Uniaxial Compressive Modulus Research Articles

Mechanical Properties and Damage Characteristics of Coal Samples under Water Immersion Pressure

Abstract Uniaxial compression tests were conducted on four groups of coal samples under dry condition and 0, 3, and 5 MPa water immersion pressure using acoustic emission, scanning electron microscopy, and a digital scattered spot deformation monitoring system to investigate the mechanical properties and damage characteristics of coal samples under the action of water pressure. The results showed that as the water immersion pressure increased, the water–rock interaction intensified and aggravated the internal damage of the samples. The uniaxial compressive strength, elastic modulus, and peak strain decreased by 71.4%, 43.3%, and 34.74%, respectively. The deformation localized zone of the samples first appeared at the original crack in the deformation and failure process. As the water immersion pressure increased, the coal sample deformation localization zone appeared earlier, the displacement dislocation momentum of the deformation localization zone increased, and the deformation failure gradually increased, furthermore, the samples transitioned from brittle to plastic failure, and their failure mode changed from tensile to tensile–shear hybrid failure. The acoustic emission activity of the samples corresponds to their failure processes. The initial compaction stage of the samples was prolonged as the water immersion pressure increased. In the stage before the peak point, the plastic damage to the samples caused a gradual increase in the intensity and frequency of the acoustic emission signal, and multiple sudden increase points appeared. After the peak point, the active degree of the acoustic emission decreased, and the signal changed from “single tremors” to “swarm tremors” type fluctuation. The water immersion pressure promoted the water–rock interaction in the samples, the water flow continuously eroded the samples, and many water marks appeared on the fracture surface. The average porosity of the fracture surface increased, and the defect size of the coal samples increased.

Mechanical properties and deformation mechanism of stratified cemented tailings backfill under unconfined compression

Studying the deformation mechanism of stratified cemented tailings backfill (SCTB) is of great significance to determining the reasonable strength of the backfill and reducing the filling cost. In this study, the SCTB samples with 0,1,2,3, and 4 stratified surfaces are prepared. Mechanical tests were carried out on the samples, in which the digital image correlation method and scanning electron microscopy (SEM) analyses were conducted on the layered backfill samples. The findings indicated that the existence of stratified surfaces causes a significant deterioration in the uniaxial compressive strength and elastic modulus of the SCTBs, but increases its deformation capacity. During the loading process of the SCTBs, the stratified surfaces always present a compressed state and the average strain of the SCTBs is greatly affected by the stratified surfaces. The macro–micro-cracks at the stratified surfaces could be divided into two types: cracks passing through the stratified surfaces (CPSSs) and cracks not passing through the stratified surfaces (CNPSSs). The SEM results indicated that there was a greater abundance of CNPSSs than CPSSs, meaning the stratified surfaces inhibited the propagation of micro-cracks. With the increase of the number of stratified surfaces in SCTBs, the stratified surfaces compression displacement (SSCD) of a pair of stratified surfaces decreases.

The Energy Dissipation Mechanism and Damage Constitutive Model of Roof–CPB–Floor (RCF) Layered Composite Materials

The stability of composite material that is composed of roof rock, cemented paste backfill (CPB), and floor rock has an important impact on safe mining within metal mines. In order to explore the mechanical properties, acoustic emission (AE), energy dissipation, and damage evolution of roof–CPB–floor (RCF) layered composite materials, uniaxial compression (loading rate 0.02 mm/min) AE tests on RCF materials with different CPB height ratios were performed. The test results show that: (1) the uniaxial compressive strength (UCS) and elastic modulus (ER) of the RCF material were lower than those of the roof or floor rock and higher than that of the CPB. With the increase in the CPB’s height ratio from 0.2 to 0.7, the UCS and the ER decreased from 18.42 MPa to 10.08 MPa and 3.15 GPa to 1.79 GPa, respectively, and the peak strain first decreased from 0.695 to 0.510 and then increased from 0.510 to 0.595. The UCS increased as a polynomial function with the increase in the ER. (2) The AE ring count first increased slowly, then increased rapidly, and finally maintained a high-speed increase. The AE cumulative ring count at the peak point decreased with the increase in the CPB height ratio. The energy dissipation showed that the elastic energy UE accumulated slowly at first, then the dissipated energy UD increased, and finally the UE decreased and the UD increased almost linearly. The UT, UE, UD, UE–UT ratio and UD–UT ratio showed a decreasing trend, and the UE–UD ratio showed an increasing trend at the peak point with the increase in the CPB height ratio. (3) Two damage constitutive models were established based on the AE ring count and energy principle. The damage evolution process of RCF materials can be divided into three stages: the slow damage accumulation stage, stable damage growth stage, and rapid damage accumulation stage.

Chemical-mechanical coupling effects on the permeability of shale subjected to supercritical CO2-water exposure

The permeability of shale reservoir rock and caprock is the key parameters influencing the shale gas production and the storage security of CO2. During the CO2 enhanced shale gas recovery, the issue of how the coupled chemical-mechanical process control the evolution of porosity and permeability in shale remains undetermined. In this study, multiple tests were conducted to obtained the shale properties alteration induced by ScCO2-water exposure, including the mineral compositions measured by XRD, XRF and ICP-OES, the mechanical properties measured by uniaxial compression test, the pore structure and permeability of shale measured using nuclear magnetic resonance (NMR) at different confining stresses over a range of injection pressures. After ScCO2-water exposure, the contents of carbonate and clay minerals decreased, while the contents of quartz and feldspar increased. The geochemical reaction altered the pore structure and mechanical properties of shale, resulting in the enlargement of pore, the decrease in uniaxial compressive strength and elastic modulus of shale, which in turn impact the porosity and permeability evolution in shale. At the unstressed state, the pore in shale was enlarged by the pure chemical reaction, leading to the increase in porosity and the initial permeability of shale. At the stressed condition, the porosity and permeability of shale is controlled by the chemical-mechanical coupling effects, the permeability of CO2-water treated shale sample is lower than that of the untreated shale sample, which can be explained by the increase in stress sensitivity of shale permeability induced by the mechanical weakening, as the compressibility Cf and permeability change rate Δkc were increased after ScCO2-water exposure. In addition, the stress sensitivity of permeability in shale is stress-dependent, for both untreated and ScCO2-water treated shale samples, the Cf and Δkc of shale shown a negative relation with effective stress. At a higher effective stress condition, the change in the permeability of ScCO2-water treated shale is more significantly enhanced. The results demonstrated that the ScCO2-water-shale interaction induced chemical-mechanical effect may decrease the permeability of shale at in-situ stress condition, and hence adversely affecting the efficiency of gas recovery and CO2 sequestration in shale formation.

Quantitative permeation grouting in sand layer with consideration of grout properties and medium characteristics

Proper implementation of grouting engineering projects in sand layers requires quantitative characterization of the influence of multiple parameters on the groutability and grouting reinforcement effect. A grout injection system was developed for a series of laboratory tests to determine the effects on groutability of particle size, relative compaction, water-cement ratio, clay content and grouting pressure. Based on the test results, a multivariate model was established to predict the potential groutability in sand layers. The uniaxial compressive strength, deformation modulus and permeability coefficient were selected as evaluation indices to investigate the grouting reinforcement effect. The significance of influencing factors was discussed with orthogonal tests. A series of single-factor evaluation formulas were obtained based on the relationships between the influencing factors and evaluation indices. Also developed are the quantitative characterization formulas for the grouting reinforcement effect in consideration of multiple factors and curing time.

Application of soft computing methods to estimate uniaxial compressive strength and elastic modulus of soft sedimentary rocks

Application of soft computing methods to estimate uniaxial compressive strength and elastic modulus of soft sedimentary rocks

Effects of nominal maximum aggregate size and compaction effort on the mechanical properties of hot-mix asphalt (HMA)

Considering the potential rutting distress and excellent fatigue performance of small nominal maximum aggregate size (NMAS) mixes, this study explored the function of number of design gyrations (Ndes) variable in the design, and attempted to balance the high-temperature and fatigue performance of these materials. Therefore, one experimental group (9.5 mm NMAS mixes under Ndes = 80 cycles) and three control groups (9.5 mm/13.2 mm/16 mm NMAS mixes under Ndes = 60 cycles) were designed. The uniaxial penetration shear test, the uniaxial compressive modulus test, the British pendulum test, the indirect tensile cracking test, and the indirect tensile fatigue test were then conducted. It can be found that through increasing the Ndes by 20 cycles, the rutting resistance of 9.5 mm NMAS mixes improved significantly, and can reach the same level as that of 13.2 mm NMAS mixes. Moreover, under this design, the 9.5 mm NMAS mixes can still perform a longer fatigue life than 13.2 mm NMAS mixes. In addition, increasing the Ndes also helps to improve the cracking resistance and dynamic modulus of these materials. Specifically, with the increase of Ndes by 20 cycles, the fracture energy and dynamic modulus of 9.5 mm NMAS mixes can improve by almost 516 J·m−2 and 1860 MPa (10 Hz and 21 °C), respectively. These results may indicate that selecting a suitable Ndes is helpful to balance the properties of small NMAS mixes.

Effect of Liquid Nitrogen Freeze-Thaw Cycles on Pore Structure Development and Mechanical Properties of Coal.

To improve the mining efficiency of coalbed methane, liquid nitrogen freeze–thawing experiments were performed to improve coal seam permeability and to study its influence on coal pore structure development and mechanical properties. Mechanical properties and nuclear magnetic resonance tests of coal samples were performed with 0, 5, 10, and 15 freeze–thaw cycles of liquid nitrogen. The results show that the number of freeze–thaw cycles caused the change of uniaxial compressive strength and elastic modulus of coal, and the change effect decreased significantly after 11–15 freeze–thaw cycles. Between 0 and 5 freeze–thaw cycles, the base growth rate of the transverse relaxation time T2 spectral area of the full pore of coal is 44.1%, and that of the transverse relaxation time T2 spectral area of adsorption pore is 71.5%. After 6–10 freeze–thaw cycles, the fixed base growth rate of the transverse relaxation time T2 spectral area of the full hole of coal is 269.0%, and the chain growth rate is 156.2%. In this stage, the chain growth rate of the transverse relaxation time T2 spectral area of the seepage hole is 198.4%, which is mainly the growth of seepage hole volume. After 11–15 freeze–thaw cycles, the chain growth rate of the full pore of coal transverse relaxation time T2 spectrum area is 20.1%, the chain growth rate of adsorption pore is 4.8%, the chain growth rate of seepage pore is 22.2%, and the growth rate of the pore volume is greatly reduced. Comparing the changes of pore and coal mechanical properties in different pore sizes, it can be seen that the change of adsorption pore volume has a greater impact on coal mechanical properties.

Experimental studies on the physical and mechanical properties of heated rock by air, water and high-viscosity fluid cooling

A series of tests were conducted to study the thermal shock damage effect on the physical and mechanical properties of granite and limestone in different cooling media. The dry rock specimens were heated slowly to different target temperatures ranging from 25 °C–800 °C and then quickly cooled to room temperature in air, water, and high-viscosity liquid. After the heat treatment, physical and mechanical properties, such as P-wave velocity, mass, uniaxial compressive strength, elastic modulus and microstructure, were measured. The test results showed that the P-wave velocity, uniaxial compressive strength, and elastic modulus of the heated limestone and granite were all lower after thermal shock in the three cooling media. With increasing heating temperature, the changes in physical and mechanical properties became more pronounced. Air cooling has least influence on rock behavior among the three cooling media. Compared with water cooling, high-viscosity liquid cooling caused less degradation on the physical and mechanical parameters of rocks at any target temperature, which indicates that the thermal shock effect of high-viscosity liquid cooling on rocks was slightly lower than that of water cooling. The brittleness of the rock was affected by the treatment temperature and cooling media. When the treatment temperature was low, the brittleness of the rock was obvious. The brittleness of rock was maintained well by air cooling and was weakened by high-viscosity liquid cooling. Water cooling made the rock more ductility than the other two media.

Quantification and acoustic emission characteristics of sandstone damage evolution under dry–wet cycles

Rocks are continuously affected by dry–wet cycles, which leads to intensified weathering and deterioration of the mechanical properties and often triggers engineering disasters. This study analysed the changes in the mechanical properties and microstructure of sandstone under dry–wet cycles. Based on the box dimension theory and the associated dimensions, the fractal dimension and correlation dimension were developed through digital images and time-series parameters to analyse the microstructural features and acoustic emission (AE) in MATLAB. The degree of microstructure damage under different dry–wet cycles was determined, and the relationship between the microstructure characteristics, fracture development trend, fractal dimension, and correlation dimension was established. The uniaxial compressive strength, elastic modulus, p-wave velocity, and resistivity of the sandstone decayed in distinct stages as the number of dry–wet cycles increased. The degree of microstructure damage was observed to be directly proportional to the fractal dimension. The physical and mechanical properties of the sandstone exhibited a significant relationship with the fractal dimension; it could be used to quantitatively describe the parametric characteristics of the sandstone. The change law of AE energy is consistent with the damage process of sandstone. Based on the analysis of the correlation dimension and sandstone microscale failure, its variation characteristics could be used to quantitatively evaluate the trend of sandstone fracture development and microstructural characteristics. The research results provide a reference for further exploration of the deterioration mechanism and engineering application of the sandstone during dry–wet cycles.

The Effect of Heat Shocks and Freezing–Thawing Cycles on the Mechanical Properties of Natural Building Stones

The damage to rock masses due to the action of freezing is one of the most important factors in the development of landscapes, the performance of civil structures, and the efficiency of mining operations. In this research paper, the effect has been studied on the physical and mechanical performance of seven different natural building rock samples. The testing program included an experimental study on both dry and saturated intact rock samples and therefore, the effect of saturation on the extent of damage on the tested samples has been discussed as well. Based on the obtained results, freezing–thawing cycles increase the porosity of rock samples and decrease the values of P-wave velocity, uniaxial compressive strength, elastic modulus, and Brazilian tensile strength. Moreover, the behavior of different rock types differs to some extent when exposed to weathering cycles under dry and saturated conditions. A multivariate linear regression analysis was used to predict the changes in the physical and mechanical properties of different rock types. It was been shown that with some cautions, the obtained correlations can be generalized for practical cases and can be used to predict the change of rock physical and mechanical properties during the lifetime of rock engineering projects. Such predictions have a high potential of applicability in quite different types of natural stone applications in cold climates. From the stability of structures created in rock materials to the durability of structures created by different natural stones.

Cracking Behavior and Acoustic Emission Characteristics of Rock Containing a Single Preexisting Flaw

The existence of flaws in brittle rocks or rock-like materials has an obvious influence on the material mechanical properties and cracking behavior of civil engineering projects. In this work, the two-dimensional particle flow code PFC2D was used to study the deformation and strength properties, failure processes, and acoustic emission (AE) characteristics of mudstone with a single preexisting flaw. First, the procedure to construct a parallel bond model is introduced. The Weibull distribution is used to reflect the mechanical heterogeneity of rocks. Then, the microscopic parameters used in PFC2D are calibrated to the macroproperties of mudstone obtained from laboratory tests under the uniaxial compression. The results indicate that the increases of the flaw inclination lead to the increasing uniaxial compressive strength and elastic modulus. In terms of microcrack evolution, the initiation, propagation, and coalescence of microcracks are closely related to the force chain. Specifically, an “X” shaped tension force chain concentrated area around the preexisting flaw is founded, which is the most prone area for microcracks to initiate. With an increase in flaw inclination, the b value of AE also shows an increasing trend. By incorporating the AE event numbers into a damage variable, this paper derives a constitutive model, which is verified by numerical results on brittle rocks with a single preexisting flaw under uniaxial compression.

A Comparison Among Some Non-linear Prediction Tools on Indirect Determination of Uniaxial Compressive Strength and Modulus of Elasticity of Basalt

A Comparison Among Some Non-linear Prediction Tools on Indirect Determination of Uniaxial Compressive Strength and Modulus of Elasticity of Basalt

Prediction of uniaxial compressive strength and modulus of elasticity for Travertine samples using an explainable artificial intelligence

The durability of rocks is a substantial rock property that has to be considered for designing geotechnical structures. Uniaxial compressive strength (UCS) and Young's modulus (E) are key indexes for measuring rocks’ durability. Several types of artificial intelligence (AI) methods have been used for modeling these key indexes; however, surprisingly, no explainable AI (XAI) has been considered for their model developments. An XAI is a model whose assessment is not a black box, and humans could understand its problem solution approach. This study has filled this gap and presented SHAP (Shapley Additive Explanations) as one of the most recent XAI methods for modeling UCS, and E. SHAP value could successfully illustrate intercorrelations between rock properties (porosity, point load index, P-wave velocity, and Schmidt hammer rebound number) and their representative UCS and E for each individual record and also together as variables. Results indicated that P-wave velocity has the highest importance for UCS and E prediction. eXtreme gradient boosting (XGBoost) was used as a solid predictive AI system for UCS and E estimation. Outcomes (R2> 0.99) confirmed the high accuracy of the SHAP-XGBoost model comparing with other typical AI models (Random Forest and Support Vector Regression). These results indicated XAI could be considered for illustrating complicated relationships within rock mechanics and energy-resource developments.

Variations on Reservoir Parameters of Oil Shale Deposits under Periodic Freeze-Thaw Cycles: Laboratory Tests

As one of the most important unconventional hydrocarbon resources, the oil shale has been extracted with a frozen wall to successfully increase the shale oil production and reduce environmental pollution, which results from the harmful liquids in the in situ conversion processing of oil shale. Thereby, the strength and permeability of the frozen wall are extremely critical to reduce the harmful chemicals leaching into the groundwater. However, the permeability and strength of the frozen wall can be influenced by periodic freeze-thaw cycles. In order to investigate the damage and deterioration characteristics of oil shale samples after various periodic freeze-thaw cycles, the oil shale samples were periodically frozen and thawed as many as 48 times, after which the sample mass, stress-strain, freeze-thaw coefficient, uniaxial compressive strength, elastic modulus, and longitudinal wave velocity of the oil shale samples were separately measured. According to the measured results, the number of freeze-thaw cycles greatly influenced the physical and mechanical properties of oil shale samples. The uniaxial compressive strength and elastic modulus of the oil shale samples were changed with maximum variation rates of 64% and 65%, respectively. Meanwhile, the freeze-thaw coefficient of measured oil shale samples exponentially decreased with the increased number of freeze-thaw cycles, whereas the longitudinal wave velocity of tested samples ranged from 1602 m/s to 2464 m/s as a result of the new micropores inside the oil shale sample. Research results have enormous significance to the efficient and safe in situ exploitation of oil shale deposits.

Experiment and Mechanism Investigation on Freezing-Thawing of Sandstone with Different Water Contents

Freezing-thawing cycles seriously affect the safety of underground engineering in cold regions. At present, most research studies focus on the effect of number and freezing temperature on freezing-thawing cycles. As another important factor, the mechanism of rock mass water content affecting freezing-thawing is less studied. This paper studied the influence of the water content on mechanical property, microstructure, and acoustic emission characteristics of sandstone. The results indicated that the uniaxial compressive strength (UCS) and elastic modulus (E) of sandstone after 20 freezing-thawing cycles decreased as the water content increased. However, the decreasing rate of UCS gradually decreased, while the decreasing rate of E gradually increased. Furthermore, the empirical formulas of UCS and E about water content were obtained. The porosity and plasticity of sandstone after 20 freezing-thawing cycles increased as the water content increased. The empirical formulas of UCS and E about water content were obtained. The porosity and plasticity of sandstone after 20 freezing-thawing cycles increased as the water content increased. The decreasing trend of UCS with porosity was the same as that of UCS with water content. The failure form of sandstone gradually changed from splitting failure to shear failure. The results of the acoustic emission test showed that the stress-strain curves combined with acoustic emission ring counting could reveal the damage evolution process of sandstone during loading.

Experimental Study on the Effect of Coal Particle Size on the Mechanics, Pore Structure, and Permeability of Coal-like Materials for Low-Rank Coalbed Methane Reservoir Simulation

Coal-like materials have been widely used to simulate low-rank coalbed methane (CBM) reservoirs to develop fracturing stimulation technology since the transportation and preparation of low-rank coal produce easily secondary damages, causing large dispersion of test results. However, the coal particle sizes have a significant effect on the performance of coal-like materials among many factors. Hence, to accurately simulate low-rank CBM reservoirs for promoting the development of reservoir stimulation technologies, cement, gypsum, pulverized coal, and sand were selected as similar materials to make coal-like samples with different coal particle sizes to study the influence law of coal particle size on the mechanics, pore structure, and permeability of samples. The ultrasonic wave velocity, dynamic parameters, uniaxial compressive strength, elastic modulus, and uniaxial tensile strength of samples increased with increasing coal particle size in the range of 0–2 mm. However, these parameters suddenly decreased when the coal particle size increased to 2–3 mm. The porosity and permeability of the samples exhibited an opposite trend compared to the above parameters. The fracture occurred in cementing materials, and the crack extension direction was affected by the coal particle sizes. The fracture complexity of the coal-like samples decreased with increasing particle size under uniaxial compressive testing. The failure mode was mainly a tensile failure, supplemented by shear failure. The coal-like materials can well meet the performance requirements of low-rank CBM reservoirs.

Bedding Plane Effects on Mechanical Behavior of Surrounding Rock in Mountain Tunneling

The layered rock showed the characteristics in Mountain tunnel, Yunnan. A series of uniaxial compression tests and variable angle shear tests were carried out, and the aim was to investigate the effect of the bedding on its mechanical parameters and failure modes. The test results show that the uniaxial compressive strength, elastic modulus, and Poisson's ratio of layered rock present a U-shaped distribution with the increase in bedding orientation from 0° to 90°. All of them have a maximum when the bedding orientation is 0° and a minimum when the bedding orientation is 45°. The failure modes of layered rock can be summarized into three types: the fracture tensile failure parallel to the weak plane of bedding; the shear slip failure along bedding weak plane; and tension-shear composite failure between bedding weak plane and matrix. Based on the testing data and analysis results, it can be concluded that the layered rock specimen with different bedding orientations is an important reason for the anisotropy of mechanical parameters and failure modes.

Effect of thermal cycles on mechanical properties of a moderately weathered granite

The friction between the hard rock and milling wheels of a tunnel boring machine generates high temperatures. The high-temperature bedrock was then cooled down by circulating slurry. Therefore, thermal cycles were initiated within bedrocks. Understanding the response of mechanical properties of hard rock to these thermal cycles can be helpful in improving the drilling efficiency. In this study, uniaxial compressive strength and elastic modulus of a moderately weathered granite after various thermal cycles were determined. Results show that, for the specimens treated at a temperature of lower than 300°C, uniaxial compressive strength and elastic modulus decreased with the increasing numbers of thermal cycles. For the specimens treated at a temperature of higher than 400°C, uniaxial compressive strength and elastic modulus increased in the first six thermal cycles, and the tendencies reversed as the thermal treatment was further proceeded. For a given numbers of thermal treatment, uniaxial compressive strength and elastic modulus peaked at 200°C and began to decrease as the temperature further increases. These findings can be used to optimize the rotating speed of milling wheels during excavating granite bedrock.

Degradation of Strength and Stiffness of Sandstones Caused by Wetting-Drying Cycles: The Role of Mineral Composition

Rock mechanical parameters are of great importance for the construction and design of rock engineering. Rocks are usually subjected to the deteriorating effect of cyclic wetting-drying because of the change in moisture content. The main objective of this study is to reveal the degradation effects of wetting-drying cycles on strength and modulus on varying rocks. Three kinds of sandstones with different mineral constituents are selected for testing. Artificial treatments of cyclic wetting-drying are conducted on respective specimens of the three sandstones (0, 10, 20, 30, and 40 cycles) to simulate the damage of rocks exposed to natural weathering. Uniaxial compressive tests are carried out on sandstone specimens to obtain their strength and modulus. Test results show that, for the tested sandstones, both of the uniaxial compressive strength (UCS) and modulus are reduced as the cyclic number rises. In the first ten cycles, the losses of UCS and modulus are very significant. Subsequently the changes of UCS and modulus become much more placid against cyclic number. When the cyclic number is the same, the loss percentages of rock mechanical properties of the three sandstones are very different which mainly depends on the contents of expandable and soluble minerals.