Rock Testing System Research Articles

Fractal Characteristics and Energy Evolution Analysis of Rocks under True Triaxial Unloading Conditions

To investigate the mechanical properties and energy evolution laws of rocks under true triaxial unloading conditions, a study was conducted using a true triaxial rock testing system on three different types of rocks: coal, sandy mudstone, and siltstone. The study examined the mechanical behavior, failure patterns, and fractal dimensions of these rocks under true triaxial unloading conditions. The tests revealed significant variations in stress–strain curves and failure patterns among the different rock types. Observation indicated that rocks with lower peak strength exhibited higher fractal dimensions and increased fragmentation upon failure. Subsequently, based on the experimental data of siltstone, the impact of the unloading rate and particle size distribution on the energy evolution under true triaxial single-sided unloading paths was further investigated using the three-dimensional particle flow software PFC3D 6.0, revealing the micro-mechanisms of rock energy evolution. The study revealed that when the initial stress unloading level was low, the total energy and strain energy at the peak strength exhibited a strong linear relationship with the unloading rate. Before the stress peak, the dissipative energy was mainly composed of frictional energy. After the stress peak, the dissipative energy consisted of frictional energy, damping energy, and kinetic energy. The heterogeneity of rock significantly affected the distribution of dissipative energy, with an increase in rock heterogeneity leading to a decrease in frictional energy and an increase in kinetic energy.

Study on pore-crack evolution and connectivity of coal subjected to controlled electrical pulse based on CT scanning technology

Controlled electric pulse (CEP) technology is a novel method for enhancing coal seam permeability. To study the pore-crack evolution and connectivity of the coal body by CEP, a self-built CEP cracking and enhancing permeability of coal rock test system was used to test five groups of coal samples under different breakdown voltages (BVs). We established a three-dimensional crack structure and equivalent pore network model for before and after penetration using high-precision micro-CT technology to quantitatively study the evolution and connectivity of the pore and crack structures of the coal body. The results showed that with rising BV, coal body crushing intensifies, leading to an increased cracking ratio and fractal dimension. This summarised the three stages of coal sample crushing by CEP. In addition, the proportion of larger equivalent pores and throat radii in the breakdown coal body was significantly increased compared to that of raw coal, and the connectivity structure was improved to different degrees compared to that of raw coal. Eventually, a three-dimensional crack network structure formed inside the coal samples, which was broken at both ends and complex in the middle, providing a transport channel for coalbed methane mining.

Hydromechanical characterization of gas transport amidst uncertainty for underground nuclear explosion detection

Given the challenge of definitively discriminating between chemical and nuclear explosions using seismic methods alone, surface detection of signature noble gas radioisotopes is considered a positive identification of underground nuclear explosions (UNEs). However, the migration of signature radionuclide gases between the nuclear cavity and surface is not well understood because complex processes are involved, including the generation of complex fracture networks, reactivation of natural fractures and faults, and thermo-hydro-mechanical-chemical (THMC) coupling of radionuclide gas transport in the subsurface. In this study, we provide an experimental investigation of hydro-mechanical (HM) coupling among gas flow, stress states, rock deformation, and rock damage using a unique multi-physics triaxial direct shear rock testing system. The testing system also features redundant gas pressure and flow rate measurements, well suited for parameter uncertainty quantification. Using porous tuff and tight granite samples that are relevant to historic UNE tests, we measured the Biot effective stress coefficient, rock matrix gas permeability, and fracture gas permeability at a range of pore pressure and stress conditions. The Biot effective stress coefficient varies from 0.69 to 1 for the tuff, whose porosity averages 35.3% ± 0.7%, while this coefficient varies from 0.51 to 0.78 for the tight granite (porosity <1%, perhaps an underestimate). Matrix gas permeability is strongly correlated to effective stress for the granite, but not for the porous tuff. Our experiments reveal the following key engineering implications on transport of radionuclide gases post a UNE event: (1) The porous tuff shows apparent fracture dilation or compression upon stress changes, which does not necessarily change the gas permeability; (2) The granite fracture permeability shows strong stress sensitivity and is positively related to shear displacement; and (3) Hydromechanical coupling among stress states, rock damage, and gas flow appears to be stronger in tight granite than in porous tuff.

Experimental Study of the Mechanical and Acoustic Emission Characteristics of Sandstone by Using High-Temperature Water-Cooling Cycles

In order to study the physical and mechanical properties of sandstone under high-temperature water-cooling cycling conditions, an RMT-150B electrohydraulic servo rock testing system and a DS-5 acoustic emission detection and analysis system were used to conduct uniaxial compression acoustic emission tests on sandstone after high-temperature water-cooling cycles. The deformation, strength, and acoustic emission characteristics of sandstone were analyzed under different temperatures and cycle times. The results show that the high-temperature water-cooling effect caused changes in the physical properties of sandstone. The volumetric expansion rate of the rock samples first decreased, then increased in temperature, and the strength first increased, then decreased, whereas the number of cycles had less of an impact on the physical properties. At 200 °C, with increased cycle number, the elastic modulus increased by 20.1%, and the compressive strength increased from 63.9 MPa to 71.46 MPa. At 300–600 °C, the elastic modulus and compressive strength of sandstone gradually decreased with increases in the temperature and cycle number, with reductions of 6.04%, 7.24%, 28.7%, 35.57%, 17.6%, 18.2%, 20.4%, and 60.5%, respectively. With increased temperature and cycle times, the acoustic emission ringing counts increased, ringing counts and cumulative energy appeared earlier, the rock samples entered elastic deformation earlier, the yield stage length increased, and the samples showed a tendency to transition from brittle to ductile damage.

Deformation Damage and Energy Evolution of Basalt Fiber Reinforced Concrete under the Triaxial Compression

To explore the law of energy evolution and the change of damage before and after specimen failure, the conventional triaxial compression tests (5, 10, 15, 20, and 30 MPa) of basalt fiber reinforced concrete (BFRC) with different fiber volume fractions (0, 0.2% and 0.4%) were carried out by MTS816 rock testing system, and the cyclic loading and unloading tests of BFRC with a fiber content of 0.2% were carried out. The experimental results show that the peak strength and strain of BFRC increase with the increase of confining pressure. Tensile failure occurs under low confining pressure, and shear failure occurs under high confining pressure. The best volume fraction of fiber is 0.2%. Under different confining pressures, the input energy, elastic energy, plastic properties, and dissipated energy of the samples first increase and then decrease to a stable level. The elastic energy and dissipated energy reach the maximum near the peak stress, while the input energy and plastic properties reach the maximum at the peak. At the same time, the damage increases continuously with the input of load under different confining pressures, indicating that the failure of the specimen is a process of energy accumulation.

A multifunctional rock testing system for rock failure analysis under different stress states: Development and application

The stress state in a rock mass is complex. Stress redistribution around underground excavation may lead to various failure modes, including compressive-shear, tensile-shear, and tensile failures. The ability to perform laboratory tests with these complex stress states is significant for establishing new strength criteria. The present paper introduces a new rock testing system with “tensile-compressive-shear” loading functions. The device includes bi-directional and double-range hydraulic cylinders, auxiliary loading equipment, and roller rows that can perform direct compressive-shear tests, direct tensile tests, and direct tensile-shear tests. The testing system provides maximum vertical and lateral loading forces of 2000 kN and allows testing cubical rock specimens with dimensions of 0.5 m × 0.5 m × 0.5 m. The performance of the testing machine was evaluated by testing a rock-like material based on cement mortar under compressive-shear, tensile, and tensile-shear stress states. The failure process and deformation characteristics were monitored during loading using acoustic emission (AE) transient recorder, piezoelectric AE sensors, a high-speed camera, and a thermal infrared camera. The failure mechanism was investigated by analyzing AE counts, AE amplitude, strain, and temperature changes on the rock specimen surface. The test results confirmed that the testing system could successfully simulate the abovementioned stress path. The AE counts and amplitude responses were influenced by different failure modes. The temperature response during the compressive-shear test indicated the development of a high-temperature band on the rock specimen surface. In contrast, a negligible temperature change was observed during the tensile and tensile-shear tests. The newly developed multifunctional rock testing system allows laboratory tests under various failure modes. The monitoring results of multiple variables during rock failure tests provide valuable information on failure characteristics. • Develop a novel rock testing system with tensile-compressive-shear loading function. • Investigate the failure behavior of rock-like specimens under different stress states. • Analyze the various attributes response characteristics in different failure modes.

Constitutive model of concrete creep damage considering the deterioration of creep parameters

The MTS815.03 rock test system is used to carry out mechanical property and triaxial compression creep tests on concrete specimens of the same specification under different confining pressure conditions to study the creep characteristics of concrete structures in deep high-stress level areas. The relationship between the creep strain rate of the concrete specimen and the stress level and the creep characteristics of the concrete specimen in the high-stress level area are analyzed. Based on the establishment of a creep damage model in the high-stress level zone of a concrete specimen, a method for determining creep parameters is proposed on the basis of the characteristics of the creep test curve. The degradation law of creep parameters under different stress levels is analyzed, and the relationship between creep parameters and stress is constructed. Finally, the expressions between each creep parameter and stress are substituted into the creep damage model, and an improved creep damage model is obtained. The correctness of the established model is verified by comparing the test curve and the model curve. Result shows that the agreement between the established creep model curve and the accelerated test curve is higher than that between the traditional Nishihara model curve and the accelerated test curve. The established creep damage creep model can be used to predict the deformation trend of surrounding concrete over time after chamber or tunnel excavation and provide theoretical basis for concrete support and lining construction.

Incompatible deformation and damage evolution of mixed strata specimens containing a circular hole

Analysing the incompatible deformation and damage evolution around the tunnels in mixed strata is significant for evaluating the tunnel stability, as well as the interaction between the support system and the surrounding rock mass. To investigate this issue, confined compression tests were conducted on upper-soft and lower-hard strata specimens containing a circular hole using a rock testing system, the physical mechanical properties were then investigated. Then, the incompatible deformation and failure modes of the specimens were analysed based on the digital speckle correlation method (DSCM) and Acoustic Emission (AE) data. Finally, numerical simulations were conducted to explore the damage evolution of the mixed strata. The results indicate that at low inclination angles, the deformation and v-shaped notches inside the hole are controlled by the structure plane. Progressive spalling failure occurs at the sidewalls along the structure plane in soft rock. But the transmission of the loading force between the soft rock and hard rock are different in local. At high inclination angles, v-shaped notches are approximately perpendicular to the structure plane, and the soft and hard rock bear common loads. Incompatible deformation between the soft rock and hard rock controls the failure process. At inclination angles of 0

Energy analysis-based core drilling method for the prediction of rock uniaxial compressive strength

The uniaxial compressive strength (UCS) of rock is a basic parameter in underground engineering design. The disadvantages of this commonly employed laboratory testing method are untimely testing, difficulty in performing core testing of broken rock mass and long and complicated onsite testing processes. Therefore, the development of a fast and simple in situ rock UCS testing method for field use is urgent. In this study, a multi-function digital rock drilling and testing system and a digital core bit dedicated to the system are independently developed and employed in digital drilling tests on rock specimens with different strengths. The energy analysis is performed during rock cutting to estimate the energy consumed by the drill bit to remove a unit volume of rock. Two quantitative relationship models of energy analysis-based core drilling parameters (ECD) and rock UCS (ECD-UCS models) are established in this manuscript by the methods of regression analysis and support vector machine (SVM). The predictive abilities of the two models are comparatively analysed. The results show that the mean value of relative difference between the predicted rock UCS values and the UCS values measured by the laboratory uniaxial compression test in the prediction set are 3.76 MPa and 4.30 MPa, respectively, and the standard deviations are 2.08 MPa and 4.14 MPa, respectively. The regression analysis-based ECD-UCS model has a more stable predictive ability. The energy analysis-based rock drilling method for the prediction of UCS is proposed. This method realized the quick and convenient in situ test of rock UCS.

Effects of Unloading Rate on Energy Evolution Mechanism in the Single‐Side Unloading Failure of Highly Stressed Marble

Highly stressed rock masses continuously exchange substances and energy with the outer environment during single‐side unloading, which is a damage evolution process with energy dissipation. The true triaxial unloading disturbance rock test system was utilized to perform single‐side unloading tests of marble specimens at different unloading rates and axial pressures. The unloading failure characteristics of the marble specimens were revealed from the perspective of energy conversion. Results indicated the following: (1) The marble specimen was more likely to be damaged under the single‐side unloading condition than under the loading condition, with the unloading failure strength reaching 80% of the loading failure strength. (2) Crack propagation under single‐side unloading was intermittent. (3) A high single‐side unloading rate corresponded to a high release rate of the energy concentrated inside the rock mass, a small amount of energy dissipated due to second failure, insufficient crack propagation, and occurrence of only tensile failure. (4) A large amount of axial strain energy indicated a great amount of energy aggregated inside the rock mass and the generation of tensile stress due to unloading dilatancy. Consequently, a high critical value for splitting the failure of the rock specimen implied large failure‐induced energy dissipation and a high failure degree. The study of failure energy evolution mechanism in the single‐side unloading of highly stressed rock masses at different unloading rates will provide great guiding significance in controlling underground engineering disasters and guaranteeing construction safety.

Experimental Study on Mechanical Properties of Deep Buried Granite under Different Confining Pressures

Brittle failure of hard rock poses a serious threat to the stability of surrounding rock in deep underground engineering. In order to study the deformation and failure characteristics of deep buried granite under high confining pressure cyclic loading and unloading, MTS815 electro‐hydraulic servo rock test system was used to conduct cyclic loading and unloading tests under confining pressures of 15 MPa, 35 MPa, 45 MPa, and 55 MPa, and the corresponding stress‐strain curves and deformation failure characteristic curves were obtained. The experimental results show the follows: (1) under the same confining pressure, the peak strength, crack initiation stress, crack damage stress, and Poisson’s ratio of the specimens under cyclic loading and unloading are larger than those under conventional triaxial loading and unloading, and the unloading elastic modulus is smaller than that, under conventional triaxial compression; (2) the results show that, under different confining pressures, the granite samples show obvious brittle failure characteristics, the elastic modulus and crack initiation stress increase first and then decrease with the confining pressure, the peak strength and crack damage stress of the samples increase linearly with the confining pressure, and Poisson’s ratio increases first and then remains unchanged with the confining pressure; (3) under the two kinds of stress conditions, the macroscopic failure of the samples is mainly shear failure. The deformation and failure law of granite samples revealed in this study has significant reference value for the selection of rock mass mechanical model of surrounding rock stability of underground engineering, the formulation of surrounding rock support countermeasures, and the evolution law of mechanical parameters with damage variables.

Experimental study and modelling of coal stress induced by gas adsorption

In practice, adsorption stress significantly influences the effective stress of a coalbed and thus the permeability during coalbed methane (CBM) recovery. However, in current coalbed permeability models, this stress is simply calculated from the adsorption strain and is rarely determined by experimental methods. In this paper, a tri-axial rock test system was used to measure this stress. Based on the experimental results, a Langmuir-like equation is able to accurately describe the changes in adsorption stress with gas pressure, and the adsorption stress with respect to adsorption amount is well described using a linear relationship. In addition, an effective horizontal stress model under uniaxial strain conditions is established. These models are directly established based on the experimental results and do not contain any uncertain parameters, and they are able to truly reflect the coal stress induced by gas adsorption and gas pressure. Finally, the evolution of coal adsorption stress with gas pressures and adsorption amount were analyzed, and the coal seam anisotropy was discussed. The models proposed in this paper shed light on the development of coalbed permeability models.

Deformation and Energy Dissipation Characteristics of Joints Under Cyclic Confinements

Using the rock testing system, MTS-815, we conducted cyclic-confinement loading tests on the joints, made of Maokou Limestone. Then, by analyzing the characteristics of the confinement-normal displacement curve of the joints with various roughness coefficients, initial openings and confinements, we investigated the energy dissipation law of the plastic hysteresis loop, the elastic–plastic law and the stiffness characteristics of the joints. The results show that the increase in the confinement cycle shrunk the area of the plastic hysteresis loop; whereas the increase in the roughness of the joint surfaces enlarged the area, and further promoted the dissipated energy. In addition, plastic deformation and elastic deformation subsequently occurred on the joint with the increase in the confinement cycle. Moreover, the increase in the confinement cycle promoted the normal stiffness. According to the characteristics of the normal deformation, an index, β, was introduced to modify the power function model for the normal stiffness of the joint after specific loading cycles. The fitted curves, obtained from the modified model, agree well with the laboratory curves.

Testing of Sealing Elements for FIB-1 Apparatus Designed to Liquidate Open Eruption by Drilling Tools

Abstract Petroleum and natural gas still have their place among the most important resources in many industrial areas. Their global consumption influences an increasing demand on the quality of drilling works and the efficiency of extraction. Nonetheless, even in this field of human activity, we can encounter exceptional events and accidents. One of the most serious kinds of accidents during exploration is the so called open eruption of extracted medium. The specific case of this accident is an open eruption caused by drilling tools when a working crew is not capable of securing drilling workplace. In order to solve this emergency situation, Main Mining Rescue Station Hodonin (HBZS Hodonin), in cooperation with researchers from Faculty of Mining and Geology at VSB - Technical University of Ostrava, designed and developed specialised apparatus DPRP (Drill Pipe Rescue Press), working designation FIB-1. This apparatus enables the liquidation of eruption by pressing the drilling pipe. The residual crack, which remains following the pressing, must be eliminated by sealing materials. This paper reviews the testing of sealing elements (materials), designed by our team, in residual crack of circle shape with help of hydraulic press MTS 816 Rock Test System.

Mechanical Behavior and Damage Evolution for Granite Subjected to Cyclic Loading

This paper focuses on the mechanical behavior and damage evolution of Huashan granite subjected to cyclic loading. Four levels of confining pressure (0, 15, 25, and 35 MPa) were applied during cyclic axial loading by using a Rock Test System (MTS815) along with an acoustic emission (AE) monitoring device. Experimental results indicate that the number of AE activities is higher under cyclic triaxial loading compared to that under cyclic uniaxial loading. The measured stress‐strain curves of both uniaxial and triaxial tests under cyclic loading are concave‐up, but the degree of concavity is mild for the latter. As the cycle number rises, elastic modulus of the granite sample under different confining pressures increases. The slope of the peak strength versus confining pressure plot for the cyclic loading is larger than that for the monotonic loading. Besides, it is found that the dissipated energy increases with the increase of cyclic stress, but it hardly increases in proportion with the confining pressure. The damage parameters defined in terms of the plastic strain can be extended for the whole cyclic loading process, and they agree well with the energy‐based damage parameters.

Limit of crustal drilling depth

Deep drilling is becoming the direct and the most efficient means in exploiting deep mineral resources, facilitating to understanding the earthquake mechanism and performing other scientific researches on the Earth's crust. In order to understand the limit of drilling depth in the Earth's crust, we first conducted tests on granite samples with respect to the borehole deformation and stability under high temperature and high pressure using the triaxial servo-controlled rock testing system. Then the critical temperature-pressure coupling conditions that result in borehole instability are derived. Finally, based on the testing results obtained and the requirements for the threshold values of borehole deformations during deep drilling, the limit of drilling depth in the Earth's crust is formulated with ground temperature.

Physical and mechanical behavior of granite containing pre-existing holes after high temperature treatment

To understand the high temperature effects on the mechanical and failure behaviors of rock, uniaxial compression tests were carried out on granite specimens containing three pre-existing holes using a rock testing system. Based on the experimental results, the influences of testing temperature on the physical and mechanical parameters of granite were analyzed in detail. An obvious color change of tested granite occurs from gray at room temperature to reddish after 450°C and to red-brown after 900°C high temperature treatment. The granite volume increases, mass decreases and density decreases with increasing testing temperature. As the temperature increases, the peak strength first increases and then decreases, while the elastic modulus decreases. However, the peak strain changes slightly before 450°C, increases dramatically up 450°C. As the bridge angle increases, the mechanical parameters of granite specimens first decrease and then increase. And then, the crack initiation, propagation and coalescence behavior of granite specimens after high temperature exposure was investigated using an acoustic emission (AE) and photography monitoring technique. The cracking process shows that the propagation of crack from the surface of holes leads to the coalescence between adjacent holes. A large AE count and a stress drop are observed during the crack initiation and propagation. The failure modes can be generally classified into three categories: splitting mode, shear mode and mixed mode and they are closely related to heat treatment temperature and bridge angle. Finally, the mechanism causing the differences in the mechanical parameters observed with increasing temperature was discussed based on the SEM observations.

Grain Size Distribution Effect on the Hydraulic Properties of Disintegrated Coal Mixtures

In order to better understand groundwater influx and protection in coal mining extraction works, an in-house water flow apparatus coupled with an industrial rock testing system, known as MTS 815.02, were used to study the effects of grain size mixtures on the compaction and flow properties of disintegrated, or non-cemented, coal samples. From the Reynolds number evaluation of the samples with different grain mixtures, and the relationship between the water flow velocity and pore pressure gradient differences, it was found that seepage through the mixtures are of non-Darcy flow type. The porosity of coal specimens was found to be highly affected by compaction, and the variations of the porosity were also influenced by the samples’ grain size distribution. It was found that the sample porosity decreases with increasing compaction and decreasing grain sizes. Grain crushing during compaction was observed to be the main cause of the appearance of fine grains, and the washing away of fine grains was consequently the main contributing factor for the weight loss due to water seepage. It was observed that during the tests and with the progression of compaction, permeability k decreases and non-Darcy factor β increases with decreasing porosity φ. The k-φ and β-φ plots show that as the sizes of disintegrated coal samples are getting smaller, there are more fluctuations between the porosity values with their corresponding values of k and β. The permeability value of the sample with smallest grains was observed to be considerably lower than that of the sample with largest grains. Non-Darcy behavior could reduce the hydraulic conductivity. It was found that the porosity, grain breakage and hydraulic properties of coal samples are related to grain sizes and compaction levels, as well as to the arrangement of the grains. At high compaction levels, the porosity of disintegrated coal samples decreased strongly, resulting in a significant decrease of the permeability at its full compression state; Non-Darcy flow behavior has the slightest effect in uniform samples, therefore, indicating that disintegrated coal in uniform grain size mixtures could be treated as an aquicluding (water-resisting) stratum.

Coal permeability and crack distribution characteristics in unloading confining pressure experiments under different water pressures

This paper addresses the mechanical behavior and permeability characteristics of the unloading coal under different water pressures in the TOP2518-comprehensive- rock-testing system. With the use of the computed tomography scan tests, the 3-D crack distribution characteristics of the fractured coal are obtained and reconstructed. The difference in the 3-D crack distribution of unloading coal induced by different water pressures is also analyzed in detail. The results show that, as the water pressure changes from 0 MPa to 8 MPa, the peak stress of unloading coal gradually decreases, and that the variation degree of permeability of the coal samples increases significantly comparing with the initial permeability.

Variations of hydraulic properties of granular sandstones during water inrush: Effect of small particle migration

The evaluation of the hydraulic properties evolution of granular sandstones in relation with groundwater inrush within faults is an important issue for mining engineering applications. This paper presents the results of an experimental investigation of small particle migration from granular sandstone samples under different original porosities, particle size compositions and water flow pressures. A new rock testing system has been setup to carry out the tests. Based on the results, it is observed that the overall permeability evolution during the tests can be divided into four different phases, including i) re-arrangement of large rock fragments, ii) water inrush with substantial particle migration, iii) continued moderate particles seepage, and iv) steady state water flow. The crushing of edges and corners of large rock fragments, and the evolution of the fracture network has mainly been observed in the first two phases of the tests. The results indicate that the migration of small particles has an essential effect on permeability and porosity increase during water inrush through fractured sandstone. The samples with higher original porosity, higher percentage of fine particles in their formation and under higher water flow pressures, achieve higher permeability and porosity values when the test is complete. Furthermore, using the measured data, the performances of a number of empirical models, for permeability evolution in fractured porous media, have been studied. The prediction results indicate that not all of the fractures in a sample domain contribute in small particle migration. There are parts of the fracture network that are not effective in particle flow, a sample with less original porosity, more fine particles and under lower water pressure shows less ineffective fractures. Therefore, using the concept of the effective porosity (fracture) is sufficient enough for the flow calculation.