Rock Mechanics and Rock Engineering

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  • Guangdi DengGuangdi Deng
  • Heping XieHeping Xie
  • Mingzhong GaoMingzhong Gao
  • [...]
  • Zhiqiang HeZhiqiang He
High-intensity longwall top coal caving (LTCC) mining of super-thick coal seam in high stress conditions often leads to significant movement and failure of overburden. The competent roof above the super-thick coal seam is easy to form large unstable structures, and the breaking and movement of the roof strata often induce high weighting strength and large dynamic disasters which severely affect the mine safety. The key to control the behavior of competent roof is to understand the fracturing law of the roof strata. In this paper, a case study based on microseismic (MS) monitoring was performed at LW8309 working face in Tongxin coal mine, where super-thick coal seam under competent sandstone roof in high stress conditions was mined by LTCC method. The spatial distribution characteristics of MS events were studied and four impact zones in the horizontal direction and ‘four-zones’ in the vertical direction were identified. Multifractal analysis of MS events was carried out to study the multifractal characteristics of the fracturing of competent overburden. In addition, a large and small periodic law of fracturing of competent roof was revealed from the temporal distribution of MS events and on-site monitoring of hydraulic support pressure. A key stratum (KS) structural model was then proposed to explain the strong behaviors of competent strata during high-intensity mining. The research results are of great significance for safe and efficient mining of super-thick coal seam under competent roof in high stress conditions.
  • Mahdi HaddadMahdi Haddad
  • Peter EichhublPeter Eichhubl
The combined effects of oil and gas production and saltwater disposal in stacked reservoirs can result in poroelastic-stress changes that affect fault stability and induced seismicity but that are not captured by models that consider disposal only. While the significance of these combined effects has been demonstrated in site-generic geomechanical simulations, their significance is yet to be quantified for specific sites of observed induced seismicity. We conducted 3D monolithically coupled poroelastic finite-element simulations for a site-specific geomechanical analysis to assess the potential for reactivation of basement-rooted faults in response to saltwater injection and hydrocarbon production near Venus, Johnson County, Texas. Earthquake activity with magnitudes as high as Mw 4.0 primarily occurred in the basement section of a listric normal fault extending from basement across the Ellenburger disposal reservoir and into the overlying gas-producing Barnett Shale. We find that using the best estimates of in-situ stress and fault orientation, and fault frictional coefficient of 0.6, do not hindcast fault reactivation. Increasing the maximum horizontal stress azimuth by 10° and the basement fault dip by 5°, both within the uncertainty space of the input parameters, and lowering the friction coefficient of the fault in basement to 0.35, leads to fault reactivation in basement. Using the same model geometry but a friction coefficient of 0.6 leads to fault reactivation within the Ellenburger disposal reservoir, which is inconsistent with observed hypocenter depths. Including the effects of production from Barnett reduces the potential for fault reactivation compared to simulations of disposal only. Comparing simulations with only five disposal wells to results of simulating 35 wells, we demonstrate the sensitivity of fault reactivation to selected number of wells. In addition to showing the sensitivity of simulation outcomes on the availability of high-quality field parameters, these results demonstrate the need for coupled poroelastic simulations unlike common hydrogeological and reservoir engineering simulations that may significantly over- or under-estimate the potential for fault reactivation and thus for induced-seismicity hazard.
This work presents the spatio-temporal variations of microcracking and damage accumulation during the progressive failure in a low-porosity sandstone under uniaxial compression, using active ultrasonic surveys and passive acoustic emission monitoring. The combination of both techniques allows us to perform a joint inversion of source locations and P-wave velocity tomography of the specimen during the failure process. Based on the P-wave first arrivals from both the active surveys and numerous passive AE events, three-dimensional (3D) P-wave velocity structure and its temporal variations can be obtained in high resolution in spite of the limited active ray coverage. The reconstructed P-wave velocity structure indicates that the specimen becomes more heterogeneous due to damage accumulation. Within such a highly anisotropic medium, the ray paths are found to be curved, rather than straight as assumed in a homogeneous material. The curved ray paths are used to calculate the travel times of each AE event so as to obtain the accurate source location. We calculate the energy of the detected AE event and estimate the energy distribution. It shows that about 50% of the events account only for about 2.5% of the total energy while 1% of the events release 47.3% of the total energy. The events with large energy, albeit few, allow to trace the fractures. We also investigate the variations of the P-wave velocities along different characteristic ray paths during the failure process. Based on the joint inversion, the variations of the 3D P-wave velocity structure with deformation have been further investigated. It is found that the emergence of the low-velocity zone (LVZ) does not indicate the onset of fractures and that the LVZ is highly unpredictable in a spatio-temporal sense. The spatial correlation between the LVZ and AE events suggests that AE events usually appear on the edge of the LVZs, where there are high-velocity contrasts. This work suggests that the quantification of P-wave velocity changes has the potential for extracting timely precursory information of pillar bursts.
Coal seams are often affected by gas effects in addition to static and dynamic superimposed loads in the process of coal mining. Understanding the mechanical properties and energy dissipation of impacted coal under different initial gas pressures is extremely important. Therefore, dynamic compression experimentation of coal samples was conducted using a self-developed observable combined dynamic and static loading test system of gas-bearing coal. Furthermore, based on the crack volume increment, the crack energy dissipation density (CEDD) was defined to measure the energy required for a unit volume of cracks under different gas pressures, and the following conclusions were drawn: the deformation of gas-bearing coal can be divided into the elastic stage, elastic‒plastic stage, plastic stage and failure stage, and the dynamic strength and secant modulus decrease with increasing initial gas pressure. The impacted coal contains annular parallel cracks that run through the cross section and axial splitting/thoroughgoing cracks, and the two forms of cracks become more obvious with increasing initial gas pressure. The reflected energy of gas-bearing coal increases and the transmitted energy, dissipated energy and energy dissipation ratio decrease with increasing gas pressure. In addition, the CEDD decreases with increasing gas pressure because free gas and desorbed gas participate in external expansion work at the moment of impact, which results in a decrease in the CEDD required for gas-bearing coal failure. These conclusions enrich the basic theory of the dynamic coal–rock–gas disaster induction mechanism and can provide theoretical support for the monitoring, early warning and prevention technology of dynamic disasters in composites.
The application of microwave irradiation in low permeability coal seams is considered to be an effective method for increasing the permeability of coalbed methane (CBM). Microwave irradiation not only improves the pore structure of coal, but also affects the mechanical properties. To evaluate the impact of microwave irradiation on the mechanical behavior of coal, a series of uniaxial compression tests of coal after microwave irradiation with different parameters were carried out. The results suggested that the mechanical properties of coal were significantly reduced by microwave irradiation with different parameters. Microwave irradiation changed the deformation behavior and fracture properties of coal. After microwave irradiation, the stress–strain curve of coal tended to be flat, and the failure mode changed from shear failure to tensile failure. Peak strength, initial modulus, and elastic modulus were negatively correlated with microwave energy, while peak strain was positively correlated. The energy evolution of coal under uniaxial compression was closely related to the deformation behavior. The energy absorption, storage and dissipation capacity of coal showed a downward trend with the increase in input microwave energy. Moreover, the energy distribution ratio (\({U}_{p}^{e}/{U}_{p}^{d}\)) of the stress peak point was changed after irradiation. Under compression, the deformation and failure characteristics of coal showed significant enhancement of plasticity and ductility. The brittleness index (B1, B2) first increased and then decreased with the increase of microwave energy, and the microwave energy of 90 kJ was taken as the threshold boundary. It was observed that the mechanical behavior of coal after microwave irradiation is related to the degree of microwave thermal damage. The average decay rate of normalized mechanical parameters with microwave energy is 0.0026 kJ−1.
The effect of temperature (T) on fracture characteristics of granite was investigated in this study under four mode mixites. Notched semi-circular specimens were prepared and thermally treated for 24 h at T up to 700 °C. The growth of thermal fractures was monitored and then three temperatures were adopted for the original tests; room temperature (RT = 25), 250, and 500 °C. A special electrical circuit was used to measure crack tip speed at different locations and determine the areas for crack acceleration and deceleration. The results focused on the effect of T and notch inclination angle β (mode mixity ratio KII/KI) on the fracture characteristics;displacement − load curves, fracture toughness, fracture envelope, and crack tip speed. The experimental outcomes showed that there are two regimes for temperature − fracture behavior. The first regime is for T between RT and 250 °C, where SCB specimens behave in a brittle manner and all results are directly proportional to the temperature. The second regime is for T between 250 and 500 °C, where all fracture characteristics dramatically decreased by increasing the T to 500 °C. For T = 500 °C, β = 0.0° (pure mode I) and 15° (KII/KI = 0.3), SCB specimens behave in a ductile manner and the crack tip speed was very low (< 0.005 m/s) so the crack growth can be seen with the naked eye. By increasing β to 30° (KII/KI = 0.75) and 50° (pure mode II) under the same temperature (T = 500 °C), the rock sample becomes stronger and returns to brittle behavior.
Mohr-Coloumb failure criterion is vastly employed to understand the risk of collapse of the highly-stressed boreholes in oil and gas industries. It is, thus, important to measure the shear strength parameters of Mohr-Coloumb criterion for reservoir rocks in terms of simple input data to easily mitigate the risks of collapse. For this purpose, 59 core plugs from four carbonate oil fields located in Abadan Plain, were tested under triaxial stress to measure shear strength parameters (internal friction angle and cohesion). Then, these parameters were estimated based on a novel approach using petrographic properties. Fuzzy clustering (FCM) were initially imployed to cluster the input data followed by developing the Adaptive Neuro-Fuzzy Inference System (ANFIS) and interconnecting ANFIS to particle swarm optimization algorithm (PSO-ANFIS) and Genetic Algorithm (GA-ANFIS). The input parameters of the developed models are petrographic features including rock texture (Tex.), sedimentary environment (SE), quartz and pyrite content (QP), micrite index (MI), and porosity (Por.), obtained from optical microscopy and XRD analyses. To compare the performance of the models, some statistical metrics were measured. A credibility assessment of the prediction performances reveals that the combination of evolutionary algorithms with ANFIS can be a practical technique to reduce the uncertainties related to shear strength parameters approximation. Therefore, PSO-ANFIS is considered as a promising method for the prediction of shear strength parameters of the studied limestones with R2>0.93 and RMSE<3.15. Finally, the estimated shear strength parameters are compared and validated with the literature results.
To better understand the bonding failure at the contact between the bolt shank and anchoring agent, we developed a constitutive model of fully bonded rock bolts, in which the relative movement of the contact was considered. A bond coefficient factor was proposed to calculate the longitudinal displacement of the anchoring agent at the contact between the bolt shank and anchoring agent. The credibility of the developed model was demonstrated with three independent experimental pulling tests. A highlight of this model is that most input arguments can be acquired and calibrated directly from the experimental pulling data. Based on the developed model, a sensitivity analysis was performed. The influence of the bond coefficient and residual bond strength on the anchorage performance of bolt shanks was evaluated. The results revealed that the bond coefficient had a direct relationship to the anchorage performance of bolt shanks. The larger the bond coefficient is, the higher the bond capacity of the bolt shanks. However, there was a certain influencing range for the bond coefficient. Once the bond coefficient was beyond that range, the bond coefficient had a marginal effect on the anchorage performance of the bolt shanks. Moreover, when the residual bond strength equalled the bond strength, fully bonded bolts were likely to show constant resistance behaviour.
The Opalinus Clay (OPA) formation is considered a suitable host rock candidate for nuclear waste storage. However, the sealing integrity and long-term safety of OPA are potentially compromised by pre-existing natural or artificially induced faults. Therefore, characterizing the mechanical behavior and microscale deformation mechanisms of faults and the surrounding rock is relevant for predicting repository damage evolution. In this study, we performed triaxial tests using saw-cut samples of the shaly and sandy facies of OPA to investigate the influence of pressure and mineral composition on the deformation behavior during fault reactivation. Dried samples were hydrostatically pre-compacted at 50 MPa and then deformed at constant strain rate, drained conditions and confining pressures (pc) of 5–35 MPa. Mechanical data from triaxial tests was complemented by local strain measurements to determine the relative contribution of bulk deformation and fault slip, as well as by acoustic emission (AE) monitoring, and elastic P-wave velocity measurements using ultrasonic transmissions. With increasing pc, we observe a transition from brittle deformation behavior with highly localized fault slip to semi-brittle behavior characterized by non-linear strain hardening with increasing delocalization of deformation. We find that brittle localization behavior is limited by pc at which fault strength exceeds matrix yield strength. AEs were only detected in tests performed on sandy facies samples, and activity decreased with increasing pc. Microstructural analysis of deformed samples revealed a positive correlation between increasing pc and gouge layer thickness. This goes along with a change from brittle fragmentation and frictional sliding to the development of shear zones with a higher contribution of cataclastic and granular flow. Friction coefficient at fault reactivation is only slightly higher for the sandy (µ ~ 0.48) compared to the shaly facies (µ ~ 0.4). Slide-hold-slide tests performed after ~ 6 mm axial shortening suggest stable creeping and long-term weakness of faults at the applied conditions. Our results demonstrate that the mode of fault reactivation highly depends on the present stress field and burial history. Fault slip behavior of sandy and shaly facies samples of Opalinus Clay is highly pressure sensitive.Brittle localization behavior is limited to the confining pressures at which fault strength exceeds matrix yield strength.With increasing confining pressure, gouge layer width increases and anastomosing shear plane networks develop during shear dominated by cataclastic (sandy facies) and granular (shaly facies) flow.Restrengthening of shaly and sandy facies Opalinus Clay is minor. Fault slip behavior of sandy and shaly facies samples of Opalinus Clay is highly pressure sensitive. Brittle localization behavior is limited to the confining pressures at which fault strength exceeds matrix yield strength. With increasing confining pressure, gouge layer width increases and anastomosing shear plane networks develop during shear dominated by cataclastic (sandy facies) and granular (shaly facies) flow. Restrengthening of shaly and sandy facies Opalinus Clay is minor.
Pressure solution, a mechanism that involves tight coupling between the geometry and thermal-hydro-mechanical-chemical (THMC) processes, plays an important role in diagenesis. In this study, we make the first attempt to conduct microscale THMC modeling to understand and quantify the impacts of geometry and temperature on pressure solution, taking natural salt rock as an example. This modeling capability is achieved by expanding a novel MC code that we developed previously (Hu et al. J Geophys Res: Solid Earth 126:e2021JB023112, 2021) to include temperature effects. We first conduct a simulation of an example that involves a single brine inclusion within a single halite grain and find that the temperature impact is limited for that case. We then extract geometry from an image of a natural salt rock and conduct simulations with different cases: (A) only temperature and no stress, (B) only stress and no temperature, and (C) with both stress and temperature. These different cases result in quite different phenomena. In case A, dissolution and precipitation occur across the entire system due to isolated pore space reaching a localized mass balance between dissolution, precipitation, and diffusion. In case B, intense geometric features (e.g., major asperities, inclusions) in one area undergo stress concentration, thus dominating pressure solution in that area. In case C, pressure solution is spread out at contacting highly stressed geometric features close to the hotter side. We conclude that geometric features dominate stress distribution, thus dominating pressure solution in a natural salt rock that may be affected by the temperature if a sufficient temperature gradient is applied.
Coal strength and deformability anisotropy are one of the important contents of deep rock mechanics, as well as a crucial aspect influencing the secure and efficient exploitation of deep coalbed methane. However, there are few anisotropic studies on coal. To survey the effects of anisotropy and confinement on strength and deformability of anthracite, a suit of triaxial compression tests up to high confinement were completed on anthracite samples at four confining pressures and five bedding orientations. Then, strength and deformability anisotropy were analyzed. Furthermore, anisotropic failure criteria for the prediction of the strength were evaluated. Finally, the influences of anisotropy and confining pressure on mechanical parameters and failure patterns were systematically discussed. The results show that the strengths of loaded vertically to bedding plane above parallel to bedding plane can be explained by the fracture of different bonds. The Saeidi and modified Hoek–Brown criteria can reflect anisotropy strength better. Empirical linear relationships between the peak axial strain and confining pressure under different bedding orientations are proposed. Young′s modulus of anthracite is independent of bedding orientation under high and ultrahigh stress and remarkably influenced by bedding orientation under low and moderate stress. Furthermore, Young′s modulus with increasing confinement can be distinguished into three stages at all bedding orientations and becomes approximately constant above high pressure except 30°. The fracture pattern is independent of confining pressure and controlled by the shear failure mechanism in the coal matrix at 30°, and is dominated by the bedding plane under arbitrary confining pressure at 45° and 60°. The stress level of 15 MPa is the transition stress value at 90° from tensile fracture to shear fracture.
To reveal the shear-seepage coupling characteristics of fractured specimens under cyclic loading and unloading, the specific test device and test method were designed in this study. The cyclic loading and unloading shear-seepage coupling test on the fractured rock mass under different confining pressures and seepage pressures was carried out by processing “double L-shaped” specimens, and the change laws of the shear characteristics and seepage characteristics of fractured specimens with different roughness were experimentally investigated. The results indicated that the peak shear stress, residual shear stress, and shear stiffness of rough fractures all increase with increasing confining pressure, while the change in normal dilatation displacement is the opposite. Under a constant normal stress, the permeability of rough fracture decreases, increase, and then stabilizes with increasing shear displacement. The peak shear stress of the smooth fracture is 3.7 times lower than that of the rough fracture with the same shear displacement, and the smooth sandstone specimens are all in a shear shrinkage state, with the normal shrinkage displacement of less than 1.0 mm. In addition, during unloading, permeability increases to some extent but cannot recover to the original value. The confining pressure causes permanent damage to the permeability of fractured rock mass. The permeability of sandstone specimens changes primarily in the early loading stage and late unloading stage. Based on the test results, the relationship between permeability and confining pressure follows a negative exponential function under cyclic loading and unloading conditions.
The whole life cycle evolution process of roadway surrounding rocks of deep mines is the energy competition evolution process. Therefore, a “three-stage” stress path was proposed to obtain the energy evolution characteristics and impact damage mechanism of deep-bedded sandstone. Meanwhile, conventional triaxial loading and “three-stage” triaxial loading and unloading mechanical tests of deep-bedded sandstone with various bedding angles were conducted. The results show that: (1) within β∈(30°, 45°), the decay rate of the peak strength in the two stress paths was signifcant, (2) the failure degree of deep-bedded sandstone under “three-stage” loading and unloading condition (TLUT) was weak failure, but that under conventional triaxial loading condition (CTLT) was medium failure and strong failure, and the proportion of medium failure degree was relatively high, (3) there was a signifcant linear relationship among the peak elastic energy, peak strength, and peak strain; in addition, the linear energy storage law of rocks from the perspective of triaxial conditions was verifed, (4) the accumulated energy in the tips of the weakest structural plane corresponding to the high stress concentration zone or high state gradient zone continuously impacted the low state gradient zone in terms of the “Energy Flow”. Subsequently, the weakest structure plane was gradually and continuously expanded, and extension and connection accumulated and eventually resulted in failure. The obtained conclusions provide a theoretical basis for the stability control of deep roadways and the prevention and control of rock burst.
As a novel rock-breaking technology, the high-voltage electro pulse rock-breaking boring (EPB) technology has a broad prospect for high efficiency and low energy consumption rock-breaking boring. But the mechanism of rock-breaking under the action of high voltage electro pulse is not clear. In this study, First, red sandstone is selected as the boring sample, the high-voltage electro pulse breaking experiment on red sandstone is carried out with an electrode bit with a diameter of 80 mm and the electrode spacing of 35 mm, the red sandstone is broken down by the shock wave from the electric blasting in plasma channels of the red sandstone, and the complete drilling in the red sandstone is realized. Second, the electric current is collected during the experiment, in addition, the parameter of the shock wave is obtained based on the discharge circuit model and the shock wave model developed in the previous study. The parameters of shock wave and discharge energy are obtained based on the electric discharge circuit model and the shock wave model established in the previous study. Finally, the breakdown process of red sandstone by the shock wave is simulated, and the high-voltage EPB mechanism is analyzed, the simulation result is consistent with the experiment result. Experimental and simulation results indicate that the breakdown process of red sandstone under the action of high-voltage electro pulse includes the electric blasting occurrence stage and the energy dissipating stage, the red sandstone is broken down by the resultant mechanisms of shear failure and tension failure, but the shear failure is dominant.
To study the shear behaviors of jointed rocks reinforced by basalt fiber-reinforced polymer (BFRP) bars and steel–FRP composite bars (SFCBs), we conduct laboratory tests and numerical simulations to analyze the shear strength, shear stiffness, energy dissipation, and bolt failure modes. Our results show that the shear stiffness of the BFRP bolted specimen is lower than that of the specimens bolted by steel bars and SFCBs, but the residual shear strength is higher. SFCB-reinforced jointed rock has the highest peak shear strength, and its residual strength is similar to that of the steel bar bolted specimen. The total energy absorbed by the BFRP bolted specimen is comparable to that absorbed by the steel bolted specimen. When the bolt inclination angle is 60°, the shear strength of the BFRP bar bolted specimens is higher than that of the steel reinforced one. The failure characteristics of BFRP bar bolted rocks can be categorized as resin matrix fracture, resin matrix and fiber shear, and fracture of resin matrix and rocks. The failure modes of the SFCB divided into surface FRP failure and steel bending. Based on numerical results, BFRP bars have larger axial force than conventional bolts, but lower shear stress. The axial stress of the BFRP bar increases as the bolt inclination angle decreases. Moreover, the BFRP bar is more likely to cause shear cracks at the interface between the rock and the bolt.
The shale oil reservoirs of the Lucaogou Formation in the Jimsar Sag developed a large number of natural fractures and thin interlayers. The expansion of hydraulic fractures is full of randomness and unknowingness due to this complex geological feature. Since the multi-cluster fracturing process with small well spacing is widely used, the growth of hydraulic fractures requires a combined consideration of competition between perforation clusters and interference between wells. From the perspective of engineering application, this paper summarizes the fracture propagation law under single-fracture, multi-cluster fracturing, and adjacent well interference from simple to complex based on three-dimensional discrete element methods and a large number of field data. Then the key controlling factors of fracture group morphology were quantitatively analyzed. Comparative analysis reveals that under complex geological conditions, a single crack can still be described by existing theories, but the expansion law of the fracture group is not a simple superposition of single fractures. The competitive expansion of multiple fractures is reflected in the Fracture Surface Area (FSA) differences between clusters, asymmetry, and irregularity. Moreover, under inter-well stress interference, the hydraulic fractures of the posterior-construction well will tend to expand in the opposite direction of the first-construction well. The optimal well spacing of the Jimsar shale oil reservoir is 200–300 m.
Excavation-damaged zones (EDZs) are caused by repeated blasting and can significantly influence the overall performance of underground excavation engineering. The determination of the extent of blast-induced damage is, therefore, a significant concern for engineers in terms of safety and costs. A comprehensive study was conducted to assess the effects of repeated blasting on the accumulative damage of interlaid rock to better understand the characteristics of EDZs that develop during the excavation of small clearance tunnels in underground railway stations. The results of ultrasonic tests show that rock mass wave velocities are influenced by blasting to a depth of approximately 1.6 m. The first blast causes near-field damage and affects the range of the fractured zone, and the subsequent blasts cause far-field damage of the exposed tunnel that subsequently accumulates. Field tests indicate that the influence of repeated blasting should be taken into consideration to properly assess blast-induced damage in tunnels. The damage accumulation process caused by millisecond delay blasting is simulated using commercial software LS-DYNA. The effect of the initial damage on the rock mass mechanical properties in interlaid rock is also considered in the analyses. The simulation results provide instructive insight on the rock damage mechanism and evolution law of accumulative damage caused by drilling and blasting.
A laboratory experimental program has been conducted to investigate three hinge buckling failure (THB) using a newly developed THB testing apparatus and methodology. The novel apparatus differs from buckling experimental programs used in the past in that it allows for direct control of axial load throughout the test similar to a horizontal in situ stress. The objective of the study was to gain quantifiable insight into the THB failure mechanism under a range of controlled axial stress conditions to inform future development of stability analysis methods. Specimens (two block) of the relatively homogeneous Wallace sandstone formation were selected and a total of seven tests were conducted to parametrically explore a range a specimen thickness (3.2 to 5 cm) and two levels of axial ‘clamping’ stress (10 and 15 MPa). In addition to load and displacement monitoring, high-speed camera images and acoustic emission (AE) data was collected during each test. The tests revealed relatively consistent characteristic behaviour with identifiable thresholds of yield, peak and failure (collapse). A brittle-ductile response transition was observed based on the tested parameters, although not clearly identified from the limited testing. A clear relation was developed for specimen thickness and axial stress. A calibrated distinct element modelling simulation of the experiment was conducted that showed a reasonable match to the characteristic experimental behaviour. This provided greater insight into the THB failure mechanism and showed that this method of simulation is suitable for this mode of failure.
Rotational bi-planar sliding is the most common type of failure in high bedding rock slopes (HBRSs). Prestressed anchor cables are often used to reinforce HBRSs because of their low cost and high efficiency. In this work, a new limit equilibrium method was proposed for evaluating the stability of such cable-reinforced HBRSs (CHBRSs) with respect to rotational bi-planar failure. Tilt-test physical modeling and discrete numerical simulations were first conducted to investigate the mechanism underlying the occurrence of rotational bi-planar failure. A no-block-division mechanical model was then established and the method used to assess the stability of the CHBRS was proposed based on formulae describing force and moment equilibria. Four physical models and traditional ‘slice’ methods (e.g., the Morgenstern–Price and Spencer methods) were subsequently used to validate the feasibility and accuracy of the proposed method. Finally, the effects of various parameters on the magnitude of the reinforcement effect (anchor location, pretension value, anchorage angle, and joint strength) were investigated. The results obtained using the proposed method are in good agreement with those obtained via tilt-test models and traditional slice methods. The slope has a higher safety factor when the prestressed anchor cable is located closer to the lower part of the slope; however, the anchorage angle of the cable has almost no effect on the safety factor. The proposed method can be used as a useful design aid to help protect HBRSs from rotational bi-planar failure.
UT-Austin’s Devine Fracture Pilot Site, 50 miles southwest of San Antonio, Texas, has been targeted for a comprehensive, multidisciplinary development of fracture diagnostic techniques that are cross-validated by ground-truth data acquisition near a recently created, 175-ft-deep, horizontal hydraulic fracture (Ahmadian et al. 2018 Demonstration of proof of concept of electromagnetic geophysical methods for high resolution illumination of induced fracture networks. In Proceedings of the SPE Hydraulic Fracturing Technology Conference and Exhibition, The Woodlands, Texas, USA, 23–25 January 2018. SPE-189858-MS.). To evaluate the fracture diagnostic methods at this site, we conducted injection tests with a predefined volumetric flow-rate profile, resembling a diagnostic fracture injection test on September 2020. Subsequently, we developed hydrogeological and geomechanical models based on flow-rate and bottomhole-pressure measurements. History-matching efforts using a simplified layer-cake hydrogeological model resulted in the field-scale formation permeability of 9.87 × 10–15 m2 (10 mD) and Darcy-scale fracture permeability. The analysis of the bottomhole pressure and injection-rate history showed that (1) the newly created horizontal fracture was closed adjacent to the injection well pre-injection and (2) the initial pump-pressure increase at a nominal volumetric injection rate led to near-well fracture reopening, fluid conductivity increase, and abrupt injection-rate increase. To overcome hydrogeological-model limitations of predicting fracture reopening throughout injection, we extended the modeling to a finite-element, poroelastic analysis of horizontal-fracture growth using a cohesive-zone model. Using this fracture-reopening model, we improved the history match of the transient-pressure response during the experiment by adjusting the hydromechanical properties. Post-injection pressure transient analyses helped reduce uncertainty in the overburden-stress gradient, and the initial hydraulic-fracturing simulation verified the plausibility of achieving the surveyed propped fracture area.
In rock engineering, size effects have been a topic of extensive research since the early 1960s, and despite many advances over the years, our understanding of size effect remains incomplete, especially for weak, porous, homogeneous rocks. Indeed, the vast majority of studies related to size effect have specifically considered low porosity rocks (generally crystalline). To bridge this gap in knowledge, we conducted unconfined compression tests on cubic limestone blocks ranging in size from 0.1 to 0.9 m. Texas Cream Limestone, which is a porous, homogeneous, weak rock, was chosen for this study. As this rock has not previously been studied in the literature, conventional compression tests and indirect tensile strength tests on cylindrical specimens were completed prior to testing the cube specimens. For the largest specimens, 3D digital image correlation (3D-DIC) was employed to track the surficial displacements as a function of the applied load. The tests revealed a lack of size effect for the entire range of block sizes considered. To evaluate size effects more broadly, data from prior studies on sedimentary rocks were compiled, and a tendency for the magnitude of the size effect on strength to decline with increasing porosity was noted. Some hypotheses regarding this trend are presented and evaluated based on strain-field heterogeneity metrics obtained from the 3D-DIC analysis.
Rock joints in faulted zones are usually filled with large amounts of fault gouge, in which clay-rich fillings are very common. Fault water can significantly influence the mechanical parameters of clay-rich fillings and affect the shear mechanical behaviour of rock joints. Based on a tunnelling project passing through a faulted zone in Yunnan Province of China, this paper investigates the mechanical behaviour of rock joints with clay-rich fillings. A series of direct shear tests are conducted on rock joint samples made from rock blocks and fillings collected from a fault zone. The shear stress‒displacement curves and shear failure characteristics of rock joints are obtained, and several key factors of the fillings are considered, such as particle size, mineral composition, clay content, and water content. Furthermore, the peak and residual shear strengths are analysed by considering the influence of the water content and normal loading pressure. The test results show that water content of fillings has a great influence on shear strength of clay-filling rock joints; there is a trend to a negative correlation between the shear strength of rock joints when water content (w) exceeds the plastic limit (wp). Finally, a shear strength model for rock joints with clay-rich fillings is proposed based on the JRC-JCS strength criterion. Influence of water content on the shear strength of rock joints with clay-rich fillings is studied. A series of direct shear tests are conducted on rock joint samples based on a tunnelling project. The shear stress–displacement curves of rock joints with clay-rich filling are obtained. A shear strength weakening model of clay-filled joints is proposed.
One of the complexities of rock engineering is predicting the fracture, especially in cases where the rock material exhibits anisotropy in both elastic and fracture properties. In this study, a fracture criterion based on a stress averaging procedure using a combination of the extended finite element method (XFEM) and the cohesive zone model (CZM) is proposed to predict the fracture growth in anisotropic rocks subjected to mixed mode I/II loadings. The precision of the proposed model is scrutinized by comparing its predictions for fracture load, fracture initiation angle and fracture path with the experimental results conducted on Grimsel granite. Finally, a discussion is presented on the effects of the important model parameters such as the mesh size dependency and the radius of the averaging zone. The proposed model is proven to be robust and reveals very good estimations for fracture parameters.
Understanding the force state of disc cutters in advance is of great practical significance to predict the dynamic response during mechanical excavation and provide data information for design and arrangement of cutterhead. The sandstone-breaking behaviour of single disc cutter is studied based on the three-dimensional Particle Flow Code (PFC3D) software and rock rotational cutting tests. A novel platform of rotational cutting, which enables circular cutting up to 3.3 m in diameter, is developed to compensate for the shortcoming of existing platforms. The five mechanical and structural parameters (penetration depth, rotational speed, tip width, cutter spacing and installation radius) are changed to investigate the characteristics of cutting force. The numerical simulation reveals that the cutter ring breakage is not only attributed to normal impact, but also caused by side force, which decreases with the growth of installation radius. Meanwhile, the average cutting force and the number of bond breakages increase with the growth of penetration depth and tip width due to the increasing contact area between rocks and disc cutters. It is found that the optimum cutter spacing is 100 mm based on the variation of specific cutting energy. The rotational speed has a weak effect on the cutting force. The trend of average cutting force obtained from the experiment is similar to that of numerical results, and the maximum error does not exceed 15% by comparing the both results. This verifies the accuracy of numerical results.
An understanding of the creep behavior of clayey rocks is considered fundamental for further development in the fields of nuclear waste underground disposal, underground mine design, strata control, and many other geological phenomena occurring in the earth’s crust. In this research, we performed a series of long-term triaxial compressive tests on a typical clayey rock, which confirmed its high creep potential beyond the creep threshold. Further analysis of the creep mechanism of this clayey rock indicated that two effects—strengthening and damage—are accompanied by the creep process. Additionally, based on the overstress theory and Drucker–Prager cap model, we developed a novel constitutive model considering two creep reference surfaces—cohesion and consolidation. Meanwhile, laws to reflect the strength and damage effect during creep were established and applied to the above constitutive model. Finally, the above theoretical studies were implemented in the finite element method software ABAQUS FEA using the subroutines CREEP and USDFLD. Some theory parameters were verified through back analysis. A comparison between the experimental results and numerical simulations confirmed the superiority of the established theoretical model.
In this study, cyclic loading–unloading experiments on coal samples with different moisture contents were conducted. The damage mechanism was investigated through acoustic emission (AE) monitoring. The validity of the average frequency and rise angle (RA–AF) correlation for the fracture evolution of water-bearing coal samples under cyclic loading was verified. The damage evolution equation of coal samples based on energy dissipation theory was proposed. The results show that at the same growth rates of damage variables for the three water-bearing coal samples, there is a characteristic stress σ' (σ' = 5.36 MPa, N = 3) in the evolution of damage variables of the samples. Under cyclic loading, the moisture content was negatively correlated with the damage variable during the initial damage stage, while the two were positively correlated during the accelerated damage stage. The total strain energy of the water-bearing coal samples increases with the increase in the loading cycles, and the higher the moisture content, the greater is the total strain energy. The results of cyclic loading–unloading experiments demonstrate that there is a correlation between the damage process caused by moisture content and stress in coal samples, and the evolution of dissipation energy and the development of hysteresis loops are both related to the presence of the characteristic stress σ'. The RA–AF scatter plot and probability density plot feedback of each period correspond to the characteristics of damage evolution. This study verifies the feasibility of RA–AF qualitative evaluation of fracture type evolution mode of water-bearing coal samples under cyclic loading.
Cyclical impact loads will transform into low-frequency fatigue loads during propagation; furthermore, the long-term disturbance of such loads induces the prestress loss of far-field anchored joint rock masses (AJRMs). Laboratory tests of AJRMs under different fatigue shear loads were conducted. The yield pattern of anchors, failure pattern of rock blocks and failure mechanism of joints were analysed. The prestress loss characteristics of anchors were studied in great detail. Based on the experimental results, a prestress loss constitutive model of anchors was explored. The results revealed that the morphology of the yielded zone of the anchor is z-shaped, while the failure patterns of the anchored rock blocks were mainly body cracking, and the stable rock blocks presented exfoliation of the cement mortar. In addition, microfissure growth and macrocrack nucleation under shear forces and fatigue forces resulted in the formation of macroscopic cracks and joint block spalling. A wavy ascent stage in the prestress loss caused by the fatigue load occurred, and the prestress loss increment and rate increased gradually as the fatigue loads increased. The Nishihara model and an elastic body were coupled to establish a coupled model that considers the influence of fatigue load, and a prestress relaxation equation of anchors was derived. When compared with data measured during a real engineering project, the results of the prestress relaxation equation exhibited good accuracy.
Thermally induced deformation and fracturing in rocks are ubiquitously encountered in underground geotechnical engineering and they are highly influenced by the material anisotropy. In the present manuscript, a superposition-based PD and FEM coupling approach is proposed for simulating thermal fracturing in orthotropic rocks. In this approach, the critical regions with possibility of cracks are encompassed by the non-ordinary state-based peridynamics (NOSBPD) model, while the entire problem domain is discretized by a fixed underlying finite element (FE) mesh. The thermal balance equation is fully approximated by the underlying finite elements without any contribution from the NOSBPD model. The NOSBPD model and FE model are coupled based on the superposition theory. The mechanical anisotropy, thermal anisotropy as well as the hindering effect of the insulated crack on the thermal diffusion are considered in this coupled model. A staggered solution scheme is employed to solve the coupled system. The performance of the coupled method for thermomechanical problems with and without damage is evaluated by two numerical examples. After validation, thermal fracturing in an orthotropic rock specimen under high surrounding temperature is systematically studied. The parametric study shows that the inclination angle of the cracks and the major axes of the elliptical shape of the isotherms are generally consistent with the principal material first axis. Both the mechanical and thermal anisotropy highly affect the thermal fracturing in orthotropic rocks.
Tunnel Boring Machines (TBMs) have been used in many underground coal mines in China with high constructive efficiency, sound equipment integration and low cost. A recent application of TBM in Yuandian No.2 Coal Mine in Huaibei, China, has shown that the surrounding rockmass spalled severely in stratum impacted by multiple joint sets. To analyze this problem, a new 3D ubiquitous-multiple-joint (UMJ) model was proposed in the present research, which comprised a rock matrix and multiple sets of joints according to geological conditions. Then, the yielding and deformation of the roadway excavated by TBM were analyzed. The results showed that the plastic zones, stress states and displacement of surrounding rockmass were significantly affected by the orientation of joint sets. The results could well indicate failure characteristics of the in-situ roadway and showed the advantages of UMJ model to simulate the excavation in jointed rockmass. The key factors influencing the roadway stability were discussed by the UMJ model. Based on the simulation results, suggestions on excavation-support schemes were proposed for TBM excavating roadways.
A series of freeze–thaw (F–T) cycle and multilevel fatigue loading tests are carried out on sandstone samples to explore rock mass's fracture behavior and energy evolution characteristics under the coupling action of F–T cycles and fatigue loads. First, the energy evolution characteristics of the sample are analyzed by the image integration method, and the law of energy storage and energy dissipation of the sample are further discussed. Subsequently, a coupled damage model is established based on the Lemaitre strain equivalence hypothesis. Finally, based on the b value and AF-RA waveform theory, the sample's crack evolution process and failure mode are analyzed using acoustic emission (AE) technology. The results show that the specimen's elastic energy and total energy density under the coupling action increase step-like with increasing the upper limit stress. The dissipated energy density decreases rapidly and stabilizes after the first cycle of each stage. The energy evolution process of the sample obeys the linear energy storage law and the two-stage energy dissipation law, in which the energy dissipation law is transformed from linear to exponential in the accelerated energy release stage. The coupling damage of the sample increases exponentially with the number of cycles, and the damage growth rate is slow at first and then fast. In addition, the crack propagation process of the specimen exhibits a 3-stage characteristic. As the number of F–T cycles increases, the proportion of shear cracks in the sample increases significantly, the failure mode transitions from X-conjugate failure to shear failure with a single oblique section, and the fatigue-softening effect is enhanced.
Novel experimental method was developed to conduct transient excavation experiments.A special plug is designed to achieve the transient unloading process.The stress, displacement and vibration of the surrounding rocks after transient excavation of a tunnel are systematically observed in the laboratory.The process of the stress redistribution is revealed during the transient excavation. Novel experimental method was developed to conduct transient excavation experiments. A special plug is designed to achieve the transient unloading process. The stress, displacement and vibration of the surrounding rocks after transient excavation of a tunnel are systematically observed in the laboratory. The process of the stress redistribution is revealed during the transient excavation.
We conducted laboratory direct shear tests on two parallel coplanar intermittently jointed rock to investigate the effect of joint roughness and loading conditions on the shear behavior and acoustic emission characteristics. Experimental results show that the joint roughness, shear rate, and normal stress positively correlate with the peak shear strength of intermittently jointed rock. For intermittently jointed specimens with different roughness on both sides, the roughness significantly influences the mechanical properties of the jointed rock. The stress concentration at the loading end mainly affects the accumulative acoustic emission (AE) energy before the peak strength and the growth rate of the peak shear strength. In the residual stage, the influence of normal stress on the residual shear strength is greater than that of the shear rate. The damage rate of intermittently jointed rock shows an increasing trend with the increase in roughness, shear rate, and normal stress. However, the effect of roughness on the damage rate is less than that of the shear rate and normal stress.
In this paper, the Pull-Off Test (POT) was analyzed as a method for determining the mode I fracture toughness (KIc) of rock and concrete. POT is submitted to tensile load (which causes a simpler stress state than tests submitted to bending and compression load) and its execution is easy, replicable and can be performed in laboratory and in situ (being its great advantage). A ratio between the dimensions of the POT was proposed to meet the Linear Elastic Fracture Mechanics requirements and the POT literature recommendations. Parametric studies were performed using numerical analysis and it was concluded that the POT is suitable for KIc testing, since mode I prevails in the test, and an equation was proposed for determining KIc in homogeneous rock and concrete (micro-concrete and mortar). A practical example of the method application was provided and KIc was determined for some homogeneous rocks. A similar result was achieved between POT and Semi-Circular Bending test.
Foliated rocks are often encountered in underground engineering, and the spatial orientation relationship between its foliation and in situ stress controls the stability of the surrounding rock. This is closely related to the anisotropy of the mechanical properties of foliated rocks. The anisotropic mechanical properties of thin foliated rock under the influences of foliation and stress can be obtained by laboratory testing; however, there have been few experimental studies on foliated rocks under true triaxial compression at present. Foliated rocks are in a three-dimensional unequal stress state in deep excavation engineering; thus, to evaluate the stability of surrounding rock and scientifically guide the support design, a systematic true triaxial test considering the loading orientations (β, ω) of schistosity for a foliated gneiss was conducted. The results show that the strength and failure of the gneiss are greatly affected by inherent structure and stress conditions. More specifically, the larger the ω and σ2, the greater is the strength, and the failure mode tends to be controlled by the differential stress. Finally, a new empirical true triaxial anisotropic failure criterion was proposed according to the variations of strength with loading angle and stress conditions. This criterion can reflect the tendency and sensitivity of the change in the strength to σ2 at different ω and β, and can satisfy the need to model degradation from the true triaxial stress state to the conventional triaxial stress state. This criterion provides a new approach to characterize the strength of anisotropic rocks and improve the design of engineering works in practice.
Conventional borehole pressure relief technologies cannot consider roadway anchorage support and pressure relief simultaneously, which is a disadvantage in that the integrity of the rock mass and supporting structure in the shallow surrounding rock anchorage zone is damaged when relieving and transferring stress. Therefore, this study proposes a new “anchorage + pressure relief” collaborative control technology to realize a trade-off between roadway anchorage support and pressure relief and to study its influencing factors. The proposed technology strengthens anchorage using bolt-cable-grouting in the shallow surrounding rock and excavates large-diameter holes for pressure relief in the stress peak zone outside the anchorage zone; both of these can help transfer the stress peak zones of the roadway sides to the deep surrounding rock without destroying the rock mass in the shallow anchorage zone. The stress evolution characteristics of the hole-making diameter, hole-making angle, and dynamic pressure coefficient on the pressure relief effect are analyzed via numerical simulations. A similarity simulation method is used to verify the effectiveness of the numerical simulation results and prove the feasibility of the proposed technology. Field engineering practice suggests that the maximum convergence of the two sides is 115 mm, and the stress of the anchor cable is < 200 kN after pressure relief via the internal hole-making operation. The deformation of the surrounding rock and stress of the anchor cables are within the safety restrictions. The study results help provide a new method and technical means for the continuous large-deformation control of the surrounding rock of the deep soft broken coal roadway under dynamic pressure disturbance.
Fragmentation of blocks upon impact is commonly observed during rockfall events. Nevertheless, fragmentation is not properly taken into account in the design of protection structures because it is still poorly understood. This paper presents an extensive and rigorous experimental campaign that aims at bringing insights into the understanding of the complex phenomenon of rock fragmentation upon impact. A total of 114 drop tests were conducted with four diameters (50, 75, 100, and 200 mm) of rock-like spheres (made of mortar) of three different strengths (34, 23 and 13 MPa), falling on a horizontal concrete slab, with the objective to gather high-quality fragmentation data. The analysis focuses on the fragment size distribution, the energy dissipation mechanisms at impact and the distribution of energy amongst fragments after impact. The results show that the fragment size distributions obtained in this campaign are not linear on a logarithmic scale. The total normalised amount of energy loss during the impact increases with impact velocity, and consequently the total kinetic energy after impact decreases. It was also found that energy loss to create the fracture surfaces is a constant fraction of the kinetic energy before impact. The trajectories of fragments are related to the impact velocity. At low impact velocity, the fragments tend to bounce but, as the impact velocity increases, they tend to be ejected sideways. Although testing mortar spheres in normal impact is a simplification, the series of tests presented in this work has brought some valuable understanding into the fragmentation phenomenon of rockfalls.
At present, hydrofracture technology is mainly used for shale gas exploration and exploitation, and the effect of volume fracturing is tightly linked to the mechanical properties (including anisotropy and brittleness) of shale. Meanwhile, Sichuan Basin is also an important area for unconventional natural gas exploration and development in China. This work analyzed mineral composition and microstructure of outcrop shale matrix in Lower Silurian Longmaxi Formation through XRD test, slice observation, and scanning electron microscope (SEM). How anisotropy affects shale deformation, strength and failure characteristics at different bedding angles (0°, 30°, 45°, 60°, and 90°) under diverse confining pressures (0 MPa, 25 MPa, 50 MPa, 75 MPa, and 100 MPa) in conventional uniaxial/triaxial experiments were discussed. The acoustic emission (AE) evolution was introduced to reveal the cracks closure and propagation of Longmaxi Formation shale and qualitatively characterized the brittleness of the shale. What’s more, based on the existing brittleness index, this study proposed an indicator BInew that considers the shape of the stress–strain curve before and after the peak stress to evaluate the brittleness of the Longmaxi Formation shale. The index BInew can better describe the changes of brittleness with confining pressure and bedding angle. Finally, this work elaborately explained the relationship between AE characteristics, brittleness, and failure modes of shale. AE characteristics can not only qualitatively characterize shale brittleness but also show the failure process and modes of shale samples. Change of failure modes rests with the difference in brittleness and also reflects the level of brittleness. The change of brittleness with failure modes can be expressed as shear along bedding plane (Sh-Al) > splitting through bedding plane (Sp-Th) > conjugate shear failure (Co-Sh) > shear along bedding plane (Sh-Al) > shear through bedding plane (Sh-Th).
The hydraulic conductivity of rock joints is an important parameter controlling fluid flow in various rock engineering applications. The shearing and normal loading have significant effects on hydraulic conductivity of rock joints, the property of which is mainly controlled by hydraulic aperture. Despite the importance of hydro-mechanical behaviour of rock joints, the fundamental micro-scale processes leading to macro-scale observations remain unexplored partly due to difficulties with in situ measurement of hydraulic aperture and its complex relation to roughness and contact area. Therefore, in this study, a series of experiments coupling fluid flow with normal deformability and direct shear are performed on joints with varying controlled roughness at different normal stresses. Along with measuring stress and flow rate, the time-lapse X-ray micro-computed tomography is carried out to explore the evolution of joint aperture and contact area during the experiments. The results of the normal deformability experiments show that the joint conductivity is well correlated to the mean hydraulic aperture of joint profiles. Such correlation, however, is not apparent for the shearing experiment where under high normal stresses, the flow rate decreases continually indicating that damaged asperities hinder the fluid flow. Despite the trend in the average mechanical aperture not following the flow rate in some cases, the trend in the contact area follows the flow rate very closely throughout the shearing process. In addition, the results reveal that despite an increase in contact area with increase in normal stress, it is not physically possible to reach full contact even for the artificially well-mated samples at a high normal stress of 10 MPa. Finally, a new correlation is proposed to relate the hydraulic aperture to joint average mechanical aperture, contact area and roughness. The correlation estimates the experimental flow rates at both normal and shear loading conditions with good accuracy.
The yielding support has proven to be one of the most effective measures to control the large deformation of the tunnel, as it could well stabilize tunnel support structure by releasing the deformation energy in the surrounding rock and exerting the self-supporting ability of the surrounding rock. To explore the damage mechanical behavior of shotcrete linings with yielding supports in large-deformation tunnels, an elastic–plastic damage model of concrete was developed, and a numerical method for simulating the yielding support based on the double-node beam element and interface element was proposed. Then, the influences of the position and resistance of the yielding element on the behaviors of shotcrete lining were analyzed and discussed. The results indicate that the position and resistance of the yielding element have a significant effect on the distribution of damage, principal stress, axial force, and bending moment of the lining. In the tunnel dominated by horizontal deformation, the yielding structure with constant resistance symmetrically arranged at the waist of the tunnel sidewall has the best yielding effect. The damage and maximum axial force are minimal, and the maximum bending moment is relatively large but lower than the maximum bending moment that the lining structure can bear. Assuming that the yielding element position stays the same, the maximum lining axial force rises with increasing yielding resistance, the maximum bending moment shows an initially upward and subsequently downward trend as the yielding resistance rises, and the maximum damage shows a trend of first declining and then growing as the yielding resistance rises. It is also found that the resistance of the yielding component should be controlled within a reasonable range, neither too small nor too large, for the lining to be in good working condition.
Hard roofs are a major cause of mining accidents such as rock bursts. To address the issue of hard roofs which do not collapse in time with coal mining workings, we propose directional fracturing of hard rock by high-temperature thermal shock. Experiments were conducted using a large true triaxial testing machine and a homemade heating system working with thermal shock. The results indicated that high-temperature thermal shock caused directional cracking in granite when it was heated to 500–550 °C at the heating rate of 114 °C/min. The temperature and temperature gradients along the heating direction were larger than those in the vertical heating direction at the same distance from the center of the borehole. The temperature gradient within 1.5 cm of the borehole in the heating direction was as high as 6300 °C/m. Under bidirectional compression conditions, we discovered that the horizontal stress difference played a dominant role in the initiation location of directional cracking by thermal shock. During the tests, the minimum horizontal principal stress was kept constant at 10 MPa and the maximum horizontal principal stress gradually increased. When the horizontal stress difference was smaller than 5 MPa, thermal shock-induced cracking occurred in the heating direction. When the horizontal stress difference was 6–7 MPa, thermal shock-induced cracks were generated both along and perpendicular to the heating direction. The results suggested that high-temperature thermal shock can produce cracks perpendicular to the maximum horizontal principal stress direction within a range of horizontal stress differences. These outcomes provide an essential guide for manually controlling hard roofs.
Barrier holes are often utilized around blasting excavations to reduce vibration. Understanding the vibration reduction effect of barrier hole is significant for the optimal design of barrier hole. In this study, a series of blasting tests were conducted on mortar block with barrier holes and opening. The horizontal first-peak strains and peak particle velocity (PPV) were recorded to investigate the effect of barrier hole parameter on stress wave attenuation, as well as vibration reduction effect at both ground surface and adjacent opening. The test results showed that barrier hole diameter, spacing, and the number of barrier hole rows all have significant impacts on the first-peak compressive strain on the adjacent opening wall, PPV on the top surface of the mortar block, and the vibration–isolation rate across the barrier hole screen. With the increase in barrier hole diameter and the number of barrier hole row or decrease in barrier hole spacing, the first-peak compressive strain of the adjacent opening and PPV on the block surface decrease, and the vibration–isolation rate increases. Among all barrier hole parameters, the barrier hole diameter has the greatest impact on the vibration–isolation rate. Correlations between barrier hole parameters and vibration isolation effect were established. The findings in this study can provide a framework for the design of barrier hole arrangements.
Deformation measurement is crucial for understanding the mechanical responses of rock masses; however, full-scale tests on rock masses are difficult and costly. With the development of 3D printing technology, small-scale tests provide a promising solution. Nevertheless, traditional DIC codes usually fail due to the existence of discontinuities such as cracks and shear bonds, and the correlation also rapidly drops when large deformation occurs, which is an obstacle for further application of the DIC in small-scale tests on rock. In this work, a discrete digital image correlation (DDIC), based on the principle of DIC and the discrete finite element method (DFEM), is introduced to address discontinuities and large deformations. The triangular mesh with shrunken internal nodes was selected to address the discontinuities. A scheme of updating the reference image based on the preset threshold was employed to solve the decorrelation caused by large deformation. The rigid rotation of the strain field was removed by introducing polar decomposition in DDIC. The effectiveness and reliability of the proposed DIC were verified through several small-scale tests including dislocation, separation, and large deformation. The results showed that the updated triangular mesh can adequately trace and adapt to the discontinuous deformation and large deformation of specimen, while correlation coefficients can still fall within a relatively high confidence interval. Overall, DDIC with small-scale tests might provide a promising solution to the difficulty of preparing large specimens in rock mechanics studies and provide measurement data for the verification and calibration of discontinuous numerical methods.
Water is a crucial factor that influences the mechanical behavior of rock and can induce failure. The aim of this study was to investigate the effects of water content and water distribution on the mechanical behavior of sandstone subjected to triaxial compression. Sandstone samples under different water states, including dry, unsaturated, saturated, and long-term saturated, were prepared. Some saturated samples were dried in air for different periods to prepare samples that were dry on the outside but remained wet on the inside. The triaxial compression tests were performed on the samples under four confining pressures: 5, 10, 15 and 25 MPa. The results indicated that the water state of a sandstone sample can be characterized by its water content, soaking time, water distribution, and long-term saturation. The different water states can affect remarkably the strength, deformation, and failure pattern of the sandstone samples subjected to triaxial compression. Several empirical equations were established to describe the evolution of the mechanical behavior of sandstone with increasing water content and soaking duration. The nonuniform distribution of water promotes the generation of tensile cracks, while increasing confining pressure promotes the development of shear cracks. The strength of sandstone samples with dry outside and wet inside are lower than that of samples with wet outside and dry inside. What is more, a long-term saturated state increases the strain-softening of the sandstone samples, thereby increasing their deformation and lowering their strength.
Mining, drilling, blast excavation and large-scale explosive disturbance effects can all threaten the safety of constructed tunnels. Aiming to clarify the failure mechanism of deep-buried cavern under dynamic disturbance, a true triaxial deep cavern ground impact effect simulation test device was used to realize the whole process from cavern excavation to engineering disaster induced by impact load. Based on the similarity theory, similarity indexes of the whole model test were obtained. Cubic specimens of 1.3 m were used to simulate the failure process of the cavern with the length similarity ratio of 50:1. The test adopted sequence of loading, cavern excavation and plane impact disturbance. Dynamic and static pressure sensors, fiber Bragg grating sensors and dynamic displacement meters were used to monitor the changes of stress, strain and displacement. Evolution laws of stress and strain during the excavation and impact process were obtained. During the test, plane dynamic and static coupling loading were applied on the top of specimen. Results showed that with the increase of impact disturbance, the displacement and strain of surrounding rock increase, and so did the failure of cavern. The attenuation law of stress wave in the structure with cavity was obtained by analyzing the stress variation from the top to the bottom of the specimen. In addition, block activation was detected during the test. Finally, engineering disasters were simulated successfully. Findings are helpful for improving understanding of the mechanism of deep cavern disaster under dynamic disturbance.
In this study, a suite of tests was conducted to investigate the microscale mechanical, elastic, and microstructural alterations due to rock–microbial interactions in dolomitic rocks (dolostones) at elevated treatment (temperature and pressure) conditions. Dolostone samples were treated with a microbial media, and the microscale mechanical and elastic responses and the unit weight changes due to the rock–microbial interaction in dolotstones were quantified. Subsequently, a new relationship for estimating dynamic Young’s modulus (E) of rocks from scratch test was proposed. The microstructural alteration during the process was reported using scanning electron microscopy (SEM). The results of rock–microbial interactions in dolostone indicate potential: (i) mechanical alterations with distinct low and peak values in investigated core samples; (ii) enhancement in mechanical and elastic properties and the mechanical integrity (−37% Poisson’s ratio, ν; +320% scratch-derived Young’s modulus, Escr; +80% unconfined compressive strength, UCS; +223% scratch toughness, Ks); (iii) increase in pore throat clogging and unit weight (+ 33%) due to mineral precipitations, which can occlude and cement the pore spaces in dolostones and increase the intergranular-bonding integrity. The results also suggest a greater rock–microbial impact in dolostones relative to shales, due to higher inherent pores and permeable fractures in dolostones which might have accelerated microbial actions at farther reach. Further, this study provides new insights into understanding the coupled rock–microbial interaction in dolostones, and proposes Escr as an alternate method for estimating the Young’s modulus of sedimentary rocks from scratch tests.
Microseismic/acoustic emission (MS/AE) location technology is a powerful means to study the spatial evolution characteristics of rock fracture and early warning of geological hazards. This paper investigated the variation in MS/AE location accuracy and the spatial evolution characteristics of granite fracture in complex stress conditions by using the velocity-free MS/AE source location method. Results show that the variation of wave velocity caused by granite fracture is a key factor for the variation of location accuracy. It is expected to improve the location accuracy by dynamically correcting the iterative wave velocity. The evolution process and results of granite microcracks in uniaxial and biaxial stress conditions show consistency and difference. The consistency is that the microcracks start from the edges and corners at both ends of the rock and gradually develop to the central side of the rock. The distribution of AE events changes from scattered to clustered, nucleated, and finally to banded distribution. The difference is that the advance of rock damage strain point and the macroscopic fracture surfaces are mostly perpendicular to the minimum principal stress direction in biaxial stress conditions. This paper is not only a useful supplement to the MS/AE location methods and theories, but also provides a reference for the disaster-causing mechanism of rock instability as well as disaster prevention and control.
The measured swelling pressures against tunnel linings range between a fraction of one MPa and 6–7 MPa. A strong spatial heterogeneity is often observed. The paper integrates these considerations into a procedure to design tunnel linings in anhydritic formations. Three-dimensional effects and proper consideration of heterogeneity is shown to be consistent with monitoring data of lining reinforcement stresses. The calculation methodology is illustrated in the case of the Lilla tunnel lining, which was monitored for more than 6 years. The described procedure leads to a rational design away from the conservatism of the assumption of uniform pressures against lining and two-dimensional modelling of tunnel cross-section.
Frost crack evolution induced by cyclic freeze–thaw is responsible for rock deterioration in cold regions and poses major threats to public safety, engineering structures, and alpine slope stability. This paper presents experimental and numerical works aimed at investigating the frost crack evolution in fissured rock masses, as well as the interaction between frost cracks. A series of laboratory freezing experiments are conducted on rock-like specimens with various pre-existing fissures. Experimental results show that frost cracks initiate at the pre-existing fissure tips and propagate under the freeze–thaw treatment. Moreover, the frost crack evolution is significantly influenced by external stress conditions and frost crack interactions, forming several typical propagation patterns (e.g., deflection, coplanar and butterfly shape, etc.). Then, numerical simulations with a low-temperature thermal–mechanical coupled model, where the water/ice phase transition and hence volume expansion are explicitly simulated, are conducted to reproduce the experimental observation. The numerical results are consistent with the experimental observations and help to reveal the underlying mechanisms of the frost crack growth and frost crack interaction. This experimental and numerical investigation helps to improve the understanding of frost cracking mechanisms that can inform engineering design in cold regions with fissured rock masses.
Bolts-grouting reinforcement method can reduce the water inflow and strengthen the surrounding rock in the tunnel with a quantity of water seepage. However, there are few analytical theories of this bolts-grouting reinforcement considering seepage for these water-rich tunnels. This paper proposed a closed-form solution for the bolts-grouting reinforcement of surrounding rock under axisymmetric conditions. Based on Darcy's law and Terzaghi’s effective stress principle, the force of the rockbolts on the surrounding rock and pore water pressure was substituted into the equilibrium equation of the surrounding rock, and the effect of grouting reinforcement was considered by the shear modulus and permeability coefficient of the surrounding rock. Besides, the “smeared” method was used to calculate the system effect of rockbolts. The results obtained with the closed-form solution were shown to be equivalent to the results of the same problem solved by finite element methods. The effect of bolts-grouting reinforcement parameters on the maximum shear stress and displacement at the inner edge of the surrounding rock under different support pressure and impermeability pressure was discussed. It is more conducive to controlling the maximum shear stress and deformation of the surrounding rock with the increase of the number of rockbolts and the decrease of the length of each rockbolt under the same total length of rockbolts. The impermeability pressure has a negative impact on the control of the maximum shear stress and deformation of the surrounding rock. However, the support pressure was beneficial to reinforce the surrounding rock. A bolts-grouting design included pre-grouting, and optimization method has been developed for a section of the Gaoligongshan tunnel with the average RMR-value of 51 and RMRTBM of 63 based on our closed-form solution. The monitoring data suggested that the optimum design method can ensure the safety of the tunnel while making full use of the tensile properties and reducing the length of the rockbolts. A closed-form solution considering both rockbolts and grouting reinforcement under seepage was proposed.The increase of the number of rockbolts and decrease of the length of each rockbolt have a better reinforcement performance for the surrounding rock.The impermeability pressure has a negative impact on the control of the stress and deformation in the surrounding rock.An optimal design method of bolts-grouting parameters is proposed based on our analytical solution. A closed-form solution considering both rockbolts and grouting reinforcement under seepage was proposed. The increase of the number of rockbolts and decrease of the length of each rockbolt have a better reinforcement performance for the surrounding rock. The impermeability pressure has a negative impact on the control of the stress and deformation in the surrounding rock. An optimal design method of bolts-grouting parameters is proposed based on our analytical solution.
Top-cited authors
Xia-Ting Feng
  • Chinese Academy of Sciences
Xibing Li
  • Central South University
Feng Dai
  • Sichuan University
Abbas Taheri
  • Queen's University
Sheng-Qi Yang
  • China University of Mining and Technology