True Triaxial Compression Tests Research Articles

Influence of acoustic emission sequence length on intelligent identification accuracy of 3-D loaded rock’s fracture stage

Accurate prediction of impending disasters in underground projects is crucial and requires the identification of rock fracture stages. Currently, rock fractures are commonly analyzed using microseismic parameter statistics or individual waveform features for engineering disaster early warning systems. However, rock fracturing is a continuous process, and waveform sequences contain a wealth of information on fractures, which is often overlooked by existing research that generally neglects the information within continuous waveforms. In this study, we leverage acoustic emission (AE) data and employ a transfer learning approach with a convolutional neural network (CNN) to identify rock fracture stages under three-dimensional (3-D) stress paths induced by true triaxial compression tests. Failure experiments were performed on seven sandstone specimens under various 3-D stress paths. To fully utilize the characteristics of the crack rupture sequence, we introduce the concept of AE waveform sequence length. This concept integrates the discrete features of AE time–frequency images, thereby improving the CNN model’s performance. Utilizing waveforms of six different lengths (1, 2, 3, 5, 10, and 15) to train the neural networks, our findings reveal that a sequence length of 10 enables the CNN to effectively identify rock fracture stages under 3-D stresses with an accuracy rate of up to 90.4%. This demonstrates that appropriately increasing the sequence length to process the discrete features of AE waveforms structurally is a viable strategy to enhance CNN identification accuracy. Our results underscore that rock fracturing is a sequential process with significant inter-sequence correlations, which critically influence the CNN model’s ability to accurately identify rock fracture stages. These insights offer valuable theoretical contributions to the automatic monitoring of rock fracture stages in deep engineering projects.

Damage characteristics of rock induced by unloading in hole under true triaxial boundary stresses: Experimental study

In order to study the effect of unloading of underground rock excavation on the stability of surrounding rock, the true triaxial compression (TTC) and coupled true triaxial compression and in-hole loading-unloading tests (TTC-HU) were conducted on hollow cubic rock (100mm × 100mm × 100mm). The experimental results indicate that the compressive strengths of the specimens increase as confining pressure increases in the equal biaxial stressand increase with increasing intermediate principal stresses in the unequal biaxial stress. At low confining pressure (σ < 30 MPa), the internal stress at unloading damage in the hole of hollow cubic specimen increases with the increase of the three-directional boundary stresses. At high confining pressure (σ ⩾ 30 MPa), the specimen will not be damaged during the entire unloading process in the hole. The damage mode of the specimen is dominated by tensile damage. The shape of the holes in the specimens did not significant change, and hole wall of specimen have layered destruction along the circumference in low confining pressure. In high confining pressure, the hole area of the specimen undergoes significant deformation damage, forming a V-groove along the direction of the minimum principal stress, and the wall of the hole is completely peeled off. This study can reasonably predict the actual engineering damage patterns and guide the optimization of surrounding rock support.

Anisotropic characteristics and creep model for thin-layered rock under true triaxial compression

The failure phenomenon of thin-layered rock tunnels not only exhibits asymmetric spatial characteristics, but also significant time-dependent characteristics under high in-situ stress, which is attributed to the time-dependent fracture of thin-layered rocks. This paper conducted a series of true triaxial creep compression tests on typical thin-layered rock siliceous slate with acoustic emission technique to reveal its anisotropic time-dependent fracture characteristics. The anisotropic long-term strength, creep fracturing process, and fracture orientation characteristics of thin-layered rocks under different loading angles (β, ω) and intermediate principal stress were summarized. A three-dimensional (3D) non-linear visco-plastic creep model for thin-layered rock was developed to simulate its anisotropic creep behavior. The time-dependent fracturing of rocks during true triaxial creep loading is reflected through the change of equivalent strain based on an improved Euler iteration method. By constructing the plastic potential function and overstress index related to loading angles and stress state, the anisotropic time-dependent fracturing process and propagation of thin-layered rocks under different loading angles and intermediate principal stress are expounded. The model was validated experimentally to show it can reflect the long-term strength and creep deformation characteristics of thin-layered rocks under true triaxial compression.

Experimental investigation of the intensity and conditions for strainburst triggering in intact hard rocks

Strainburst occurs during deep hard rock excavations. A series of single-free-face true triaxial compression (TTC) tests were conducted on seven hard rock types to investigate strainburst intensity and triggering conditions. Five of the seven rock types failed by strainburst, and two failed by spalling. The differences between the strainburst intensities are attributed to the mineral composition and structure. High-hardness minerals and mosaic-type microstructures facilitate stress conduction, which promotes rock microcrack initiation and propagation, causing severe rock fragmentation and fragment ejection, as observed for the Jinzhou granite. However, if the proportions of high- and low-hardness rock minerals are high and the microstructure is stable, such as in the triangular microstructure of the Chifeng basalt, then microcrack initiation and propagation are inhibited, decreasing the likelihood of rock fragmentation. The triggering conditions for strainburst were further analysed by considering the source of kinetic energy. For intact rock specimens, the kinetic energy for rock fragment ejection mainly originates from the intrinsic potential energy (Wbk) released by the specimen itself; thus, strainburst triggering requires that the applied maximum principal stress exceeds the bearing capacity and that the rock Wbk exceeds zero. However, a high Wbk is needed to trigger strainburst for structural rock specimens. Based on these findings, the possibility of triggering strainburst during excavation is greater when the Wbk (>9.1 kJ/m3), rock quality designation (>75%), and in situ stress of the rock are all high.

Influence of orientation of the intermediate principal stress on fracture reactivation in granite

Fracture (fault) reactivation can lead to dynamic geological hazards including earthquakes, rock collapses, landslides, and rock bursts. True triaxial compression tests were conducted to analyze the fracture reactivation process under two different orientations of σ2, i.e. σ2 parallel to the fracture plane (Scheme 2) and σ2 cutting through the fracture plane (Scheme 3), under varying σ3 from 10 MPa to 40 MPa. The peak or fracture reactivation strength, deformation, failure mode, and post-peak mechanical behavior of intact (Scheme 1) and pre-fractured (Schemes 2 and 3) specimens were also compared. Results show that for intact specimens, the stress remains nearly constant in the residual sliding stage with no stick-slip, and the newly formed fracture surface only propagates along the σ2 direction when σ3 ranges from 10 MPa to 30 MPa, while it extends along both σ2 and σ3 directions when σ3 increases to 40 MPa; for the pre-fractured specimens, the fractures are usually reactivated under all the σ3 levels in Scheme 2, but fracture reactivation only occurs when σ3 is greater than 25 MPa in Scheme 3, below which new faulting traversing the original macro fracture occurs. In all the test schemes, both ε2 and ε3 experience an accumulative process of elongation, after which an abrupt change occurs at the point of the final failure; the degree of this change is dependent on the orientation of the new faulting or the slip direction of the original fracture, and it is generally more than 10 times larger in the slip direction of the original fracture than in the non-slip direction. Besides, the differential stress (peak stress) required for reactivation and the post-peak stress drop increase with increasing σ3. Post-peak stress drop and residual strength in Scheme 3 are generally greater than those in Scheme 2 at the same σ3 value. Our study clearly shows that intermediate principal stress orientation not only affects the fracture reactivation strength but also influences the slip deformation and failure modes. These new findings facilitate the mitigation of dynamic geological hazards associated with fracture and fault slip.

Temperature effect of rockburst in granite caverns: insights from reduced-scale model true-triaxial test

To investigate the influence of thermal treatment on the rockburst in granite caverns, true-triaxial compression tests were conducted on pre-heated granite cubic samples containing a circular through-hole using a true-triaxial test system, and the micro camera was used to monitor and record the rockburst process in realtime. Test results show that the uniaxial compressive strength and elastic modulus first increase and then decrease as the temperature increases, which reach the maximums at 200 °C and sharply decrease at temperatures from 400 to 600 °C. The density and P-wave velocity decrease and the peak strain increases with increasing temperature. The main failure mode is X-shaped shear-tension failure at 25 and 200 °C, and single-slope shear failure at 400 and 600 °C. Thermal treatment exhibits slight effect on the rockburst incubation process in granite caverns. The stress required for rockburst decreases when the temperature increases or decreases from 200 °C. The higher the temperature, the lower the required stress. At 25 and 200 °C, rockburst is violent, and fine particles and large slabs are mainly produced; at 400 and 600 °C, the intensity of rockburst is relatively weak, and fine particles are mainly generated. Thermal treatment exhibits double effects on rockburst. The mechanism of thermal treatment on granite rockburst is the strengthening effect caused by water loss and the weakening effect caused by thermal expansion cracking. This study can provide theoretical guidance for the stability analysis and rockburst prevention of temperature-affected granite caverns.

Failure process and characteristics of deep concrete-supported arch tunnel under true-triaxial stress

In deep underground rock engineering, tunnel structures after concrete support are often subjected to failure because of stress variation. In this study, to understand the failure process and characteristics of deep concrete-supported tunnel, true-triaxial compression tests were conducted on concrete-supported and unsupported tunnel model specimens using a true-triaxial test system. An AE system and a microcamera were used to capture the AE events and failure evolution process of the hole sidewalls in rock specimens, respectively. The test results show that after concrete support, the failure characteristics of the hole sidewalls are converted from dynamic rockburst failure to static spalling failure, and the initial vertical failure stress of the hole sidewalls is significantly increased. The failure of the hole sidewalls in the concrete-supported tunnel model specimen experiences the calm stage, the cracking and buckling of concrete-support layer stage, the spalling of concrete-support layer stage, and the spalling of concrete-support layer and rock layer stage. The cumulative AE counts of the concrete-supported tunnel model specimen are less than those of the unsupported one at any same time during loading. Comparing the failure processes and characteristics of hole sidewalls in the two types of specimens, the curbing effect of concrete support on the failure of tunnel is confirmed. The results of this study contribute to the understanding of the failure behavior of deep rock tunnels with concrete support.

Intermittent disturbance mechanical behavior and fractional deterioration mechanical model of rock under complex true triaxial stress paths

Mechanical excavation, blasting, adjacent rockburst and fracture slip that occur during mining excavation impose dynamic loads on the rock mass, leading to further fracture of damaged surrounding rock in three-dimensional high-stress and even causing disasters. Therefore, a novel complex true triaxial static-dynamic combined loading method reflecting underground excavation damage and then frequent intermittent disturbance failure is proposed. True triaxial static compression and intermittent disturbance tests are carried out on monzogabbro. The effects of intermediate principal stress and amplitude on the strength characteristics, deformation characteristics, failure characteristics, and precursors of monzogabbro are analyzed, intermediate principal stress and amplitude increase monzogabbro strength and tensile fracture mechanism. Rapid increases in microseismic parameters during rock loading can be precursors for intermittent rock disturbance. Based on the experimental result, the new damage fractional elements and method with considering crack initiation stress and crack unstable stress as initiation and acceleration condition of intermittent disturbance irreversible deformation are proposed. A novel three-dimensional disturbance fractional deterioration model considering the intermediate principal stress effect and intermittent disturbance damage effect is established, and the model predicted results align well with the experimental results. The sensitivity of stress states and model parameters is further explored, and the intermittent disturbance behaviors at different f are predicted. This study provides valuable theoretical bases for the stability analysis of deep mining engineering under dynamic loads.

Behavior of ultra-high performance concrete under true tri-axial compression

Confinements have been adopted to alleviate the brittleness of ultra-high performance concrete (UHPC). However, as the foundation of confinement mechanism, the research on behavior of UHPC under tri-axial compression is rather limited. Furthermore, the confinement mechanism of PE fiber-strengthened UHPC (PE-UHPC) has not been reported yet. To this end, conventional and true tri-axial compression tests were performed for 22 UHPC cubes with different PE fiber lengths and various lateral confining stresses. The typical axial stress-strain curve has a long elastic ascending stage, a relatively short plastic ascending stage and a steep post-peak decline stage. Increasing the lateral confining stresses could extend the elastic ascending stage, and make the plastic ascending stage longer and clearer. Furthermore, the reinforcement effect of longer PE fibers became more pronounced with increasing lateral confining stresses. However, only increasing intermediate principal stresses with minor principal stresses unchanged had minor effect on the axial strain-volumetric strain behavior.

Cracking property and brittleness evaluation of granite under high-temperature true triaxial compression in geothermal systems

Cracking property and brittleness are critically important to the drillability of injection and production wells of enhanced geothermal systems. This paper is devoted to evaluating the cracking property and brittleness of the Gonghe granite under high-temperature true triaxial compression conditions through a series of laboratory tests. Thermal–mechanical coupled true triaxial compression tests were conducted on Gonghe granite samples under four representative temperatures (20 °C, 120 °C, 180 °C, and 240 °C) and three different minimum (5, 10, and 30 MPa) and intermediate (40, 60, and 75 MPa) principal stresses that correspond to the in-situ conditions of Gonghe geothermal reservoir. The strength, deformation, and cracking characteristics of the Gonghe granite are quantitatively evaluated from macro- to micro-scales based on the experimental results. Meanwhile, a novel brittleness index evaluation method considering both energy and failure surface roughness is proposed to accurately assess the brittleness strength of the Gonghe granite. We found that the strength of Gonghe granite is reduced by more than 20% when the temperature increases from room temperature to 240 °C. Higher temperature contributes to a smoother fracture surface and reduced brittleness of the Gonghe granite under thermal–mechanical coupled true triaxial compression. Our findings provide new insights for brittleness evaluation of the Gonghe granite formation and assist in efficient wells drilling design of enhanced geothermal systems.

A novel anisotropic deterioration mechanical model for rock ductile–brittle failure under 3D high-stress and its application

After the excavation of deep underground engineering, the three-dimensional (3D) stress state redistributes, leading to a significant change in the mechanical properties of the surrounding rock; however, the mechanical properties and parameter evolutions of rock under true triaxial stress are not clear, and theoretical mechanical models characterizing the above rock properties for surrounding rock stability analysis are very deficient. Therefore, in this research, a series of true triaxial compression tests and cyclic loading and unloading tests were carried out to investigate 3D stress-induced anisotropic behaviours, ductile–brittle characteristics and mechanical parameter evolutions. The determination methods of the mechanical parameters of Young’s modulus Ei, cohesion c, internal friction angle φ and dilation angle ψ in the rock failure process under true triaxial stresses were established, and the evolution functions of the mechanical parameters were proposed. A 3D degradation mechanical model for rock ductile–brittle and anisotropic behaviours under true triaxial stress was established. A numerical calculation program of the proposed model was implemented, and the simulation results agreed with the experimental results. The engineering application of the proposed model was further investigated with an improved index of rock fracture degree for the localized asymmetric failure of deep surrounding rock after excavation.

Experimental study on failure evolution mechanism of clastic rock considering cementation and intermediate principal stress

The study of clastic rock failure evolution under true triaxial stress is an important research topic; however, it is rarely studied systematically due to the limitation of monitoring technology. In this study, true triaxial compression tests were conducted on clastic rock specimens to investigate the effect of cementation and intermediate principal stress (σ2) on the failure mechanism. The complete stress–strain curves were obtained, while the acoustic emission (AE) was monitored to indirectly evaluate the evolution of tensile and shear cracks, and crack evolution under true triaxial compression was imaged in real time by a high-speed camera. The results showed that the deformation and failure characteristics of clastic rock were closely related to the cementation type and intermediate principal stress. On the basis of the distribution characteristics of the ratio of rise time to amplitude (RA) and the average frequency (AF) of AE signals, tensile cracks of the contact cementation specimen propagated preferentially. Meanwhile, the enhancement of specimen cementation promoted the evolution of shear cracks, and the increase in σ2 promoted the evolution of tensile cracks. Moreover, the mesoscale cracking mechanism of clastic rock caused by cementation and σ2 under true triaxial compression was analyzed. The failure patterns of clastic rock under true triaxial compression were divided into three modes: structure-induced, structure-stress-induced and stress-induced failures. This study confirms the feasibility of high-speed camera technology in true triaxial testing, and has important implications for elucidating the disaster mechanism of deep tunnels in weak rocks.

The effect of the fracture geometries from different stratification angle on the imbibition recovery of Jimusar shale oil: A comprehensive experimental study

Hydraulic fracturing technology is one of the most effective stimulation means in the development of shale oil. Due to the stratification structure of shale, the fracture geometries after hydraulic fracturing will be seriously affected by the stratification angle, which will further effect the imbibition recovery of shale oil. At present, scholars have carried out a large number of research on the influence of stratification angles on fracture morphology. However, few scholars have studied the influence of fracture morphology on imbibition recovery under different stratification angles. Based on the Jimsar shale, this paper comprehensively explored the fracture morphology under four stratification angles and its influence on imbibition recovery. Twelve shale cores were subjected to true triaxial compression test, non-destructive CT scan and NMR-based imbibition test to obtain acoustic emission signals and energy during fracturing, quantitative information of CT scan fracture morphology and corresponding imbibition recovery. The results showed that, first of all, shale rock samples with stratification angles below 30° have strong brittle failure characteristics under 0 MPa confining pressure. When the stratification angle is higher than 60°, the brittleness of the rock sample is weakened under 20 MPa confining pressure. Secondly, when the confining pressure is 0 MPa and 10 MPa, the fracture volume decreases first and then increases with the increase of stratification angle, showing a “V” shape change rule. When the confining pressure is 20 MPa, the fracture volume decreases with the increase of stratification angle. At the same time, the imbibition experiment verifies that the oil recovery of rock sample with dominant seepage channel is 47.61% higher than that without dominant seepage channel. Finally, for shale reservoirs, hydraulic fracturing perforation along the angle of stratification development can effectively improve the efficiency of energy utilization, increase the effective fracture volume of fracturing, and further improve oil recovery.

Influence of Spherical Caverns on the Failure Characteristics of Neighboring Tunnels under True Triaxial Conditions: Insights from an Experimental Test and Discrete Element Simulation

Caverns are generally formed by a combination of regional geological action and groundwater, and their improper treatment will inevitably lead to dangerous conditions in underground works. To detect the specific failure mechanism of tunnel-surrounding rock induced by invisible caverns, a true triaxial compression test is conducted, accompanied by acoustic emission technology and an internal borehole camera, for monitoring the acoustic response and visible secondary cracks, and a corresponding DEM simulation is carried out to reveal the meso-mechanism. The results indicate the following: (1) The invisible cavern demonstrates a negative influence on the stability of the tunnel and leads to a 25.82% reduction in the peak z-axis load of the specimens. (2) The acoustic emission results show that the relatively severe dominant failures mainly occur near the peak stress in all types of specimens, and the speed and intensity of the cavern-existing specimen is significantly greater than that of the cavern-free specimen. (3) The cavity-free tunnel shows mirror-symmetric splitting failure on the left and right sidewalls, while the secondary cracks appear earlier and show asymmetrical distribution in the cavern-existing specimen, and the volume of broken rock blocks near the free surface is larger. (4) The cavern directly changes the failure process of the tunnel-surrounding rock (intermediate rock failure occurs earlier than splitting failure), the distribution of principal stress, and the corresponding mechanism of secondary failures. (5) Application of the displacement and velocity trend fields helped to reveal accurate failure procedures in the true triaxial test.

Ductile–brittle quantitative evaluation of rock based on post-peak properties under true triaxial stress

To quantitatively evaluate the influence of high true three-dimensional stresses on the ductile–brittle behaviours of rock in deep underground engineering, a series of true triaxial compression tests with different stress levels were carried out on marble and four kinds of granite. The influences of true triaxial stress states (σ2, σ3) on the post-peak characteristics were analysed, and a new normalized ductile–brittle evaluation index was proposed based on post-peak energy conversion characteristics of rock under true triaxial stresses. The ductile–brittle behaviours of rock were divided into four qualitative levels, namely, ductile-brittleness, transitional, brittleness and super-brittleness, and the influences of true triaxial stress states on the ductile–brittle behaviours of rock were quantitatively investigated. The research shows that as σ2 increases or σ3 decreases (that is, the differential stress between σ2 and σ3 increases), the brittleness of rock increases, and its increase rate gradually decreases and tends to be stable, transforming from ductile-brittleness to transitional, brittleness and super-brittleness and resulting in super-brittleness being easily induced by low-σ3 and high-σ2 conditions. When the differential stress between σ2 and σ3 is small, the intrinsic characteristics of rock itself have an obvious influence on ductile–brittle behaviours. When the differential stress between σ2 and σ3 is large, all kinds of rocks can exhibit super-brittle behaviour. The change of stress controls the evolution of rock ductile–brittle behaviours, and high-stress controls rock brittleness. The rock brittleness under true triaxial stress is significantly higher than that under conventional triaxial stress at the same σ3. σ2 induces an increase in rock brittleness and causes the decay rate of brittleness to decrease with increasing σ3, and σ2 increases the upper limit of σ3 for brittle failure of rock. The enhancement effect of σ2 on rock brittleness must be considered when evaluating the brittle failure of deep surrounding rock under high-stress conditions; otherwise, the risk of brittle failure may be underestimated.

Brittle failure modes of underground powerhouses: an insight based on true triaxial compression tests

Brittle failure modes of underground powerhouses: an insight based on true triaxial compression tests

A constitutive model for caprock siltstone subject to three-dimensional compression with emphasis on plastic potential and hardening law

Investigation of the strength, deformation and constitutive model of rocks under true three-dimensional stress is of much significance to scientifically and accurately predict the integrity of underground gas storage caprock. In general the intermediate principal stress and the minimum principal stress to which the caprock is subjected are not equal, and there is uncertainty in the in-situ stress test. In this study, the siltstone is taken from the caprock of the Hutubi gas storage in Xinjiang, with a buried depth of 3600 m. A series of true triaxial compression tests on siltstones were carried out using a true triaxial testing system for rocks, and the strength and deformation characteristics of siltstones under different intermediate principal stress conditions were studied. The elastoplastic constitutive model of rocks has been established: in the plastic part, the Mogi-Coulomb yield criterion has been modified so as to obtain the triple-mean-normal-stress yield function and triple-mean-normal-stress potential function suitable for rocks under the true three-dimensional stress state. The hardening law has been determined by fitting the relationship between the plastic deviatoric shear strain and the deviatoric stress in the true triaxial compression test. The constitutive model and the test data of siltstones have been verified by fitting. Finally, the influence of true triaxial stress on the permeability and breakthrough pressure of caprock siltstones has been discussed. The result indicates that with the increase of intermediate principal stress within a certain range, the peak strength of siltstones will also increase, and deformation in the direction of the intermediate principal stress will be restrained. Compared with the potential functions in other constitutive models, the triple-mean-normal-stress potential function proposed in this paper can accurately describe the plastic flow direction of siltstones under the true three-dimensional stress state. And the hardening law truly reflects the plastic deformation of siltstone specimens under different stress states during the loading process. The calculation result of the constitutive model is in good agreement with the test result. Under the true three-dimensional stress, the permeability of siltstones shows anisotropy. With the increase of the intermediate principal stress, the minimum value of siltstone permeability decreased by 8%, and the maximum value of breakthrough pressure increased by 4.3%.

Experimental Study on the Mechanical Behavior of Sandy Soil Reinforced by Disposable Face Mask Chips under Different Stress Paths

Since 2020, with the global spread of major respiratory infectious diseases, such as COVID–19, the demand and consumption of personal protective equipment, such as masks, have increased dramatically worldwide. The environmental pollution caused by numerous waste disposable face masks has gradually attracted people’s attention. In this study, the mechanical properties of mask–chip–reinforced soil are evaluated from a new perspective, through the uniaxial, biaxial, conventional triaxial, and true triaxial compression tests on reshaped sandy soil samples mixed with different contents of mask chips. The experimental results show that the mechanical properties of the sandy soil can be improved by the mask chips. With the proper content of mask chips, the failure strength is substantially improved, and the failure of soil is delayed. Meanwhile, the strength and stiffness are significantly affected by the stress path and the content of mask chips, even if the soil samples with the same mask–chip content can also show different mechanical properties under different stress paths. Additionally, the mechanical properties of soil are not necessarily improved constantly with the increasing content of mask chips. The failure strength of sandy soil samples under conventional and true triaxial stress paths decreases when the mass content of mask chips exceeds 0.3% and 0.5%, respectively. This study confirms the potential of mask chips applied to subgrade, slope, and other engineering construction fields in a sustainable way.

Coupled Thermo-Mechanical Study of Crack Initiation Stress Threshold in Porous Sandstone

The increasing depth of tunnels and underground mining operations worldwide has resulted in the rise of deep mines at temperatures well above 50°C. Such unwanted thermal gradients not only adversely impact the physical and mechanical behaviours of the rock mass but could also affect the safety of underground mines leading to temporarily and permanently shutting down the operation. The combination of high temperature and high-stress conditions further culminates in explosion-like fractures like rockburst and spalling, which are the main problems in deep mining and tunnel constructions. Spalling is identified as an abrupt explosion-like rock failure that occurs spontaneously and can endanger tunnelling and mining operations, causing injuries and fatalities, and necessitating investigation of the effect of temperature on rock behaviour. It is known that the spalling strength is correlated with the internal crack initiation in low-porosity and brittle crystalline rocks. This paper investigates this phenomenon and examines the influence of coupled thermo-mechanical conditions on the crack initiation stress level of sandstone samples. True triaxial compression tests were carried out on 50 mm sandstone cubes from room temperature (25°C) to 100°C using a high temperature true triaxial testing facility. The results suggest that temperature can have a significant impact on crack initiation and the spalling strength of rock, particularly at low confinement.

Constitutive relations, mechanical behaviour, and failure criterion of microcapsule-based self-healing concrete under uniaxial and triaxial compression

Microcapsule-based self-healing concrete can autonomously release repairing agents to bond and seal cracks when the material is damaged. It is seen as a promising construction material from the perspective of a sustainable society. In addition, it is important to study mechanical behaviour under complex stress conditions to promote the application of self-healing concrete in engineering structures. This study investigated the constitutive relations, strength and deformation, and failure criterion of the epoxy/urea-formaldehyde microcapsule-based self-healing concrete through uniaxial compression, conventional triaxial compression and true triaxial compression tests, in which the influence of different microcapsule dosages, confinement pressures and the principal stress ratios on the mechanical behaviours was considered. The results showed that the ultimate strength of the specimen decreased with increasing microcapsule dosage under all loading conditions. The lateral pressure significantly promoted the ultimate strength and deformation capacity of the specimen, and this increase mainly depended on the maximum principal stress (σ1) rather than the intermediate principal stress (σ2). By visual checking the failure modes of the specimens, more cracks were observed under the conventional triaxial compression than those under the true triaxial compression. This multi-crack failure mode allowed more core material of microcapsules to enter the microcracks, which accordingly increased the strength of the microcapsule-based concrete.