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Rock specimens of the reservoir's core material from the Chayanda oil and gas condensate field (OGCF) were tested using the Triaxial Independent Load Test System of the Ishlinsky Institute for Problems in Mechanics, Russian Academy of Sciences (TILTS) in order to experimentally study the effect exerted by desalinization of reservoir rocks on their strain, strength, and filtration characteristics. According to the study data, desalinization considerably increases a rock permeability. Strain and strength properties of the studied rocks change toward softening (except for an internal friction angle), nevertheless remaining relatively high. Based on the experimental physical modeling of the pressure reducing process at the bottom of a horizontal well using TILTS, a slight decrease in elastic and strength properties of the Chayanda OGCF reservoir rocks after their desalinization should not affect the stability of the well's walls. The results obtained make it possible to draw the important practical conclusion: when operating the field at the stage of salt washing out due to water filtration under the operation process, a dramatic increase in the risks related to reservoir destruction and higher sand production should not be expected. [ABSTRACT FROM AUTHOR]

The elastic modulus of rocks is a measure of the ability of rocks to resist elastic deformation. It is related to the size of rocks and can effectively measure the internal physical and mechanical strength of rocks. The development of joint fractures is the main reason for the size effect of rocks. Therefore, exploring the influence of joint roughness on the elastic modulus of rocks of different sizes is of great significance in mining rock mechanics. The article investigates the size effect of joint roughness on elastic modulus of rocks by establishing simulation schemes for 30 working conditions. By analyzing the stress-strain curves of rocks with different roughness and sizes, the deformation and failure patterns of rocks with different sizes were obtained. Research has found that the elastic modulus of rocks is in a power function relationship with joint roughness, while the elastic modulus of rocks is negatively exponentially related to rock size; The characteristic elastic modulus of rocks is in a power function relationship with joint roughness. The above relationships not only reveal the variation of rock elastic modulus with size, but also reveal the influence of joint roughness on elastic modulus, providing important basis for understanding the stability of mining rock engineering. [ABSTRACT FROM AUTHOR]

The shear performance of the binary interface formed by mortar and rock cementation is a key factor influencing the stability and safety of basic engineering projects related to livelihood and economy since concrete has become one of the most widely used materials in engineering. Therefore, it is an urgent practical problem to further explore and clarify the shear failure mechanism of the mortar–rock binary interface. In response to the current research objective focused on fully bonded interfaces, this paper constructed binary interface models with different bonded conditions to perform direct shear experiments using numerical simulation methods, and the effect of bonded conditions on the shear behavior of the mortar–rock binary interface was analyzed. The results indicate that the bonded conditions have a significant influence on the shear mechanical behavior of the mortar–rock binary interface, which is mainly reflected in the stress-displacement curve characteristics, the shear strength, the fracture development and the stress distribution state. The research findings are of great theoretical significance for the further study of shear mechanics at the mortar–rock binary interface and of great practical significance for safe construction, resource conservation and disaster warning. [ABSTRACT FROM AUTHOR]

Rock naturally contains numerous defects, and both loading rate and defect geometry affect the loading capacity of rock structures when subjected to dynamic impact loads. Understanding the rate-dependent mechanical response and characterizing geometry-related fracture parameter of defected rocks are crucial to safety assessment of rock engineering. In this paper, the effects of loading rate and notch parameter on dynamic brittle fracture behavior of Fangshan granite weakened by a blunt V-notch are experimentally studied. First, the dynamic fracture experiments on blunt V-notched semi-circular bend (BVNSCB) specimen of Fangshan granite with different notch geometries are performed using the split Hopkinson pressure bar (SHPB) system. Second, the dynamic fracture processes of blunt V-notched rocks are recorded using an ultrahigh-speed camera, and the mechanical responses to different impact loading conditions are obtained via digital image correlation (DIC) method. Third, the effects of loading rate and notch parameter on the dynamic fracture parameters including notch fracture toughness, crack propagation toughness, and crack velocity are discussed to reveal the fracture mechanism of blunt V-notched rocks subjected to impact loads. It is found that dynamic notch fracture toughness obtained from DIC method is usually larger than that measured directly from conventional maximum impact load method. The maximum tangential stress (MTS) criterion eliminating the effect of the fracture process zone (FPZ) at blunt V-notch tip is used to correct the apparent notch fracture toughness value measured from the maximum load method. The results show that corrected notch fracture toughness values are in good agreements with those obtained from the DIC method. This study is significant for dynamic fracture analyses and strength assessments of rock masses containing notches or other defects with complex geometry. Highlights: Effects of loading rate and notch geometry on dynamic fracture behavior of blunt V-notched rocks are studied. Dynamic notch fracture toughness and crack propagation toughness are simultaneously determined. DIC method is used to track the development of FPZ at blunt V-notch tip and quantify the FPZ length. [ABSTRACT FROM AUTHOR]

To investigate the dynamic response of fractured rock under cyclic loading and unloading, a WHY-300/10 microcomputer-controlled electro-hydraulic servo universal testing machine was used to conduct uniaxial cyclic loading and unloading tests. Simultaneously, acoustic emission (AE) and a CCD high-speed camera were employed to monitor the fracturing characteristics of sandstone. The mechanical properties, energy evolution, AE characteristics, and deformation of 45° sandstone were analyzed. The results indicate that as the load cycle level increases, both the elastic modulus and deformation modulus exhibit a "parabolic" increase, with a rapid rise initially and a slower rate of increase later. The damping ratio generally shows a decreasing trend but tends to rise near the peak load. The total energy, elastic energy, dissipated energy, damping energy, and damage energy all follow exponential function increases with the load level. The b-value fluctuates significantly during the stable crack propagation phase, unstable crack propagation phase, and peak phase. When the FR (Felicity ratio > 1), the rock is relatively stable; when the FR (Felicity ratio < 1), the rock gradually extends towards an unstable state. The Felicity ratio can be used as a predictive tool for the precursors of rock failure. Shear fractures dominate during the compaction and peak phases, while tensile fractures dominate during the crack propagation phase, ultimately leading to a failure characterized by tensile fracture. High-speed camera observations revealed that deformation first occurs at the tips of the prefabricated cracks and gradually spreads and deflects toward the ends of the sandstone. This study provides theoretical support for exploring the mechanical behavior and mechanisms of fractured rock under cyclic loading and unloading, and it has significant practical implications. [ABSTRACT FROM AUTHOR]

Rock engineering achieves the secondary stress balance through rock mass structure adjustment, where energy conversion is throughout and associated closely with rock deformation and damage. In this study, a series of triaxial compression tests were conducted on red sandstone to investigate these features. The results showed that the damage state of red sandstone specimens presented five stages under different confining pressure, corresponding to the multistage evolution characteristic of the energy conversion. In the case of the dissipation energy conversion ratio (η), it showed five stages: a gradual increase, decreasing gradually and reaching a minimum value, increasing gradually, increasing with growth rate, and accelerated growth, therein the strong nonlinearity reflected the stability and instability of the internal structure of the rock and had the basic characteristics of the mutation theory, therefore the damage state warning model was established on just that. The relation between the η and time fitted by a four‐rank potential function had a fitting parameter (R2) larger than 0.9, and the bifurcation set of the η calculated by the damage state warning model had twice stages less than 0. The second stage, which occurred near the minimum value of the η and run through the plastic deformation stage, could be used to predict rock damage and fracture, and it was proven feasible by acoustic emission (AE) precursor and better than AE warning. This research can enrich the methods for identifying rock damage state and provide reference for revealing the occurrence and development mechanism of various rock instability disasters. [ABSTRACT FROM AUTHOR]

A data-driven approach is used to show the significant discrepancy between the coefficients of permeability that are estimated through packer field tests and those back-calculated from the actual discharge pumped out of randomly fractured rock masses (i.e., masses that do not have predominate oriented fracture sets) using either dewatering or pumping well systems. The presented data, some of the richest of its kind, are used to develop a novel correlation between the estimated and back-calculated coefficients of permeability. A procedure is proposed to evaluate the permeability of the rock masses that considers two different but frequently encountered scenarios: (1) lack of any field permeability test data; or (2) only packer test results are available. Numerical examples are presented to illustrate how the suggested procedure could be used. It constitutes an advance in the state of practice because the commonly used procedures to estimate the permeability of randomly fractured rock masses employ the direct use of packer field test results, which, as the data shows, could be orders of magnitudes out. The proposed procedure could be particularly useful for practitioners when faced with cases where field pumping tests are not feasible due to budget, time constraints, or both. It gives them a simple and reliable approach (based on case histories) to produce preliminary designs of dewatering systems in randomly fractured rocks. [ABSTRACT FROM AUTHOR]

Most recent tunnel designs rely on more thorough analyses of the intricate rock interactions. The three principal techniques for excavating rock tunneling are drill-and-blast for complete or partial cross-sections, TBM only for circular cross-sections with full faces, and road header for small portions. Tunnel-boring machines (TBM) are being utilized to excavate an increasing number of tunnels. Newer studies have demonstrated that subterranean structures such as tunnels produce a variety of consequences during and after ground shaking, challenging the long-held belief that they are among the most earthquake-resistant structures. Consequently, engineering assessment has become crucial for these unique structures from both the geotechnical and structural engineering standpoints. The designer should evaluate the underground structure's safety to ensure it can sustain various applied loads, considering both seismic loads and temporary and permanent static loads. This paper investigates how adding elastic, soft material between a circular tunnel and the surrounding rock affects seismic response. To conduct the study, Midas/GTS-NX was used to model the TBM tunnel and the nearby rock using the finite element (F.E.) method to simulate the soil–tunnel interactions. A time–history analysis of the El Centro (1940) earthquake was used to calculated the stresses accumulated in the tunnels during seismic episodes. Peak ground accelerations of 0.10–0.30 g, relative to the tunnel axis, were used for excitation. The analysis utilized a time step of 0.02 s, and the duration of the seismic event was set at 10 s. Numerical models were developed to represent tunnels passing through rock, with the traditional grout pea gravel vs. isolation layer. A parametric study determined how isolation material characteristics like shear modulus, Poisson's ratio, and unit weight affect tunnel-induced stresses. In the meantime, this paper details the effects of various seismic isolation materials, such as geofoam, foam concrete, and silicon-based isolation material, to improve protection against seismic shaking. The analysis's findings are discussed, and how seismic isolation affects these important structures' performance and safety requirements is explained. [ABSTRACT FROM AUTHOR]

This study has carried out three-dimensional nonlinear elastoplastic analyses to understand the behaviour of tunnels constructed in rock having reinforced concrete lining when subjected to impact loading. The Lagrangian analysis tool has been used to create geometry and mesh the different parts of the present rock tunnel model through Abaqus/Explicit software. 3D lagrangian elements have been used to model the rock and concrete lining. The hoop and longitudinal bars of the reinforcement cage have been modelled using a beam element. The impact resistance behaviour of rock tunnel constructed in three different stages of weathered Quartzite has been simulated to understand the stability characteristics of the tunnel. The three-dimensional missile has been considered a rigid material to incorporate the maximum penetration in the rock when it hits the tunnel surface from 100 m. The four different missile weights i.e. 50 kg, 100 kg, 150 kg and 200 kg have been considered to contemplate the extent of deformation and damage In the tunnel. Moreover, the effect of overburden depth variation has been incorporated in terms of H/D ratio, which is defined as the ratio of overburden depth to diameter of the tunnel. The Ducker-Prager constitutive model with strain-rate dependence and dynamic increase factor is considered to simulate the behaviour of Quartzite rock. Johnson-Cook material model is used for steel bars of reinforcement cage and Ducker-Prager model with strain-rate and dynamic increase factor has been considered for reinforced concrete lining of the tunnel. The analyses results were studied for deformation, particle velocity, particle acceleration and damage in rockmass surrounding the tunnel and its reinforced concrete lining. It has been observed that reinforced concrete lining experiences a significant amount of compression and tension damage at the crown of the tunnel and this damage remains concentrated in the middle one-third section of tunnel length irrespective of change in weight of missile and H/D ratio. It has been concluded that a slightly weathered Quartzite rock tunnel has 18% more impact resistance than a medium weathered rock tunnel and 41% from a highly weathered rock tunnel. Moreover, the particle velocity is found to be inversely proportional to the strength of the surrounding rockmass. Further, particle acceleration generated due to impact loading changes proportionally with the rise in the degree of weathering in rocks. Also, it has been concluded that rock tunnels having a H/D ratio of 2.0 has 4-times higher impact resistance in terms of tension damage than the rock tunnels having a H/D ratio of 0.5. [ABSTRACT FROM AUTHOR]

With the development of data collection and storage capabilities in recent decades, abundant data have been accumulated in geotechnical engineering fields, providing opportunities for the usage of machine learning approaches. Thus, a rising number of scholars are adopting machine learning techniques to settle geotechnical issues. In this paper, the application of three popular machine learning algorithms, support vector machine (SVM), artificial neural network (ANN), and decision tree (DT), as well as other representative algorithms in geotechnical engineering, is reviewed. Meanwhile, the applicability of diverse machine learning algorithms in settling specific geotechnical engineering issues is compared. The main findings are as follows: ANN, SVM, and DT have been widely adopted to solve a variety of geotechnical engineering issues, such as the classification of soil and rock types, predicting the properties of geotechnical materials, etc. Based on the collected relevant research, the performance of random forest (RF) in sorting soil types and assessing landslide susceptibility is satisfying; SVM has high precision in classifying rock types and forecasting rock deformation; and backpropagation ANNs and Hopfield ANNs are recommended to forecast rock compressive strength and soil settlement, respectively. [ABSTRACT FROM AUTHOR]

The properties of fresh ballasts collected from five quarries around Thailand were investigated in the laboratory. The investigation includes properties specified by the State Railway of Thailand (SRT) for ballast, i.e., rock type, sieve analysis, flat and elongation, and Los Angeles abrasion. Furthermore, cementing value, water absorption, specific gravity, sulfate soundness, and petrographic analysis were also conducted. The obtained rock properties were then compared with the requirements for use as a ballast specified by various national standards, including the Australian Rail Track Corporation (ARTC), the American Railway Engineering and Maintenance of Way Association (AREMA), and the State Railway of Thailand (SRT). The results show that all rocks generally comply with all standards. However, Buriram basalt is considered the best. Saraburi limestone has low abrasion resistance and high cementing value due to its carbonate nature which tends to give higher cementing strength. Igneous and metamorphic rocks usually have plagioclase as their parent mineral which gives low cementing potential. A large abundance of plagioclase in igneous rock, such as Buriram basalt, Nakhon Sawan dacite, and Prachinburi andesite, results in lower cementing values. Rock with high cementing value may hinder its usage as a railway ballast; therefore, the cementation property of ballast by cementing value test, initially suggested by Raymond [1], is strongly recommended as additional criterion in the selection of a ballast. [ABSTRACT FROM AUTHOR]

This work presents the results of a FEA (Finite Element Analysis) concerning the propagation of the fracture trajectory during the detachment of rock masses. The analysis considers mechanical anchoring parameters, including the depth of anchoring and the friction between the anchor head and the brittle material. The analysis involved varying the effective anchorage depth hef = 50, 100, and 150 mm, along with considering different coefficients of friction (µ) for the anchor head against the rock, including 0.15, 0.3, and 0.45. The results indicated that within the chosen range of anchorage depths, the extent of damage increases with the depth of anchor embedment. However, the depth of anchoring, considering the studied range of this parameter and the assumed mechanical properties of the rock, did not have a significant effect on the value of the angle cone (a0) during the initial phase of the medium's destruction. For the friction coefficient µ = 0.15, there is a clear deep penetration into the fracture at its initial stage of development (at the initial angle of a0 = -20°), promoting further spread of the fracture on the free surface. For larger values of the friction coefficient (µ = 0.3 and 0.45) - the trend is less pronounced (a0 = -2°÷-8°) resulting in a decrease in the crack range. [ABSTRACT FROM AUTHOR]

In the ultra‐deep reservoir or other deep‐buried subsurface engineering, the in situ stress is high enough to make the rock transition from the brittle to the plastic. In such situation, the fracturing process always goes with the plastic deformation together. Therefore, to develop an elastoplastic constitutive model which can simultaneously account for both fracture and plastic deformation is critically important. In this paper, an elastoplastic augmented virtual internal bond (EP‐AVIB) model is developed. In EP‐AVIB, a hyperelastic AVIB is developed to characterize the elastic deformation of rock, in which the micro bond is linear elastic in compression whereas is hyperelastic in tension. The Drucker‐Prager yield criterion is employed to characterize the plastic deformation. Thus, the EP‐AVIB can combine the fracture and plastic deformation together. The simulation examples show that the EP‐AVIB can simulate the elastoplastic mechanical behaviors and the fracturing process of rock subjected to compression. It provides a simple approach to the elastoplastic failure simulation of rock. Its perspective in deep reservoir and deep‐buried subsurface engineering simulation should be inspiring. [ABSTRACT FROM AUTHOR]

Acoustic signals emitted during rock fracture constitute an important tool for rock damage evaluation. To investigate the evolutionary characteristics of acoustic emission (AE), microseismic (MS), and sound signals produced by hard rock fracture, uniaxial compression tests on granite specimens observed by AE, MS and sound monitoring were carried out. The evolution characteristics of the acoustic signal index, including its waveform, fractal dimension, b value, main frequency, energy proportion of signal frequency bands, and signal activeness, were analysed. The results indicate that there are significant differences in some characteristics of the AE, MS, and sound on the eve of granite failures, such as the waveform amplitude density, the average decline rate of the b value, the distribution of the main frequency, and the evolution of the energy proportion of the advantage frequency band. The three types of acoustic signals can characterize different scales of rock fracture under uniaxial compression. AE is sensitive to small-scale rock fractures, and MS and sound are sensitive to large-scale rock fractures. In addition, a unified damage evolution equation established by acoustic signals is proposed to quantitatively describe the damage process of granite specimens during uniaxial compression tests. [ABSTRACT FROM AUTHOR]

In order to study the dynamic stress distribution and the fracture area of rock around the tunnel under different orientations of blasting disturbance, AUTODYN finite difference method software was used to conduct numerical simulation research. Gauge monitoring points were set around the numerical model of the tunnel to conduct real-time monitoring of the stress distribution, displacement and fracture area of the tunnel. Based on the analysis of the stress wave propagation law, the following conclusions are obtained: (1) under the condition of the same blasting loads, the stress and displacement of the tunnel is relatively small when the blasting disturbance source is located above the roof, i.e., the stress state of the tunnel is relatively stable and the fracture area around the tunnel is minimal; (2) from the uniaxial stress around the tunnel and the tunnel peripheral displacement, it can be seen that the displacement caused by horizontal direction stress of the tunnel is the largest, and the deformation is mainly concentrated above the floor and at the shoulder, while the vertical wall part has almost no deformation; (3) for brittle materials such as rock, the arch-shaped stress-bearing surface is more likely to disperse stress, while the straight wall and flat floor of the tunnel cannot well disperse stress, resulting in uneven stress on the stress-bearing surface, uncoordinated deformation and ultimately, failure. [ABSTRACT FROM AUTHOR]

The failure characteristics of geotechnical materials are quite complex, among which there are obvious differences between soil materials and rock materials. In order to describe the failure characteristics of soil materials and rock materials uniformly, a new failure and yield criterion, called VML criterion, is proposed in this paper. This strength criterion has the following characteristics. (1) On the deviatoric plane, the criterion can be expressed as a power parameter expression of von Mises criterion and spatial mobilized plane (SMP) criterion, so it can be used to describe cohesive materials such as metals or frictional materials such as sand. (2) On the meridian plane, another power parameter expression can be used to consider tensile stress strength. On the meridian plane, the material strength envelope can be expressed as a curve, so the hydrostatic pressure effect of soil material can be reasonably considered. (3) In order to consider the volumetric compression yield properties of soil materials under isotropic compression, a yield criterion considering the volumetric compression yield properties is also proposed on the deviatoric plane. Since geotechnical materials are often in plane strain condition in engineering, a VML criterion expression considering plane strain condition is proposed in this paper. The criterion shows that the internal friction angle of soil under plane strain is usually 1.1 times of that under conventional triaxial compression. The comparison between the predicted and tested data shows that the VML criterion proposed in this paper can be widely applied to the failure simulation of rock and soil materials, and the plane strain VML criterion can also be applied to the failure prediction with plane strain state. [ABSTRACT FROM AUTHOR]

Findings. The function of breaking deformations within the rock mass in the bottomhole of a hydromechanical drilling well is dependent on various technological means and methods. A sequential analysis has been conducted to identify the most influential factors in this process. Positive features of hydromechanical drilling have been outlined from the viewpoint of effective intensification of basic technical and economic parameters of the process of well construction with different purposes. Complete operational similarity and technological interconnection of a mechanism of the formation of different parts of a hydromechanical drilling well have been shown in terms of their stipulation by the properties of rock formations and mode support of a well construction process. Top-priority of a hydromechanical drilling type has been proved to generate as many parameters of dynamic effect on rock mass, which results in the increasing scope of bottomhole breaking processes. Attention has been paid to the study of the problem of tool support for drilling operations from the viewpoint of tracing the nature of bottomhole processes running in terms of different technical and technological factors. The possibility and necessity of using surface-active substances (SAS) as the main activators of positive deformation interactions in the "metal pellets - rock" pair have been proved and substantiated; use will be based on the developed methodological approaches of rational selection of a component-concentration composition of a breaking medium. Originality. The efficiency of a hydromechanical drilling type is stipulated by the degree of dynamic effect on the rock bottomhole; depending on its geological-mineralogical and physicomechanical features, it can be intensified by increasing frequency of impacting, interpretation of the effecting mechanism, variation of the cleaning agent type as well as directed activation of the manifestation of surface and interphase interactions. Practical implications. The represented results of analytical and laboratory-experimental studies are the basis for the development of methodological foundations to elaborate the mode parameters of the technology of hydromechanical drilling for the construction of wells. They belong to the basic initial data applied while developing the design and working characteristics of the corresponding modernised operating members. [ABSTRACT FROM AUTHOR]

The work is devoted to the development of an electric pulse method for processing rocks and other solid dielectric and semiconducting materials. For the first time, experimental studies of the electric pulse method of cutting rocks in water were carried out using a high-voltage pulse generator with a pulse repetition rate of up to 25 pulses per second and a voltage of up to 160 kV. The results showed the possibility of using high-voltage generators with low pulse energy and high repetition rate in the electric pulse technology when using water as a flushing liquid. At the same time, the used mechanical system for moving the electrodes and washing needs to be improved to increase the productivity of electric pulse processing. [ABSTRACT FROM AUTHOR]

Purpose. To rationalize well washing by considering the influence of the annular channel limitation, granulometric composition of the mud, limitation of the joint movement of mud particles, drill string rotation, shape of the movement channel, and rheological properties of a cleaning agent. Methodology. Applied laboratory studies on the properties of drilling fluids are carried out using modern methods of analytical analysis and experimental studies, in particular, by using general principles of mathematical and physical modelling, methods of processing the results of studies in the EXCEL, MATNSAD environment, control and measuring devices of the laboratory of washing liquids of the Department of Oil-and-Gas Engineering and Drilling of Dnipro University of Technology (hydrometer, viscometer, plastometer, stalagmometer), and materials (technical water, clay bentonite powder, reagents, stabilizers and activators, surfactants as well as equipment and laboratory base of “Ukrnafta” PJSC to carry out certain stages of research. Findings. The properties of surface-active substances and activated washing liquids are analyzed. The main technological properties of drilling fluids are studied. Comprehensive information on the physical and chemical ability of flushing drilling fluids is provided. The main technological measures ensuring the stability of drilling process using activated-surfactant drilling fluids are considered in detail. The main technological principles of determining rational formulations of a selected cleaning agent in the circulation of Newtonian liquids are formulated and substantiated. A detailed description of surfactants, their properties and methods to study the formed solutions is given. Originality. The process of a rock-breaking tool on the well bottom will be stable in the conditions of dynamic equilibrium of the rock-breaking processes and removal of broken products into the annular space. Therefore, for a correct understanding of wellbore washing issues, basic regularities are established of the specified processes and the influence of various factors on these processes. Practical value. It has been proven that in order to give a drilling fluid the necessary properties that, on the one hand, will ensure an increase in the efficiency of rock breaking on the well bottom, and, on the other hand, will create favourable conditions for stable removal of cuttings onto the surface and maintenance of the well walls in a stable condition, it is necessary to apply different methods, in particular chemical activation as well as magnetic, thermal and electrochemical activation, in its different varieties. [ABSTRACT FROM AUTHOR]

This paper has proposed a new perspective of studying internal structure-based tests, the results of which will improve the present experimental methods and enrich our understanding of rock structure-based modeling without any core preparations, with low cost in a short time. Pore volume compressibility (PVC) is an important feature of rock and is related to mechanical and structural behavior of porous rock sample. An accurate evaluation of pore volume compressibility depends on experimental test which is time-consuming and costly. This paper outlines new method for evaluation of PVC of rock cores using a computed tomography (CT) scan-based finite element method (FEM). The verification studies were performed on a series of porous rock cores which were extracted from deep oil reservoirs in Iran. In order to construct a finite element model, a relationship between spatial elastic properties of samples and CT-scanned data images was derived. The samples were scanned by a conic beam computed tomography (CBCT) machine, and the scanned data were converted into a model to simulate PVC tests. The pore volumetric strains were obtained from a linear elastic analysis for each stress and pore pressure step. To validate the finite element analysis (FEA) results, a series of experimental PVC tests were conducted on the pre-scanned samples and PVC curves were extracted. As a result, the predictions calculated from the CT scan-based numerical models have shown a good correlation with the results obtained from laboratory experiments. The results revealed that it is possible to simulate PVC tests using this numerical proposed evaluation method in such a way that the cost and time of the tests were lowered. [ABSTRACT FROM AUTHOR]

Under conditions of low or medium stress, rocks that are in the compression state exhibit perceivable nonlinear elastic characteristics. After a comprehensive review of the existing nonlinear elastic models of rocks and joints, we proposed a new unified nonlinear elastic model. This new model contains two parameters with clear definitions, namely, the initial elastic modulus Eni and the modulus change rate m. This model covers a variety of existing models, including the simple exponential model, BB model and two-part Hooke's model, etc. Based on the RMT experimental system, a large number of uniaxial compression tests for dolomite, granite, limestone and sandstone were performed, and their nonlinear deformation stress‒strain curves were obtained and fit with the unified nonlinear elastic model. The results show that the rocks have obvious nonlinear elastic characteristics in their initial compression stage, and that the unified nonlinear elastic model is able to describe these phenomena rather well. In addition, an empirical formula for predicting the uniaxial compressive strength of the rock was constructed, corresponding to the unified nonlinear elastic model. [ABSTRACT FROM AUTHOR]

This paper presents a machine learning-based approach to estimating the compressive strength and elastic modulus of rocks. A hybrid model, GWO-ELM, was built based on an extreme learning machine network optimized by the grey wolf algorithm. The proposed model was carried out on 101 experimental datasets, and four commonly used models were used as benchmarks to evaluate the accuracy of the proposed model. The results showed that the proposed hybrid model can accurately achieve the prediction of elastic modulus and compressive strength with high correlation coefficients and small prediction errors. The prediction performance of the hybrid model is significantly better than the other four original models, and it is an alternative model for predicting the compressive strength and elastic modulus of rocks, which is recommended as an auxiliary tool for real-time prediction of rock mechanical properties. [ABSTRACT FROM AUTHOR]

A study of dolomite rock material failure using a simple small-scale blast setup is presented. Laboratory tests were conducted using disc specimens drilled with a borehole in the center. A detonation cord and a blasting cap were fitted inside the borehole to induce cracking and fracturing of the specimens. The specimens were inserted between two steel plates, which were compressed against the specimen using bolt screws. Prior to testing, the most suitable screw torque for constraining the vertical displacement of the specimen surfaces without compressing the specimen was selected based on numerical simulations. Then, the experimental tests with the blasting cap were simulated using the Johnson--Holmquist II (JH-2) material model, and the properties of the blasting cap were determined and verified in two special tests with a lead specimen. Possessing the validated model, the influence of specimen thickness on the cracking patterns was finally analyzed. This paper presents a relatively easy method for studying rock material behavior under blast loading and for validating the numerical and constitutive models used for rock simulations. [ABSTRACT FROM AUTHOR]

Quasi-brittle fracture in geo-materials plays a crucial role in geotechnical engineering, and numerical methods represent a valuable approach for modeling this complex problem. This study introduces a mixed bilinear failure model considering bond effects in a modified Discontinuous Deformation Analysis (DDA) framework. Several benchmark crack-propagation problems for different materials and geometries, including three-point beam, L-shaped structure, semi-disc test and cemented granular packing, are presented to validate the novel DDA method proposed in this study. The simulation results show the accuracy of the model in predicting the mixed-mode failure of quasi-brittle materials with complex structures and assess its great potential for investigating the mechanical properties of geo-materials. [ABSTRACT FROM AUTHOR]

Depending on the type of failure mechanism, the strength of intact rock may be susceptible to the effects of scale. The influence of scale is apparent in the spalling of rock around underground openings. To reconcile field-scale observations, an understanding of the unconfined rock strength has to be appreciated from the standpoint of energy instability, particularly for instances in which the mechanism of failure involves propagation of fractures. Fracture energy principles were used to create a link between laboratory-measured uniaxial compressive strength (UCS) and rock failure as observed in spalling around circular openings. An Instability Line (IL) is proposed to predict the stresses above which spalling could occur with different opening diameters. Good comparisons were obtained for a range of dimensions from small boreholes to large tunnels. Based on field observations and analytical study, the results suggest that two conditions are required for spalling to occur. The first is that the crack initiation stress has to be exceeded, and the second is that the stress needs to be above the IL. The findings suggest that the effects of scale are limited by the crack initiation stress. For large-diameter openings, instability occurs immediately at crack initiation stress, whereas for smaller-diameter openings instability occurs at higher stresses post-crack initiation stress. Highlights: This paper aims to explain the discrepancies in the rock mechanics literature to predict spalling for small-diameter boreholes as against large-diameter tunnels. The failure mechanism of spalling is studied based on the framework of energy stability. An Instability Line is proposed based on the analytical study. The results of the analytical study compared well with published laboratory and field observations. The results suggest that two conditions are required for spalling to occur. The first is that the crack initiation stress has to be exceeded, and the second is that the stress needs to be above the Instability Line. The findings suggest that the effects of scale are limited by the crack initiation stress. [ABSTRACT FROM AUTHOR]

Circular/spherical rigid particle models that were initially applied to rock fracture studies were not able to match the ratio of the compressive strength to tensile strength that occurs in rock. In addition, the predicted macroscopic friction angle was much lower than the known hard rock experimental values. Several enhancements have been proposed to address these issues, namely the use of a clumped particle logic or the adoption of polygonal/polyhedral grain structures, either rigid or flexible. In this work, a flexible 2D DEM based particle model (PM) that allows deformable particles to interact in a simplified way is presented. The proposed flexible PM model keeps the contact interaction simplicity and the reduced computational costs characteristic of circular rigid particle models. The PM model is tested using biaxial tests and Brazilian tests. A discussion regarding the influence of the grain deformability on the macroscopic elastic and strength response is presented. It is shown that, when compared with a rigid model, the proposed flexible PM model predicts more reasonable indirect tensile strength to direct tensile strength ratio and requires a smaller value of contact fracture energy to give a good agreement with known experimental data. It is also shown that the proposed flexible PM model can predict a behaviour similar to that obtained using a flexible PM model through inner particle discretization that is more computationally demanding. [ABSTRACT FROM AUTHOR]

Tensile strength is a vital mechanical property of rock and the deformation and failure process of rock under tension is of much significance in rock mechanics and engineering. In this study, a micro analysis on rock direct tensile failure was made based on the newly introduced quantitative analysis of acoustic emission (AE) waveforms. Direct tensile tests of common rocks were carried out, accompanied by real-time AE monitoring. The spatial and temporal distributions of AE signals were studied over the whole loading process. Distribution and evolution laws of dominant frequencies of AE waveforms with normalized applied stress were statistically analyzed. The statistical relationship between the energy ratios of AE waveforms distributed in low and high dominant frequency bands (L-type and H-type waveforms) and peak strengths of rock specimens was built. Microstructural observations with SEM were further conducted. Results show that the dispersion degree of rock tensile strength corresponds to the complexity of mineral composition. Initial release moments of L-type waveforms appeared earlier than H-type waveforms. L-type waveforms of the same proportion carry more energy compared to H-type waveforms in rock subjected to tension. There is an overall downward trend for peak strength of rock specimens with the increasing energy ratios of L-type waveforms. The tensile strength of rock obtained by fitting in this study is smaller than that obtained by averaging. There exist micro shear fractures during the macro tensile failure process of rock according to the microstructural observations with SEM. The peak strength corresponding to the case that L-type waveforms (or micro tensile failures) account for 100% can be considered as the "ideal" tensile strength from the microscopic perspective. The determined tensile strength by fitting can be used as a conservative design parameter for rock engineering. Highlights: Statistical relationship between energy ratios of AE waveforms distributed in low and high dominant frequency bands and peak strengths of rocks under tension is built. There exist micro shear fractures during the macro tensile failure process of rock. Peak strength in the case that L-type waveforms (or micro tensile failures) reach 100% can be considered as the "ideal" tensile strength. The determined tensile strength by fitting can be used as a conservative design parameter for rock engineering. [ABSTRACT FROM AUTHOR]

This paper investigates the dynamic fracture resistance of blunt V-notched rock samples using the experimental and theoretical methods. The semi-circular bend sample, made of Fangshan granite, weakened by an edge blunt V-notch is selected as the testing rock sample. An empirical formula for calculating the dynamic notch fracture toughness of blunt V-notched samples is established by the stress extrapolation method in the finite-element method (FEM). According to the established empirical formula, the dynamic notch fracture toughness of the testing rock samples under split Hopkinson pressure bar (SHPB) impact loading condition are obtained from the dynamic strain signals in pressure bars. The experimental results reveal that the dynamic notch fracture resistance depends heavily on the opening angle, notch radius, and loading rate. A modified mean stress (MS) criterion is proposed to assess the dynamic fracture onset of blunt V-notched rock samples, which agree well with that obtained from the experimental data. These conclusions show that the proposed experimental and theoretical methods are capable of accurately assessing the dynamic strength of blunt V-notched specimen being tested, which may have significant implications to assess the dynamic fracture in notched rock samples and other brittle components. Highlights: The dynamic notch fracture toughness of blunt V-notched rock specimen under SHPB impact loading is investigated. An empirical formula for calculating mode-I notch fracture toughness of blunt V-notched rock specimen is derived. Dynamic notch fracture toughness is assessed using a modified mean stress criterion considering the effect of loading rate. [ABSTRACT FROM AUTHOR]

This paper suggests a simple loading configuration for conducting the SCB (semi‐circular bend) tests on brittle materials to produce mixed mode I/III loadings. In the suggested test setup, the SCB specimen containing an inclined crack is loaded using a conventional three‐point bending fixture, but the bottom supports touches nearly half of the specimen thickness rather than the entire thickness. It is found that various mixed mode I/III loadings can be provided by rotating the crack, and especially the SCB specimen experiences pure mode III when the crack angle is set to be 90°. Different values of the crack length, span and crack angle are then considered in the numerical analyses to further study the proposed SCB test setup. Finally, several experiments are successfully conducted on the rock samples under various mixed mode I/III and pure mode III loadings to evaluate the SCB test setup in practical point of view. Research highlights: A simple loading configuration is suggested for conducting the SCB tests.Mixed mode I/III and pure mode III tests can be performed using the SCB specimen.Effects of crack length, span and crack angle in the SCB tests are numerically explored.Mode III has a negative effect on the fracture resistance of rock materials. [ABSTRACT FROM AUTHOR]

Understanding the internal structures of rock and their subsequent evolution is essential across multiple disciplines. Fractures and mineral grains are significant components of internal structures. These components influence natural processes and human activities, such as oil migration and extraction. With advancements in non-destructive evaluation (NDE) techniques, especially X-ray computed tomography (CT), in-depth analyses of internal structures of rock have become feasible. However, the data-intensive nature of CT scans necessitates more efficient post-processing algorithms. This research uses the Residual Network-Visual Geometry Group-UNet (Res-VGG-UNet) deep learning framework in conjunction with two threshold segmentation methods to capture components like fractures, the drilled hole, and minerals. The performance of Res-VGG-UNet is evaluated against DeepLabv3+ and Weka3D. Key findings include the capability of the deep learning technique to distinguish between various components and the performance of single-slice-based versus multi-slice-based threshold segmentation methods. VGG16 and VGG19 outperform DeepLabv3+ and Weka3D in the performance of fracture classification. Moreover, a 3D reconstruction technique is introduced, providing enhanced insights into internal features of the rock. We conclude this paper by showing the potential to use the outcomes of the deep learning and multi-slice-based threshold segmentation methods to develop discrete element models, accurately reflecting the physical characteristics of rock specimens. • The Res-VGG-UNet framework effectively and accurately segments CT images of rock. • Single-slice and multi-slice threshold segmentation methods are compared for mineral identification. • 3D numerical models reflecting the physical features of rock has been developed. [ABSTRACT FROM AUTHOR]

Fractures commonly exist in rock masses; the coalescence of fractures provides fluid flow pathways in a fractured rock mass and greatly increases the flow capacity of fractured rock. This work aims to study the characteristics of nonlinear flow in fractures. A series of tests were conducted and indicated that the Forchheimer law performed well when describing the nonlinear relationship between hydraulic gradient and flow. The test results also indicate that higher water pressure may induce stronger nonlinearity. Additionally, the linear and nonlinear coefficients of the Forchheimer law increase with a decrease in the particle size of the filling material in fractures. On the basis of the laboratory results, the classical Forchheimer law was modified by considering the influence of stress on the variation of fracture aperture. A hydro-mechanical coupling model for fractured rock masses was built and programmed with a subroutine through ABAQUS. Furthermore, a random discrete fracture network was generated and simulated to prove that a high flow velocity will result in a nonlinear flow, not only in a single fracture, but also in a fracture network. The numerical results from fractured rock masses show that a ratio of the flow to the hydraulc gradient will change the flow from linear to weak nonlinearity and, finally, to strong nonlinearity with an increase in the hydraulic gradient. It also shows that the linear and nonlinear coefficients increase with an increase in the confining pressure and that they decrease with an increase in the aperture. Due to the complexity of fracture channels, a nonlinear flow is likely to occur in a fractured rock mass. Finally, the developed model was applied to simulate the flow behavior of underground engineering; the results show that the smaller the hydraulic aperture is, the higher the water pressure is required to be in order to change the flow regime from linear to nonlinear. [ABSTRACT FROM AUTHOR]

With our device, triaxial tensile tests can be performed on rock specimens with a confining pressure of at least 100 MPa. All the component materials of the device are made from high-strength steel, and the elastic modulus, tensile strength, and stiffness values are significantly greater than those of the rock specimens, thus meeting the triaxial tensile test requirements. Keywords: Triaxial extension; Tensile device; Extensile fracture; Tensile-shear failure; Shear failure; SEM; Rock EN Triaxial extension Tensile device Extensile fracture Tensile-shear failure Shear failure SEM Rock 1069 1077 9 02/23/22 20220201 NES 220201 Introduction Laboratory testing is one of the most important means to determine the basic parameters of rock materials and analyze/simulate the failure process or mechanism of rock masses. [Extracted from the article]

Tunnels have become an essential part of metro cities, therefore, always been under threat of destruction due to explosive used for causing blasts. To understand the behaviour and blast resistance of underground rock tunnel under internal blast loading a 3D model of rock tunnel of 5 m diameter has been developed in Abaqus using finite element technique. Nonlinear elastoplastic material models are adopted through different constitutive material models, for rock, concrete, reinforcement, inside-air of tunnel and trinitrotoluene (TNT). The focus is placed on the effect of weathering in rock tunnel. Four different stages of weathering of rock Granite has been considered. Trinitrotoluene has been used to simulate in-situ event of internal blast loading. The TNT sphere and air surrounding it, have been modelled using coupled Eulerian–Lagrangian technique. Further, the steel reinforced cement concrete lining behaviour has been analyzed in terms of tension–compression damage. The behaviour of surrounding rock along with weathering has been studied in conjunction with blast effect. The results extracted in terms of deformation, particle velocity, vibration, energy, pressure and damage has been used to understand response of weathered rocks. The rocks in general and granites in particular can show sudden variation in their strength due to different stages of weathering. Along, linear directions varying degrees of weathering can creep in due to presence of shear zones or very closely spaced joints. It has been observed that weathering of rock causes significant reduction in blast-resistance of tunnel. A progressive deformation in case of blasting has been observed from fresh granite to highly weathered granites through slightly and medium weathered ones. However, highly weathered rock has shown unique behaviour and has failed miserably. [ABSTRACT FROM AUTHOR]

Fracture geometry, density, and distribution control fluid and thermal transport in low porosity rocks, as well as their seismic attributes. The creation of cracks throughout the matrix can result in large and disparate reductions in P and S wave velocities with negligible porosity development, which breaks the classic velocity‐porosity relationship. Properly interpreting these signatures in seismic surveys from stimulated geothermal, oil, and gas reservoirs or seismogenic areas requires a rock physics model that uses realistic elastic properties of fractures. Conventional models for cracked rocks employ specific inclusion shapes such as spheroids with small aspect ratios. However, all these exhibit the same normal‐to‐shear compliance ratio—regardless of aspect ratio—and therefore impact the bulk and shear moduli in the same qualitative way. As such they are inadequate for describing many of the complementary evolutions of P and S wave velocities that transpire with the generation of cracks. Using a differential model of dimensionless compliances, we analyze published velocities for granite, basalt, and limestone to quantify typical compliance ratios and find that they are often different from those assumed in conventional crack models, which then affects crack density estimates. We also investigate how the total normal and shear compliances change due to certain thermo‐hydro‐chemo‐mechanical processes that generate large crack densities, and conclude that the final ratios often still deviate from that of conventional models. Finally, we demonstrate how the model can be used to describe scatter in both P and S wave velocity‐porosity trends. Implications for energy extraction and seismic monitoring are discussed. Plain Language Summary: Tight rocks are fractured, or need to be fractured in order to serve some purpose. The cracks that form in tight rocks—whether from natural or human‐driven processes—end up controlling not only how fluids move throughout them, but also how the rock as a whole interacts with seismic waves. In order to understand what seismic surveys are really telling us, it is important to utilize a model containing realistic parameters that describe the elastic properties of the fracture network. In this article, we demonstrate the power of one published model to reliably extract an important and useful property, the normal‐to‐shear compliance ratio, from ultrasonic velocity measurements on dry rock cores of granite, basalt, and limestone. We show how this property varies across rock types and how it can change as these rocks undergo heat‐driven cracking, hereafter defined as damage. Finally, we demonstrate how the model can be used to understand velocity changes following damage processes (whether known or unknown) that are incompatible with conventional crack models. Inferences from our findings are made in the context of the stimulation (fracturing) of geothermal, oil, or gas reservoirs, as well as monitoring seismogenic zones. Key Points: A full picture of cracked rock elasticity is achieved by plotting the P and S wave velocities in the plane of normalized K and G moduliCrack compliance ratios in thermo‐hydro‐chemo‐mechanically altered, tight, dry rocks are usually less than those of penny cracksScatter in velocity‐porosity crossplots can be readily described by tuning the compliance ratio, but not by tuning inclusion aspect ratio [ABSTRACT FROM AUTHOR]

This paper presents the results of the analysis of acoustic emission signals (AE) recorded during the propagation of sound waves in rock samples. The sources are excited by breaking a 3-mm-thick pencil lead (Hsu-Nielsen) on the surface of the rock samples at different AE source locations. Ten repetitions of pencil lead fracturing were performed at a given location. A VS150-M piezoelectric sensor with a frequency range of 100–450 kHz was attached to the upper surface of the rock samples. The results showed that the AE energy and the average frequency of the acoustic emission signals in the rock samples decreased with increasing distance from the AE source locations. These results can be used to establish a relationship between the characteristics of the acoustic emission signals and different rock types at different AE source locations. [ABSTRACT FROM AUTHOR]

Fractures provide preferential flow paths and establish the internal "plumbing" of the rock mass. Fracture surface roughness and the matedness between surfaces combine to delineate the fracture geometric aperture. New and published measurements show the inherent relation between roughness wavelength and amplitude. In fact, data cluster along a power trend consistent with fractal topography. Synthetic fractal surfaces created using this power law, kinematic constraints and contact mechanics are used to explore the evolution of aperture size distribution during normal loading and shear displacement. Results show that increments in normal stress shift the Gaussian aperture size distribution toward smaller apertures. On the other hand, shear displacements do not affect the aperture size distribution of unmated fractures; however, the aperture mean and standard deviation increase with shear displacement in initially mated fractures. We demonstrate that the cubic law is locally valid when fracture roughness follows the observed power law and allows for efficient numerical analyses of transmissivity. Simulations show that flow trajectories redistribute and flow channeling becomes more pronounced with increasing normal stress. Shear displacement induces early aperture anisotropy in initially mated fractures as contact points detach transversely to the shear direction; however, anisotropy decreases as fractures become unmated after large shear displacements. Radial transmissivity measurements obtained using a torsional ring shear device and data gathered from the literature support the development of robust phenomenological models that satisfy asymptotic trends. A power function accurately captures the evolution of transmissivity with normal stress, while a logistic function represents changes with shear displacement. A complementary hydro-chemo-mechanical study shows that positive feedback during reactive fluid flow heightens channeling. [ABSTRACT FROM AUTHOR]

Microwave technology is increasingly used in laboratory tests and field applications as an effective rock breaking technique. The underlying mechanism of rock fracturing induced by microwave energy, however, has not been well addressed. In this study, we employ experimental and numerical methods to investigate the global and local damage of rock in microwave radiation direction, including the fracture formation process. Our analysis focuses on three damage indicators under microwave irradiation, namely P-wave velocity attenuation, temperature distribution and fracture pattern. For a diabase sample, our experimental results indicate that the microwave power and irradiation time has a substantial influence on these three indicators. With increasing power level and irradiation time, we measured a lower P-wave velocity, higher temperature, and more fractures within the rock substrate. We observed that under the same energy, heating at higher power levels for shorter durations has a better weakening effect. Our numerical results show that thermal stress mismatch in the local high-temperature area is the main reason for crack initialization. Nucleation and propagation of microcracks depend on the thermal stress induced by global temperature increase, the geometry of the sample and existing fractures. [ABSTRACT FROM AUTHOR]

• Totally 4300 h relaxation tests were conducted on argillaceous sandstone to investigate its time-dependent behaviors. • The strength degradation of rock induced by relaxation is firstly analyzed quantitatively. • Verified that strength degradation of rock induced by relaxation is less than that of creep behavior. The underground engineering time-dependent stability is affected by the surrounding rock combination behaviors of creep and relaxation. A series of triaxial relaxation tests under 10–30 MPa confining pressure was conducted on argillaceous sandstone to investigate the relaxation-induced strength degradation. Based on tested data, an empirical equation is proposed to describe the rock strength degradation quantitatively. The rock strength after relaxation is approximately 67–83 % of the instant strength. While the rock strength after the creep process is only about 40 ∼ 60 % of its instant strength, the results indicate the rock strength degradation induced by relaxation is much lower than that of creep behavior. [ABSTRACT FROM AUTHOR]

The determination of fundamental rock mechanical properties, uniaxial compression strength (UCS) and elastic modulus (E), constitutes a pivotal facet of rock engineering design. However, deriving these properties directly from standard laboratory tests on rock core samples can be challenging, especially when dealing with deep high-stress rock formations and weak fractured strata. Thus, it is crucial to establish a cost-effective and practical approach for predicting the macro-mechanical properties of rocks in situ. In this study, a machine learning approach was proposed to predict UCS and E by upscaling meso -mechanical parameters at particle scale in low-porosity crystalline rocks. To expand the correlation database of rock meso -macro mechanical properties, the meso -mechanical parameters, including the fracture toughness, tensile strength of the rock crystal interface, and the elastic modulus of rock crystal, were accurately measured, using a newly designed mechanical apparatus and a nanoindentation device. The grain-based models implemented in the combined finite discrete element method (FDEM-GBM) were developed based on these experimental results, and their reliability was validated though standard tests. Subsequently, a database, including 225 groups of data, was established using the numerical method. Five machine learning algorithms were applied to develop prediction models for UCS and E through data training in the database. Excellent performance improvement was achieved through the application of the grid-search method. The results indicate the optimized kernel ridge regression (KRR) and gaussian process regression (GPR) models demonstrated excellent performance with relative average errors of 4.9% and 1.1% in predicting UCS and E , respectively. Finally, the predicted values of UCS and E were compared with the experimental results, validating the feasibility of the optimized KRR and GPR models. [ABSTRACT FROM AUTHOR]

This paper investigates the relationships between hillslope stability and fabric anisotropy of brittle rock materials and the implications for landscape shaping. We use discrete element models to study the stability and failure modes of slopes made of transverse isotropic rock materials, focusing more particularly on the influence of the material orientation relative to the topographic slope. After validating the numerical approach with a limit equilibrium analytical solution in the case of an isotropic material, we modify our numerical slope models to simulate the rheological features of anisotropic gneissic rocks. Systematic exploration of the transverse isotropy plane's orientation in two dimensions (dip angle) reveals that slope collapse requires strength values that are highly dependent on the orientation of the material relative to the slope. For a 1000 m high escarpment, the stability of a slope with a fixed gradient requires strength that is one order of magnitude greater in a configuration where the isotropy plane is slightly less inclined than the topographic slope (i.e., cataclinal overdip configuration) than in a configuration where the isotropy plane is perpendicular to the slope (i.e., anaclinal configuration). Mirroring this highly variable stability according to the relative orientation of the material, four modes of deformation or gravitational instability are observed: in order of appearance, when the transverse isotropy plane orientation goes from 0 to 180° with respect to the horizontal (going from cataclinal to anaclinal configurations), the slope collapses respectively by sliding, buckling, toppling and crumbling. The crumbling mode corresponds to a very stable configuration for which the preferred ground movements will be rock falls from the cliff compared to the structurally controlled, deep-seated deformation modes leading to sliding and toppling. Despite the simplifications inherent to the numerical approach, our study highlights the essential characteristics of landslides occurring along slopes cut in transverse isotropic materials and reproduces the various instability modes observed in natural slopes. It also enables assessing their respective kinetics as well as the volumes of material they mobilize. Finally, by comparing our findings on the azimuthal variations in hillslope gradients observed along the central Himalaya (Nepal), in an area characterized by the relatively uniform orientation of the anisotropy in gneissic and mica-schist formations, we show that, even though multiple environmental factors come into play, landscape shaping is indeed strongly controlled by material anisotropy. • Anisotropic rock strength affects slope stability by foliation dip angle. • Stability and failure modes are investigated by means of discrete element models. • Sliding and buckling failures occur in cataclinal slopes. • Toppling and crumbling failures occur in anaclinal slopes. • Himalayan slopes confirm how anisotropy controls landscape evolution. [ABSTRACT FROM AUTHOR]

The paper focuses on the effect of the gas content in rock macropores on a hydraulic fracturing process. The process was simulated by combining the discrete element method (DEM) with computational fluid dynamics (CFD) under two‐dimensional (2D) isothermal conditions. The mechanical behavior of the rock matrix was simulated with DEM, and CFD was used for describing the behavior of laminar two‐phase fracturing fluid (liquid and gas) flow in pre‐existing and newly developed fractures. Geometry changes of pores and fractures in the rock matrix were precisely reproduced. Fully coupled hydro‐mechanical simulation tests were carried out with a rock segment of simplified particulate mesostructure under plane strain compression. The rock segment contained one or two injection slots. The effect of the initial gas‐phase content in macropores on the propagation of hydraulic fractures was estimated in numbers for different initial rock porosities. In addition, the influence of different pre‐existing discontinuities in the rock segment was studied. The main characteristics of a fractured rock segment due to a high‐pressure fluid injection were realistically reproduced with a proposed approach. The initial gas‐phase fraction in macropores and pre‐existing discontinuities were found to be strongly influential in the course of hydraulic fracturing. [ABSTRACT FROM AUTHOR]

Energy absorption and dissipation are mainly involved in the rock breaking process which is affected by many factors, such as impact strength, number of cycles, and rock material characteristics. In this study, a novel method was proposed to reveal the mechanical properties and energy dissipation law of surrounding rock under the cyclic impact. A split Hopkinson pressure bar (SHPB) test device was used to obtain the energy dissipation characteristics of granite samples. The stress-strain evolution process was compared with the energy dissipation process to analyze the characteristics of incident energy, transmitted energy, and reflected energy in each stage of the impact process. A relationship between impact strength and energy dissipation was established. Results demonstrate that under the impact load, the rock is primarily an axial tensile failure, and the dissipated energy is mainly in the form of surface energy in crack propagation. With the increase in impact, the number of active cracks, the ratio of incident energy to strain energy, and the ratio of reflected energy all increase, whereas the ratio of transmitted energy decreases. The larger the impact load, the larger the proportion of strain energy absorption, and the average value increases from 28.49% to 30.51%. The proportion of single strain energy absorption under cyclic impact conditions is far less than 40.57% of single impact failure. This study contributes to the optimization design of blasting engineering and the production safety improvement in metal mining. [ABSTRACT FROM AUTHOR]

Summary: A new three‐dimensional (3D) Hoek–Brown (HB) failure criterion based on an elliptical Lode dependence is proposed to describe failure of rocks and concrete under multiaxial stress states. This criterion not only inherits all benefits of the classical HB criterion that is developed for the triaxial compression (TXC) of rocks but also accounts for the effect of the intermediate principal stress. It is capable of representing the strength difference between the triaxial extension (TXE) and TXC with the introduction of an additional coefficient k (0.5 ≤ k ≤ 1.0), which can be derived from TXE tests or taken as 0.53 for rocks in cases where the TXE test data is unavailable. Other two material constants (mi and σci) involved in this criterion can be obtained from TXC tests. Additionally, the failure surface of this criterion is smooth and convex on the deviatoric stress plane when 0.5 < k ≤ 1.0. The new criterion achieves very good fit to the test data of TXC/TXE, biaxial compression, and polyaxial compression (PXC) on a wide variety of rock materials and concrete, reported in the literature. Comparison of the new criterion with an existing 3D HB criterion based on the same Lode dependence has demonstrated that the new criterion performs better than the latter for test data of rock and concrete under multiaxial stress states except for PXC test data of one rock type. Finally, the influence of values of k on the accuracy of the new criterion is discussed. [ABSTRACT FROM AUTHOR]

In this paper, an empirical relationship between the Unconfined Compressive Strength (UCS) of intact rock and the unit shaft resistance of piles penetrating rock is investigated. A growing number of civil engineering projects are utilizing steel piles driven into rock where a significant portion of the pile capacity is derived from the shaft resistance. Despite the growing number of projects utilizing the technology, little to no guidance is offered in the literature as to how the shaft resistance is to be calculated for such piles. A database has been created for driven piles that penetrate bedrock. The database consists of 42 pile load tests of which a majority are steel H-piles. The friction fatigue model is applied to seven of the pile load tests for which sufficient UCS data exists in order to develop an empirical relation. The focus of this paper is on case histories that include driven pipe piles with at least 2 m penetration into rock. [ABSTRACT FROM AUTHOR]

The Brazilian method experiments were applied to investigate effects of loading rate on tensile strength of gabbro-diorite, marble, granite. The tensile strength was measured over a wide range of loading rate, σ ˙ = 10 - 3 - 10 2 MPa s. Also the fracture toughness of marble, gabbro-diorite, dolerite were measured over a wide range of loading rate from 5 × 10−4 to 25 MPa m1/2/s by three-point bending of beams with a central narrow cut. The strength and fracture toughness of studied rocks are related to the loading rate by typical equations: σ = a + e ln σ ˙ and K 1 c = c + d ln K ˙ 1 in the range of loading rates of 10−3–102 MPa/s. A comparison of the characteristic fracture parameters for both types of tests showed the initial activation energy of fracture has the same values. The parameters of proportionality were compared on the basis of the integral strength criterion. The obtained parameters of the studied rocks allow extrapolating the results of measurements of strength and fracture toughness for longer periods, it is important for estimating the long-term stability of rock structures. The methodology for determining the characteristics of rock fracture from the loading rate in static fracture of rocks can be easily applied to other brittle materials. [ABSTRACT FROM AUTHOR]

Discontinuity‐controlled slopes constitute potentially fatal risks during exploitation activities in open‐pit mines and severely threaten the safe mining of collieries. To investigate the dynamic mechanical properties of rock with structural planes under periodic cyclic dynamic disturbance, this study took the north side of the Anjialing open‐pit mine as an example. Uniaxial compression tests were carried out on rock samples containing structural planes with different dip angles (θ). The impacts of θ on the strength, deformation characteristics, energy dissipation characteristics, and fracture rules of rock samples under different disturbance stress amplitudes (Δσ) are studied. The test results show that the structural plane affects the mechanical properties of rocks under dynamic perturbation. A closed hysteresis loop curve of rock is formed in each loading and unloading cycle, thus forming three stages: a sparse stage, a dense stage, and another sparse stage. θ, Δσ, and the number of disturbance cycles (n) impact the rock hysteresis loop curve and deformation modulus. The plastic deformation of rock mainly includes an initial deformation stage, an isokinetic deformation stage, and an accelerated deformation and overall failure stage. Moreover, there is a disturbance threshold for Δσ′ of the rock failure, and the evolution law of plastic strain accumulation and n near the disturbance threshold conform to the negative exponential function and the inverse function of the Langevin function, respectively. Based on damage theory, a constitutive model of the structural plane considering a disturbance threshold is proposed. [ABSTRACT FROM AUTHOR]

This paper aims to demonstrate that the elastic stiffnesses and the anisotropic parameters of rocks can be accurately predicted from geophysical features such as the porosity, the density, the compression stress, the pore pressure and the burial depth using relevant machine learning methods. It also suggests that the extreme gradient boosting method is the best method for this purpose. It is more accurate, extremely faster to train and more robust than the artificial neural networks and the support vector machine methods. Very high R‐squared scores was obtained for the predicted elastic stiffnesses of a relevant dataset that is available in the literature. This dataset contains different types of rocks, and the values of the features are in large ranges. An optimal set of parameters was obtained by considering an appropriate sensitivity analysis. The optimized model is very easy to implement in Python for practical applications. [ABSTRACT FROM AUTHOR]

It is shown that rigidity of the medium is a more strain-sensitive parameter than rock density for the elastic wave velocity with the increase in the depth. For assessing the effect of intergrain and fracture porosity, the ratio (in percent) of the measured P-wave velocity in the rock to the velocity in the solid matrix—the "normalized velocity"—is used. The results of the experimental studies have shown that the effect of fractures (faults, rock joints) and intergrain porosity on the propagation velocity of the elastic P-waves is different. The increase in fracture porosity more strongly reduces the P-wave velocity than the increase in intergrain porosity of identical magnitude. It is shown that the propagation velocity of the elastic P-waves in the rocks and model materials depends on the presence and number of cracks (fractures). The possibility of selective assessment of intergrain and fracture porosity in reservoir rocks of the oil and gas fields during their development is demonstrated. [ABSTRACT FROM AUTHOR]

The pull-off test (POT) is a tensile strength testing method that is widely used in concrete/mortar and was recently applied to rock mechanics. The POT can be easily performed in both the field and the laboratory. This study uses the extended finite-element method to analyze the POT fracture mechanism under different test conditions and to assess the factors influencing the tensile strength of homogeneous rock. The numerical results were validated with experimental data, and a good agreement was obtained. An ideal test geometry was proposed (the depth and the width of the partial core were 2.5 and 0.4 cm, respectively). The POT can overestimate or underestimate the tensile strength of the rock, depending on the cohesive fracture energy, and a correction equation was proposed for visually homogeneous rocks and concrete. [ABSTRACT FROM AUTHOR]