Conventional Triaxial Compression Research Articles

Experimental study on failure mode and fracture evolution characteristics of red shale in Kaiyang Phosphorus mining area

In order to study the failure mode and fracture evolution characteristics of red shale in Kaiyang Phosphorus mining area, conventional triaxial compression mechanical tests of red shale with different bedding dip angles were carried out by using DSTD-1000 electro-hydraulic servo rock mechanics experiment system. Based on the laboratory test results, the conventional triaxial particle flow simulation of red shale samples with different bedding dip angles was carried out using discrete element PFC2D. The results show that: (1) the failure mode of red shale is controlled by bedrock when the bedding dip angle is 0° and 60° ~ 90°. When the bedding dip angle is 15° ~ 45°, the rock failure mode is controlled by bedding. The compressive strength of rock is the minimum when the bedding dip angle is 30°and the maximum at 0°, which is about 2 times of the minimum. (2) In the failure process of red shale, the cracks with different bedding dip angles show slow growth stage, accelerated growth stage and stable stage with axial strain. The whole failure process is dominated by tensile cracks, accompanied by a few shear cracks. (3) The type of displacement field varies with the bedding dip angle: tensile failure and shear failure are the main displacement field types at 15° ~ 45°, and mixed failure is often the main mode at 60° ~ 90°and 0°. The research results provide the basis and reference for the safety control of red shale roadway.

Experimental study on the triaxial mechanical properties of hybrid steel fiber–reinforced self-compacting concrete

In this study, the triaxial mechanical properties of hybrid steel fiber–reinforced self-compacting concrete (HSFRSCC) were investigated. Based on previous research, a mixture of HSFRSCC that satisfies the requirements of compression and workability performance was determined. Conventional triaxial compression tests were performed at confining pressures of 0, 4, 8, 12, and 16 MPa. The results showed that all the specimens had excellent deformation ability, and both the steel fibers and confining pressure improved the ductility of HSFRSCC. With an increase in the confining pressure, the triaxial peak stress and strain of concrete were increased. The differences in the damage characteristics of HSFRSCC and plain concrete were compared based on scanning electron microscopy test results. Finally, the applicability of the Mohr–Coulomb criterion with that of the Willam–Warnke (W–W) criteria to HSFRSCC was compared. the results showed that the nonlinear W–W criterion had a better fitting effect. Hence, the relationship between triaxial peak stress, peak strain, and confining pressure was established, and the triaxial compression meridian formula for HSFRSCC was obtained.

Study of Rock Damage Constitutive Model Considering Temperature Effect Based on Weibull Distribution

The deformation and damage process of rocks is accompanied by crack extension and penetration. The rock strength criterion, as a macroscopic characterization of the rock strength microelement, is the basis for establishing the damage constitutive modeling of rock. Aiming at the problem of the Hoek–Brown (H–B) strength criterion having a large strength prediction value under high confining pressure, the H–B strength criterion is corrected by considering the influence of the initial cracks on the development of the rock strength, and its applicability is verified. Based on the damage theory, assuming that the rock strength microelement obeys the Weibull distribution and considering the influence of residual strength, the damage correction coefficient is introduced, and a thermal damage statistical constitutive model that can reflect the whole process of the development of initial cracks inside the rock is established. The degree of penetration up to the damage is established, and the method of determining the parameters of the model is given. The theoretical curves of the established model are compared and analyzed with the curves of a conventional triaxial compression test of rock samples, and the study shows that the statistical constitutive model of the thermal damage of rock, established based on the modified H–B strength criterion, can better simulate the stress–strain relationship of rock under a conventional triaxial test. It also verifies the reasonableness and applicability of the model, which is expected to provide a basis for the exploitation of deep resources and the safety assessment of underground engineering.

Investigation on failure mechanism and time-delay fracturing behavior of hard-rock tunnel under extremely high geostress state

To investigate the failure mechanism and time-delay fracturing behavior of hard-rock tunnel under extremely high geostress state, a series of conventional triaxial compression tests and time-delay loading–unloading mechanical tests on rocks were conducted. Combining testing results and on-site data of deep-buried tunnels, the generation mechanism and failure process of time-delay rockbursts were analyzed. The results indicate that with the increase of confining pressure, the failure mode of hard-rock exhibits a transition from axial splitting type and shear-slip type to special squeezing-expansion type. The progressive damage of rocks under extremely high stress state mainly manifests as aging lateral and volumetric expansion. Under time-delay compression tests, the long-term strength of hard-rock is lower than the threshold stress of σeb. In extremely high stress state, the fracture and dissipated energy evolution of rocks has significant time-dependent characteristics. Under extremely high stress conditions, rocks can store a large amount of releasable energy internally and gradually generate numerous cracks without the need for external work, ultimately leading to sudden unstable failure. Affected by the superposition of time-delay pressure and loading–unloading effects, the hard-rock also exhibits the same time-dependent fracturing behavior as soft-rock under high stress state, but its deformation mechanism is dominated by lateral expansion and crack volumetric dilatancy. When the stress level is far from reaching the ultimate strength required for rockmass failure, it is sufficient to cause the propagation of internal microcracks. Under the action of time-delay loads, the spalling and peeling failure of surrounding rock is significantly reduced, but the scale of unstable blocks is larger. Based on the failure process and microseismic monitoring date of rockburst in hard-rock tunnels, the entire evolution process of time-delay rockbursts can be divided into the following six stages: before excavation, rockmss excavation, aging cracking, squeezing-expansion, instability and spalling, time-delay rockburst. Time-delay rockburst has an obvious acoustic emission calm period before its occurrence, making its incubation process has strong suddenness and randomness.

Experimental investigation on the influence of NaCl concentration on triaxial mechanical behaviors of a low-clay shale

During a hydraulic fracturing operation, a large quantity of salinity fracturing fluids will be retained in the shale reservoir, which may cause the problem of wellbore instability. Previous studies mainly focused on the change mechanism of clay-rich shale after brine immersion. However, the influence of NaCl concentration on mechanical behaviors of a low-clay shale is still unclear. Therefore, in this investigation, conventional triaxial compression tests were carried out on low-clay shale specimens saturated with NaCl solutions at different concentrations (0%, 5%, 15%, 25%) under different confining pressures (0 MPa, 15 MPa, 30 MPa, 45 MPa), and their mechanical properties were analyzed systematically. Optical microscopy, polarized light microscopy and scanning electron microscopy were used to understand the microscopic changes in shale samples after saturation in NaCl solutions. The brittleness indices of saturated shale specimens were evaluated based on rock strength properties and the energy evolution law. According to the experimental results, compared with the water-saturated specimens, the peak strength, elastic modulus, cohesion and internal friction angle of specimens saturated with 5%NaCl solution all showed a downward trend. However, for the specimens saturated with the NaCl solution at concentrations of 15% and 25%, higher NaCl concentration showed a greater rebound effect on the mechanical parameters compared with the 5%NaCl saturation. The microscopic observation showed that the brine could invade the interior of shale specimens from micropores, microcracks and bedding fractures, which were mainly parallel to the bedding direction. And the bedding fractures and some of micropores seemed to be filled by deposited NaCl crystals and mineral particles. The brittleness index of the brine-saturated shale specimen decreased at the low-concentration (5%) and increased at the high-concentration (15% and 25%). This investigation could provide a theoretical basis and mechanical parameters for analyzing the stability of shale wellbore affected by the salinity fracturing fluids.

Experimental study on the triaxial compression mechanical performance of basalt fiber-reinforced recycled aggregate concrete after exposure to high temperature

The use of basalt fiber-reinforced recycled aggregate concrete (BFRRC) can effectively treat waste concrete and reduce the consumption of natural resources. BFRRC is often in complex triaxial compression in engineering applications and may exposure to high temperature conditions. To evaluate the conventional triaxial compression mechanical performance of BFRRC after exposure to high temperature, 324 cylindrical specimens were produced. The specimens were designed and fabricated by varying several experimental parameters: the recycled coarse aggregate replacement ratio, basalt fiber dosage, temperature, and lateral confining pressure. The specimens previously exposed to high temperature were subjected to conventional triaxial compression tests and the effects of various experimental parameters on the mechanical performance indexes of BFRRC were analyzed. The results show that the peak stress and elastic modulus of BFRRC all raise with greater fiber dosage and confining pressure but decrease with higher replacement ratio and temperature. The maximum increase of the peak stress and elastic modulus is 352.45% and 224.59%, respectively. The peak strain of BFRRC has two variation trends of gradual increase and gradual decrease with the replacement ratio rising. And the peak strain gradually grows with the temperature and confining pressure increasing. The fiber dosage, however, does not exert the considerable impact on the peak strain. The maximum increase of the peak strain is 132.22%. The research provides a scientific basis for the triaxial mechanical performance, fire resistance design, and post-disaster assessment of BFRRC, and promotes the popularization and application of BFRRC in building structures.

Experimental study on mechanical behavior, AE response and energy evolution of fissure-filled sandstone from mine roadway under conventional triaxial compression

Engineering reliability exhibit sophistication and unpredictability for mining from all the regions, largely due to the random combinatorial morphology of natural discontinuities associated with the ubiquitous fissures in the rock formation. This study determined the effects of intersecting fissure angle (none, 0°, 30°, 45°, 60°, and 90°) and confining pressures (4, 8, 12, and 16 MPa) on its mechanical behaviors by triaxial compression tests targeting in fissure-filled sandstones from Lu-Xi coal in Shandong Province, China. Mechanical and AE results indicated that fissure inclination and confining pressure were the origin of the variation in the mechanical behaviors and AE parameters of the sandstone specimens. With increase of pre-existing flaw angle, the peak strength, elastic modulus, peak strain and AE counts presented a U-shape trend. With rise of confining pressure, these parameters displayed a linearly upward trend. However, no significant correlation was found for the Poisson’s ratio. Besides, energy analysis showed that the energy values of the specimens at the peak stress reflected well the variation of the mechanical parameters of the fractured sandstone. This study provided implications for stability of surrounding rock in mine roadway.

Characteristic stress and strain precursor information for fine-grained granite during failure process under triaxial loading and unloading conditions

The unloading effect by excavation may cause irreversible and severe damage to the surrounding rock masses in underground engineering. In this paper, both conventional triaxial compression (CTC) tests and triaxial unloading confining pressure (TUCP) tests were conducted on fine-grained granite to study its triaxial compression failure processes due to unloading. Based on the crack volumetric strain (CVS) method, the crack axial strain (CAS) method and crack radial area strain (CRAS) method were proposed to identify the failure precursor information (including stress thresholds and axial strain at the initiation point of crack connectivity stage) during the rock failure processes. The results of the CTC tests show that the stable crack development stress σsd, unstable crack development stress σusd, and crack connectivity stress σct identified by the CAS method are 6%, 74%–84%, and 86%–97% of the peak stress, respectively. For the TUCP cases, as the confining pressure increases, the stress thresholds, axial pressure at failure and axial strain at the start of the crack connectivity stage increase, while the time ratio of the crack connectivity stage to the entire unloading stage decreases. This indicates that fine-grained granite is prone to generate more cracks and leads to fail suddenly under high confining pressure. Furthermore, this new method demonstrates that the point at which the derivative of the radial crack area strain transitions from stable to a sudden increase or decrease is defined as the precursor point of rock failure. The results of axial strain at the starting point of the crack connectivity stage are very close to those predicted by the AE method, with β1 no more than 11%.

Experimental Study on the Effect of High Temperature on the Physical and Mechanical Properties of Sandstone with Different Bedding Angles

As a typical sedimentary rock, the number of beddings in the horizontal direction of sandstone is far greater than that in the vertical direction, leading to its physical and mechanical properties showing obvious anisotropy with changes in bedding angle. After high temperature exposure, bedding damage further transforms the change rule of the physical and mechanical properties of sandstone with the bedding angle. This study tested the appearance, wave velocity, uniaxial compression, and conventional triaxial compression properties of sandstone with five bedding angles before and after high temperature exposure. The results show that (1) the longitudinal wave velocity, shear wave velocity, elastic modulus, and cohesion decreased, while the internal friction angle increased slightly. At the same temperature, when the dip angle of sandstone was 30° or 60°, the mechanical properties were optimal, and when the dip angle was 45°, the mechanical properties were the worst. (2) High temperature increases the development degree of micropores and microfractures in the sandstone bedding plane and matrix, thus increasing the anisotropy degree of the physical and mechanical properties of sandstone with different bedding angles. (3) With increasing temperature, the rock samples gradually transitioned from brittle failure to ductile failure. Sandstone with a bedding angle of 0° presented splitting failure that vertically penetrated the bedding plane at different temperatures. Sandstone with dip angles of 30° and 40° presented shear failure that penetrated the matrix and bedding plane. A failure plane along the bedding plane appeared at the end. Sandstone with dip angles of 60° and 90° was more prone to failure along the bedding plane, showing shear failure along the bedding plane and tensile failure along the bedding plane, respectively.

A new damage constitutive model for frozen-thawed sandstone under triaxial conditions: Considering the characteristics of pre-peak compaction and post-peak residual strength

The study of the constitutive model under stress loading for rock after undergoing freeze-thaw weathering cycles (FTWCs) treatment has important theoretical significance for the site operation and numerical calculation of rock mass engineering in cold regions. In this work, a series of FTWCs treatment tests of sandstone were carried out, and the evolution characteristics of the T2 spectrum distribution curve for sandstone before and after FTWCs treatment were analyzed utilizing nuclear magnetic resonance (NMR) technology. The schematic diagram of freeze-thaw damage evolution for sandstone was drawn, and the damage evolution mechanism was analyzed. Evolution laws of the stress-strain curves, peak and residual strength, peak and residual strain, and Young's modulus of sandstone under different FTWCs and confining pressures were analyzed by conventional triaxial compression tests. A piecewise constitutive model under triaxial stress loading for sandstone after undergoing FTWCs treatment was established, and the model considered the significant influence of FTWCs on the nonlinear deformation in the compaction stage and residual strength. By introducing error analysis indexes, the proposed model and published models were compared with the experimental data, it was found that the proposed model's performance is better than the published models, which indicated that the proposed model has good performance and strong universality.

Microscopic mechanism analysis and failure criterion study of glass fiber recycled concrete under three‐dimensional compressive stress state

AbstractThis study aims to address the complex stress state commonly observed in concrete structures in practical engineering, as well as the challenges posed by resource scarcity and environmental pollution. To achieve this, a total of 168 cylinder specimens of glass fiber recycled coarse aggregate concrete (GFRAC) were designed for conventional triaxial compression tests. The analysis of the test results reveals that the application of lateral restraint force induces a shift in the failure mode of GFRAC from longitudinal splitting failure to plastic deformation failure, occurring either in the middle or at the end of the specimen. The lateral constraint force can effectively improve the stress and strain at the peak point of GFRAC and the modulus of elasticity of the specimen. The glass fiber increases the peak stress of the specimen, and the maximum growth rate is 20%. On the contrary, the glass fiber reduces the peak strain, and the maximum reduction is 49.66%. Increasing the replacement rate of recycled coarse aggregate will lead to a decrease in peak stress, with a maximum decrease of 19.17%. Glass fiber (GF) limits the appearance and propagation of cracks by bridging, pulling out or breaking. Finally, three different failure criteria are used to analyze the triaxial compression state of GFRAC from a macro perspective.

Stress Path Efforts on Palm Fiber Reinforcement of Clay in Geotechnical Engineering

Sixteen Reduced Triaxial Compression (RTC) triaxial tests were conducted to investigate the reinforcement effect of fibered clay in this paper. Palm fiber with four different fiber lengths (5 mm, 10 mm, 15 mm, and 20 mm) and four different fiber contents (0.3%, 0.5%, 0.7%, and 0.9% in mass) were utilized. Accordingly, three additional groups of triaxial tests were performed to analyze the stress path effects with four different stress paths, including RTC, Conventional Triaxial Compression (CTC), Reduced Triaxial Extension (RTE), and isotropic Triaxial Compression (TC). Three samples were tested, including fibered clay with a fiber length of 10 mm and a fiber content of 0.7% (referred to as 10 mm 0.7%), fibered clay with a fiber length of 20 mm and a fiber content of 0.5% (referred to as 20 mm 0.5%), and bare clay, which was used to reveal the fiber reinforcement of clay. All samples were tested under consolidated undrained conditions. The test results showed that in RTC conditions, the deviator stress increased to a greater extent with 0.3% mass content of fibers according to the same higher confining pressures of bare clay. Fibers primarily increased the cohesion of fibered clay, a shear strength parameter, in terms of total stress, whereas they also increased the friction angle of fibered clay in terms of effective stress. For short fibers, the coefficient of strength reinforcement of the fibered clay increased with fiber content. However, for long fibers, this reinforcement may lead to a weakening of the clay’s strength, as the long fibers may cluster or weaken along their longitude. Among the four stress paths (CTC, TC, RTC, and RTE) examined, the reinforcement took effort mainly in the CTC condition. In contrast, in unloading conditions, the fibers had little contribution to reinforcement. Consequently, in unloading conditions, such as deep excavating and slope cutting, the stress path should be considered to obtain a reliable parameter for geotechnical engineering applications.

Experimental study on the mechanical behavior of different lithologic shales in Qingshankou group under in-situ geological conditions

To investigate the mechanical behavior of shale under in-situ geological conditions, a novel experimental methodology called in-situ stress restoration triaxial compression (ISRTC) was proposed for testing shale under in-situ geostress. Then the ISRTC and conventional triaxial compression (CTC) tests were conducted on four different lithological shale core samples from the Qingshankou Formation reservoir to explore the differences in shale mechanical behavior between conventional laboratory conditions and in-situ geostress conditions. Compared to CTC tests, shale samples subjected to ISRTC tests demonstrated a noteworthy decrease in pre-peak elastic deformation, alongside an intensified and increased nonlinear deformation. These findings indicated that in-situ geostress restoration led to reduced brittleness and increased ductility of shale. Moreover, shale cores in the ISRTC test showed higher peak strength, residual strength, Young's modulus, and Poisson's ratio compared to CTC tests. The elastic energy, dissipative energy, and total energy in ISRTC tests were also greater than those in CTC tests. Furthermore, in-situ geostress restoration promoted numerous microcracks in shale, resulting in the release of more energy during failure, leading to a more complex failure mode. In CTC tests, shale damage concentrated in the stage of unstable crack propagation, whereas in ISRTC tests, the damage developed progressively throughout the entire loading stage. Additionally, the abundance of hard-phase minerals and scarcity of weak-phase minerals in shale had a smaller influence of in-situ stress restoration on peak strength and residual strength but a greater influence on the elastic modulus. This study provides valuable insights into the in-situ mechanical characteristics of different lithologies of Gulong shale in the Qingshankou group and serves as a scientific basis for the efficient development of shale oil.

Mechanical characteristics and energy evolution in a rock mass with a weak structural plane

The present aims to investigate the mechanical characteristics and energy evolution in rock masses containing weak structural planes under conventional triaxial loading conditions. Using a fluid–solid coupling test system of coal rock, numerous conventional triaxial compression tests were performed on rock masses at various dip angles of the structural plane. The obtained empirical outcomes revealed that the deviatoric stress–strain curve of the weak structural plane rock mass with an inclination angle greater than 20° rises step-by-step. On the macro level, slip-stability occurs on the upper and lower parts of the rock mass on the weak structural plane. Then mechanism of the slip-stability phenomenon is explored by analyzing the stress level in the rock mass with various inclination angles. It is found that the energy evolution during deformation and failure reflects the damaged state of the rock. Accordingly, the concept of ‘slip dissipation energy’ is proposed, and the values of each energy are calculated. The results have a good correspondence with the deviatoric stress–strain curve. Furthermore, it was found that the energy evolution of rock mass with a weak structural plane can be primarily classified into four stages, including storage of the initial energy, slip dissipation, abrupt increase in the pre-peak dissipation energy, and sudden drop in post-peak energy. Rock masses with various levels of dip angles exhibit similar elastic strain energy and dissipation energy at the peak point, demonstrating that energy evolution is dominated by energy storage and dissipation. At the same time, a negative correlation is observed between the structural plane dip angle and the occurrence of instantaneous impact instability failure in rock masses, indicating that a greater dip angle makes the rock mass less prone to experiencing instantaneous impact instability failure. This article provides a new idea for analyzing the geological disasters caused by external disturbances.

Experimental Study of Compact Sandstone Deformation Under Axisymmetric Triaxial Loading Along Specific Paths in Stress Space

The deformation of rocks well below their ultimate strengths is frequently described through the constant stiffness moduli of the linear elasticity theory. In addition, the isotropic material approximation is usually used as the basic approach. Particularly in sedimentary rocks, local inhomogeneities, anisotropy, and irreversible processes, mainly related to the gradual breakage of grain joints, can affect their deformation behavior from the beginning of the loading. Consequently, the stiffness moduli measured under triaxial conditions are effective parameters, which may depend on the loading path in the stress space. Therefore, a complex experimental study of the deformation response of compact sedimentary rock (Brenna sandstone) along various paths in triaxial stress space was prepared to understand this dependence. In addition, a detailed analysis of the rock composition and structure of this compact sandstone was carried out. This paper presents an initial experimental study that is based on a loading method using alternative stress paths that correspond to different modes of the monotonic increase in differential stress from an initial isotropic compression state. In the experiment, the dependence of stiffness moduli on the loading path was found. Differences in rock deformation for conventional triaxial compression and extension can be attributed to a slight rock anisotropy originating from the stratification. The different behavior of deformation and related stiffness moduli along so-called reduced triaxial paths and conventional paths indicates that irreversible processes must be taken into account.

DEM study on the loading rate effect of marble under different confining pressures

Abstract Based on the conventional triaxial compression test of marble, the microscopic parameters corresponding to the macroscopic mechanical properties of marble were obtained by PFC2D, and on this basis, discrete element method models were established to conduct numerical tests of triaxial compression of marble under different confining pressures and loading rates to study the effect of loading rates on the rock specimens. Under the same confining pressure, brittle damage occurs in marble when the loading rate is low, and the damage type transforms from brittle to ductile as the loading rate increases. The peak strength, cracking stress, damage stress, cohesion, and internal friction angle of marble appear to increase with the increase in the loading rate under the same confining pressure. The characteristic stresses and strength parameters are linearly related to the loading rate. The influence of the loading rate on the peak strength is the most significant. With the increase in the confining pressure, the fitting coefficient of the linear relationship between initiation stress and loading rate decreases from 18.9 to 15.4 for different confining pressures, indicating that the growth rate of initiation stress decreases with the increase in the confining pressure when the loading rate is increased, and the increase in confining pressure suppresses the growth rate of initiation stress with loading rate.

Numerical simulation of true triaxial unilateral unloading effect of fractured rock

PurposeThe interaction between rock mass structural planes and dynamic stress levels is important to determine the stability of rock mass structures in underground geotechnical engineering. In this work, the authors aim to focus on the degradation effects of fracture geometric parameters and unloading stress paths on rock mechanical properties.Design/methodology/approachA three-dimensional Particle Flow Code (PFC3D) was used for a systematic numerical simulation of the strength failure and cracking behavior of granite specimens containing prefabricated cracks under conventional triaxial compression and triaxial unilateral unloading. The authors demonstrated the unique mechanical response of prefabricated fractured rock under two conditions. The crack initiation, propagation, and coalescence process of pre-fissured specimens were analyzed in detail.FindingsThe authors show that the prefabricated cracks and unilateral unloading conditions not only deteriorate the mechanical strength but also have significant differences in failure modes. The degrading effect of cracks on model strength increases linearly with the decrease of the dip angle. Under the condition of true triaxial unilateral unloading, the deterioration effect of peak strength of rock is very significant, and unloading plays a role in promoting the instability failure of rock after peak, making the rock earlier instability failure. Associating with the particle vector diagram and crack coalescence process, the authors find that model failure mode under unilateral loading conditions is obviously distinct from that in triaxial loading. The peak strain in the unloading direction increases sharply, resulting in a new shear slip.Originality/valueThis study is expected to improve the understanding of the strength failure and cracking behavior of fractured rock under unilateral unloading.

Mechanical Properties and Strength Characteristics of Rock–Coal–Rock Assemblages under Different Peripheral Pressures

To investigate the deformation and damage characteristics of internal coal bodies of small pillars under different pressures, rock–coal–rock assemblage samples were subjected to the conventional triaxial compression test to analyze the mechanical behavior characteristics under different pressures. The results showed that, with the increase in peripheral pressure, the peak strength and modulus of elasticity of the assemblage specimens increased, the range of fracture compaction stage gradually decreased, and the specimen was gradually transformed from brittle to ductile. With an increase in peripheral pressure, the residual strength gradually increased, and the strength decay coefficient gradually decreased. The strength decay coefficient decreased the most at 0–10 MPa, and this decrease slowed down after exceeding 15 MPa. When the peripheral pressure was 0 MPa, the damage degree of the coal pillar was larger. With the increase in peripheral pressure, the number of cracks in the coal column increased, the damage degree increased more, and mixed damage characteristics of tension–shear were found. Based on the Hoek–Brown criterion, the strength criterion applicable to the specimen of rock–coal–rock combination was obtained through numerical fitting iteration, which provides an experimental and theoretical basis for realizing the stability control of small coal columns.

Experimental and DEM Simulation Study on the Mechanical Characteristic and Strain Energy Evolution of Longmaxi Shale under a Confining Pressure Unloading Path

Drilling vertical and horizontal wellbores in the shale reservoir may trigger the in-situ stress release around the wellbore walls and change the original stress equilibrium state, leading the wellbores to instability. This stress change in the wellbore corresponds to the stress paths of confining pressure unloading and axial stress loading under laboratory conditions. In this paper, according to the conventional triaxial compression test results, laboratory experiments and DEM simulations by PFC2D were conducted to deeply study the strength, failure, strain energy evolution, and micro-crack damage mechanism of shale specimens under confining pressure unloading conditions. The shale specimens at different bedding inclinations were tested under different initial axial stress levels and confining pressure unloading rates, with fixed initial unloading confining pressure. This research revealed that confining pressure unloading induces greater plastic deformation, more micro-crack damage and strain energy dissipation, and a more complex failure pattern. The strain energy dissipation and dilatation under confining pressure unloading conditions are mainly induced by the generation and accumulation of tensile cracks. Moreover, the unloading rate has a significant effect on the mechanical properties, and the high unloading rate enhances the failure strength and induces more strain energy dissipation and micro tensile cracks. For the wellbore drilling in shale formations, when the buried depth and vertical stress are fixed, the lower the lateral stress is, the easier it is to form tensile failure around the wellbore wall in the drilling process, and the more induced fractures will be generated in the formation around the wellbore.

Cold recycling mix design approach targeting permanent deformation resistance

Cold recycling technology offers a sustainable construction pathway for pavements with a potential of recycling up to 100% of the existing in-situ materials. This study introduces a new approach to design emulsion-based cold recycling mixtures. The proposed mix design approach channels the design efforts to optimize the mixture’s components and evaluate its long-term performance. The elements of the proposed design include altering aggregate gradation to achieve optimum packing; a method to selecting an optimum moisture content that is derived from both liquid components in the mixture; and selecting the residual asphalt content based on the mixture’s resistance to permanent deformation. To evaluate the reliability of the proposed mix design method, an optimum mixing water content was derived for an optimum RAP gradation. The effect of varying the emulsion content while keeping the mixing water content constant was evaluated on the mixture’s volumetrics, strength, and permanent deformation behavior. A conventional triaxial compression stress path was applied on samples compacted with several residual asphalt contents. The triaxial test results were compared with results obtained from Hamburg Wheel Tracking Test and Asphalt Pavement Analyzer rutting device. The increase in residual asphalt content at optimum mixing water content led to a systematic increase in strength and reduction in voids that stabilizes as the content increases. The permanent deformation experiments showed the importance of imposing a deformation criterion to select the residual asphalt content in the mix design stage. Additionally, the triaxial repeated load tests demonstrated the importance of using anticipated stress states to characterize permanent deformation resistance which cannot be captured by simple performance tests. Representative stress states in the CR layer should be taken into account during the mix design stage where higher stresses are anticipated in the recycled layer.