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Wellbore drilling is the main activity to access oil reservoirs for oil production. This study aimed at presenting a safe drilling mud window using two criteria for estimating the minimum and maximum drilling mud pressure in order to prevent tensile and shear failure in oil wells. To this end, data on oil wells was first collected. To calculate the lower limit of the mud window, the oil well was modeled at different drilling mud pressures with the help of the finite difference software FLAC2D. By plotting the normalized yield zone area (NYZA) versus the mud pressure, NYZA = 1 was considered as the shear failure threshold, the corresponding pressure to which was considered as the lower limit for the drilling mud weight. To calculate the upper limit of the mud window, the wellbore was modeled with the help of the finite element software, Abaqus. To this end, the drilling mud pressure was gradually increased, and according to the increase in the crack length in the borehole, the pressure causing tensile failure was considered as the upper limit of mud weight. Ultimately, two linear relationships were proposed using the multivariate linear regression analysis in SPSS. In the first relationship, the minimum drilling mud pressure to prevent a shear failure is estimated using pore pressure, minimum and maximum horizontal stresses, cohesion, and internal friction angle. The second relationship estimates the maximum drilling mud pressure to prevent a tensile failure using pore pressure, minimum and maximum horizontal stresses, and maximum and minimum tensile strengths. Combining these two relationships, a new correlation was obtained to estimate the optimal drilling mud pressure. As its main innovative aspect, this study aimed at providing a safe window for drilling mud using two criteria for estimating the minimum and maximum drilling mud pressures in order to prevent shear and tensile failures in oil wells in south Iran oilfields.

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.

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.

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.

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 μe and − 1182.6 μe, 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The effect of the normal stress and joint persistency on the mechanical behavior of granite samples containing discontinuous joints was investigated in the laboratory through direct shear tests. The test results revealed three different failure modes occurring at the rock bridge: (a) shear failure, (b) compressive-shear failure, and (c) tensile failure. The failure mode depends on the joint persistency and normal stress level and influences the peak shear strength. Moreover, the joint persistency and normal stress level were also found to have a considerable influence on the post-failure behavior such as the fracture roughness and residual shear strength based on rock bridge area. The conducted investigation provided better understanding of the effect of a rock bridge on mechanical behavior of granitic discontinuous joints towards estimating the resistance capacity of rock bridges and hence to allow evaluation of stability of rock masses in both surface and underground excavations in rock engineering.

During transition from open pit extractions to underground mining of an orebody, often both the open pit and the underground workings operate simultaneously, before the former is closed. To avoid the risk of inundation, the underground workings connecting or driven closer to the open pit are isolated using bulkheads. In this paper, the authors reviewed some of the theoretical equations and norms followed worldwide for determining the safe dimensions of a bulkhead to withstand water pressure. It is found that the theoretical equations are insufficient to represent the actual mode of failure and the ultimate pressure–bearing capacity of a bulkhead, as they were developed based on only one mode of failure of the dam construction material. For better representation of the bulkhead failure and its strength determination, it is found prudent to conduct strain-softening numerical modeling simulating a real mining scenario. Mode of dam failure and effect of parameters such as dam thickness and roadway dimensions on the ultimate pressure–bearing capacity of an arched bulkhead are studied. Numerical modeling studies show that the failure initiates with tensile cracking of the dam surface, but the bulkhead ultimately fails in a combination of tension and shear yielding. On comparison, it is found that the tensile failure theories underestimate the pressure-bearing capacity of a dam, while the shear strength– and crushing strength–based equations overestimate the same. Further, an application of the numerical modeling technique for design of water-retaining dams at an underground mine for its safe isolation from the open pit is presented.

The extensive bedding and cleats have a great influence on the mechanical properties of coal. The previous research mainly focused on the bedding effect, and few studies took the influence of both bedding and cleats into consideration. The scanning electron microscopy (SEM), uniaxial compression experiments, and acoustic emission (AE) monitoring were conducted on one hard coal specimen with different bedding and cleat angles. The experimental results show that for the bedding dip of 0° and face and butt cleat dip of 90°, the properties of coal specimens were characterized by the highest peak strength, the drastic failure process, the post-peak brittle behavior, the long initial quiet stage, and the short violent booming stage of AE hits number. For the bedding and face cleat dip of 45°, and the butt cleat dip of 90°, the properties of coal specimens were characterized by the lowest peak strength, the slip failure mode, the step-like drops of the post-peak stress-strain curve, and the several peaks of AE hits number. On the other hand, for the bedding and butt cleat dip of 90°, and the face cleat dip of 0°, the properties of coal specimens were presented as tensile failure mode with plate cracks, the short non-linear stage and the slight step-like drops of stress-strain curve, the short initial quiet stage of AE hits number, and the large value of elastic modulus. Then, a theoretical model of coal containing bedding and cleats was established in this paper. The simulation results of peak strength and failure mode verified the proposed model. The simulation of coal strength shows that bedding had the decisive influence on the uniaxial compressive strength of coal, while the weakening effect of cleats was non-negligible. When bedding and cleats were taken into consideration together, the minimum peak strength was reduced and the bedding dip related to the minimum peak strength changed.

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.

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 × 10−5 to 8.33 × 10−2 mm s−1). 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.

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.

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.