Search

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Uniaxial and triaxial compression tests are commonly used in rock engineering to achieve strength and shape variations parameters. However, loading conditions in real projects like excavation, underground reservoirs, and even high-magnitude earthquakes play a vital role in these experiments. Therefore, to represent the actual loading conditions on Earth, a true triaxial test ( σ 1 ≥ σ 2 ≥ σ 3 ) is performed on rock samples. In this regard, choosing the right shape of samples has significant effects on stress distribution. Cylindrical samples are more common in experimental tests because they represent better behaviour under stress and can compensate for some cubic drawbacks. Accordingly, this paper proposes a new simulation of a true triaxial test on cylindrical samples to present an efficient and economical procedure to understand the behaviour of rocks in a true triaxial state. Particle flow code in three dimensions (PFC3D) based on the discrete element method (DEM) and flat joint model were selected in this study. First, based on the data from experimental tests, a simulation of rock was performed in PFC3D software. Then, the micro-parameters were calibrated based on the macro-parameters of Inada granite test results, and the effect of those micro-parameters in this step was investigated. Next, the cylindrical true triaxial test machine was simulated in PFC3D, and loading conditions on the sample were simulated by applying various stresses in x-, y-, and z-directions. At last, the effect of unequal confinement pressure on the ultimate sample strength and failure has been investigated to predict the sample behaviour under true triaxial conditions. [ABSTRACT FROM AUTHOR]

The physical and mechanical properties of the surrounding rock on the geothermal wellbore will be deteriorated by high temperature, resulting in reduced stability. In order to investigate the tensile properties and damage mechanism of sandstone after the action of different temperatures, direct tensile tests were conducted on square sandstone after the action of 20 °C, 200 °C, 600 °C, and 1000 °C and tested the change rule of wave velocity in the process of rock gradual cracking and analyzed the influence of temperature on roughness. Meanwhile, based on the Weibull distribution and the statistical damage principle, a statistical damage constitutive model for square sandstone, considering high temperature and tensile effects, is constructed, and the parameter expressions are determined. The result showed that the tensile stress–strain curve of square sandstone has four-stage properties, and the tensile strength and elastic modulus of square sandstone gradually decrease with the increase of temperature. The tensile strength and elastic modulus of the square sandstone gradually decrease, and they decrease by 35.55% and 76.43% after the action of 1000 °C relative to that of 20 °C. The overall wave velocity of the rock also tends to decrease with strain and temperature. And roughness gradually decreases with increasing temperature. The comparison of the model curves reveals that the calculated curve and the test curve have a high degree of agreement, indicating that the constitutive model can reflect the tensile stress–strain process of the square sandstone and can reflect the reasonableness of the model. The coupled damage variables D after different temperature effects showed an S-shaped monotonic increasing trend with strain. The research results of this paper can provide some theoretical guidance for the stability assessment of geothermal mining engineering. [ABSTRACT FROM AUTHOR]

Abandoned coal mine roadways are used to construct compressed air energy storage (CAES) caverns. However, the surrounding rock of the roadway is subjected to high internal pressure, which changes the stress state of the surrounding rock and causes rock deformation that directly affects the roadway stability. In this study, numerical simulation software FLAC3D was used to calculate the displacement and plastic zone volume of surrounding rock under 5–10 MPa internal pressures. Effects of buried depth, surrounding rock grade, and in situ stress state on surrounding rock deformation under high internal pressure were analyzed. Results show that the displacement and plastic zone volume of the surrounding rock decrease with the increase in buried depth, and the deformation of shallow surrounding rock is greater than that of the deep surrounding rock. The deformation of grades IV and V surrounding rocks is greater than that of grades I, II, and III surrounding rocks. The surrounding rock deformation is the largest under σv > σH > σh, followed by σH > σv > σh, and the smallest under σH > σh > σv. The research results play an important role in the safe construction of CAES caverns on abandoned roadways. [ABSTRACT FROM AUTHOR]

Wellbore instability in shale formations has always been a concern in oil exploration field. According to the Mohr–Coulomb criterion, a new model for evaluating and classifying the collapse of horizontal wellbore in shale formation is established. In this model, the temperature field and the pore pressure field affected by hydraulic pressure, thermal potential, and chemical potential difference between the drilling fluid and formation fluid are considered. Using the model, the influences of the temperature and solute concentration of drilling fluid on the failure degree of horizontal wellbore in shale formation with weak planes are simulated and analyzed. Based on the research results, weak planes that exist in shale formation will cause a significant wellbore collapse and using cool drilling fluid with high solute concentration can reduce the collapse degree of horizontal wellbore. The model established may help drilling engineers in practice to take effective measures for reducing wellbore instability and avoiding possible wellbore collapse during drilling in shale formations. [ABSTRACT FROM AUTHOR]

Initially used as foundations for transmission towers, helical piles are now used for solar power plants, wind turbines, boardwalks, retaining walls, retrofitting works, and even residential buildings. Crucial progress has been made over the last few decades using various numerical solution approaches, field tests, and software analyses to improve the design of such piles, as they have numerous benefits over conventional piles. A few countries have developed codes for their design such as Acceptance Criteria 358 (USA), Australian Standard AS 2159-2009, British Standard BS 8004: 2015 (UK), and Canadian Foundation Engineering Manual CFEM 2006. Helical piling field practices are also done based on design manuals formulated by companies manufacturing them. However, the investigation is still in progress for perfecting their design and accurately predicting their uplift, lateral, and compressive capacities. In this paper, the most important and widely used methods for predicting the axial load carrying capacities and the essential design parameters influencing them have been discussed. A detailed review of the finite element analysis conducted on helical piles has been presented, which will be helpful for further analyses on such piles using software. A design scheme has also been outlined for the analysis of the behavior and capacity of helical piles. [ABSTRACT FROM AUTHOR]

This paper aims to utilize different stochastic numerical procedures for designing a rock tunnel lining, namely, strength classification (SCM) and random field (RFM) methods. The procedures are implemented in a finite difference scheme, while a stochastic process based on the Monte Carlo simulation technique is followed for rock mass characterization, considering its property variability. Moreover, a performance function is defined to investigate the lining serviceability based on the interactions between bending moment, thrust, and shear forces. The effects of coefficient of variation and correlation length on the results are also investigated. Finally, the probabilistic analysis results are compared with the deterministic ones and the in situ measurements. The results show that the effect of the coefficient of variation on the spatial variability is more significant than the effect of the correlation length. It can also be observed that the SCM models calculate lower support requirements than the RFMs when CoV is smaller than 30%. Moreover, the cross-correlated models lead to lower PoF, which can be related to the assumed negative cross-correlation between c and φ . [ABSTRACT FROM AUTHOR]

When using the Hoek–Brown strength criterion or Mohr–Coulomb strength criterion for tunnel collapse analysis, the effect of intermediate principal stress is always ignored. In order to study the collapse mechanism considering the influence of intermediate principal shear stress, the collapse mechanism of deep buried tunnel subjected to pore water pressure in conjunction with generalized nonlinear unified strength theory is established. The analytical solutions of two-dimensional detaching curve f x of circular and rectangular tunnels are deduced; therefore, the range and shape of collapsing blocks can be clearly obtained by substituting the practical rock mass parameters; the results obtained by the presented approach are approximate with the numerical results provided by finite difference code, which proves that the proposed method in this work is valid. Furthermore, the impacts of the influence factors are discussed in the analytical solution; the results show that the intermediate principal shear stress influence coefficient b has significant effect on the collapse of deep buried tunnel, which is one of the non-negligible parameters in collapse analysis, ignoring that the intermediate principal shear stress effect would underestimate the strength properties of the rock material. When setting the coefficient b to 0, the results show great consistency with other strength theory and verify the validity of present approach. In addition, the intermediate principal stress coefficient is functionally related to instantaneous friction angle in plane strain state, indicating that the instantaneous friction angle also has an impact on tunnel collapse, and the influence regularity is discussed accordingly. The results of this paper may provide workable guidelines for the prediction of potential collapse extent and support designs in minimizing rock detachment above tunnel roofs in practice. [ABSTRACT FROM AUTHOR]

The macroscopic mechanical properties of granites are affected by inhomogeneous factors such as mineral shape, size, and distribution. Microscopic parameters of granites from the Songliao Basin were calibrated in PFC2D using the grain-based model approach. The effects of mineral grain size on strength, deformation, and microcracking behavior were analyzed by conducting the uniaxial compression test and triaxial compression test under confining pressure of 50 MPa. Research showed that the strength of the rock gradually reinforced as the grain size increased and was positively correlated with the average grain-disk size ratio of 2.5~16.87. The results of numerical simulations on grain size effects conflicted with the results obtained in laboratory experiments. To reflect the true mechanism of the grain size effect, the relative percentage method of microscopic strength parameters was proposed, and the strength variation law after the discounted reduction matched well with the conclusion of the laboratory test, indicating that the method has considerable feasibility. Research on the microscopic parameters of granites will be of great guidance for geothermal research in the Songliao Basin. [ABSTRACT FROM AUTHOR]

With the demand for many engineering facilities on coral reefs, it is urgent to study the physical and mechanical properties of reef limestone to meet the needs of coral reef construction. In this paper, real-time high-resolution X-ray micro-CT was used to scan reef limestone sample experimentally under unconfined axial compression for the first time. The rock samples from 286 m below sea level in S Reef were made into cylindrical specimens with diameter of 3.871 mm and height of 8.443 mm to study progressive failure characteristics. Through 2D vertical slices and reconstructed 3D stereograms, the microstructure and pore morphology of the samples were extracted and quantitatively characterized corresponding to each loading stage. These data show that biological components, coral gravel, and algae bonding surface are weak parts in the reef limestone, which reduces its tensile strength and enhances the heterogeneity of the internal structure. In addition, the relationship between the void volume, solid volume, and total volume of reef limestone CT images with the axial stress-strain curve was discussed. [ABSTRACT FROM AUTHOR]

Landfill cover is one of the most important elements of any modern landfill, the integrity and stability of which is obviously a major issue in the design and construction of landfills. Tapered landfill cover is an attractive technique to enhance the stability of landfill cover. In most of design analyses of landfill cover, the simultaneous effects of reinforcement (root and artificial layer), seepage, and seismic forces are ignored, and only the impact of each of these forces on the landfill cover stability has incompletely investigated. Basically, simply neglecting the simultaneous effects of mentioned forces can lead to an overestimated factor of safety (FoS). A novel approach based on limit equilibrium method (LEM) was proposed to analyze the stability of tapered landfill cover, which is supported by artificial reinforcement layer in addition to the root system under seismic and seepage loading conditions. The results indicated that the reinforcement parameters (Kc/Kt and tR) remarkably affected the stability of tapered landfill cover system, specifically when the landfill cover is under extremely adverse conditions, and the slopes should remain steep and uninterrupted along their length to ensure optimal use of the landfill area; the application of proposed reinforcement mechanisms can significantly enhance the stability. [ABSTRACT FROM AUTHOR]

This paper designed a special-shaped cross-section hollow self-drilling bolt with discharge grooves, in which Mindlin's solution of displacement was used in the elastic solution of a wholly grouted bolt. Theoretically, the ultimate pulling force and the residual pulling force were analyzed by considering the average distribution assumption of residual stress. The bolts' performance with 2, 3, and 4 discharge grooves was compared with circular cross-section bolts, which experimental results are consistent with the theoretical results. The ultimate pulling force and the residual pulling force of the special-shaped cross-section bolt improved significantly according to the circular bolt. The bonding properties of special-shaped cross-section bolts present significant difference to circular section bolts. The residual pulling strength was contributed both by the bar-grout and grout-grout frictions. Upgrading the grout strength may further enhance the pulling strength of special-shaped cross-section bolts. [ABSTRACT FROM AUTHOR]

In hydraulic fracturing for fractured wells, the pressure of cyclic loading and unloading in the casing can easily lead to integrity failure of the cement sheath in the fracturing section, representing a production safety hazard. The integrity failure evaluation experiments of cement sheath were carried out for fractured wells under the condition of alternating loading. The cause of integrity failure was analyzed based on the fatigue cumulative damage theory and failure criteria. The integrity failure law was obtained for cement sheath under a confining pressure of 30 MPa and alternating loading. The results demonstrate the following findings: firstly, under alternating pressure, the internal structure of the cement is damaged, and its mechanical properties (such as compressive strength) decrease with increases in the number of alternating pressure cycles; the maximum decrease can be 40%. Secondly, the bond strength of the casing–cement interface is low, concentrated between 0.45 and 0.65 MPa, and the plastic accumulation of the cement sheath during fracturing leads to inconsistency in the interface strain between the casing and the sheath. The interface separates and forms a micro-annulus when the interfacial tensile stress exceeds the interfacial bond strength during pressure relief. Thirdly, under the condition of a confining pressure 30 MPa and high alternating pressure, micro-annulus formation is the primary mode of failure regarding cement sheath integrity. Increasing the elasticity and deformation ability of the cement sheath helps prevent this failure. The conclusions of this research may provide a technical reference for improving the cement slurry and fracturing operation of fractured wells and help ensure the integrity of the cement sheaths of fractured wells. [ABSTRACT FROM AUTHOR]

In the Changning shale gas development blocks of the Sichuan Basin, casing deformation and uneven gas production rates between wells seriously restricted the efficient development of shale gas reservoirs. Heterogeneous distributions of in situ stresses are one of the important factors that influence the occurrence of these unfavorable engineering phenomena. This paper presents the assessment of present-day in situ stress distributions on one of the blocks in the Changning shale gas development area through field diagnostic fracture injection test (DFIT) and the development of a 3D in situ stress model. To accurately model the distributions of in situ stress in the Longmaxi shale gas formation, more than 11 DFITs were conducted on a 192-km2 shale gas development block. Minimum principal stress and pore pressure were interpreted from the DFIT data by using pressure transient analysis (PTA) methods. The 3D in situ stress model was constructed by using the finite element (FE) method based on a site-specific 3D geological model. The stress model was calibrated and constrained by the results from field DFIT stress measurements. The reservoir is highly over-pressured with pore pressure gradients ranging from 16 to 19 kPa/m within the development block. The distributions of in situ stress are highly variable within the reservoir, and the shear stresses in some areas are significantly higher than that in other areas. The authors compiled the casing deformation and well production data from more than 40 wells. It was found that (1) the casing deformation frequently occurred in the area with large shear stress, especially in the case of the non-uniform shear stress distribution along a horizontal well, and (2) the gas production rates were positively correlated with the shear stress level (SSL). The higher the SSL, the greater the gas production rates are. Because many previous studies about in situ stress modeling have been based on well log-interpreted stresses, this study emphasizes the importance of constructing in situ stress models based on direct field stress measurements and constraining the stresses through multiple sources and methods. The methodology and the data repository are potential sources for future geomechanical studies on similar projects. [ABSTRACT FROM AUTHOR]

To investigate the characteristics of the acoustic emission and microseism occurring during rock shear failure, we conducted a series of direct shear tests on the cubic specimens under different normal stress conditions. First, the tested specimen shear strength, deformation characteristics, and crack during rock shear failure are investigated. Then, the waveform, fractal, and spectrum of microseism (MS) are analyzed. The deformation and stress characteristics of the tested specimen during the direct shear test are consistent with those obtained in the previous studies, indicating that the experimental results from our direct shear test are reasonable. In addition, a "quiet period" can be observed in the waveform of MS before the overall shear failure under low normal stress conditions for all the types of specimens. With the increase of normal stress, the relative length of the "quiet period" decreases, especially for the granite specimens, the "quiet period" will disappear. Otherwise, it can be obtained that before the overall fractal dimension exhibit a rapid increase. Furthermore, investigation on the frequency-amplitude distribution of MS indicates that high-amplitude MS signals occur during the rapid stress decreasing stage. Meanwhile, with the increases of the normal stress, more high-amplitude MS signals arise. The distribution of these high-amplitude MS signals changes from punctiform-shaped to bar-shaped and then clustered shape. It is concluded that the "quiet period" of MS waveform, the rapid increase of fractal dimension, and the appearance of high-amplitude MS before the overall shear failure can be taken as the precursor information to the overall shear failure. [ABSTRACT FROM AUTHOR]

The development and application of rock-breaking technologies, such as laser, hydraulic and acoustic, have provided more possibilities for traditional mechanical rock-breaking tunnel boring machine (TBM) to improve tunnel construction efficiency and realize construction in complex geological environment and large-scale tunnel excavation. At present, the research and development of high-performance TBM coupled with multiple rock-breaking technologies is still in the exploratory stage, and there is still a lack of understanding of the synergistic operation mode of different rock-breaking technologies. According to the differences in the implementation characteristics and scope of action of different technologies, the rock-breaking process can be classified into three modes: borehole treatment, slotting treatment and surface broken treatment of rock wall. And the synergistic operation characteristics of the three modes for rock breaking are analysed. Taking the slotting treatment mode as an example, the free surface is formed on the rock mass by prefabricated kerfs, and the penetration tests of the sandstone with slotting treatment are carried out with single and double indenters. The results show that the slotting treatment is of great significance for reducing the rock-breaking force and improving the rock-breaking efficiency of TBM mechanical disc cutter, and the feasibility of the application of slotting treatment mode in TBM for rock breaking is verified. It shows that the coupling of various rock-breaking technologies and synergistic operation with different treatment modes are of great significance to the renovation and upgrading of TBM equipment. [ABSTRACT FROM AUTHOR]

To evaluate the influence of confining pressure on the rock-breaking force required by the cutter and provide a new possible method for ground stress prediction, the rock breakage mechanism under TBM cutters is studied through theoretical analysis and numerical simulation. A theoretical model for peak penetration force required for rock breakage prediction is established with confining pressure considered. The rock breakage by cutters under different confining pressures is simulated by PFC and compared with the theoretical analysis results. The numerical simulation results are in good agreement with the theoretical analysis, which verifies the validity of the established mechanical model. The peak penetration force increases significantly with the increase of the confining pressure that is perpendicular to the cutting direction and increases slowly with the increase of the confining pressure that is parallel to the cutting direction. Considering the effect of biaxial confining pressure, the impact factor is introduced to modify the theoretical formula for peak penetration force prediction. Besides, we discuss the variation of specific energy and found that increasing confining pressure improves rock fragmentation efficiency at high confining pressure though the effect is less evident when it exceeds a critical value. [ABSTRACT FROM AUTHOR]

The rock mass of underground engineering is often faced with tensile–shear failure disaster due to excavation. However, the failure mechanism of flawed rocks under tensile–shear stress state is not well understood at present and there is no relevant experimental investigation due to the difficulty of test technology. To reveal the crack propagation mechanism of flawed rocks under tensile–shear stress, experimental investigation using a developed tension–shear auxiliary device and numerical simulation were conducted in this study. The results obtained from direct shear tests of single-flawed sandstone specimens under constant normal tensile stress indicate that the flaw inclination angle and normal tensile stress significantly affect the shear strength and crack pattern. The cracks are mainly subjected to tensioning for different flaw inclination angles. It is easy to develop secondary cracks to produce the nucleation failure with the primary cracks when the flaw inclination angle is an acute angle. Digital image correlation (DIC) analysis suggests that the strain concentrations are greater gradually near the two tips of the flaw and the cracks grow along the strain concentration areas. Furthermore, the crack propagating process and stress field evolution in single-flawed and double-flawed specimens were examined by DEM simulations. The primary cracks are initiated at the tensile stress concentration areas of flaws, and then the concentration areas transfer to the crack tips and impel the crack propagating. The nucleation failure formed by two wing cracks is a main rock bridge failure pattern in double-flawed specimens. The nucleation failure area becomes small and maybe disappeared due to the increase in normal tensile stress. The conclusions can provide the theoretical reference for the stability evaluation of underground rock mass excavation. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing of tight shale gas reservoir is considered an emerging high-efficiency production method, which is especially suitable for extremely low permeability reservoir. A complex crack network intertwined with the natural crack in the unconventional shale reservoir can be generated by hydraulic fracturing techniques, so as to form a high-permeability domain in the low-permeability reservoir. Vertical well fracturing is one of the typical approaches for increasing production of unconventional reservoirs in recent years. It is widely implemented in coalbed methane (CBM) extraction, geothermal development, unconventional oil and gas production, and marine hydrocarbon development engineering. In this paper, more attention is paid to whether the artificial echelon cracks will develop of a high-quality crack network in hydraulic fracturing through hybrid finite element method (FEM) and discrete element method (DEM). This problem is profoundly investigated through engineering-scale numerical simulation and the establishment of a simple theoretical echelon model. Through the quantitative description of the ultimate crack area, we find that the appropriate spacing, such as the actual engineering spacing mentioned in this paper, i.e., 67.6 m, is regarded as the optimal spacing. It is conceivable that there will also be corresponding "dessert spacing" in the practical engineering. In addition, the microseismic inversion algorithm is used to obtain the location and magnitude of microseismic events with different crack failure types, including contact slip events and damage events. [ABSTRACT FROM AUTHOR]

The objective of this work is to develop a new constitutive model based on a modified Mohr–Coulomb (M-C) criterion in order to reflect the brittle–ductile transition behavior of crystalline rock. The innovative contributions rest on that the post-peak strain softening and brittle–ductile transition behaviors of rock materials can be described by integrating an exponential function of damage variable and confining pressure into the yield criterion. Then, the numerical integral formulation of the model is deduced in detail based on the return mapping algorithm. And the iterative updating of plastic deformation and damage evolution is numerically calculated by the plasticity and damage decoupling correction algorithm (PDDC). Besides, the method of model parameters identification is presented based on laboratory test data. Finally, the consistency between numerical simulation and experimental results from crystalline marble validates the performance of the proposed model. The constitutive model is also versatile to describe the fundamental mechanical characteristics of the like crystalline rock. [ABSTRACT FROM AUTHOR]

High plastic clays (fat clays) instigate failures like rutting and fatigue cracks in flexible pavements and sometimes make the pavements unserviceable when subjected to wet-dry cycles of the environment. This study focuses on the strengthening potential of polypropylene fiber (PP) on the engineering properties of fat clays. Different fiber percentages (0.2, 0.4, 0.6, and 0.8% by oven-dried soil weight) were used in clay samples and then compaction properties, unconfined compressive strength (UCS), elastic modulus (E50), California bearing ratio (CBR), and swell-consolidation properties of reinforced samples were investigated. Also, desiccation tests were carried out to assess the efficacy of fibers in enhancing the cracking resistance of clay. The test results show remarkable strength improvement, as the UCS value of 0.4% reinforced soil increased by 356%, E50 by 109%, and soaked CBR by 225%, resulting in a better-quality subgrade for pavement construction. The compression and rebound indices were decreased by 49.5% and 78.8% for fiber content of 0.8%, respectively, as well as the percent swell and swell pressure were also lowered by 75.4% and 70%, respectively, shifting the medium swelling soil to a low swelling class. Furthermore, the scanning electron microscopic (SEM) studies revealed that the frictional interface between fiber-clay particles appears to be the primary mechanism influencing the reinforcing effect. Economically, stabilizing fat clay reduces the required pavement thickness by an average of 38%. Hence, PP fiber as a low-cost material provides a long-term stabilizing solution for widespread fat clays. [ABSTRACT FROM AUTHOR]

Aiming at the frequent occurrence of borehole wall collapse and other accidents in the drilling process of deep shale horizontal wells in the Luzhou block, the effects on mechanical properties of brittle shale were studied and analyzed by carrying out plugging test of drilling fluid and loading and unloading test of confining pressure. Considering the weak plane of bedding fracture and according to the shape of borehole caving, a mathematical model of ground stress inversion was established. The research results show that the deep shale in the Luzhou block has a high content of brittle minerals, and the bedding and microfractures are relatively developed. The plugging performance of the drilling fluid has a significant impact on the compressive strength and elastic modulus of deep brittle shale. The good sealing performance of drilling fluid can effectively improve the failure strength of borehole wall rock. In a complex downhole pressure environment, a too fast tripping speed will cause the effective bottom hole pressure to drop, reduce the effective support of the drilling fluid on the wellbore, and induce the damage of the wellbore rock along the bedding and microfractures. Under the condition of formation heterogeneity, the collapse shape of the wellbore is no longer the ideal round or oval shape but more of a four-leaf shape. With the known shape of the borehole after the collapse, a mathematical model of inverse in situ stress was established, and the accuracy of the mathematical model of inverse in situ stress was verified through the true triaxial test. The research results provide a certain theoretical basis for the safe and efficient development of deep shale gas. [ABSTRACT FROM AUTHOR]

To explore the rule of pore-fracture propagation in the course of hydraulic fracturing of a single pore shale under the action of fluid–structure coupling, a numerical model was established for 8 groups of single pore shales present in the Niutitang Formation of the northern Guizhou area. Different bedding dip angles and different hydraulic pressures were used with the numerical software RFPA2D-Flow. The results showed that: 1) the elastic modulus and compressive strength of shale showed pronounced anisotropy due to the influence of different water pressures and bedding dip angles; 2) for the same water pressure, the curve for compressive strength was V-shaped as the bedding dip angle rose; 3) with the same bedding dip angle, the compressive strength of single pore rock samples was negatively correlated with the increase in hydraulic compressive strength; 4) based on the distribution of cracks in samples with different bedding inclination angles and hydraulic pressures, the samples were divided according to three final failure modes, namely inclined type I (0°/30°/60° 10 MPa, 60° 8 MPa), inclined type N (0°/30°/ 90°, 8 MPa) and inclined type H (90°, 10 MPa). The results of this paper provide crucial theoretical guidance for exploitation of Lower Cambrian black shale with hydraulic fracturing in the Feng'gang No. 3 block of northern Guizhou. [ABSTRACT FROM AUTHOR]

In this paper, the dynamic fracture characteristics and mechanism of the medium with compound defect under blast loading are studied by varying the relative positions of the void and pre-crack in the compound defect. A transmission caustics experimental system was used, while polymethyl methacrylate (PMMA) was selected as the experimental material. The void and crack were integrated into the same defect form, defined as a compound defect. The experimental results indicate that the end of the pre-crack is the weakest point of the defect, while the cracks are more likely to appear in the stress concentration area. By combining the interaction mechanism between the explosion stress wave and the end of the pre-crack, two modes of action for the medium containing compound defect under the blast loading were identified. One is that the stress wave directly acts at the end of the pre-crack, and the other is that the stress wave acts at the end of the pre-crack after circling around the void. As the relative positions of the void and the pre-crack change, the two forms of initiation and non-initiation were observed at the end of the pre-crack. The difficulty of initiating cracks at the end of prefabricated cracks is constantly changing which is indicated by the differences in crack tip initiation time and the stress intensity factor (the peak stress intensity factor when the crack had not initiated). At the same time, combined with the LS-DYNA finite element numerical simulation, the JH-2 material damage model is used to reproduce the dynamic damage evolution process of the composite defect, and the experimental results of the model are highly restored. The pressure values of the end element of the pre-crack under different defect structures are extracted from the numerical simulation results, and the possible reasons for the defect initiation in the composite are explained from the numerical simulation perspective. In future research, the JH-2 damage model can be widely used to solve issues pertaining to brittle materials such as PMMA. In practical blasting design processes, the relative position of the blasthole and compound defect should be considered as the basis of blasting parameter design. [ABSTRACT FROM AUTHOR]

This work presents the experimental results of the fracture behaviors for Tibet hornfels rock obtained from an open pit with different natural fracture ratios under multi-level cyclic shear loads. A newly developed rock dynamic shear testing apparatus was employed to perform the increasing-amplitude cyclic loads on the samples; the digital image correlation (DIC) technique was used to monitor the full-field displacement and the principal strain evolution for all tested samples. The results show the shear strength, deformation, full-field displacement, and the final failure pattern are influenced by the inherent natural fractures. The DIC results reveal the full-field displacement evolution and the characteristics of strain concentration. The stimulation of the natural fractures is impacted by the relations between shear direction, natural fracture orientation, and its cement characteristics. High strain concentration and large displacement occur near the natural fractures. For the sample with low natural fracture volume or the sample whose natural fracture orientation is parallel to the shear direction, the morphology of failure surface is the simplest. It is interesting to notice that for the sample whose fracture orientation is almost vertical to the shear direction, the multiple strain concentration zones were identified. Overall, this study confirms the pronounced effect of rock structure on the deformation and fracture evolution of the natural rock mass exposed to cyclic shear loads. [ABSTRACT FROM AUTHOR]