile failure" mode was put forward based on the spatial relationship between the tunnel and the landslide. Secondly, field monitoring methods were adopted to monitor the surface displacement of the slope, the deep-seated displacement of the landslide, and the propagation of cracks in the tunnel lining. Finally, three-dimensional numerical models were established to investigate the stability of the slope and the tunnel under natural conditions, tunnel excavation conditions, and rainfall conditions. The field investigation results, field monitoring results, and numerical simulation results illustrated that: (1) The tunnel traversed the traction segment of the landslide body in parallel, and tensile failure or shear dislocation failure would occur at different stages of the interaction between the tunnel and the landslide. (2) Two sliding layers were discovered in the landslide, the shallow creep sliding layer and the deep creep sliding layer, which corresponded to the tensile failure and shear dislocation failure modes proposed in the "landslide traction segment-tunnel longitudinal tensile failure" mode, respectively. (3) The slope was in an unstable state under natural conditions. The tunnel excavation disrupted the initial stress equilibrium of the slope, resulting in stress release of the surrounding rock mass. Both excavation and rainfall would exacerbate the deformation of the landslide and the tunnel. Eventually, control measures based on the control grouting technology of the steel floral tubes were suggested to counter with landslide-tunnel deformation problems. [ABSTRACT FROM AUTHOR]

The borehole method is quite commonly utilized in underground excavation engineering, such as for mining and tunneling, to release high pressure in brittle rock materials for safety control. In this paper, with the purpose of better understanding the failure behavior of brittle rock material with en echelon boreholes, a series of uniaxial compression tests were carried out on sandstone samples in the laboratory. The geometric layout of boreholes was the focus in this study and varied in terms of the number of borehole rows and orientation. In accordance with the experimental results, three failure modes were observed to occur in the sandstone samples, which are tensile failure across the borehole row, tensile failure along the borehole row, and a mixed failure mode. The strength and deformability of sandstone samples are found to significantly depend on the layout of boreholes. The variation in the brittleness of the rock materials was also analyzed with respect to the ratio of the initial stress over peak stress. The findings of this study would enhance the understanding of the borehole method and provide guidance for underground excavation engineering. [ABSTRACT FROM AUTHOR]

The existence of joints has a significant influence on the failure characteristics of rock mass. In this paper, based on two-dimensional (2D) particle flow code (PFC2D), uniaxial compression tests are conducted on the granite specimens with a single joint at different dip angles to investigate the evolution laws of macroscopic and mesoscopic failure characteristics. The results demonstrate that macro-mechanical characteristics of rock are influenced by the dip angle of the joint. The peak strength, peak deformation, and elastic modulus are positively correlated with the dip angle of the joint, while Poisson's ratio is negatively correlated. With the increase in the dip angle of the joint, mechanical characteristics of rock finally tend to be consistent with those of an intact rock specimen. In addition, when the dip angle of the joint is small, the rock specimens show obvious plastic characteristics before reaching the peak and there are multiple peaks being found on stress–strain curves. As the dip angle of the joint rises, the number of stress fluctuations gradually decreases and the curve near the peak becomes straighter and sharper. Moreover, the rock specimens exhibit more obvious brittle characteristics. The increase in the dip angle of the joint enhances the tendency of azimuth angle of microcracks to develop along the direction of the maximum principal stress. In the meanwhile, the distribution of the azimuth angle is gradually similar to that of microcracks in the intact specimen. With the rise of the joint dip angle, the failure modes of rock specimens change from tensile failure to shear failure to tensile failure, and finally tend to be consistent with the intact rock. [ABSTRACT FROM AUTHOR]

A series of triaxial compression and cyclic loading tests were conducted to reveal the nonlinear deformation behavior of compact sandstone. The strength, deformation, damage, and failure mode were analyzed. The experimental results indicated that the strength and elastic modulus increased nonlinearly with the increase of confining pressure, while Poisson's ratio changed inversely. As the confining pressure increased, the failure mode changed from tensile failure to shear failure. The peak strength obtained during the triaxial compression test was greater than that observed during the triaxial cyclic loading test. Based on the experimental results, an elastoplastic damage constitutive model considering the weights of elastic energy and plastic hardening energy was established to describe the mechanical behavior of compact sandstone, and the corresponding parameters of the model were determined. The validity of the model was corroborated by comparing the results of the numerical model with the experimental data. [ABSTRACT FROM AUTHOR]

Clarifying the influences of anchorage parameters on rock mass deformation and failure under impact dynamic load is the key to the control of surrounding rock in rock burst roadways. However, the existing research mainly focuses on the influence of anchorage parameters on rock mass deformation and failure under static load. However, the research on the dynamic failure mechanism of rock mass remains insufficient. According to the deformation and failure characteristics of anchor structure in practical engineering, the dynamic response law of the anchored rock mass under lateral impact load was studied through indoor similar physical model tests. Besides, the effects of anchor material, prestress and anchorage method on crack propagation mode and dynamic mechanical characteristics of anchored rock mass were analyzed. The lateral impact mechanical model of the anchored rock mass was established. Furthermore, rational suggestions on anchorage support in deep rock burst roadways were put forward based on the impact failure mechanism of prestressed anchored rock mass. The following research results were yielded: Under impact load, the cracks of the anchored rock mass are concentrated around the impact point, and the rock sample mainly undergoes shear failure and tensile failure. The size of cracks is controlled by the anchor material, the anchoring method and the prestress level. The axial force of the bolt increases with the increase of steel strength. The axial force platform segments of HRB400, HRB500, and CRM700 bolts are 20.84kN, 20.95kN, and 45.65kN, respectively. The material of the bolt has a significant impact on the impact resistance of the rock mass. The high strength of the bolt increases the stiffness of the anchored rock mass, and the impact force and axial force of the bolt are more sensitive to the impact load. When the pre-stress of the bolt increases from 15 to 45kN, the impact force action time decreases from 47 to 16ms, and the impact force platform value increases from 24.54 to 30.60kN. The impact force change is not significant, but the impact force action time changes more significantly. High pre-stress can improve the deformation resistance of the rock mass, but it will reduce the bearing capacity and impact resistance of the anchor structure. There is also a significant difference in the axial force of the bolt between the full length anchoring and end anchoring methods. The axial force of the bolt under the full-length anchoring method shows a slow increasing trend, and the axial force of the bolt under end anchoring is similar to its static load mechanical characteristic curve. compared with end anchorage, the rock mass is of greater stiffness and deforms less severely under full-length anchorage; meanwhile, the bolt can absorb massive energy through its elongation, but it tends to break under dynamic load. For deep rock burst roadways, it is suggested to choose "three-high" bolts (cables) as the supporting material, reduce the dynamic and static loads of surrounding rock and improve the mechanical properties of surrounding rock, to weaken the influence of dynamic load on the supporting structure of the anchored surrounding rock. The research results provide some theoretical guidance for the design of anti-burst bolt support in deep rock burst roadways. [ABSTRACT FROM AUTHOR]

Mudstone is a typical weak rock that fails to provide sufficient base resistance in the bored pile. The belled bored pile can replace the super-long bored pile in the super high-rise building due to the higher base resistance and lower settlement. However, ignoring the displacement-softening behavior of the concrete-mudstone interface probably overestimates the vertical bearing capacity of the belled bored pile in the mudstone. Thus, this study investigates its load transfer considering the interfacial softening behavior. The interfacial softening constitutive model is derived based on the in-door direct shear test. Full-scale simulation results show that the interfacial softening weakens the bearing capacity of the belled bored pile. The belled section's side resistance is mobilized earlier than the lower half of the uniform cross-section. Its interfacial tensile failure accelerates the side resistance at the lower half to the peak and softening stages rapidly. This adjustment effectively resists most settlements over 30 MPa compared to the bored pile. Moreover, parametric sensitivity analysis reveals the significance of interfacial cohesion. This study is valuable for researching the analytical solution of side resistance by providing verification or comparison. It also indicates the belled bored pile can be gradually popularized into the super high-rise building in the mudstone stratum, saving cost and ensuring structural safety. Highlights: The displacement-softening model for concrete-mudstone interface is derived based on in-door shear test. The side resistance of belled section is mobilized earlier than the lower half of uniform cross-section. The interfacial tensile failure rapidly mobilizes the side resistance at 15–20 m to peak and residual stages. The belled bored pile effectively resists the failure settlement over 30 MPa caused by interfacial softening. [ABSTRACT FROM AUTHOR]

The experimental tests and discrete element method were adopted to study the interaction between concrete-gypsum interface and internal notch. Gypsum and concrete semi circular ring type samples with diameter of 10 cm and heigth of 10 cm were prepared seperatly. Then one gypsum sample and one concrete sample were attached to each other with glue. The midlle space between gypsum-concrete interface dose not have any glue so one joint was built in this space. The lengths of joints were 2cm, 4cm, and 6 cm. In experimental tests, the joint direction related to loading axis changes from 0° to 90° with an increment of 30°. Totally 12, experimental test were performed. in numerical tests, the joint direction related to loading axis changed as 0° to 90° by 15° increments. As a whole, 21 numerical tests were simulated. This study demonstrates that the failure and fracturing processes in the cracked or jointed specimens containing interfaces are mostly governed by the joint or crack direction related to general loading axis. The tensile failure of various specimens containing interfaces mainly dependens on the failure and fracture mechanisms of the weakness planes due to discontinuities and the gupsum-concrete interface direction. Increasing the joint or crack length causes a decrease in the tensile strength of the concrete-gypsum specimens. It is shown that the experimental and numerical results are very close to each other. Therefore, the applicability and usefulness of the present analyzes can be easily attested. [ABSTRACT FROM AUTHOR]

In underground coal mining, frequent dynamic disturbances adversely affect the stability of coal pillars. Thus, it is essential to understand the dynamic mechanical behaviour of coal under repeated impact loads. In this paper, a cone-shaped striker bar with a length of 200 mm was designed for a split Hopkinson pressure bar (SHPB) system, and repeated impact experiments of coal were carried out with this system. Based on three-wave theory, original voltage signals were recorded during the tests and then processed to study the changes in the dynamic stress-strain curves with the number of impacts. Relationships between the dynamic compressive strength, peak strain, and the number of impacts were analysed. Meanwhile, the failure mode and fracture mechanism of coal under repeated impact loads were discussed. It was found that as the number of impacts increases, the peak strain increases but the dynamic strength decreases. Furthermore, the studied coal specimens present two typical failure modes under these conditions, including axial splitting failure and hoop tensile failure. Based on the ultimate fracture modes of the specimens, two crack group failure models are established to reveal the fracture mechanism of coal under repeated impact loads. This study can make a deep understanding about the failure mechanism of coal pillar's instability induced by impact load. [ABSTRACT FROM AUTHOR]

The methods commonly used to prepare fractured structures can be used to form only 2D cracks or holes in rock specimen. In engineering, however, the geological structures considered usually have prominent 3D characteristics. To prepare specimens with 3D internal structures, a method to form real cavity in a hard-brittle rock-like model is proposed in this paper. The uniaxial compression test was then conducted on it using three cylinder samples (solid, internal fracture, and internal cavity), and main conclusions were obtained: (1) By using the volume loss method and super absorbent polymer as material in a small-scale laboratory test, rock-like samples with internal open-type flaws and cavities were prepared such that they were not in contact with the outer surfaces of the samples. (2) The results of the compression test show that the internal fractures reduced the uniaxial compressive strength as their volume increases. However, the size of the internal fracture had little influence on the elastic modulus and peak-time axial strain of the samples over a certain range. (3) Unlike the final failure surface obtained in 2D studies, the splitting surfaces shown in this test always extended diagonally with respect to the internal structures and were vertical to the preset fractures. (4) Because the horizontal section of the internal cavity/flaw was square in shape (the diagonals were perpendicular to each other), the direction of extension of the tensile failure surface was not along the horizontal diagonals of the internal structures but perpendicular to the corresponding diagonals. [ABSTRACT FROM AUTHOR]

Fractures enhance secondary porosity and permeability of tight sandstone and in turn promote fluid migration and well recovery. The developmental characteristics and dominant factors of tight sandstone reservoir fractures in the lower Permian Shanxi Formation, southern Qinshui Basin, were systematically studied by combining qualitative observations, quantitative characterizations, logging interpretations, and tectonic analysis. The results show that fractures are extensively developed in the Shanxi Formation tight sandstone. The primary factors controlling these fractures include tectonic position, proximity to faulting, rock brittleness, single sand body thickness, formation anisotropy, and diagenesis. In crest or flank portions of the anticline and the bottom or low regions or well-developed faults, the fracture density is generally greater than 2 per meter. The scale and intensity of faulting both have a significant impact on the fracture development. Near some faults, "crushed zones" or "weak zones" were observed in the cores. These areas have a moderate- to low-angle or near-horizontal dips of less than 15° and widths of less than 50 cm. The rock rupture of the crushed zone typically occurred in a certain direction. It was found that the first fractures to form in a tight sandstone reservoir are related to tensile failure or shear-tensile failure. A negative exponential correlation exists between the linear fracture density and the single sand body thickness. When the single sand body thickness is less than 3 m, the linear fracture density is generally higher than 4 per meter; when the single sand body thickness is greater than 6 m, the fracture density is generally lower than 2 m−1. The influence of fracture density on rock anisotropy is stronger than that of the geostress. Minerals with unstable chemical properties, such as carbonate cements and feldspar, provide favorable conditions for the migration of acidic fluids and the formation of dissolution fractures. [ABSTRACT FROM AUTHOR]

To research the influence of different confining pressures on the damage evolution of defective sandstone, the sandstone numerical model with real mesostructure is established by RFPA3D using CT scan, and then carried out a triaxial compression test. The two-dimensional and three-dimensional mesoscale fracture box dimension algorithm using the digital image is compiled by MATLAB to quantitatively analyze the fractal characteristics. According to the research, as confining pressure increases, the stress corresponding to each characteristic point increases noticeably, and the specimen follows the law of transition from brittle to plastic failure under high confining pressure. The 0 MPa specimen is a tensile failure, the 5 MPa specimen is a tensile-shear mixed failure, and the 10 MPa and 15 MPa specimens are shear failures. Two types of fractal dimensions assess sandstone damage based on fracture development and fragmentation. The greater the fractal dimension, the more complete the fracture mode, and the greater the damage degree. Compared with other specimens, the mechanical characteristics and damage evolution of 0 MPa specimens are greatly different, and the fractal dimension is the largest. [ABSTRACT FROM AUTHOR]

The distribution of cracks around a perforation, as the initial condition for hydraulic fracturing, has a non-negligible influence on the fracturing process. This work presents a perforating experiment to investigate the crack distribution around the perforation tunnel. The distribution of the number of cracks on samples was statistically analyzed. Then, a 2D cross-section was selected as the research target for numerical simulation. And a numerical model was introduced to describe the rock cracking in the cross-section during perforation based on FEPG (Finite Element Program Generator) software. The meso-mechanical parameters were set to be randomly distributed to ensure a random distribution of cracks. The tensile failure criterion, Mohr-Coulomb criterion, and the fracture toughness criterion were used as the failure criterion. The modulus reduction method was applied to show element cracking. The simulation results show that the damage zone can be divided into four parts after perforation. According to the main causes of cracked elements, the four zones were named as the central crushed zone, the compression-shear damage zone, the tensile damage concentration zone, and the tensile damage propagation zone. The variations of the distribution of cracks under different perforating charge sizes and confining pressures have been analyzed. The reliability of the numerical model was verified by comparing the results of the numerical simulation to the physical experiments. [ABSTRACT FROM AUTHOR]

In order to reveal the influence of bedding angle on the fracture mode and mechanical response of carbonaceous slate, Brazilian split tests were carried out on a disc specimen with bedding, and the systematic numerical study was carried out based on beam-particle model (BPM). The results show that the bedding angle has an important influence on the tensile strength and microscopic failure mode of slate. Compared with our published work, the main differences lie in the following points: (1) The Divider type is added on the basis of the five bedding inclination samples, which fully shows the load-displacement response and rupture of the samples under different bedding angles. (2) In terms of modeling methods, through the mesh size function, the physical model composed of multiple parallel lines under different bedding conditions is constructed. (3) Through parameter calibration, the critical strain and critical rotation angle of the beam in the BPM solver are obtained. Moreover, the number of bedding and the ratio of bedding to matrix strength have a greater impact on the failure mode and peak load of engineering structures in rock mass. With the gradual increase in the number of bedding, the corresponding peak load generally shows a gradual decrease trend, and cracks are more likely to occur along the bedding surface; as the strength ratio of the bedding to the matrix increases, the peak load generally shows a gradually increasing trend, and cracks are more likely to occur cleavage tensile failure through the matrix. Through comparative analysis of test load-displacement response and failure mode with numerical simulation, the BPM solver can better reproduce the split evolution process of layered slate under different bedding angles. [ABSTRACT FROM AUTHOR]

Floor heave disasters easily occur along tunnels in deeply buried, weak, anisotropic rock masses with high in situ stress. The previously presented floor heave mechanical models (buckling failure, shear dislocation failure, and slip line field theory) do not consider the processes of stress release, transfer, and concentration, and only the initial in situ stress state can be analyzed with these models, i.e., they are static mechanical models. Therefore, the combined finite–discrete element numerical method (FDEM) is employed here to study floor heave tunneling in anisotropic rock masses to propose a new floor heave mechanical mechanism based on progressive fracture and swelling processes and the influences of the in situ stress (lateral pressure coefficient) and the layer thickness are also investigated. The simulation results indicate that (1) the essential mechanical mechanism of floor heave is X-shaped conjugate shear failure accompanied by tensile failure caused by the concentrated maximum horizontal stress present after tunneling; the shallow rock fragments cannot bear the enormous concentrated horizontal stress, and thus, this stress is transferred deeper into the intact rock mass, further causing the shear and tensile fractures to propagate downward (a progressive evolution); (2) for anisotropic floor rock masses, in addition to the X-shaped conjugate shear fractures, shear fractures parallel to the bedding plane and tensile fractures perpendicular to the bedding plane form; (3) with the change in the in situ stress lateral pressure coefficient, the failure mode directly under the floor remains almost unchanged, but it will have a significant impact on the rock mass on both sides of the floor; and (4) the layer thickness has a weak influence on the failure mode of the floor rock mass. [ABSTRACT FROM AUTHOR]

The joint geometry configurations significantly affect the mechanical properties and failure behavior of intermittent jointed rock models under uniaxial compression. Combining laboratory experiments with discrete element method (DEM) simulations, this paper comprehensively investigated the influence of four joint geometrical parameters (i.e., dip angle, joint spacing, persistency, and density) on the mechanical properties and progressive failure behavior of intermittent jointed rock models. Our experimental results indicate that the uniaxial compression strength (UCS) of multi-jointed rock models linearly decreases with the increase of the four geometrical parameters, while the elastic modulus nonlinearly varies with geometrical parameters. Compared with the joint spacing, the other three geometrical parameters affect the mechanical properties of jointed rock models more significantly. Three basic cracks on the surface of jointed specimens are observed in our tests, and six coalescence patterns are classified according to the combination of these basic cracks. Moreover, two failure modes of the jointed rock models occur in the present study, namely, the tensile failure mode and the tensile/shear mixed failure mode. In addition, the characteristics of microscopic energy evolution and the progressive failure behavior of multi-jointed rock models under uniaxial compression are numerically revealed using open-source DEM code ESyS-Particle. Our numerical results indicate that the total input energy is mainly restored as strain energy at the early stage, and then dissipated as friction energy and kinetic energy at the post-peak stage. The progressive failure processes of the multi-jointed rock models can be divided into five stages by characterizing the spatial development of micro-cracks and the maximum principal stress field. [ABSTRACT FROM AUTHOR]

Seismic liquefaction of sand is an important research topic in geotechnical engineering. Most of the previous researchers have simulated liquefaction process under earthquake by sinusoidal load dynamic test, that is, the actual seismic wave is simplified to equivalent sinusoidal wave through Seed's method of equivalent cycle number. However, it cannot reflect the actual situation of soil seismic performance. To overcome this issue, in this paper, a method for applying the actual seismic wave to test is proposed, which is used for conducting the dynamic triaxial test to study the liquefaction properties of calcareous sand under actual seismic loading. Based on the equivalent cycle number method, the actual seismic waves are equivalent to sinusoidal waves, and the liquefaction properties of calcareous sand under equivalent sinusoidal loads are compared with that of actual earthquake loads. After that, the earthquake performance of calcareous sand under different loading modes is analyzed to obtain the equivalent reduction coefficient conforming to calcareous sand. The results demonstrate that the pore water pressure development of calcareous sand under actual seismic loading has the characteristic of "stepped." The strain failure type is a tensile failure, and there is a threshold cyclic stress ratio of liquefaction failure. Compared with the actual seismic load, the pore water pressure and strain accumulation of calcareous sand under sinusoidal load have more regular shapes. When studying the dynamic liquefaction properties of calcareous sand, the pore water pressure failure criterion is preferred for actual seismic load, while the strain failure criterion is preferred for the sinusoidal load. The equivalent reduction coefficient of calcareous sand under seismic loading is slightly less than 65%, and it is supposed to be selected according to different loading periods. [ABSTRACT FROM AUTHOR]

Water is an important factor affecting rock properties. In this paper, through uniaxial compression experiments on marble, granite, and sandstone with different water immersion times, the difference in the failure mechanism and time effect of three types of hard rock during water immersion condition were studied. The results indicate that different rock components have different sensitivities to water immersion. Under loading, with increasing water immersion time, the failure mode of marble and granite gradually transitions from tensile failure to shear failure, while the failure mode of sandstone transitions from tensile-shear composite failure to tensile-dominated failure. For marble, water reduces the probability of rockburst to some extent, but it has little influence on the intensity. The accumulated strain energy of granite decreases slightly, which may limit the intensity of potential rockburst. The strain energy storage capacity of sandstone significantly reduces, resulting in a significant decrease in the potential failure intensity. Most studies are limited to the mechanical properties of a single rock type before and after immersion, but difference analysis and comparison of the mechanical properties and energy evolution of different hard rock types after different periods of water immersion are rarely reported. The research results are of theoretical value to the in-depth study of the mechanism of water–rock interaction and have practical guiding significance for the practice of controlling rockburst with water treatments in engineering applications. [ABSTRACT FROM AUTHOR]

Field research, theoretical analyses, numerical simulations, and industrial tests were employed to solve the serious problems of anchor cable breakage and for controlling the stability of roadway surrounding rocks of roadway along goaf under a composite roof in a short-distance coal seam under goaf. Renlou Coal Mine 7324 N airway in the Wanbei Mining Area was the research object, and we proposed the stability control technology for the surrounding rocks. We concluded that the main failures of the cable body and the anchor cable tail are due to composite tensile failure and composite shear failure, respectively. Based on this conclusion, we increased the pre-tightening force of the bolt (cable) and substituted a short anchor cable for bolt support of the surrounding rock control technology. Through the simulation software FLAC3D 6.0, we identified that effective anchoring layer thickness and strength could be enhanced after using the optimized surrounding rock control scheme, thus effectively improving the supporting effect of the roadway. The results from industrial tests have shown that, compared with those from the original support method, the roadway surface displacement, roof separation, anchor cable stress, and failure rate of the on-site anchor cable during the service period were greatly reduced. The test results are consistent with the theoretical analysis results. Our results provide a reference for the stability control of roadway surrounding rocks of the same type and further enrich the technical system of coal mine support. [ABSTRACT FROM AUTHOR]

The additional water pressure caused by the compression of fracture water in rock mass will have a great impact on the strength characteristics of fractured rock mass. Based on three different fracture theoretical criteria, the strength calculation formula of fractured rock mass considering the influence of additional water pressure is deduced. Prefabricated single fracture rock specimens with different dip angles (0°, 30°, 45°, 60°, and 90°) are poured with a special mold, and stress seepage coupling tests are carried out under different confining pressures (6.0 MPa, 8.0 MPa, 10.0 MPa) and seepage pressures (2.0 MPa, 3.0 MPa, 4.0 MPa); the following conclusions are obtained: (1) for prefabricated single fracture rock specimens with dip angles of 30°, 45°, and 60°, under the action of external load, shear failure occurs first along the prefabricated fracture, and then tensile failure occurs along the rock block, with two peak strengths; for the prefabricated single fracture rock specimens with dip angles of 0°and 90°, the overall shear failure finally occurs, and there is only one peak strength; (2) the strength of prefabricated single fracture rock specimens with 45°inclination is the lowest, and the strength of prefabricated single fracture rock specimens with 0°and 90°inclination is the highest; (3) with the increase of confining pressure, it can inhibit the failure of fractures in rock, and the strength increases gradually; with the increase of seepage pressure, the normal stress on the fracture surface will decrease, the effective shear driving force on the fracture surface will increase, and the strength will decrease; (4) the theoretical value of fracture strength of fractured rock mass is compared with the test results; the results show that when the influence of additional water pressure is considered, the inclination range of fracture failure of fractured rock mass along prefabricated fractures will become larger, and it is more consistent with the test results, which proves the rationality and correctness of the theoretical derivation results. [ABSTRACT FROM AUTHOR]

During deep drilling in the high in situ stress area, the core discing phenomenon often occurs. Numerical simulations have been used to consider the core discing process; nevertheless, they are all regarding the core discing as a whole. To address this problem, core discing process was separated into drilling and lifting phases in this article, and the changes of stress–strain, force chain/fabric, and fracture in different processes were studied. The results revealed that: (1) the distribution of principal stress and fabric in drilling and lifting stages were different; and the number of fractures increased with the decrease of contact forces. (2) The fracture dips were mainly small angle (≤ 30°) and micro fractures (2.24 × 10−4 m ~ 2.64 × 10−4 m). Moreover, the length of fractures had power law relation with the quantity of fractures. (3) Although connected fissures were formed in the lifting stage, core discing mainly occurred in the drilling stage. Meanwhile, the theoretical model of local stress around the fractures and the fracture scale during the two processes was also established according to the numerical results. The theoretical model of local stress around the fractures and fracture scale was in good agreement with the numerical results, which showed that the core discing is a time-dependent tensile failure phenomenon. [ABSTRACT FROM AUTHOR]

The mechanical response law of underground utility tunnels passing through active faults is complicated, and the structures suffer severe damage. An experimental model test with a scaling ratio of 1:30 has been designed to simulate thrust fault dislocation with the 45° dip angle. Through the monitoring and analysis of the lining structure vertical displacement, strains, contact pressure, and failure patterns, the mechanical behavior of the utility tunnel structure with flexible joints passing through the active thrust fault with the 45° dip angle is investigated. The results show that the compression failure occurs mainly in the joint area of the segmental lining. The surrounding rock forms a triangular shear zone as a result of thrust fault displacement. After the fault dislocation is completed, the longitudinal direction of the tunnel is bent into a Z-shape, forming a compression zone and void zone near the fault trace to accommodate the displacement of the thrust fault. The cracks, fragments of concrete peeling off and dislocations are mainly concentrated in the vicinity of the fault trace; especially, the segments B–D are the most damaged. The three segments B, C, and D appear to have an obvious dislocation at the flexible joint, and the average displacement is 6.9 mm (actually 0.207 m) and 4.3 mm (actually 0.129 m). The maximum tensile and compressive strains are distributed in 3# with values of1716.5 με and − 1182.6 με, respectively. Compression and tensile failure occur. The segments with flexible joints improve the overall anti-disconnection performance. When the thrust fault displacement exceeds 50 mm (actually 1.5 m), the damage length of the lining structure within the hanging wall and the footwall is 3.5S (S is the maximum span of the tunnel) and 2.4S, respectively. [ABSTRACT FROM AUTHOR]

Fractured rock masses possess defects that are extensively developed in nature. Studying the deformation and instability process of fractured rock masses is of great significance for an in-depth understanding of the deformation process and instability modes of slopes with fractured rock masses. In this paper, through field survey of fracture distribution statistics and laboratory triaxial compression tests on field-cored rock specimens, the fracture distribution parameters and the basic physical and mechanical parameters of the rock mass were obtained, and a discrete element model of the fractured rock mass based on the representative element volume (REV) size was developed. The meso-scale deformation and failure characteristics of fractured rock masses under different levels of confining pressure were studied. The results show that the deformation process of fractured rock can be divided into fracture closure stage, quasi-elastic stage, unstable stage of new crack initiations, new crack propagation stage, and fracture crack coalescence stage. As the confining pressure increases, the lateral deformation of the fractured rock mass was impeded, and the overall ductility and strength were improved. Further, the failure mode of the fractured rock mass transitioned from overall tensile failure to shear failure, while new cracks were mainly initiated during the quasi-elastic stage of the stress-strain curve due to the bonding failure of the original fracture surface. In essence, the deformation and failure of fractured rock mass are attributable to the initial bonding failure of the original fracture surface, followed by the failure of the rock mass and the subsequent overall instability of the fractured rock mass. From a mesoscopic perspective, the stress-strain response of a fractured rock mass is the macroscopic manifestation of the evolving interaction between internal normal and tangential stress components. The fabric evolution of the fractured rock mass during the deformation process corresponds to distinct deformation stages. The deformation and failure characteristics of the fractured rock mass resemble and indicate those of the slope, and the design parameters of the slope can be calibrated from those of the fractured rock mass. The findings of this paper are of theoretical and practical significance to better understand the deformation and instability process of slopes with fractured rock masses and obtain design parameters of slope stability. [ABSTRACT FROM AUTHOR]

In the present study, the two-dimensional blast model has been simulated using finite element software Abaqus/CAE. The John-Wilkins-Lee equation of state has been used to calculate the pressure caused by the release of the chemical energy of the explosive. Detonation point from center of hole has been defined for the traveling path of explosive energy. Elastoplastic dynamic failure constitutive with kinematic hardening model was adopted for rock mass responses under high explosive pressure to understand the mechanism of blast phenomena. In this model, it is assumed that failure of rock occurs under tensile failure when yield plastic stress exceeded to its static tensile strength. The hydrostatic pressure was used as a failure measure to model dynamic spall or a pressure cut off. Variation of detonation velocity has been measured in terms of simulation blast output energies index results. [ABSTRACT FROM AUTHOR]

Freeze–thaw (F-T) cycles are one of the prime reasons for the performance degradation of pervious asphalt roads in seasonally frozen regions. With the aim of studying the high- and low-temperature performance of permeable asphalt mixture under F-T cycles, in this study, low-temperature splitting test and high-temperature dynamic creep test were carried out on steel slag, basalt, and recycled aggregate pervious asphalt concrete (PAC) under a sufficiently wide range of F-T cycles. Acoustic emission technology was used to map the influence of the number of F-T cycles on the low-temperature cracking resistance and high-temperature rutting resistance of the three PACs. Furthermore, the performance of recycled material aggregate as an alternative to pervious asphalt mixture coarse aggregate is assessed. The results show that by the twentieth F-T cycle, the tensile modulus of the three types of PAC decrease by 80–90%, and the rate of transitioning of the materials from primary creep to steady-state creep is increased by about 90%. Moreover, by comparing the performance of the three mixtures under low-temperature as well as high-temperature regimes, it is concluded that steel slag can serve as a viable basalt replacement in PAC preparation. [ABSTRACT FROM AUTHOR]

In order to restore the normal development of nitrogen drilling technology, it is significant to gain a deep and comprehensive insight into the bottom-hole rock failure mechanism induced by gas-induced fracture. In this paper, the mechanism of bottom-hole rock failure induced by gas-induced fracture was analyzed by means of conventional analysis method of wellbore instability. The research suggests when the bottom-hole is gradually approaching the gas-induced fracture, the difference between circumferential stress and radial stress of the crack face gradually increases. The increasing shear stress caused by the difference between principal stresses leads to the compression-shear failure; then, the failure mode evolved from single-shear failure mode to combined mode of shear and tensile failure with the failure zone extending. If the wellbore is connected with the fracture, then the high-pressure gas in fracture carrying plenty of debris is injected into the wellbore, releasing lots of energy. The mechanism analysis of bottom-hole rock failure can explain the abnormal behaviors of logging data in well QL1 located in Baimamiao structural belt, Dayi County, Sichuan Province. The results not only provide a theoretical basis for the prevention and control of bottom-hole rock failure, but also provide a new perspective for studying the dynamic instability of deep rock in the field of petroleum engineering. [ABSTRACT FROM AUTHOR]

The drill and blast method (D&B) is perhaps the most common excavation method for rock mass, and intense blasting vibrations would induce an excavation damage zone (EDZ) around the excavated space. The tensile failure of rock mass in EDZ at diverse rupture velocities results in various geological disasters in engineering practices. The objective of this paper is to investigate the effect of blasting on the tensile strength of sandstone rock and the influence of loading rate on the disk specimens affected by blasting. We firstly performed a D&B exercise on a sandstone block with a size of 600 mm × 600 mm × 120 mm. Then, a total number of 49 standard disk specimens were prepared from large fragments of this blasting sandstone block and an undamaged block. A series of Brazilian split tests was carried out using these specimens to determine their indirect tensile strength, and to assess the effects of the distance from the blasting source and loading rate (varying from 1.67 × 10 to 8.33 × 10 mm s). The results show that the tensile strength of specimens exhibits an upward trend with increasing distance from the blasting source, to approach that of undamaged rock, following a power function with a positive exponent (0~1). The loading rate affects the tensile mechanical behaviors of disks, in terms of the convergence of microscopic defects, the main load-bearing area, and the absorbed energy at the fracture moment of specimens. Both the tensile strength and absorbed energy have positive linear correlations with the natural logarithm of the loading rate. In addition, the fragmentation degree of disk specimens also increases due to an increasing brittleness of sandstone with the loading rate. [ABSTRACT FROM AUTHOR]

The deep soft rock shaft with non-uniform stress is a technical challenge for the selection of support parameters. In order to obtain the stress distribution, failure mode, and control characteristics of shaft surrounding rock under non-uniform stress field (NSF), a shaft model under non-uniform stress field was established by numerical simulation software of FLAC3D. The characteristics of maximum stress-bearing circle and plastic zone were investigated using theoretical analysis and numerical simulation. The maximum stress-bearing circle and plastic zone increased with the increase of the NSF coefficient. The peak stress in the y-axis direction increased, while the peak stress in the x-axis direction decreased. However, the change of the plastic zone in the x-axis was not obvious. The negative Poisson's ratio (NPR) cable was proposed to control the stability of the shaft, and the optimal support schemes with space of 1000 mm and length of 8 m were obtained by comparing the plastic zone, maximum stress-bearing circle, and shaft failure. The results indicated that the optimal support scheme reduced the plastic zone and failure significantly, which indicated that NPR cable support could effectively improve the bearing capacity of the shaft. The optimal NPR cable support scheme was also applied to control the stability of the shaft in field test. According to 200 days of monitoring data, the deformation of the surrounding rock is very small, which provides guidance for shaft support. [ABSTRACT FROM AUTHOR]

Considering the structural complexity of rock mass, shale and other unconventional energy mining problems exist widely. The dynamic mechanical behavior and the constitutive relationship of shales with different bedding (0°, 30°, 45°, 60°, 90°) were comprehensively investigated by split Hopkinson pressure bar (SHPB) technology. Basic dynamic mechanical properties of shales, such as the dynamic compressive strength, the failure strain, the dynamic elastic modulus, and the absorbed energy of shales, were analyzed. The high-speed photography was conducted to capture the dynamic failure patterns of shale samples. The test results showed that the dynamic mechanical parameters of shale materials with different bedding are affected by strain rates significantly. The failure patterns of shale at the high strain rate levels could be divided into three major types: splitting tension failure, shear failure, and mixed failure of splitting tension and shear. The energy absorption characteristics were similar to the strength characteristics and had a positive correlation with the degree of failure. In addition, the establishment of improved constitutive equation of shales was based on the ZWT viscoelastic constitutive theory, which simplified the low-frequency term and took into account material damage characteristics. [ABSTRACT FROM AUTHOR]

Voids and lining defects can be regarded as typical diseases in tunnel structure. In this paper, three-dimensional numerical modeling was conducted to explore the safety state and the failure behavior of degraded tunnel structure with compound diseases. At first, the three-dimensional distribution of lining inner force was explored. Subsequently, the distribution of safety factors with the reinforcement of the fiber reinforced plastic (FRP)-polymer cement mortar (PCM) method was evaluated. Finally, the plastic failure behavior of tunnel lining under various deterioration degrees and ground class was comprehensively investigated. The results indicate that the safety factors exhibit the positive relation with the grade of the FRP grids. In addition, plastic failure characteristics are not only affected by the lining deterioration degree but also the ground class. The higher the lining deterioration degree, the easier it is for tensile-shear plastic failure within the compound disease zone; otherwise, the more likely it is for tensile plastic failure. The lower the grade of the ground class, the easier it is for tensile-shear plastic failure within the compound disease zone. [ABSTRACT FROM AUTHOR]

Borehole instabilities during drilling are more common in bedding plane rocks than in most other rock formations. Bedding plane rocks make up more than 80 % of rocks in siliciclastic environments, and about three quarters of borehole problems are caused by bedding plane rocks instability. The assessment of in situ stress and analysis of borehole failure due to instability and weak bedding plane represents one of the most critical factors when evaluating borehole stability that causes borehole failure. This paper is based on elastoplastic and isotropic model for stresses around the wellbore, with the aim of trying to understand the general behavior of inclined boreholes due to anisotropy. It was found that borehole collapse was caused predominantly mainly not only by shear but also by tensile failure. It is seen that bedding exposed depends not only on inclination but also on dip of the formation, attack angle, and azimuth. The numerical analyses presented in this paper were carried out using a three-dimensional numerical program. The effects of several dips and dip directions of rock mass layering and angles of well and three different field stress conditions have been investigated. It is known that the differential stress has an important influence on wellbore instability. Also, the effect of a high differential stress is exacerbated by the layer geometry and well angles. In other words, some dip and dip direction of bedding plane causes maximum displacement toward wellbore and significantly affect wellbore stability during drilling. [ABSTRACT FROM AUTHOR]

Roof fracture has been a persistent threat to coal mine safety. In this paper, a physical modeling system was established to explore the fracture mechanism of the hard roof. The characteristics of acoustic emission (AE) signals during the process of hard roof failure were also studied. Results indicate that shear failure first occurs in the two ends of the hard roof beam due to the comprehensive effect of ground stress and mining-induced stress. After this failure occurs, the bending moment moves quickly toward the middle of the beam. This movement will cause tensile failure in the middle part of the beam. Broadband frequency signals are produced when a hard roof is fractured. When compared with AE energy, the AE count shows an increasing trend during a short period before each hard roof fracture. AE signals, especially for AE energy, increase steeply, reaching a peak value at the moment rock fracture occurs. These signals then drop rapidly, ending with a weak level until the next turn. Both the periodic characteristics and evolution process of AE signals can reflect not only the stress state but also the damage degree of the roof strata. These results could offer some thoughts and reference for forecasting and monitoring rock bursts caused by hard roof failure. [ABSTRACT FROM AUTHOR]

Rock tensile strength is a factor that affects formation fracturing. Most of the formation fracture pressure prediction models treated it as a constant or ignored it due to its small magnitude. However, the rock tensile strength was observed increasing with elevated crustal stress, so it might be influential at a large depth. In this work, hollow cylinder tensile test method was adopted to measure the tensile strength of two types of sandstones under different confining pressures. Sample in this method has the similar stress state compared with the borehole wall rock, which is important because tensile strength varies with the stress state. The result revealed that there was a significant linear enhancement in rock tensile strength with the increase of confining pressure for both types of sandstones. The effect degree of confining pressure on the tensile strength of two different sandstones was nearly the same. [ABSTRACT FROM AUTHOR]

In order to study the triaxial compression creep characteristics of rock samples containing cracks, indoor triaxial compression creep tests of intact rock-like specimens and rock-like specimens with cracks were carried out. Accordingly, the effects of confining pressure, axial stress level, and the inclined angle of prefabricated cracks on the axial creep strain, axial strain rate, long-term strength, and fracture morphology of the specimens were analyzed. Based on the soft element proposed by others according to the fractional derivative theory, the nonlinear rheological model of rock-like specimens with a crack was obtained by combining it with the existing element models. The PFC2D program was developed again, and a new nonlinear rheological model was put into the PFC2D program. The PFC2D program was used to perform the numerical tests of triaxial compression creep characteristics of intact rock-like samples and rock-like samples with cracks. Based on the laboratory test, the effects of crack length on the axial instantaneous strain, axial creep strain, axial strain rate, long-term strength, and fracture morphology of specimens were further analyzed. The results showed that when the applied axial stress was the same, the average axial strain rate of the specimen first increased and then decreased with the dip angle β of the crack. In addition, the long-term strength of the sample increased with the increase of the dip angle β of the crack. The new nonlinear viscoelastic plastic creep model was used to fit and analyze the uniaxial creep test curves of intact specimens. The theoretical results agreed with the test results, showing that the model was reasonable. When the crack length and axial stress level were the same, with the confining stress increase, the axial instantaneous strain and creep strain decreased, and the long-term strength increased. When the confining stress and axial stress levels were the same, with the crack length increase, the axial instantaneous strain and creep strain would increase, and the long-term strength would decrease. The main research achievement of this study is to obtain the variation of triaxial compression creep characteristics of rocks containing cracks with influencing factors. This research predominantly furnishes theoretical backing for the design of support systems for deep underground tunnels being constructed in western China. [ABSTRACT FROM AUTHOR]

The Zubair formation is a significant producing reservoir in southern Iraqi oil fields. During the drilling of this formation, many wellbore instability issues were encountered, including shale caving, tight hole, lost circulation, and pipe sticking. These challenges significantly lead to an increase in non-productive time (NPT). The drilling data analysis indicated that the wellbore shear failure was the principal cause of these issues. The main objective of this study is to build a one-dimensional geomechanical model using core and well logging data from offset wells. Thus, the safe mud weight necessary to prevent the initiation of the shear and tensile failures against the Zubair formation was determined by employing the Mogi-Coulomb failure criterion. The results revealed that the horizontal and strongly deviated wells are less secure and stable than vertical and slightly deviated wells (less than 30°). This is based on a sensitivity analysis of the wellbore at a specific depth. The recommended mud weight for drilling wells having angles of inclination that vary from 0 to 30° is 11.9 to 12.3 ppg. A 140° NW-SE azimuth is the optimum azimuth to drill the deviated and horizontal wells which is analogous to the orientation of the minimal horizontal stress. The research's findings can be applied to planning upcoming wells in the study region's vicinity. [ABSTRACT FROM AUTHOR]

The past few decades have been marked by a rapid acceleration in Municipal Solid Waste (MSW) generation because of industrialization, urbanization, and economic growth combined with a high population growth rate. Thus, the most economically effective alternative to developing new landfills is the construction of reinforced soil walls (RSW) in existing MSW landfills to allow for their vertical expansion. This paper presents a structure for the system reliability-based seismic design optimization (SRBSDO) approach for landfills by incorporating the variability associated with design parameters under seismic conditions. The formulation considers a three-part wedge mechanism based on the pseudo-static limit equilibrium method. The influence of the horizontal seismic acceleration coefficient (kh) on the stability against external and internal modes of failure is studied. A target reliability index of not less than 3.0 is adopted to obtain the optimum dimensions of RSW required to achieve stability against three modes of failure (bearing capacity, sliding, and eccentricity). The dimensions of RSW are presented with respect to vertically expanded heights in the form of design charts. The results of the internal stability analysis of an RSW are provided to obtain the optimum number of geosynthetic reinforcement layers required for the stability of RSW under seismic loading. It is reported that kh has a substantial influence on the reliability of RSW, and higher values of mean and coefficient of variations of kh may cause instability. It is observed that the total length of the reinforcement (B2) should not be less than 0.67 and 0.91 times the height of the RSW (H2) for kh = 0.1 and 0.2, respectively. The minimum required number of reinforcement layers to maintain overall stability are 13 for kh = 0.10 with minimum B2/H2 of 0.66 and 19 for kh = 0.20 with minimum B2/H2 of 0.83. [ABSTRACT FROM AUTHOR]

Oil and gas resources covered under gravel layer have had much attention. The mechanical behaviors and failure mechanism of conglomerate, main component of gravel layer, are still unclear for the complex distribution characteristics in gravel size, shape, and content. A numerical model considering gravel particles distributed randomly was established to study the effect of gravel distribution characteristics using a discrete element method. Results indicate that increasing the gravel size or content, or overlapping of the particles can reduce the number of independent gravel blocks, thus enhancing the heterogeneity of conglomerate and changing the failure types and crack development trend. The failure strength shows a parabolic trend with rising gravel content, while the elastic modulus increases linearly, causing the ability of specimen to resist deformation to improve. The mutual extrusion brings about the increase of failure strength rapidly and the reversal of peak strain and Poisson's ratio when gravel content is over 70%. The cohesion and internal friction angle show a parabolic tendency, based on which the strength prediction model of conglomerate considering gravel content was established with high precision. This work provides a kind of numerical method for studying the physical and mechanical characteristics of the strongly heterogeneous rock considering the components. [ABSTRACT FROM AUTHOR]

Cemented paste backfilling (CPB) in underground mines is a widely accepted backfilling technique. It has plenty of scope for application in Indian underground mines. Both the uniaxial compressive strength (UCS) and tensile strength (TS) are the essential CPB design parameters. During adjacent stope extraction, minor principal stresses in the backfill mass increase; to overcome these minor principal stresses, the CPB should have sufficient TS. There are limited studies reported in literature about TS of CPB both in lab and in situ conditions, correlation between TS and UCS of CPB and tensile strength prediction models. In this article, the tensile strength development in CPB prepared using lead–zinc mill tailings with different binders (cement, fly ash, and slag) was investigated and statistical tensile strength prediction models were developed. Moreover, the correlation between TS and UCS of CPB was statistically determined with a developed correlation model. Further, the in situ strength of paste backfill specimens was determined and compared with the lab-determined strength. The in situ backfill specimens showed a shrunk of 28-day UCS and TS by 9% and 7%, respectively, as compared to lab-prepared specimens. The findings of this study would help in bulk environment-friendly tailings disposal in underground mines, better backfill strength design, and further excavation processes in lead–zinc underground mines. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO2) fracturing as an alternative to hydrofracturing is one of the most effective fracking techniques for shale gas extraction. As the water-based fracturing fluid can cause formation damage, clay swelling, and water blocking in shale, it is essential to replace it with CO2 fracturing fluids. The CO2 fracturing technique utilizes comparatively fewer chemicals and water, and has a high capacity for adsorbing CO2 with CH4. The CO2 fracturing technique is an appropriate fracturing technique as compared to other fracturing fluids. As the shale reservoirs are heterogenous, fracturing techniques are controlled by several parameters. Therefore, the shale reservoirs need to analyze adequately at a different scale. 3D modeling is essential to understand the efficacy of fracturing in unconventional reservoirs. The present study demonstrates the 3D geo-mechanical model through reservoir simulation techniques, and we estimate the gas recovery for a shale reservoir. The multi-stage fracturing tests were conducted on the unconventional reservoir using the fracture model to assess the heterogeneity, and change in the distance between the fractures. This study also analyzes the effects due to the space distance and fracture conductivities. The simulation study is conducted under different conditions by using CO2 as a fracturing fluid. The results suggest that the shale gas recovery rate increased by approximately 3 to 7% with the changes in fracture half-length, spacing, and fracture numbers. [ABSTRACT FROM AUTHOR]

Studying the effects of the micro- and meso-scale characteristics of coal-bearing sandstone on its mechanical properties can provide basic data and accumulated experience for the development of technology for the in situ testing of the strength of rock masses. In this paper, the micro- and meso-structures, mineral composition, and elemental contents of 16 kinds of sandstone from three coal mines were studied through X-ray diffraction and polarizing microscope analysis. The stress–strain evolutional characteristics of different sandstone samples were obtained through uniaxial compression, tension, and shear tests under acoustic emission monitoring, and the effect of various micro- and meso-characteristics on the mechanical properties and failure characteristics of the sandstones was investigated. The results show that the strength, elastic modulus, cohesion, and friction coefficient all increase with increasing quartz content and degree of particle contact, and decrease with increasing plagioclase and clay mineral content, especially kaolinite content. The failure mode of sandstone samples is mainly shear failure during uniaxial compression, the larger the particles and the lower the quartz content, the higher the RA value generated near the peak, indicating that more tensile failure occurs. Furthermore, the strength damage model and damage constitutive model are established by acoustic emission measurement data. These results could provide useful reference for the development of intelligent systems for the in situ testing of the mechanical properties of coal and rock masses. [ABSTRACT FROM AUTHOR]

Jointed rock mass stability is a constant focus in underground engineering applications. In this study, the uniaxial compression test is carried out on defective red sandstone with different defect angles by using cement to simulate the defects. The physical characteristics of the strain and the acoustic emission energy released during the failure of the jointed rock mass are monitored by means of strain and acoustic emission methods. The experimental results indicate that with the difference in the defect angle, the failure mode of the jointed rock sample changes, and the instability signal released during the failure process is also different. When the angle of the defective part is small, within 15°, the failure modes of the defects are similar to those of the intact rock sample. When the defect angle is between 15° and 60°, the failure modes of the defects begin to transition from tensile failure to shear failure. With the increase in the defect angle, the failure process of the jointed rock sample becomes faster and it becomes more difficult to capture the instability signal. When the angle of the defect part is smaller than 45°, the overall instability of the jointed rock sample can be determined by measuring the physical signal released in the static sub-instability stage. When the angle of the defect part increases, the instability can only be determined based on the physical signal released from the previous stage, that is, the strong deviation from the linear stage. The experimental method used in this paper could be used to identify failure precursors of jointed rock masses and to develop technology for disaster prevention in rock engineering. [ABSTRACT FROM AUTHOR]

The weathering-induced changes contribute significant alteration to mechanical and failure characteristics in the rock matrix. It is crucial to evaluate the rock fracture characteristics and mechanical response under different weathering grades because prestressing rock is extremely susceptible to the effects of weathering. Consequently, this study investigates the failure characteristics and numerous generated sandstone fractures at various weathering degrees under quasi-static loadings. Brazilian tests were conducted under different loading rates (LR= 0.001 kN/s, 0.01 kN/s, 0.1 kN/s and 1 kN/s). The findings demonstrated that sandstone tensile strength is equally susceptible to loading rate and weathering grade. The tensile strength of fresh, slightly weathered and moderately weathered sandstone increased by 20%, 54%, and 52%, respectively, with an increase in loading rate from 0.001 to 1 kN/s. The fracture angle (FA), fracture maximum deviation distance (FMDD) and fracture deviation area (FDA) of fresh and moderately weathered sandstone decrease with an increase in loading rate. FMDD of slightly weathered sandstone increases slightly as compared to FA and FDA. Additionally, the results indicate that the fracture length (FL) of slightly and moderately weathered sandstone increases as the loading rate increases, whereas the FL of fresh sandstone decreases as the loading rate increases. Overall, it can be concluded that sandstone strength is affected not only by the loading rate but also by the weathering grade. Based on the laboratory failure characteristics, the geometrical trajectory is divided into single central line fracture, double central line fracture, central-non-central line fracture and non-central line fracture. [ABSTRACT FROM AUTHOR]

Borehole instability problems lead to increased non-productive time, essentially during the drilling of shale formations. Past drilling experiences exhibited several problems: mud losses, tight hole, caving, mechanical stuck pipe, and extensive hole collapse. The study aims to construct a mechanical earth model (MEM) to analyze the wellbore stability. Thus, MEM was employed to define a safe mud weight window and improve the drilling performance on development wells. The specific analysis of wellbore instability showed that the primary cause of those instability problems was applying inadequate mud weight to drill the Tanuma shale formation. In addition, the minimum required drilling density to drill through the Tanuma formation is a strong function with a wellbore inclination. Thus, for the Tanuma formation, the model indicates that wellbore inclinations above 50° should be avoided. NW–SE azimuth provides less risk in terms of hole collapse in the Tanuma formation. Also, the sensitivity points showed that the appropriate mud weight for horizontal wells in Mishrif formation was 12.5 ppg. [ABSTRACT FROM AUTHOR]

Water presence causes a dramatic mechanical degradation of rock not only under compression but also under tension. Here, we reported a series of uniaxial compression and tension tests, coupling with acoustic emission (AE) monitoring, performed on marble rock under dry and water-saturated conditions, that provides new insight into the mechanical influence of water on rock strength and fracture behavior. The clustering analysis of RA-AF values distribution conducted by the Gaussian mixture modeling (GMM) algorithm and AE b-values evolution were used to evaluate rock micro failure under different loading regimes quantitatively. In general terms, tests and analyzed results showed that under tension, the saturated specimens exhibit greater strength reductions than those under compression, i.e., 28.86% and 20.38%, respectively. Given the two stages of the rock failure process determined by the dramatic growth of AE count rates, water saturation is considered to make the "AE quiet period" longer and the AE b-values more and greater, supporting the delay and weakening of rock crack propagation. Under compression, the fracturing process of marble rock in a saturated state is dominated by tensile failures compared to the shear failures in a dry state, whereas tensile loading determines the dominant role of tensile failures for marble rock in both dry and saturated states. In addition, the compressive mechanical degradation can be attributed to the conjunction of pore water pressure and friction weakening effects, with the former being more obvious. The main reason for tensile mechanical degradation is friction weakening induced by the water film, and the pore water pressure is very limited due to the loading regime. [ABSTRACT FROM AUTHOR]

The effect of the curing temperature of cement mortar on the mechanical and deformation characteristics of the full encapsulated rock bolting system was not considered in most previous studies. To reveal the temperature-dependent behavior of cement mortar and bolt-grout interface in terms of strength and deformation characteristics, extensive tests over a curing temperature range of 2–60 ℃ were carried out in this study, including uniaxial compression tests of cement mortar, direct shear tests of cement mortar under both constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions, and direct shear tests of the bolt-mortar interface under CNL boundary conditions. The results showed that the uniaxial compression and shear strength of cement mortar increase with curing temperature at 2–40 ℃ and reduce slightly at 40–60 ℃. Moreover, the effect of CNS boundary conditions was discussed. Furthermore, the evolution of shear strength and dilation limit of bolt-mortar interface with curing temperature was revealed. Based on the experimental results, suggestions for supporting the design of rock bolting systems under high- or low-temperature geological environments are given. [ABSTRACT FROM AUTHOR]

The continuous development of cities caused a deficiency of land in metro regions which arises various transportation problems. The construction of underground structures is extremely effective in solving these problems, as they have solved various traffic issues in significant urban areas. The main objective of the present study is to evaluate the extent of deformation experienced in single tunnel under the effect of static loading. In the present study, the extent of deformation is measured with the help of experimental investigation. Various single tunnel models are prepared in laboratory. The strength characteristic of the rock is varied and the cover depth of the tunnel models also varied as 30 mm and 50 mm. In case of lined samples, liner material is also introduced. The results obtained from physical modelling are further compared with the numerical modelling using ANSYS software. From the results, it can be concluded that the physical and numerical modelling results are in good agreement with each other. It is also observed that if the parameters are well selected for physical and numerical analysis, it can be an easy and less time-consuming method for finding out strength and deformation behavior for analysis and design of the tunnel. Therefore, it can be concluded that this methodology can be used for the safe and economical design of tunnels that are subjected to static loading conditions. [ABSTRACT FROM AUTHOR]

The complicated process of coal and gas outburst hinders the exploration of gas outburst mechanism. In order to better quantitatively evaluate gas outburst instability of reverse fault and normal fault combination using the RFPA2D numerical simulation software to study the coal face through fault from downthrown side to uplifted side and from uplifted side to downthrown side, the results showed that the outburst process can be divided into four stages, namely, stress concentration stage, coal-rock fracturing and splitting induced by the combinations of gas pressure and in situ stress, cracks propagation driven by gas pressure and the abrupt ejection of coal and gas; Wu 9-10-20090 belt transporter tunnel outburst occurs near the fault by the tensile failure, coal seam pressure shear failure occur, due to the fault sealing formation pressure gradient, under the influence of ground stress and gas pressure, fault and broken coal outburst risk; − 320 east outstanding is due to the fault sealing effect in coal roadway, gas pressure, coal shear failure occurs, breakage of coal is relatively serious, to highlight the accident when exposed fault; face from fault plate to increase plate driving is more likely to happen to highlight the phenomenon is determined by the conditions of gas release. [ABSTRACT FROM AUTHOR]

In carbonate reservoirs, fracture evaluation is essential because of the significant effect that fractures have on reservoir permeability. Evaluation of reservoir fracture characteristics is necessary to maximize oil production. Fullbore Formation Micro Imager (FMI) is believed to be the most effective method for achieving this objective. The goal of this study is to employ FMI from the well M-120 over the interval 1561.5–2392 m, which includes the Shiranish, Kometan, Dokan, Upper Qamchuqa, Nahr Umr, Shoaiba, Najmah, and Alan Formations in the northern Kirkuk field, SW Erbil. The FMI data were processed and assessed. Different types of natural structures, including fractures (conductive, dis-conductive, resistive) and bedding planes, were found through this assessment. Different fracture sets with various orientations to the anticline axis were provided through data analysis. The structural dip was determined with the aid of all identified bed contacts. The mean dip magnitude is 6.10°, and the dip azimuth is 54.1° toward the NE. The structural dip analysis also identified four unique structural zones. At a depth of 1727 m, the highest structural dip is seen, which might be related to a fault. The total number of fractures encountered falls into three categories: dis-conductive, conductive, and resistive fractures. The majority of normal fractures are dis-conductive fractures. However, some cases also interpret conductive and resistive fractures on FMI images. Furthermore, data analysis demonstrates that the secondary porosity was formed at 1794.5–1798 m in the Dokan Formation and 1840.5–1843.5 m in the Upper Qamchuqa Formation. In the processed interval, drilling-induced fractures and borehole breakouts appeared. Breakouts develop in the NW–SE direction, while induced fractures develop in the NE-SW direction. [ABSTRACT FROM AUTHOR]