Levels Of Confining Pressure Research Articles

Triaxial compressive behaviour of ultra-high performance geopolymer concrete (UHPGC) and its applications in contact explosion and projectile impact analysis

Ultra-high performance geopolymer concrete (UHPGC) is increasingly recognised as a promising construction material in protective engineering owing to its exceptional material strength and eco-friendly characteristics. However, there is currently limited research data on the behaviour of UHPGC under complex stress states, and the applicability of some common concrete dynamic constitutive models to UHPGC requires calibration and evaluation. This paper presents an experimental study on the triaxial compression (TXC) behaviour of UHPGC under varying confining pressures with a range of 0–40 MPa. The stress-strain curves under various confinement levels and failure patterns of UHPGC are discussed. The results indicate that the peak axial stress and toughness of UHPGC increase with higher confining pressure levels, but it still exhibits noticeable softening behaviour. Under uniaxial compression, UHPGC typically fails with inclined shear and axial tensile cracks, while under TXC, particularly at confining pressures above 20 MPa, the failure mode transitions to shear failure with prominent oblique shear cracks. Additionally, existing failure criteria are calibrated to match the strength envelop of UHPGC, with the Power-law failure criterion providing the closest fitting results. Subsequently, utilising both current and pre-existing TXC data, three commonly used concrete constitutive models, including HJC, K&C and CSC, are calibrated with respect to the dominant strength parameters for UHPGC in the finite element (FE) hydrocode ANSYS/LS-DYNA. Further, numerical simulations are conducted to evaluate the effectiveness of such three calibrated concrete models in replicating the response of UHPGC panels exposed to the contact explosion or projectile impact. The simulation results demonstrate that the K&C and CSC models effectively replicate the localised damage of UHPGC panels under contact explosions, while the HJC model more accurately simulates the depth of penetration (DOP) for thick UHPGC panels under projectile impact.

Static and dynamic strength properties of highly structured clay-rich diatomite in Shengzhou, China

Diatomite is a widespread geological substance found in coastal and lake areas worldwide. Although diatomites are increasingly used in geotechnical engineering construction, a significant knowledge gap exists regarding their mechanical responses to static and dynamic loadings, particularly in high-speed railway projects. This study investigates the static and dynamic behaviors of diatomites through extensive experiments, considering both natural and remolded states. It focuses on representative diatomite specimens obtained from the Shengzhou region in China, an area intersected by the Hang-Shao-Tai high-speed railway. The results of uniaxial compression and undrained triaxial shear tests show that the structural degradation of diatomite significantly impacts its stress-strain responses, failure patterns, and mechanical parameters under static and dynamic loading conditions. The structural effects of natural diatomite tend to degrade as the confining pressure level increases, resulting in corresponding changes in its mechanical parameters. Undrained triaxial shear tests conducted on natural diatomite reveal that the inclinations of the shear band closely correspond to those predicted by the Mohr-Coulomb solution. Additionally, we present a modified hyperbolic-type model to evaluate the damping ratio of diatomites. Furthermore, a power-law model based on Hardin's formula was used to predict the maximum shear modulus of diatomites under varying stress levels and void ratios. Microstructural and chemical analyses were conducted using scanning electron microscopy and X-ray diffraction to gain insights into the observed structural effects at the microscale.

A novel constitutive law of confined grouted rock bolt under pull-out load based on a fully nonlinear bond-slip model

Rock bolts are widely applied in deep underground excavations for rock reinforcement. Here, a constitutive model that quantifies the pull-out load-displacement relationship of the grouted rock bolt has been developed though theoretically analyzing the load transfer behaviour along the bolt-grout interface. An improved nonlinear continuously yielding criteria was employed as a bond-slip model where the role of confining pressure playing in slip was accounted. The performance of the proposed constitutive model was demonstrated by comparing with three sets of laboratory pull-out tests on the fully grouted rock bolts under different confining pressure levels (including a short-encapsulated pull-out test designed in our laboratory). The input parameters in the model were determinable within the laboratory environment. The analytical simulation results of pull-out load-displacement performance, the axial stress in the rock bolt and shear stress along the bolt-grout interface agreed well with the laboratory data. We also found that the confining pressure significantly affects the critical embedment length and the higher the confining pressure, the shorter the critical embedment length. The findings enrich the understanding on the mechanical properties of grouted rock bolts and hence aid the rock engineers in rock reinforcement design in underground excavations.

Shear modulus and damping ratio of granulated rubber-sand mixtures: Influence of relative particle size

Rubber-sand mixtures (RSM) have gained recognition as a valuable, lightweight, and cost-efficient energy-absorbing material with extensive potential application in engineering construction. However, the small-strain dynamic properties and prevailing mechanisms of RSM have not been thoroughly understood. This study conducted a series of resonant column tests on the small-strain dynamic modulus and damping ratio of RSM, considering the effects of particle size ratio (PSR), rubber content (RC), and confining pressure. The results reveal that the dynamic shear modulus of RSM generally decreases with increasing RC and increases with rising confining pressure. For all RC and confining pressure levels, the maximum dynamic shear modulus of RSM reaches the minimum value as the PSR approaches 1.0. In this case, the dynamic shear modulus of RSM decreases most gently with the strain. The addition of rubber consistently augments the damping ratio, whereas the PSR initially diminishes the damping ratio before eventually enhancing it. Simultaneously, as the PSR is nearing 1.0, the damping ratio achieves its lowest point. Furthermore, new empirical equations were developed to quantitatively analyze the dynamic properties of RSM, in which the predicted values of the dynamic shear modulus and the modulus attenuation coefficient show good agreement with the observed values. This work will facilitate the preparation of artificial RSM-cored geotechnical seismic isolation layers tailored to specific engineering requirements.

Numerical study on compressive mechanical characteristics of filled jointed rock under confining pressure based on PFC

Confining pressure is an important factor affecting the strength and deformation characteristics of rock mass, it is of great significance to study the mechanical and deformation characteristics of jointed rock mass under confining pressure for the construction of deep underground engineering and the prevention of geological disasters. In order to study the mechanical and deformation characteristics of filled jointed rock under confining pressure, based on the laboratory experiment results of static uniaxial compression of filled jointed rock samples, the Particle Flow Code is used to conduct the numerical simulation. The strength characteristics, failure characteristics and micro-cracks development characteristics of filled jointed rock under different confining pressure levels, different joint inclination angles and different sample sizes are analyzed. The results show that the peak stress and peak strain increase with the increase of confining pressure level, and there is a strong linear relationship between peak stress and confining pressure level. The peak stress and initiation stress decrease first and then increase with the increase of joint inclination angle. With the increase of confining pressure level, the change law of initiation stress of filled jointed rock under different joint inclination angles is different. The confining pressure will prolong the development process of micro-cracks in filled jointed rock, which will make the distribution of micro-cracks more dispersed and the total number of micro-cracks increase. The failure mode changes from splitting failure to shear failure with the increase of confining pressure level. The change of joint inclination angle will seriously affect the failure mode and micro-cracks development characteristics of filled jointed rock.

Study on Stress Evolution and Crushing Behavior of Jointed Rock Mass under Confining Pressure and Joint Materials

To study the effects of confining pressure and joint material properties on stress evolution and fracture behavior of jointed rock mass under SHPB impact load, the numerical software LS-DYNA and the indoor SHPB impact system are used to carry out experimental research on intact rock mass and jointed rock mass. The peak stress, reflection and transmission coefficient, and specimen failure state of rock specimens under different schemes are obtained. The effects of confining pressure level and joint material properties on the propagation and attenuation law of explosive stress waves are expounded. The test results show that when the confining pressure is within a specific range, the impact resistance of the limestone specimen can be increased, and the more difficult it is to be destroyed. Moreover, if the confining pressure continues to increase after rising to the peak value, the impact resistance of rock specimens will decline. In that case, the impact resistance of the specimen will decrease—the dynamic strength of jointed rock mass changes with a change in joint material. The dynamic strength of cement jointed rock is the highest, that of gypsum jointed rock is the second, and that of epoxy resin jointed rock is the lowest. The impact damage resistance of the jointed rock has the same law as the above.

Evolution characteristics of acoustic emission and strain energy for deep granite under different damage stages

The first underground research laboratory (URL) as the research platform for the deep geological disposal of high-level radioactive waste (HLW) in China is being built in a deep granite rock mass. The damaged characteristics and mechanical properties of the deep granite under loading seriously influence the URL’s construction safety and disposal function for HLW. Therefore, this paper investigates the damaged characteristics and mechanical properties of the Beishan deep granite used in various confining pressure levels through a series of triaxial compression and acoustic emission (AE) tests. Based on the crack volumetric strain model, the crack stress thresholds of the deep granite specimens under various confining pressure levels were acquired. The damage development of the specimens used in five confining pressure conditions was analyzed, and the evolution of AE parameters and strain energy of the deep rock under different damage stages were explored. The studied results shown that the crack stress thresholds of the deep granite rose linearly with the increasing confining pressure, but their stress ratios remained relatively constant under various confining pressure levels. Under the rock loading tests, the designed confining pressure levels influenced the AE hit count and AE accumulative count development under the rock damage process. The high-frequency points of the AE hit count coincide with the AE accumulative count increasing, and the maximum value of the AE hit count of the specimens used in different confining pressure conditions appears in the postpeak stress‒strain stage. Additionally, with the axial strain of the loading rock rising in the prepeak stress‒strain stage, all the strain energy (including total energy U, elastic energy Ue and dissipated energy Ud) continues to rise, and in the postpeak stress‒strain stage, Ue stored inside the rock is quickly released, resulting in the Ue/U decreasing and Ud/U increasing.

Resilient Modulus Behavior and Prediction Models of Unbound Permeable Aggregate Base Materials Derived from Tunneling Rock Wastes.

Tunneling rock wastes (TRWs), which are often open- or gap-graded in nature, have been increasingly recycled and reused for sustainable construction of unbound permeable aggregate base (UPAB) courses with high porosity and desired drainability. However, there is still a lack of sufficient understanding of long-term mechanical stability of such TRW materials subjected to repeated applications of moving wheel loads. This paper aimed to characterize and predict resilient modulus (Mr) behavior of the TRW materials used in unbound permeable aggregate base applications. To achieve this goal, five different UPAB gradations were designed based on the gravel-to-sand ratio (G/S) concept. In order to study their Mr behavior, the laboratory repeated load triaxial tests were conducted under different combinations of confining pressure and deviator stress as controlled by the levels of the shear stress ratio (SSR). The prediction accuracy of fourteen classical Mr prediction models was comparatively analyzed, from which the improved Mr prediction model incorporating gradation and stress variables was proposed for TRW-derived UPAB materials and further validated by external database accordingly. The results show that under the same G/S value and confining pressure level, the higher the SSR is, the greater the final Mr values are, and the more significant the effect of G/S on Mr is. Under the same SSR level, the increase of confining pressure alleviates the effect of G/S on Mr. There appears to exist an optimal G/S value of around 1.6–1.8 that yields the best Mr behavior of the TRW-derived UPAB materials studied. The improved Mr prediction model was verified extensively to be universally applicable. It can potentially contribute to balancing long-term mechanical stability and drainability of TRW-derived UPAB materials through gradation optimization. The findings could provide a theoretical basis and technical reference for cost-effective and sustainable applications of UPAB materials derived from TRWs.

Experimental Investigation on Post-Peak Permeability Evolution Law of Saturated Sandstone under Various Cyclic Loading–Unloading and Confining Pressure

The permeability evolution law of saturated rock under cyclic loading–unloading after shear yield is an important basis for revealing the water resistance performance and water inrush risk of overlying rock under multiple mining conditions. In this paper, the influence of the confining pressure, the cyclic loading–unloading times (CLT), and the volumetric strain on the post-peak permeability of saturated sandstone was studied by carrying out a post-peak permeability experiment. Based on SEM images and an improved simulated annealing algorithm, the 3D internal structure characteristics of sandstone samples before and after the experiment were reconstructed. The influences of the confining pressure on pore diameter, effective porosity, connectivity, seepage path length, and tortuosity of the sandstone before and after the experiment are discussed. Research results indicated that (1) In the post-peak cyclic loading–unloading stage, the volumetric strain is negatively correlated with permeability. At the unloading and initial loading stage, the volumetric strain showed a gradually decreasing trend as the specimen was slowly compressed. However, at the middle and final loading stages, the volumetric strain curve shifted to the left and showed a decreasing trend, resulting in an obvious increase in permeability. (2) The influence of CLT on k is closely related to the confining pressure level. When the confining pressure changed from 4 MPa to 12 MPa, the volumetric strain–average stress hysteretic curve shifted to the left in turn and the corresponding permeability gradually increased. When the confining pressure increased to 16 MPa and 20 MPa, the volumetric strain–average stress hysteretic curve shifted to the right in turn and the corresponding permeability showed a decreasing trend. No matter what the value of CLT, the magnitude of sandstone permeability gradually decreased and the decreasing trend became flat as the confining pressure increased, especially for σ3 = 16 MPa and 20 MPa. (3) No matter what value of the confining pressure, the hysteresis area of the first cycle was larger than that of last three cycles, indicating that the plastic deformation generated in the first cycle was larger than that generated in the last three cycles and the recovery rate of the permeability increased with an increase of CLT. (4) As the confining pressure gradually increased, the pore diameter, effective porosity, and connectivity all approximately showed a linear decrease due to more easily compacted pores and cracks under high confining pressure, lower connectivity, and permeability, while the length and tortuosity of the seepage path increased nonlinearly, roughly due to a more significant shear failure phenomenon where the seepage path became more tortuous, that is, the greater the tortuosity, the longer the seepage path. The research results can provide an important theoretical basis for water resistance performance and water inrush risk assessment of overlying aquifer under the influence of mining stress.

Prediction of dynamic pore water pressure for calcareous sand mixed with fine-grained soil under cyclic loading

To evaluate the effect of fine-grained soil (FS) on liquefaction resistance of calcareous sand (CS), a set of undrained cyclic triaxial experiments with stress-controlled method were conducted. The cyclic triaxial tests were performed with 1 Hz frequency at three effective confining pressure levels (100, 200, 300 kPa) and three cyclic stress ratio (CSR) levels (0.1, 0.2, 0.5). Test results demonstrated that pore water pressure (PWP) developing process of pure calcareous sand (PCS) and CS-FS mixtures present three stages along with the rate of PWP ratio (PWP to effective confining pressure). The PWP ratio ru of PCS first increased rapidly then grew slowly and finally kept constant. However, the ru of mixture increased slowly first, then grew sharply and finally became unchangeable. In addition, the terminal PWP ratio ruter of all specimens were less than 1.0 which suggests that PCS and CS-FS mixtures were liquefied partly in this study. Along with the PWP behavior, the modified logarithmic model and modified Seed model were proposed to predict PWP ratio development for PCS and CS-FS mixtures respectively. Test outcomes showed that terminal PWP ratio ruter has a positive relation with effective confining pressure and CSR, and the liquefaction degree was aggravated at high stress. Nevertheless, the cycle number to cause liquefaction has a negative relation with FS content or stress level. Besides, the ruter values decreased as increasing FS percentage from 0 to 50% which indicates that FS can improve the liquefaction resistance of CS-FS mixtures.

Lifespan dynamics of cluster conformations in stationary regimes in granular materials.

We examine stationary regimes in granular materials from a dynamical systems theory perspective. The aim is to enrich the classical view of the critical state regime in granular materials, and more broadly, to improve the fundamental understanding of the underlying mesoscale mechanisms responsible for macroscopic stationary states in complex systems. This study is based on a series of discrete element method simulations, in which two-dimensional assemblies of nonuniformly sized circular particles are subjected to biaxial compression under constant lateral confining pressure. The lifespan and life expectancy of specific cluster conformations, comprising particles in force chains and grain loops, are tracked and quantified. Results suggest that these conformational clusters reorganize at similar rates in the critical state regime, depending on strain magnitudes and confining pressure levels. We quantified this rate of reorganization and found that the material memory rapidly fades, with an entirely new generation of force chains and grain loops replacing the old within a few percent strain.

Experimental Study on the Anisotropy of Layered Rock Mass under Triaxial Conditions

Weak and hard inhomogeneous rock formations are typically encountered during tunnel excavations. The physical and mechanical properties and geological conditions of these rock formations vary significantly; thus, it is crucial to investigate the mechanical characteristics of deep bedded composite rock formations. Three-dimensional (3D) scanning and 3D printing were used to prepare composite rock specimens to simulate natural rock laminae. Triaxial compression tests were conducted to determine the influence of the bedding angle, rock composition, and confining pressure on the mechanical properties of the composite rock specimens. The anisotropic strength characteristics and the damage patterns of the composite rock specimens were analyzed under different confining pressures, and the failure mechanism during triaxial loading was revealed. The results show that the damage of the composite rock specimens with a bedding structure depends on the bedding dip angle and the rock formation. The stress-strain curves and peak strengths of the composite rock specimens have anisotropic characteristics corresponding to their failure modes. As the bedding dip angle increases, the peak strength of the three groups of specimens first decreases and then increases under different confining pressure levels. The compressive strength has a nonlinear relationship with the confining pressure, and the difference between the compressive strengths of specimens with different inclination angles decreases as the confining pressure increases. The Hoek–Brown strength criterion is a good predictor of the nonlinear increase in peak strength of the composite rock specimens under different confining pressures. The specimen with a β = 60°dip angle shows the most significant increase in the strength difference with increasing confining pressure. The results can be used as a reference for testing and analyzing the anisotropic mechanical properties of bedded rock masses.

Investigation on triaxial numerical test method and dilatancy behavior of asphalt mixture

To investigate factors affecting the dilatancy behavior of asphalt mixture, more accurate prediction and reduction of rutting are required. The dilatancy law of asphalt mixture was studied using an indoor triaxial test. Based on the triaxial numerical test, the effect of meso-model parameters on the test results was studied, and relevant model parameters were determined. Then, the effects of the loading speed, confining pressure levels, and pore and aggregate characteristics on the dilatancy performance of asphalt mixture were studied. The results show that the constructed triaxial numerical simulation method correlates well with the indoor triaxial test. Increasing the loading speed and confining pressure levels can improve the dilatancy resistance of the asphalt mixture numerical model. When pores are uniformly distributed, the dilatancy deformation of the numerical model decreases as the porosity increases; the dilatancy angle increases with the increase of aggregate content; and the rougher the surface of the coarse aggregate, the larger the dilatancy angle of the specimen; when the aggregate distribution characteristics are different, the corresponding dilatancy characteristics are also different.

Unified Failure Strength Criterion for Terrace Slope Reinforcement Materials

This paper presents a study on the failure strength criterion of terrace slope reinforcement materials, such as lean cemented sand and gravel (LCSG) material, under a triaxial stress state. Cement content and confining pressure were selected as major factors to investigate their influence on the peak stress of terrace slope reinforcement materials based on experimental results and data from the literature. The mechanical properties of the LCSG samples, with cement contents of 60, 80, and 90 kg/m3, and noncemented sand and gravel materials were tested under four confining pressure levels (namely, 300, 600, 1000, and 1500 kPa). The results show that the strength of LCSG material improves as the confining pressure increases. When the confining pressure exceeds 1200 kPa, the rate of increase of the strength for LCSG material and other cemented grained materials declines generally. The material strength displays a linear increase with the growth of the cement content. When the axial load rises up to a certain value, damage will occur at the particle cemented site near the shear plane, and the resistance stress generated by the cementation shows a trend of growth first and then attenuation, and concurrently, the friction between particles increases by degrees. Based on the identified strength characteristics of LCSG material under different cement contents and confining pressures, a new strength criterion that incorporates the frictional strengths and the cementing strengths is proposed for LCSG and other similar materials. The results of this work can provide an important theoretical basis for the stability calculation of terrace slopes and LCSG dams.

An experimental investigation of the triaxial shear behaviors of silt-based foamed concrete

The silt-based foamed concrete, in which the cement is partially replaced with silt, has become a novel material in road construction projects. However, at the present time, only a few studies have been completed in which attention was paid the shear strength of the aforementioned type of material in subgrade applications. Therefore, in order to address those issues, this research investigation conducted triaxial shear tests. During the experimental testing processes, the mechanical behaviors of silt-based foamed concrete specimens were examined. Such influencing factors as wet density, silt content and confining pressure levels were considered in the experiments. As indicated by the stress-strain curves of the silt-based foamed concrete, the material had performed well in the elastoplastic model results. The stress-strain curves of the silt-based foamed concrete had displayed both hardening and softening phenomena. In addition, the peak strength and residual strength were observed to both increase to average levels of 0.82 MPa and 0.62 MPa, respectively, as the density increased to 100 kg/m3. Meanwhile, the peak and residual strength levels decreased the mean levels of 16.8% and 12.3%, respectively, when the silt content was increased by 10%. It has been found that the addition of silt will transform pore structures from uniformly distributed sphericity into irregular combined broken bubbles. In addition, increases in silt content could potentially increase the size of pores. The values of the cohesion and friction angles have been observed to greatly increase with higher density levels. The increases in silt content will decrease cohesion. However, the impacts of silt content on friction angles have been found to be complex. In the present research, the shear strength levels corresponding to different normal stress levels were determined.

Failure criteria calibration based on the triaxial compression behavior of roller compacted concrete (RCC)

Roller compacted concrete (RCC) has been widely used in large scale constructions such as hydraulic structures and heavy-duty pavements. Understanding the failure criterion based on the compression behavior of RCC under a relatively wide range of confining pressure is essential for the better analysis and design of RCC structures subjected to potential extreme dynamic loadings. However, few experimental results of RCC under confinement are available, and, to date, no study pays close attention on the triaxial behavior of RCC under relatively high confining pressure. In this study, triaxial behavior was investigated with the aim to better understand and model the constitutive behavior of RCC material. Purposely, laboratory tests were employed on 100 × 200 mm (diameter × length) cylindrical specimens, while the mix proportion and molding method were generally consistent with field construction. Triaxial tests were conducted under seven different confining pressure levels (0, 5, 10, 15, 20, 25 and 30 MPa). Consequently, the triaxial compression behavior of RCC was obtained and the confinement effect was illustrated. Compared with conventional concrete, the effect of confinement on the failure strength and failure strain of RCC was less pronounced. The failure criteria of RCC under a relatively wide range of confining pressure were discussed. Parameters of Mohr–Coulomb and William-Warnke failure criteria were calibrated and novel empirical formulas were proposed to illustrate the compressive meridian of RCC material.

Mechanical Responses of a Deeply Buried Granite Exposed to Multilevel Uniaxial and Triaxial Cyclic Stresses: Insights into Deformation Behavior, Energy Dissipation, and Hysteresis

This article presents the results for cyclic uni/triaxial tests on the deeply seated granite samples drilled from a −915 m deep tunnel in Sanshandao (SSD) gold mine. The monotonic and cyclic tests were carried out to observe the mechanical responses of the granite samples under different loading regimes. The disparities concerning the strain evolution and compressive strength of granite samples considering monotonic and cyclic uniaxial and triaxial loading are presented. Deformation behaviour, dissipated energy, and hysteresis are documented and evaluated. Quantitative correlations between strain evolution and cyclic stress levels are revealed. The amount of energy transformation during uniaxial and triaxial cyclic loading is determined. The impacts of confining pressure level on ultimate strain, energy dissipation, and stress‐strain phase shift are presented. The mechanical responses of the granite samples subjected to different stress paths and loading strategies are summarised, and corresponding interpretations are given to clarify the differences of mechanical behaviour encountered in distinct loading methods.

Shear behavior and microstructural variation in loess from the Yan'an area, China

The shear behavior of loess is closely related to its microstructural variation. Their relationship is of great significance to better understand the loess landslide mechanism. Consolidated-undrained triaxial tests were performed on natural loess from Yan'an, China, with various initial water contents under different confining pressures to investigate the shear behavior. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests were conducted on the specimens before and after the triaxial tests to study the variations in microstructure and pore characteristics and their relationship with the shear behavior. The triaxial tests revealed three different failure modes (shearing, homogeneous and plastic failure) and the corresponding stress-strain response of specimens depending on confining pressure level and initial water content. The cohesion decreases exponentially with increasing initial water content whereas the internal friction angle decreases slightly. This effect is closely related to the weakening effect of water on the inter-particle cementation and water-air interface in loess. The macroscopic shear behavior of loess is essentially the result of the continuous adjustment and change in its microstructure system under loading and wetting. This variation process is related to the confining pressure, initial water content and failure mode of the specimen. This condition is manifested by the softening, dispersion, disintegration and reassembly of cementations, particle movement and rearrangement, the reduction and mutual transformation of the inter-aggregate pore populations larger than 0.05 μm in diameter (macropores and abundant mesopores decrease, small pores increase), and the generation and development of cracks under certain conditions.

Experimental Investigation on Deformation and Strength Behavior of Marble with the Complex Loading-Unloading Stress Path

The excavation of deep tunnel in rock mass undergoes complex loading and unloading stress paths, resulting in rib spalling, flaking, and even severe rockburst disasters. Based on the variation law of the stress path of the surrounding rock, laboratory tests of rock mechanics are designed, and the deformation and strength behavior of marble with different initial confining pressure and unloading rates are systematically studied. By introducing strain increment, the characteristic stress, and the dilatancy index, the rock’s dilatancy and brittleness under different unloading conditions are quantitatively analyzed. During unloading, the energy transformation mechanism of rock is described, and the law of deformation and failure is discussed based on characteristic energy. The rock failure strength fitting formula is given by applying the Mogi–Coulomb criterion and elastic strain energy criterion. The advantages of the elastic strain energy criterion are theoretically explained. This study shows that comprehensive consideration of the complex stress paths, confining pressure levels, and the loading-unloading rates of surrounding rock is an effective way to accurately study unloading rock characteristics. The results can provide theoretical basis for stability analysis of high-stress underground engineering.

Particle breakage evolution during cyclic triaxial shearing of a carbonate sand

Particle breakage of carbonate sand under cyclic loading might be of interest in some circumstances, while few research results were reported. Drained and undrained cyclic triaxial tests terminated at different loading cycles were conducted on a carbonate sand to investigate the evolution characteristics of particle breakage during cyclic shearing. Considerable breakage was observed after successive cyclic triaxial shearing at moderate confining pressure levels (≤200 kPa). Abrasion of surface asperities and small sharp edges was found to be the primary pattern of particle breakage and the particle size distribution after cyclic shearing showed an increased mass content of fine particles. In drained cyclic tests with constant confining pressure and cyclic stress amplitude, particle breakage appeared to accumulate at a decreasing rate with increasing loading cycles, and the development of particle breakage with loading cycles can be fitted by a logarithmic equation. In undrained cyclic tests, particle breakage exhibited more complicated accumulation patterns because the effective confining pressure and cyclic strain amplitude varied greatly. A cycle-by-cycle incremental breakage evolution model was established, and the model revealed that the incremental breakage generated in subsequent loading cycles decreases with accumulated particle breakage and increases with increasing effective confining pressure and cyclic shearing intensity. The model was validated by comparing the model predictions with the particle breakage evolution observed during undrained cyclic triaxial tests.