Effect Of Confining Pressure Research Articles

A thermodynamic viscoelastic model to capture the effects of confinement pressure on asphalt mixtures

ABSTRACT The aim of this study is to develop a constitutive model capable of describing the effects of confinement pressure on the mechanical behaviour of asphalt mixtures. The model is developed within the context of a Gibbs-potential based thermodynamic framework. Functional forms are assumed for the Gibbs potential and the rate of dissipation, which is maximised to obtain the constitutive model. The Gibbs potential and the rate of dissipation are formulated to have separate components corresponding to the spherical and deviatoric parts of the stress tensor. Additionally, one of the model viscosity parameters is formulated to be pressure-dependent. The efficacy of the model is demonstrated with the aid of experimental measurements conducted at four different confinement pressures ( 0 , 70 , 140 , 380 kPa ) and two different temperatures (40 and 55 ∘ C ) (Rahmani et al. 2013, Effect of confinement pressure on the nonlinear-viscoelastic response of asphalt concrete at high temperatures, Construction and Building Materials, 47, 779–788; 2017, Constitutive modeling of coupled aging-viscoelastic response of asphalt concrete, Construction and Building Materials, 131, 1–15; Bazzaz et al. 2018, A straightforward procedure to characterize nonlinear viscoelastic response of asphalt concrete at high temperatures, Transportation Research Record, 2672 (28), 481–492). Some of the test results are used to calibrate the model, while the remaining data are used for model validation. It is found that the model reasonably predicts the mixture responses in the axial and radial directions at different confining pressures and temperatures.

Numerical Modeling of Engineering-Scale Jointed Coal Mass and Confining Pressure Effect

In coal masses, joints serve as crucial components influencing mechanical responses and failure mechanisms. The joint distribution complicates the acquisition of physical parameters for engineering-scale coal masses, and traditional laboratory or in situ tests often suffer from inaccuracies and high costs. Therefore, examination of the dynamic properties of engineering-scale coal mass is generally performed by numerical simulations. This paper proposes a model construction approach using two-dimension Particle Flow Code (PFC2D) software to study the mechanical properties of engineering-scale jointed coal mass, addressing the limitations of conventional models by integrating scale effects, accuracy, and computational efficiency. Firstly, the distribution characteristics and mechanical parameters of the joints in the coal mass were obtained based on field statistics and laboratory experiments. The parameters of the laboratory-scale model were calibrated by the numerical matching method. The discrete element model for the engineering-scale coal mass was constructed by the step-by-step matching method. The confining pressure effect on the coal mass under a biaxial loading condition was studied, while the strength change, fissure evolution, and failure mechanism under different confining pressures and fissure degrees were investigated. Based on the simulation results, a quantitative relationship was established between the mechanical parameters, fissure degree, and confining pressure under compression conditions. Ultimately, the failure zone ahead of the working face and the distribution of the abutment pressure were assessed using the mechanics parameters of coal masses with diverse joint distributions.

A deep LSTM-based constitutive model for describing the impact characteristics of concrete–granite composites with different roughness interfaces

In this work, the dynamic mechanical properties of concrete‒granite composites with various roughness interfaces were investigated via split Hopkinson pressure bar (SHPB) system to evaluate the impact resistance of the lining‒surrounding rock composite structure that is commonly present in rock engineering. The dynamic uniaxial compressive strength of the composite at an impact speed of 11.3 m/s may increase by 20.55% when the joint roughness coefficient (JRC) increases from 0 to 28.64 according to the experimental results. The JRC increased the strain rate effect of the composite but reduced the confining pressure effect. The relationships between the dynamic deformation parameters, such as the elastic modulus and critical strain, and the study variables, as well as the correlation mechanism between the macroscopic and microscopic failure morphologies of the composite and the stress‒strain curves, were investigated. A developed long short-term memory (LSTM) deep learning method was utilized to predict the dynamic stress‒strain relationships of the composites after 144 sets of experimental data were split into training and testing sets. The prediction issue of peak stress for the composites after varying time steps was presented to the recursive neural network LSTM, which was evaluated and compared with the traditional back propagation neural network (BPNN) and random forest (RF). The LSTM model showed the strongest prediction capacity when considering accuracy and predictive evaluation indicators. Compared with the BPNN model, the RF model was worse at capturing the viscoelastic plastic properties of the constitutive model while having superior assessment indicators.

Physical and mechanical depth relationships of rocks from the Rotokawa Geothermal Reservoir, Taupō Volcanic Zone, New Zealand

ABSTRACT The Rotokawa geothermal field is in the Taupō Volcanic Zone, New Zealand. It hosts two geothermal power plants, Rotokawa and Ngā Awa Pūrua, which together have a capacity over 170 MWe. The permeability of the rock mass comprising a geothermal field controls the volume of hot fluids that can be extracted for energy production. In this research we produce a stress model to estimate in-situ rock matrix permeability for a unique set of intact rock samples obtained from depths up to 2600 m in the Rotokawa geothermal field. We show that permeability generally decreases with sample depth, both intrinsically and in response to increasing confining pressure. We explore this confining pressure effect on other petrophysical and mechanical measurements, and highlight how rock texture and composition affect how the rocks respond to confining pressure. For example, we compare two altered andesite samples: a breccia with microfractures and infilled pores that tends to experience less compaction and porosity decrease than a lava with rounded, unfilled pores. We suggest that, when developing depth-models, measurements should be conducted at relevant in-situ conditions, if possible. Finally, we explore relationships between different physical parameters and provide estimating functions for those with the clearest correlations.

Rock failure deformation characteristics and mechanical parameter prediction of Xujiahe formation gas reservoir in Tianfu area, Sichuan Basin

The integrated geological-engineering reservoir reconstruction of Xujiahe Formation in Tianfu area needs accurate evaluation of rock mechanics characteristics and prediction of three-dimensional rock mechanics parameter field. The stress-strain curve characteristics, failure deformation mode, mechanical strength, and confining pressure effect are analyzed by mechanical tests. The longitudinal, lateral, and spatial distribution characteristics of rock mechanics parameters are evaluated through the combination of logging and seismic. The results show that damage morphology of mudstone is more complex than that of sandstone. With the increase of confining pressure, rock fracture pattern gradually changes from splitting failure to high angle shear failure. The mechanical strength of sandstone and mudstone increases with the increase of confining pressure, and high porosity of sandstone results in a higher confining pressure effect. The longitudinal distribution of rock mechanics parameters is significantly affected by lithology. Sandstone has high strength and little intra-layer difference, while mudstone is the opposite. The lateral distribution shows obvious sedimentary facies control characteristics, and the mechanical strength of rock in the northwest region is significantly lower than that in the southeast region. The spatial distribution is greatly affected by structural morphology and faults. The research will provide support for engineering dessert selection and fracturing reconstruction.

3D DEM investigation on macro-meso mechanical responses of gas hydrate-bearing sediments in process of unloading confining pressure

Well drilling, gas production, and reservoir stimulation can disrupt stress states of gas hydrate-bearing sediments (GHBSs) surrounding wellbores, potentially inducing wellbore instability due to confining pressure unloading. Therefore, it is very important to know about the macro-meso mechanical responses of GHBSs under an unloading confining pressure path (UCPP). Herein, a discrete element method (DEM) was used to perform triaxial tests on three-dimensional numerical GHBSs under the UCPP and a conventional drained path (CDP) to bridge the knowledge gap. Results show that GHBSs under the UCPP are more prone to failure and dilatation than under the CDP, due to the reduced bearing capacity of contact force chains, relatively unstable particle assembly, and rapid cementation breakage development. Additionally, the stress path has little effect on the cohesion and friction angle of GHBSs. Furthermore, under the UCPP, the failure and dilatation risk of GHBSs are positively correlated with unloading confining pressure rate, inversely related to hydrate saturation and initial effective confining pressure. The grey relational analysis illustrates that compared to hydrate saturation and initial effective confining pressure, the unloading confining pressure rate minimally affects the unloading confining pressure effect. These findings can serve for experimental studies and field applications (e.g., optimizing drilling mud density, sand production rate, and fracturing fluid flowback rate) concerning the unloading confining pressure responses of GHBSs.

Structural integrity assessment of a solid propellant grain considering confining pressure effect

Confining pressure has a significant effect on the mechanical properties of solid propellant, and it is crucial to develop a refined structural integrity analysis and assessment method considering confining pressure effect and nonlinearity of propellant material for the design of propellant grain of a solid rocket motor (SRM), especially for the design of high charge modulus. In this work, an existing viscoelastic-viscodamage constitutive model considering confining pressure effect was implemented as a user-defined material subroutine and verified using a few experiments under various confining pressures. The mechanical responses and safety factor of a typical variable cross-section propellant charge SRM with various charge modulus under ignition pressurization load and room temperature were analyzed and evaluated. The results show that the user-defined material subroutine can well describe nonlinear mechanical responses of solid propellant under different experimental conditions. The Von Mises stress and minimum safety factor considering the confining pressure effect are higher than the value without considering the confining pressure effect, while the Von Mises strain is opposite. In addition, regardless of whether the confining pressure effect is considered or not, the maximum Von Mises stress and strain show a nonlinear increasing relationship with the charge modulus, and the minimum safety factor decreases with charge modulus. However, when analyzing the structural integrity of high charge modulus, using two evaluation methods will yield completely different evaluation results. It is believed that this research can support structural integrity analysis and design of high charge modulus SRM.

Investigate on the mechanical properties and microscopic three-dimensional morphology of rock failure surfaces under different stress states

The macro and micro morphology of rock failure surfaces play crucial roles in determining the rock mechanical and seepage properties. The morphology of unloaded deep rock failure surfaces exhibits significant variability and complexity. Surface roughness is closely linked to both shear strength and crack seepage behavior. Understanding these morphology parameters is vital for comprehending the mechanical behavior and seepage characteristics of rock masses. In this study, three-dimensional optical scanning technology was employed to analyze the micromorphological properties of limestone and sandstone failure surfaces under varying stress conditions. Line and surface roughness characteristics of different rock failure surfaces were then determined. Our findings reveal a critical confining pressure value (12 MPa) that influences the damage features of Ordovician limestone failure surfaces. With increasing confining pressure, pore depth and crack formation connecting the pores also increase. Beyond the critical confining pressure, the mesoscopic roughness of the failure surface decreases, and the range of interval-distributed pore roughness diminishes. Additionally, we conducted a detailed investigation into the water conductivity properties of rocks under different stress states using Barton's joint roughness coefficient (JRC) index and rock fractal theory. The roughness features of rock failure surfaces were classified into three categories based on mesoscopic pore and crack undulation forms: straight, wavy, and jagged. We also observed significant confining pressure effects on limestone and sandstone, which exceeding the critical confining pressure led to increased water conductivity in both rocks, albeit through different mechanisms. While sandstone exhibits fissures running across it, limestone shows shear abrasion holes. Beyond the critical confining pressure, the rock failure surface becomes smoother, leading to decreased water flow blocking capacity. The fractal dimension of Ordovician limestone increases significantly under critical confining pressure, leading to a more complex mesoscopic crack extension route.

Effect of Lateral Confining Pressure on Shale’s Mechanical Properties and Its Implications for Fracture Conductivity

The field stress of the shale affects the proppant embedment, fracture conductivity, well production rate, and ultimately the recovery of hydrocarbons from reservoir formations. This paper presents, for the first time, an experimental study investigating the mechanical characteristics of a shale under confining pressures that simulate the in situ stress state in deep reservoirs. Bidirectional but equal confining pressures were applied to the shale sample to replicate its field stress state. Microindentation tests were conducted to assess the alterations of mechanical properties resulting from the application of confining pressures. The results demonstrate a significant increase in Young’s modulus, hardness, and fracture toughness for the samples subjected to confining pressure. Considering the effect of confining pressure, the decrease in proppant embedment is proportional to Young’s modulus of the shale. For larger-sized proppants (e.g., D = 2.50 mm), the influence of confining pressure on fracture conductivity is relatively minor. However, when smaller-sized proppants (e.g., D = 1.00 mm) are used, particularly in scenarios involving shale debris swelling due to prolonged interaction with fracturing fluid, there is a noticeable improvement in fracture conductivity. Importantly, previous computational models have tended to overestimate proppant embedment depth while underestimating fracture conductivity. The findings from this study contribute to advancing the understanding of shale’s mechanical characteristics under in situ reservoir conditions and support the optimization of proppant embedment and fracture conductivity calculation models for the efficient extraction of shale gas.

Enhanced Hoek-Brown (H-B) criterion for rocks exposed to chemical corrosion

Underground constructions often encounter water environments, where water–rock interaction can increase porosity, thereby weakening engineering rocks. Correspondingly, the failure criterion for chemically corroded rocks becomes essential in the stability analysis and design of such structures. This study enhances the applicability of the Hoek-Brown (H-B) criterion for engineering structures operating in chemically corrosive conditions by introducing a kinetic porosity-dependent instantaneous mi (KPIM). A multiscale experimental investigation, including nuclear magnetic resonance (NMR), X-ray diffraction (XRD), scanning electron microscopy (SEM), pH and ion chromatography analysis, and triaxial compression tests, is employed to quantify pore structural changes and their linkage with the strength responses of limestone under coupled chemical-mechanical (C-M) conditions. By employing ion chromatography and NMR analysis, along with incorporating the principles of free-face dissolution theory accounting for both congruent and incongruent dissolution, a kinetic chemical corrosion model is developed. This model aims to calculate the kinetic porosity alterations within rocks exposed to varying H+ concentrations and durations. Subsequently, utilizing the generalized mixture rule (GMR), the kinetic porosity-dependent mi is formulated. Evaluation of the KPIM-enhanced H-B criterion using compression test data from 5 types of rocks demonstrated a high level of consistency between the criterion and the experimental results, with a coefficient of determination greater than 0.96, a mean absolute percentage error less than 4.84%, and a root-mean-square deviation less than 5.95 MPa. Finally, the physical significance of the porosity-dependent instantaneous mi is clarified: it serves as an indicator of a rock’s capacity to leverage the confining pressure effect.

Numerical Investigation of Confining Pressure Effects on Microscopic Structure and Hydraulic Conductivity of Geosynthetic Clay Liners

A series of COMSOL numerical models were developed to explore how confining pressure impacts the microscopic structure and hydraulic conductivity of Geosynthetic Clay Liners (GCLs), taking into account the bentonite swelling ratio, mobile porosity, pore size, and tortuosity of the main flow path. The study reveals that the mobile porosity and pore size are critical factors affecting GCL hydraulic conductivity. As confining pressure increases, the transition of mobile water to immobile water occurs, resulting in a reduction in mobile water volume, the narrowing of pore channels, decreased flow velocity, and diminished hydraulic conductivity within the GCL. Mobile porosity undergoes a slight decrease from 0.273 to 0.104, while the ratio of mobile porosity to total porosity in the swelling process decreases significantly from 0.672 to 0.256 across the confining pressure range from 50 kPa to 500 kPa, which indicates a transition of mobile water toward immobile water. The tortuosity of the main flow path shows a slight increase, fluctuating within the range of 1.30 to 1.36, and maintains a value of around 1.34 as the confining pressure rises from 50 kPa to 500 kPa. At 50 kPa confining pressure, the minimum pore width measures 5.2 × 10−5 mm, with a corresponding hydraulic conductivity of 6.2 × 10−11 m/s. With an increase in confining pressure to 300 kPa, this compression leads to a narrower minimum pore width of 1.81 × 10−5 mm and a decrease in hydraulic conductivity to 5.11 × 10−12 m/s. The six-fold increase in confining pressure reduces hydraulic conductivity by one order of magnitude. A theoretical equation was derived to compute the hydraulic conductivity of GCLs under diverse confining pressure conditions, indicating a linear correlation between the logarithm of hydraulic conductivity and confining pressure, and exhibiting favorable agreement with experimental findings.

Energy characteristics of saturated Jurassic sandstone in western China under different stress paths

AbstractTo study the energy evolution and failure characteristics of saturated sandstone under unloading conditions, rock unloading tests under different stress paths were conducted. The energy evolution mechanism of the unloading failure of saturated sandstone was systematically explored from the perspectives of the stress path, the initial confining pressure, and the energy conversion rate. The results show that (1) before the peak stress, the elastic energy increases with an increase in deviatoric stress, while the dissipated energy slowly increases first. After the peak stress, the elastic energy decreases with the decrease of deviatoric stress, and the dissipated energy suddenly increases. The energy release intensity during rock failure is positively correlated with the axial stress. (2) When the initial confining pressure is below a certain threshold, the stress path is the main factor influencing the total energy difference. When the axial stress remains constant and the confining pressure is unloading, the total energy is more sensitive to changes in the confining pressure. When the axial stress remains constant, the compressive deformation ability of the rock cannot be significantly improved by the increase in the initial confining pressure. The initial confining pressure is positively correlated with the rock's energy storage limit. (3) The initial confining pressure increases the energy conversion rate of the rock; the initial confining pressure is positively correlated with the energy conversion rate; and the energy conversion rate has a high confining pressure effect. The increase in the axial stress has a much greater impact on the elastic energy than the confining pressure. (4) When the deviatoric stress is small, the confining pressure mainly plays a protective role. Compared with the case of triaxial compression paths, the rock damage is more severe under unloading paths, and compared with the case of constant axial stress, the rock damage is more severe under increasing axial stress.

Unraveling the shear behaviors of bonding interface for post-grouted sandstones considering the temperature and confining pressure effects

The flow and diffusion of grout within rock fractures, as well as the shear behavior of the grout-rock interface, are notably influenced by grouting conditions, encompassing grouting parameters and geological considerations. Regrettably, owing to constraints imposed by prevailing testing techniques, there exists a scarcity of reports investigating the shear mechanical properties of the grout-rock interface across diverse grouting conditions. This study used a low-field nuclear magnetic resonance (LF-NMR) testing system to conduct permeation grouting tests on fractured sandstones under various grouting conditions, followed by direct shear mechanical tests on the post-grouted samples. The influences of grouting pressure, temperature and confining pressure on the shear mechanical properties of the bonding interface were analyzed. Results showed that higher grouting pressure and temperature conditions led to ductile shear failures due to shear-friction action. An increase in grouting pressure (from 0.5 MPa to 1.3 MPa) increased the peak shear strength and residual shear strength of the post-grouted sample. However, increasing the temperature (from 20 °C to 50 °C) and confining pressure (from 5 MPa to 15 MPa) decreased the peak shear strength and residual shear strength. Meanwhile, with increasing the temperature, both the internal friction angle and cohesion linearly decline, while an increase in confining pressure decreased the internal friction angle but increased cohesion. A shear mechanical model was developed for the bonded interface that accounted for the influences of temperature and confining pressure. Additionally, SEM and XRD examinations yielded valuable insights into the grout filling patterns across various grouting parameters. The findings in this study shed light on the intricacies of grouting performance influenced by grouting conditions, and provide essential guidance for grouting operations in practical engineering.

Effects of including fully wraparound geogrid layers on the load-bearing capacity and settlement of a strip footing resting on sandy soil

In this study, a series of small-scale physical model tests were conducted to investigate the effects of fully wrapped geogrid sheets on the load‒settlement response of medium-dense sandy soil beneath a single-strip foundation. Recent research has provided evidence of how different parameters affect the behavior of dense sandy soil when reinforced with fully wraparound geogrid sheets. Additionally, a parametric analysis was conducted to investigate the influence of various factors on the system. These factors include the presence of a folded geogrid sheet, the depth and length of an embedded wraparound geogrid sheet, the thickness of the fully wrapped geogrid layer, the impact of tensile strains along the geogrid sheet, and the effect of additional confinement pressure. The inclusion of a single sheet of full wraparound geogrid sheets has been found to significantly affect the pressure-bearing capacity of medium-dense sandy soil and settlement beneath the strip footing. The carrying capacity increases by 280%, and the settlement ratios decrease by 50% when using one layer of full wraparound geogrid. Moreover, when using one fully folded geogrid sheet, the strain induced beneath the center of the footing decreases significantly by approximately 45.5% and the applied pressure by approximately 15.5% in comparison with two inclusions of planar geogrid layers. In addition, the stress distributions are spread over a larger region within the soil mass. One significant finding is that the presence of two overlapping parts prevents the rupture of the geogrid sheet, as opposed to the planar geogrid sheet.

Research on the zoning of blasting for ultra-deep hole cylindrical charges based on thick-walled cylinder theory

To address the challenges faced by coal miners when encountering collapsed pillars in local hard rock formations, we researched ultra-deep hole loosening blasting technology. To overcome issues related to the inapplicability of existing blasting zoning theories and the lack of a solid foundation for borehole design in ultra-deep hole loosening blasting sites, we introduced a blasting zoning model and theoretical calculation formula based on thick-walled cylinder theory. Building on this foundation, we proposed coupled and uncoupled charge calculation methods that take into account strain rate variations and confining pressure effects. Furthermore, numerical simulations using the ANSYS/LS-DYNA software demonstrated that the results obtained from this theory were consistent with the simulation results. Ultimately, based on the blast zone results derived from both theory and simulation, we determined the blasting range of individual blast holes to design effective construction plans for ultra-deep hole blasting sites, achieving satisfactory results.

Study on high temperature dynamic characteristics and mesoscopic simulation of HFRC under dynamic and static combined loading

The Φ50mm SHPB system was used to carry out three-dimensional dynamic and static combined loading tests on hybrid fiber reinforced concrete (HFRC) treated at different temperatures (25 °C, 200 °C, 400 °C) under different confining pressure ratios (0, 0.1, 0.2, 0.3). The temperature effect, confining pressure effect and failure characteristics of HFRC under impact load were studied. The effect of fiber was equivalently treated in mortar, and the meso-aggregate model of HFRC considering aggregate, mortar and interfacial transition zone (ITZ) was established. The simulation results were compared with the experimental results. The results showed that the higher the confining pressure ratio, the more significant the temperature effect of peak stress is. The higher the temperature is, the less obvious the enhancement effect of confining pressure on peak stress is. Compared with the influence of confining pressure on dynamic peak strain, the influence of temperature effect on dynamic peak strain is higher. The established three-dimensional meso-aggregate model and calculation method can accurately reflect the dynamic response characteristics of HFRC at different temperatures and confining pressure ratios. Prestress will affect the internal stress distribution of the specimen, thus affecting the compression failure characteristics of the specimen. The confining pressure can constrain the damage course of the specimen and has a good protective effect on the specimen.

Research on petrophysical properties and porosity evolution of fractured coal mass under cyclic impact for coalbed methane exploitation

In the process of coalbed methane (CBM) extraction, coal seam penetration modification is frequently subjected to several cycle impact due to drilling-blasting method and deflagration fracturing method. Therefore, the split Hopkinson pressure bar (SHPB) was utilized to investigate the impact cycle effect and confining pressure effect on dynamic behavior of coal. Furthermore, the low-field nuclear magnetic resonance (NMR) was utilized to evaluate the modification of multiscale pore before and after 5 cycles impacts. Finally, the 3D profile scanner was utilized to quantify fracture surfaces and assess fracture roughness variation. The results showed that there existed the 6 MPa critical confining pressure that altered the dynamic mechanical properties of coal. Due to the combined effect of the confining pressure and cycle impact, the damage variable based on the energy method showed a log-normal distribution. With increasing strain rate, the micropores evolved into mesopores and macropores. There was a critical strain rate that caused the ratio of effective porosity to total porosity to shift from increasing to decreasing. Furthermore, the fracture roughness was shown to be positively correlated with the ratio and negatively correlated with seepage fractal dimension. The research findings can provide theoretical guidance for the safer and more efficient CBM exploitation.

Numerical study on damage response and failure mechanism of gas-containing coal-rock combination under confining pressure effect

The research on mechanics properties and destroy behavior of coal-rock combination is extremely crucial for deep coal mining. However, there are few studies on the gas-containing coal-rock combination under the confining pressure effect, resulting in insufficient comprehension of the failure mechanism of the combination with different confining pressure. In this paper, a discrete element method (DEM) model of combination with stress-gas coupling is established. The related mechanics properties of single coal samples and combination samples under stress-gas coupling are compared and studied, such as axial stress, crack generation and failure mode. The results demonstrate that the coal mass in the coal-rock combination is continuously damaged and compacted during the loading process with different confining pressure. The interfacial effect caused by the circumferential expansion or irregular deformation of coal mass, and the improvement of compression intensity of the compacting coal mass jointly induce damage of the rock mass. The combination with confining pressure 4, 7 and 10 MPa exhibit the macroscopic failure characteristics of coal mass dominant, coal and rock mass together, and rock mass dominant, respectively. Meanwhile, the confining pressure effect also greatly determines the formation of the final failure pattern of combination samples after macroscopic failure.

Research on the Response Mechanism of Coal Rock Mass under Stress and Pressure.

In this paper, the strength and deformation failure characteristics of bearing coal rock mass are related to the confining pressure, and the SAS-2000 experimental system is used to carry out uniaxial and 3, 6, and 9 MPa triaxial tests on coal rock to assess the strength and deformation failure characteristics of coal rock under different confining pressure conditions. The results show that the stress-strain curve of coal rock undergoes four evolutionary stages after fracture: compaction, elasticity, plasticity, and rupture. With confining pressure, the peak strength of coal rock increases, and the elastic modulus increases nonlinearly. The coal sample changes more with confining pressure, and the elastic modulus is generally smaller than that of fine sandstone. The stage of evolution under confining pressure constitutes the failure process of coal rock, with the stress of different evolution stages causing various degrees of damage to coal rock. In the initial compaction stage, the unique pore structure of the coal sample makes the confining pressure effect more apparent; the confining pressure makes the bearing capacity of the coal rock plastic stage stronger, the residual strength of the coal sample has a linear relationship with the confining pressure, and the residual strength of the fine sandstone has a nonlinear relationship with the confining pressure. Changing the confining pressure state will cause the two kinds of coal rock samples to change from brittle failure to plastic failure. Different coal rocks under uniaxial compression experience more brittle failure, and the overall degree of crushing is higher. The coal sample in the triaxial state experiences predominantly ductile fracture. The whole is relatively complete after failure as a shear failure occurs. The fine sandstone specimen experiences brittle failure. The degree of failure is low, and the confining pressure's effect on the coal sample is obvious.

Evolution of thermal cracking, temperature distribution and deformation at mineral scale of Beishan granite during heating and cooling

Thermo-mechanical property of rock is of great importance for the long-term safety of the high-level radioactive waste (HLW) geological disposal engineering. Thermal cracking, temperature and meso-deformation on the mineral grain size scale of Beishan granite were investigated during heating (160 °C ∼ 200 °C) and cooling process using online temperature, deformation and acoustic emission monitoring system. During the heating process, the temperature difference in different mineral grain was evident that the highest temperature was observed in feldspar while lower temperature in quartz and mica, and the temperature difference reached 20%. Meanwhile the feldspar expanded as the quartz and mica being compressed during heating. During the temperature holding and cooling process, the temperature difference between different mineral grain reduced and stabilized around 5%. The thermal induced expansion was recovered gradually but irreversible deformation was left, which also demonstrated by the nuclear magnetic resonance measurement that size and volume of large pore in granite both decreased after heated. The thermal cracking emerged from 25 °C ∼ 35 °C, and the thermal cracking activity was active once temperature changed during heating and cooling stage, which was calm during temperature holding stage. In addition, the cracking process was investigated by numerical modelling. The ‘confining pressure effect’ was observed during heating process, and the mechanism of thermal strengthening effect was discussed. Overall, the mild high temperature caused by decay heat of HLW could play positive role for the long-term safety of the repository, since the tightness and strength could be both enhanced by mild high temperature.