Sandstone Specimens Research Articles

Physical and Mechanical Properties and Damage Mechanism of Sandstone at High Temperatures

The physical and mechanical properties of rocks change significantly after being subjected to high temperatures, which poses safety hazards to underground projects such as coal underground gasification. In order to investigate the effect of temperature on the macroscopic and microscopic properties of rocks, this paper has taken sandstone as the research object and conducted uniaxial compression tests on sandstone specimens at different temperatures (20–1000 °C) and different heating rates (5–30 °C/min). At the same time, the acoustic emission (AE) test system was used to observe the acoustic emission characteristics of the rock damage process, and the microstructural changes after high temperature were analyzed with the help of a scanning electron microscope (SEM). The test results show that the effect of temperature on sandstone is mainly divided into three stages: Stage I (20–500 °C) is the strengthening zone, the evaporation of water and the contraction of primary fissures, and sandstone densification is enhanced. In particular, the compressive strength and elastic modulus increase, the macroscopic damage mode is dominated by shear damage, and the fracture micromorphology is mainly brittle fracture. Stage II (500–600 °C) is the transition zone, 500 °C is the threshold temperature for the compressive strength and modulus of elasticity, and the damage mode changes from shear to cleavage damage, and the sandstone undergoes brittle–ductile transition in this temperature interval. Stage III is the physicochemical deterioration stage. The changes in the physical and chemical properties make the sandstone compressive strength and modulus of elasticity continue to decline, the macroscopic damage mode is mainly dominated by cleavage damage, and the fracture microscopic morphology is of a more toughness fracture. The effect of different heating rates on the mechanical properties of sandstone was further studied, and it was found that the mechanical properties of the rock further deteriorated under higher heating rates.

Microstructure Imaging and Characterization of Rocks Subjected to Liquid Nitrogen Cooling

Liquid nitrogen (LN2) fracturing is a potential stimulation method in unconventional hydrocarbon recovery, showing its merits in being water free, creating low formation damage and being environmentally friendly. The microstructure evolution of rocks subjected to LN2 cooling is a fundamental concern for the engineering application of LN2 fracturing. In this paper, pore-scale imaging and characterization were performed on two rocks, i.e., tight sandstone and coal specimens subjected to LN2 cooling using computed tomography scanning. The digital core technique was employed to reconstruct the microstructures of rocks and give a quantitative analysis of the pore structure evolution of both dry and water-saturated rocks. The results indicate that LN2 cooling has a great effect on the pores’ morphology and their spatial distribution, leading to a great improvement in pore diameter and aspect ratio. When compared to the sandstone, coal is more sensitive to LN2 cooling and thermal stresses, having a more noticeable growth in pore–throat size. The porosity growth of coal is 291% higher than that of sandstone. There is a growing trend in the irregularity and complexity of pore structures. After LN2 cooling, the fractal dimensions of the pores of sandstone and coal grow by 11.7% and 0.87%, respectively, and the proportion of pores with a shape factor > 100 increases. More bundle-like and strip-shape pores with multiple branches are generated, which causes a significant growth in the throat size and the proportion of connected pores with a coordination number ≥ 1, enhancing the complexity and connectivity of pore structures dramatically. Additionally, pore water plays an important role in aggravating rock damage during LN2 cooling, enhancing the pore space and connectivity. The porosities of the saturated sandstone and coal samples grow by 22.6% and 490.4%, respectively, after LN2 cooling, which are 5.6% and 186.6% higher than dry samples. The generation of macropores ≥ 70 μm is the primary contributor to porosity growth during LN2 cooling, although such pores account for only a small proportion of the total. These findings contribute to our understanding of the microscopic mechanism of LN2 cooling on rock damage and may provide some guidance for the engineering application of LN2 fracturing.

Effect of internal fractures on mechanical properties and failure of sandstone under multi-physical fields

Under deep mining conditions, rocks are subjected to complex multi-physical fields and can contain numerous pores and fractures. To explore the influence and correlation of these factors on the physical and mechanical properties of fractured rock samples, this study conducted triaxial compression tests on sandstone specimens under various physical conditions using a rock full stress multi-field coupling triaxial tester. Additionally, a random fracture model for multi-field coupling numerical simulation was established. This allowed the study to obtain the mechanical parameters, failure mode, and internal fracture development of rocks under multi-physical field conditions. By analyzing the complete stress-strain curve, mechanical characteristic points, and permeability, a combination of laboratory tests and numerical simulations was used to examine how temperature, seepage, and stress fields affect the development of pores and fractures in rocks. It was found that the temperature field, under conventional geothermal conditions, generates tensile force through thermal expansion and the presence of fluid, thereby promoting fracture development within the rocks. This mechanism is similar to that of seepage. The confining pressure caused by deep geo stress uniformly inhibits the expansion of pores and fissures within the rocks.

Experimental study on the strain energy evolution mechanism of thick and hard sandstone roof in Xinjiang Mining Area

For understanding the mechanical performance and strain energy evolution mechanism of thick hard roof sandstone samples, a sequence of uniaxial compression trials with acoustic emission (AE) monitoring were carried out. The results indicate: (1) The stress-strain curve of the thick hard roof sandstone specimens exhibits distinct stage characteristics. Based on the evolvement of instantaneous axial stiffness, it is separated into Fracture Closure Phase, Elastic Deformation Phase, Steady Fracture Expansion Phase, Unsteady Fracture Expansion Phase, and Post-Peak Phase. (2) The AE energy and cumulative count curves of the thick hard roof sandstone specimens also exhibit significant stage characteristics and can be mutually corroborated with the stage division of the stress-strain curve. (3) Based on the energy conservation principle, the evolution of strain energy density in the thick hard roof sandstone specimens under uniaxial compression loading was analyzed, and plastic strain energy increment was employed to study the stage characteristics of strain energy dissipation. (4) A damage constitutive model for the thick hard roof sandstone specimens was constructed, considering the characteristics of strain energy dissipation. This model effectively describes the stress-strain relationship among the samples, which undergo strain hardening, strain softening, and sudden destruction.

A novel testing method for examining mode II fracture of rock and its application

Mode II fracture often occurs in rock, which often result in violent ruptures of rock and catastrophic failure of rock engineering. However, attempts to replicate mode II crack extension on a laboratory scale generally fail due to the existence of tensile stress at the notch tip. In this study, a novel testing method named the Z-shaped centre cracked direct shear (ZCCDS) test was developed to examine the mode II fracture of rock, and a series of tests were performed on sandstone with various dislocation ratios (α1) and crack ratios (α2). The equation for calculating the mode II fracture toughness (KIIC) of rock was determined using the finite element method. The validity and reliability of this test method were verified by digital image correlation analysis and comparison with KIIC values determined by the other methods. The testing results showed that the peak shear load changes slightly with increasing α1 and decreases with increasing α2. The mode I stress intensity factor (SIF) decreases while mode II SIF increases with increasing α1 or α2. When α1 = 0.25 and α2 = 0.3, relative sliding deformation at the notch tip was captured via displacement vector analysis, corresponding to a true KIIC of 4.16 MPa∙m0.5. In addition, the effect of α1 and α2 on the failure characteristics of ZCCDS sandstone specimens is slight, and the pure mode II crack of the sandstone satisfies “self-similar extension”. The fracture surface becomes smoother with increasing contribution of the mode II component due to the wide distribution of transgranular fractures. The findings of this study can facilitate better understanding of mode II fracture behaviour of rock.

Deformation localization and crack propagation of sandstone containing different flaw inclination angles under different loading rates

Defects with varying geometric distribution in the rock mass play a crucial role in determining the stability of engineered rock masses. Previous studies have primarily investigated the initiation, propagation, and coalescence behaviors of flaws within rock masses. Nevertheless, there has been limited analysis of flawed rock masses under different loading rates. In this study, we present the deformation localization and cracking process of three types of inclined flawed sandstone specimens through conducting uniaxial compression and acoustic emission tests at four different levels of loading rate. The results supported the following findings: 1) Sandstone specimens with different flaw inclination angles exhibit the loading rate strengthening effect, and the strengthening effect gradually decreases with the increase of loading rate. 2) As the loading rate increases, the type of crack emergence changes from wing cracks to anti-tensile cracks, and the time of flaw initiation is shortened. 3) The cumulative acoustic emission counts were higher for the low-loading rate specimens than for the high-loading rate specimens. 4) Tensile cracks typically occur as the initial cracks. Anti-tensile cracks often coexist with wing cracks in rock specimens that have undergone tensile damage. Coplanar secondary cracks are the primary indication of shear damage formation in rock specimens. 5) The increase in loading rate promotes the transition of rock specimens from the mixed tensile-shear damage mode to the shear damage mode. These research results are of great theoretical significance and engineering value for understanding the failure mechanism of rock mass containing flaws and proposing effective measures to prevent cracking and ensure the safety of brittle solid structures.

Experimental study on mechanical and acoustic emission characteristics of sedimentary sandstone under different loading rates

In the field of rock engineering, complexity of stress environment is an important factor affecting its stability. Thus, in view of fracture mechanism of rock under different loading rates within the scope of quasi-static strain rate, four groups of uniaxial compression tests with different strain rates were carried out on sandstone specimens, and strength, deformation, failure modes and acoustic emission characteristics of specimens were compared and analyzed. Furthermore, the fracture mechanism was discussed from the perspective of fracture characteristics based on fractal dimension, crack propagation law inverted through acoustic emission b-value, and micro fracture morphology. The results showed that as the strain rate increased from 10 to 5 s−1 to 10−2 s−1, the fractal dimension of rock fragments increased, and the fractal dimension of rock fragments increased by 9.66%, 7.32%, and 3.77% successively for every 10 times increase in strain rate, which means that the equivalent size of fragments was getting smaller, and the fragmentation feature was becoming increasingly prominent. The crack propagation process based on acoustic emission b-value showed that with the increase of loading rate, the specimen entered the rapid crack propagation stage earlier, in order of 68%, 66%, 29%, and 22% of peak stress. Moreover, the microscopic fracture morphology showed that with the increase of loading rate, transgranular phenomenon was clear, and the fracture morphology changed from smooth to rough. That meant that the fracture of sandstone rock at high loading rates was mainly caused by the propagation of large cracks, which was different from the slow process of initiation, convergence and re-propagation of small cracks at low strain rates.

Experimental study of volumetric sand production with gas flow

The presence of sand in a production stream has been a common issue for wells drilled in high permeability sandstone formations. In the case of gas producing reservoirs, sand production can be particularly hazardous due to the high gas velocities. It can among others lead to erosion of downhole components and surface equipment and to significant production downtime. A comprehensive experimental study was conducted to investigate the effects of gas flow on sand production. This was accomplished by comparing gas flow tests to tests with oil or water flow. In these tests, hollow cylinder sandstone specimens, initially wet or at ambient humidity conditions, were isotropically compressed under simultaneous fluid injection. One-phase as well as two-phase flow experiments were carried out to simulate important processes encountered in gas fields, like water capillary flow or water-cut. The results showed that sand onset was delayed in the gas flow experiments, which agrees with experimental observations in the literature. The delayed sand onset, i.e., higher sand onset stress, was related to water evaporation due to gas flow, termed flow-through drying, which strengthens the rock. The water-sensitive strength of sandstones was also examined by completing a series of uniaxial compression tests at various water saturations. The analysis of the volumetric sand production data and the X-ray CT-scans of the tested specimens showed that the decreased capillary bonds between grains when only one fluid is present in the pores, e.g., one-phase water or gas flow, results in continuous sand production of individual grains or small-sized flakes, creating large voids in the vicinity of the hole. Under such conditions, the constantly enlarging failure zone eventually causes the total collapse of the specimen. In contrast, specimens with higher capillary cohesion due to a water-oil or water-gas interphase show post sand onset stability and lower sand rate where, due to higher grain to grain cohesion, chunks of failed rock material remain in the hole and provide additional support that prevents the rapid collapse of the specimen even as the applied stress increases.

Investigation of high-temperature effects on the strengthening and degradation of mechanical property in sandstone

The underground coal gasification (UCG) mining technology constitutes an innovative form of clean energy technology pertaining to coal resources. The sandstone specimens from the roof and floor of coal seam were collected and heat treated at 25 °C–800 °C in laboratory. According to the test results of physical parameters, 400 °C is the temperature turning point for the physical parameters such as mass, longitudinal P-wave velocity, porosity, and water absorption of sandstone specimens to change from “slow” to “drastic”. The uniaxial compression test showed that the uniaxial compressive strength (UCS) of sandstone specimens decreases by 3.22% when the temperature rises from 25 °C to 200 °C, and by 5.70% when the temperature rises from 600 °C to 800 °C, both of which show temperature degradation effect. However, in the range of 200 °C–600 °C, the average UCS of sandstone specimens increases from 122.80 MPa to 162.54 MPa, showing obvious strength strengthening effect. In order to fully elucidate the mechanism of mechanical properties strengthening and degradation of tunnel surrounding rock under high temperature, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were combined in the experiment. We concluded that these two intensity effects can be explained by physical and chemical reactions, including mineral composition thermal expansion, thermally induced microcrack and healing effects, phase transformation of the rock-forming mineral quartz, dehydration and thermal decomposition of mica, as well as dehydration and dehydroxylation of illite. The research results can provide theoretical basis for UCG roof and floor disaster prevention evaluation and tunnel support system design.

Study on dynamic characteristics of sandstone-damaged by loading and unloading

To investigate the impact of unloading on the dynamic mechanical properties of surrounding rock in roadway excavation, sandstone specimens were subjected to loading and unloading at six levels ranging from 0% to 80% of the uniaxial compressive strength (UCS). Subsequently, an improved SHPB was employed to perform dynamic compression tests, aiming to analyze the mechanical characteristics of the sandstone at various degrees of loading and unloading damage, investigate fractal attributes, and examine the dynamic failure process. The study uncovered that the stress–strain curve exhibits three distinct stages. Correspondingly, a more suitable set of dynamic elastic modulus denoted as Ed1, Ed2, and Ed3, were defined. Using 45% of the UCS as the threshold for unloading stress (75 MPa), both the dynamic elastic modulus and dynamic strength displayed an initial upward trend, succeeded by a subsequent decline. In contrast, the defined damage peak strain ratio ηεp and damage ultimate strain ratio ηεm demonstrated the opposite trend, amounting to 45.64% and 51.03% at the threshold. In addition, increasing unloading damage stress (UDS) decreases the sandstone strain rate(ε̇max and ε̇ave) gradually. Simultaneously, the fractal dimension of the cracks exhibited a linear increase, ranging from 1.159 to 1.481. Similarly, the rock exhibited the lowest proportion of reflected energy and absorbed energy at the threshold. This holds noteworthy implications for examining the dynamic load response behavior of surrounding rock in roadway unloading, ensuring the dependability of blasting unloading techniques.

Effect of freezing and thawing process on fracturing mechanism and acoustic emission characteristics of notched semi-circular bend specimens of red sandstone

Understanding the effect of freeze-thaw (F-T) cycles on the fracture mechanism of rocks under mode-I loading is exceptionally prominent in addressing the underlying factors of rock engineering under a frosty environment. A three-point bending test was carried out on sandstone specimens exposed to 0, 10, 20,40, 60 and 80F-T cycles to achieve the fracture parameters. Moreover, the digital image correlation (DIC) and acoustic emission (AE) methods were employed to reveal the progressive failure in different loading stages. The results divulged that the F-T cycles significantly influenced the fracture parameters, including fracture toughness (KIC) and fracture energy (Gf). The average KIC values have given a negative linear relation with increasing F-T cycles. The percentage of KIC drop with F-T cycles is 9.78 %, 12.20%, 18.56%, 27.2%, and 31.60% for samples treated with 10, 20, 40, 60, and 80 cycles, respectively, while the fracture energy (Gf) gives a decline-rise trend from 0 to 10 and 20 to 80 cycles. The AE signals have demonstrated an apparent stage characteristics, and a sudden rise has occurred in the AE count rate at peak loading, revealing the fracture zone of rock under loading. The DIC effectively revealed the progressive failure pattern, especially at the later loading stages. When the load progressed to around 80–90% of maximum loading, a diverse high-strain region appeared at the tipping point, and the high-strain area continued to develop. The results of this study can effectively address the complex fracturing phenomenon of rocks and provide valuable insights into construction and engineering operations in cold climates.

Numerical simulation of strength and failure analysis of heterogeneous sandstone under different loading rates

Natural rock masses often contain heterogeneous structures with varying sizes, non-uniform distributions, and strengths, which influence the mechanical response characteristics and crack propagation modes under loading. Furthermore, heterogeneous structures can affect the stability of the rock mass, in serious cases, leading to geotechnical and mining engineering disasters. In the present work, a parallel-bond model (PBM)-based numerical simulation using Particle Flow Code (PFC) was carried out to study the strength and failure characteristics of sandstone specimens with heterogeneous structures under different loading rates. The results show that the peak strength increases with the increasing loading rate. In addition, all of the initial cracks occurred at the edges of the heterogeneous structures of specimens under different loading rates. The greater the loading rate, the greater the stress concentration degree at the edge of the heterogeneous structures, the greater the dissipated energy as the sandstone specimens with heterogeneous structures suffer damage, the more intense the acoustic emission activity, and the greater the damage degree of the specimens. The number of cracks generated in sandstone specimens with heterogeneous structures increases gradually with the increasing loading rate during the initial loading stage, and gradually decreases after the specimens are damaged. Cracks propagate and develop from the upper right region to the lower right region of the specimens, forming crack groups that rapidly penetrate the specimens, leading to failure. Under different loading rates, the final failure behavior of the sandstone specimens with heterogeneous structures changes from an inverted V-type to θ-type, then gradually evolves to O-type failure.

Energy Evolution Law of Sandstone Material during Post-Peak Cyclic Loading and Unloading under Hydraulic Coupling

The sustainability of rock engineering is an emerging trend in future development, as society increasingly recognizes the importance of environmental conservation and responsible resource utilization. In this context, the field of rock engineering is undergoing a paradigm shift toward more sustainable practices. A significant aspect of this shift is the investigation of energy evolution laws specific to rocks, which assumes paramount importance in ensuring the sustainable utilization of damaged rock roadways. To investigate the impact of confining pressure and pore pressure on the energy evolution characteristics of rock beyond the peak, post-peak cyclic loading and unloading tests were conducted on sandstone specimens under hydraulic coupling conditions using the MTS815 rock mechanical test system. The study encompassed three sets of confining pressures, namely, 10 MPa, 20 MPa, and 30 MPa. Different levels of pore pressure were applied within each confining pressure group. For the 10 MPa confining pressure, the pore pressure values were set at 2 MPa, 4 MPa, 6 MPa, and 8 MPa. Similarly, for the 20 MPa and 30 MPa confining pressures, the corresponding pore pressure values were 2 MPa, 6 MPa, 10 MPa, 14 MPa, 18 MPa, and 22 MPa. The experimental findings indicate that as the confining pressure increases, both the maximum and residual elastic energy densities of the rock gradually increase. The rise in confining pressure impedes the release of elastic energy. Moreover, with increasing confining pressure, the rate of increase in the maximum dissipated energy density diminishes, highlighting the inhibitory effect of confining pressure on energy dissipation and release within the rock. Pore pressure, on the other hand, disrupts the load-bearing structure of the rock and reduces its energy storage capacity. Under a constant confining pressure, for a fixed number of cycles (axial strain), the total input energy density, elastic energy density, and dissipation energy density exhibit a negative correlation with pore pressure. With an increase in the number of cycles (axial strain), the proportion of elastic energy initially rises but subsequently declines, while the proportion of dissipated energy follows the opposite trend. Furthermore, as the confining pressure increases, the peak proportion of elastic energy also tends to increase. This indicates that higher confining pressures promote energy accumulation after rock failure, enhancing the rock’s ability to store elastic energy.

Failure behaviors of square-tunnel-structured rock specimens under various inclined boundary stress conditions

In most engineering practices, the directions of geo-stress are not always perpendicular or parallel to the center line of underground tunnels’ cross section, and the surrounding rocks are subjected to inclined boundary stresses, which is often ignored in most previous studies. The investigations on the failure characteristics of surrounding rocks around tunnels under inclined boundary stresses have great practical significance. In this study, a series of uniaxial or biaxial compression tests were conducted using cubic red sandstone specimens with different prefabricated square holes. The horizontal centerline inclinations of square holes were designed as 0°, 15°, 30°, and 45°. The test results show that the horizontal centerline inclinations of holes have a great impact on the strength of rock specimens which tend to increase firstly and followed by decreasing. The confining pressure stress σ2 amplify the horizontal centerline inclination effect of holes on the strength of rock specimens. In addition, the horizontal centerline inclination of holes also affected the failure mode and crack propagation of surrounding rock significantly, the stress concentration zone at the right-angle boundary of the hole can induce the propagation of cracks. The findings of this study may provide theoretical guidance and reference significance for the stability evaluation, support parameter and disaster control of underground tunnels.

Mechanical properties of fracture-grouted prefabricated sandstone after thermal-acid coupling treatment: An experimental study

Grouting reinforcement is an efficient approach to increase the stability of engineered fractured rock masses that may be subjected to high temperatures and acidic groundwater in complex geological environments. To reveal the mechanical properties and failure characteristics of the grout-reinforced fractured rock masses under thermal-acid coupling conditions, prefabricated fractured red sandstone specimens with different inclination angles (IAs) were firstly grouted using a new high-strength fast anchoring agent (HSFAA). Then, the grout-reinforced specimens (GRSs) were then immersed in hydrochloric acid (HCl) solutions with different temperatures (varying from 25 to 300 °C). Finally, the macroscopic mechanical properties, microscopic morphology, and crack evolution characteristics of the GRSs after thermal and acid solution treatments were investigated by the uniaxial compression test, X-ray computed tomography (CT) and scanning electron microscopes (SEM). The results confirmed that the HSFAA featured with high fluidity, rapid setting, excellent early strength, and micro-expansion, was an ideal grouted reinforcement material for fractured rock masses. The compressive and tensile strengths of the HSFAA material at a 1-day curing were 2.01 and 1.64 times those of the ordinary Portland cement 42.5 (P.O 42.5), respectively. The fractured IA, thermal-acid coupling method and temperature affected the mass loss, strength parameters, and failure modes of the GRSs to different degrees. The thermal-acid coupling treatment affected the initiation location and extension direction of cracks within the GRSs. The increasing temperature led to the larger and more varied internal cracks, as well as the more complex 3D spatial morphology within the GRSs.

Inversion study on elastic–plastic material parameters of red sandstone in uniaxial compression test

In order to obtain the real material parameters of heterogeneous rock, the material parameters of red sandstone specimens under uniaxial compression tests are inverted based on the Digital Image Correlation (DIC) method and the Finite Element Model Updating (FEMU) method. The DIC method is employed to calculate the displacement field of red sandstone specimens during uniaxial compression loading. Concurrently, a uniaxial compression elastic–plastic finite element numerical model with non-uniform material parameters is developed based on the FEMU method. The model adopts the Mohr–Coulomb yield criterion and adjusts the boundary conditions in real-time to maintain consistency with the test. The vertical displacement field of the numerical model is juxtaposed with that of the test to construct the objective function. Optimization is achieved using the Artificial Fish Swarm algorithm, which enables the acquisition of the non-uniform distribution and evolution process of the material parameters of specimens at different loading moments. The results indicate that this method can spatially obtain the non-uniform distribution field of material parameters and temporally track the evolution of material parameters during the loading process. This research lays a solid foundation for enhancing the accuracy of intelligent coal mining and dynamic disaster monitoring and early warning in coal mines.

Investigation of blast-induced rock fragmentation and fracture characteristics with different decoupled charge structures

Decoupled charge structure is widely employed in profile and production blasting of rock engineering, and its breakage effectiveness is closely linked to the utilization rate of explosion energy. In this investigation, the failure patterns and damage extent of cubic red sandstone specimens induced by blasting with various decoupled charge modes were examined through two groups of laboratory tests, and a three-parameter generalized extreme value (GEV) function was introduced to quantify the rock fragmentation size distribution (FSD) characteristics. Moreover, the Riedel-Hiermaier-Thoma (RHT) model parameters were calibrated based on the blasting test result of the R1 sample. Building upon this validated model, dynamic fracture behavior and pressure attenuation process in a dual-hole blasting finite element model with radial and axial decoupled charge structures were reproduced. Notably, the effects of delay time and initiation mode as well as coupling medium on the effectiveness of rock breakage were discussed. The results demonstrated that the three-parameter GEV function offers a more accurate characterization of the FSD features after rock blasting. Additionally, it is observed that the average fragment size decreases linearly with a reduction in the decoupling ratio, leading to a tendency for finer fragments and more uniform FSDs. Besides, through a comparative analysis of energy distribution characteristics and the volume of destruction in various coupling mediums for rock blasting, it is found that water, as a coupling medium, exhibits the highest energy transfer efficiency, followed by wet sand and dry sand, while air shows the least energy efficiency. Furthermore, the theoretical result of the stress transmission coefficient computed using the equivalent wave impedance method can proficiently reflect the extent of rock fragmentation in decoupled charge blasting.

Cracking behaviors and mechanism of pre-cracked rock specimens under coupled THM fields with chemical processes

In deep rock engineering, natural cracks in rock mass subjected to thermo-hydro-mechanical-chemical (THMC) loads have tendency to initiate and propagate, and result in potential safety hazards. Current literature on cracking behaviors and mechanism of pre-cracked rock under multi-field loads is focused on single (M), two (TM, HM) or three fields (THM, HMC), less on THMC fields. In this study, a series of self-designed triaxial compressive tests were performed to study different THMC fields influencing on mechanical behaviors and failure patterns of pre-cracked red sandstone specimens. A new THMC fracture criterion, recently proposed by our group team, is used to further analyze the crack initiation mechanism of red sandstone under THMC loads. Research results suggest that the stress-strain curve is greatly influenced by the temperature, confining pressure and acidity and alkalinity of chemical solution, but relatively less by the hydraulic pressure. As the temperature and hydraulic pressure increase, the confining pressure decreases, and the acidity or the alkalinity of chemical solutions becomes strong, both the elastic modulus, crack initiation stress and peak strength of specimen are decreased, and the failure pattern is changed from shear failure to tensile failure. The crack initiation stress and peak strength usually have the largest, middle, smallest values in the shear, tension-shear and tensile failures, respectively, while the peak strain has largest value in the mixed tensile-shear failure. The acid solution has greater weakening effect than the alkaline solution with the same concentration. Theoretical results indicate that the temperature and chemical fields influencing on crack initiation is mainly attributable to its weakening of rock fracture toughness, while seepage and stress fields on crack initiation to its change of the stress state at crack-tips. The THMC mechanism of crack initiation can be well revealed by the new THMC fracture criterion.

Experimental investigation on biaxial mechanical properties of laminated rock and floor heave mechanism of tunnel based on DIC

Floor heave at the bottom of a tunnel is a common defect during tunnel service. To study the deformation behavior and failure characteristics of the floor, laminated sandstone specimens with precracked U-shaped grooves were prepared, and a series of biaxial compression tests were carried out in the laboratory. Two typical floor structures of a tunnel (with and without inverts) and four lateral stress levels (σ2 = 0, 5, 10 and 15 MPa) were considered. The effects of lateral stress and inverted structure on the deformation and failure characteristics of the floor were investigated. The displacement, strain evolution and failure extent of the floor were analyzed based on the digital image correlation (DIC) technique. The testing results show that the stress concentration at the U-shaped grooves of the specimens can be effectively reduced by the inverted structure. The specimen with an invert can bear more axial deformation and axial stress than that without an invert. The V-shaped notch failure behavior occurred near the tunnel sidewalls for the specimen without an invert, and it occurred in the middle of the tunnel floor for the specimen with an invert. The failure intensity of the V-shaped notch was positively related to the lateral stresses. With the increasing of axial stress, the heave displacement at the tunnel floor gradually increased, while the settlement displacement of the sidewall showed a trend of first increasing and then decreasing. The tensile strain concentration area of the floor undergoes a process of increasing and then decreasing and finally transforms to compressive strain. The failure characteristics of the sandstone specimen observed in the laboratory tests are consistent with the phenomena in real tunnel engineering. In addition, the deformation and failure mechanism of the tunnel floor was analyzed. The key points for the treatment of tunnel floor heave are put forward based on the testing results.

Prediction model for sandstone strength based on Weibull distribution of micro-element strength and power-law distribution of crack length

It is of the great significance to construct the strength prediction model that considers the influence of the macro and micro defects in the rock in an overall manner to ensure the construction of smart mines and the safety of underground personnel and equipment. Based on the Weibull distribution of rock micro-element strength and the power-law distribution of crack length and the discrete element calculation model, the prediction model for strength of damaged rock specimens were established by numerical simulation and theoretical derivation respectively. Then the rationality of the model was verified by the calculated results. The results show that: 1) The weibull distribution of the micro-element strength and the power-law distribution of the crack length in the PFC2D calculation model are realized by programming, and the quantitative corresponding relationship between the macro, micro and micro damage in the rock and the corresponding distribution parameters is analyzed. 2) 400 sandstone specimens with macro, micro and micro damage are numerically established and loaded to achieve statistical analysis of the relationship between micro element strength, pre-crack information and rock strength. According to the simulation results, the four-dimensional space scatter points of uniaxial compressive strength of sandstone specimens are constructed, and the flow law of compressive strength of sandstone specimens under the influence of multiple damage parameters is obtained. 3) Combined with Mori Tanaka method and rock damage probability distribution theory, a 12 parameter rock strength prediction model is established. The model can both describe the influence of the micro-element strength and macroscopic crack information on the rock strength, and the model calculation results are highly consistent with the simulated results, with the correlation coefficient up to 0.991.