Triaxial Compressive Strength Research Articles

Investigation on Shear Wave Velocity and Triaxial Mechanical Performance of Tailings Core from Tailings Dam

The physical and mechanical parameters of tailings are important to study the stability of tailings dams (TDs). In this study, a series of laboratory experiments (shear wave velocity, triaxial compressive, and peak strain strength testing) were conducted to obtain the mechanical properties of tailings from TD. The results showed the following. (1) The linear function fitting could characterize the quantitative relationship between shear wave velocity and hole depth. (2) The corresponding static pressure coefficient increased as the confining pressure increased. The exponential fitting could characterize the quantitative function relation between the static pressure coefficient and the confining pressure. (3) The cohesion and internal friction angle of the tailings sample were 20 kPa and 41°, respectively, and the logarithmic fitting could better characterize the quantitative relation between shear peak strength and confining pressure. The results of this study can provide important references for further research on the stability of TD.

Geomechanical properties will constrain CO2 injection into the lower Ordovician Rose Run sandstone deep saline reservoir, Appalachian Basin, Kentucky, USA

The Kentucky Geological Survey (KGS) 1 Hanson Aggregates stratigraphic research well, Carter County, Kentucky, USA, was drilled to a total depth of 1474 m as a field-scale test of potential CO 2 storage reservoir properties in the Central Appalachian Basin. Geomechanical properties of the Rose Run sandstone (upper Ordovician Knox group) were tested for its suitability as a storage reservoir. A 9.8-m thick section of the Rose Run was penetrated at 1000 m drilled depth and a whole-diameter core and rotary sidewall cores were taken. Average porosity and permeability measured in core plugs were 9.1% and 44.6 mD, respectively. Maximum vertical stress gradient calculated in the wellbore was 26 MPa/km. Wellbore fractures in dolomites underlying and overlying the Rose Run follow the contemporary N53°E Appalachian Basin stress field. The Rose Run elastic geomechanical properties were calibrated to values measured in core plugs to evaluate its fracturing risk as a CO 2 storage reservoir. Mean Young's modulus and Poisson's ratio values of the Rose Run were 45 GPa and 0.23, respectively, whereas Young's modulus and Poisson's ratio values were 77.1 GPa and 0.28, respectively, in the overlying Beekmantown dolomite, suggesting the Rose Run may fracture if overpressured during CO 2 injection but be confined by the Beekmantown. Triaxial compressive strength measured in core plugs found the Rose Run and Beekmantown fractured at mean axial stresses of 156.5 MPa and 282.2 MPa, respectively, confirming the Beekmantown as suitable for confining CO 2 injected into the Rose Run. A step-rate test was conducted in a mechanically-isolated 18.6-m interval bracketing the Rose Run. Static Rose Run reservoir pressure was 9.3 MPa, and fracture gradient under injection was 13.6 MPa/km, suggesting step-rate testing before CO 2 injection, and subsequent pressure monitoring to ensure confinement. As the region around the KGS 1 Hanson Aggregates well is underpressured and adjacent to faulted Precambrian basement, further research is needed to evaluate its induced seismicity risk during CO 2 injection.

Prediction Models of Shear Parameters and Dynamic Creep Instability for Asphalt Mixture under Different High Temperatures.

This study mainly investigates the prediction models of shear parameters and dynamic creep instability for asphalt mixture under different high temperatures to reveal the instability mechanism of the rutting for asphalt pavement. Cohesive force c and internal friction angle φ in the shear strength parameters for asphalt mixture were obtained by the triaxial compressive strength test. Then, through analyzing the influence of different temperatures on parameters c and φ, the prediction models of shear strength parameters related to temperature were developed. Meanwhile, the corresponding forecast model related to confining pressure and shear strength parameters was obtained by simplifying the calculation method of shear stress level on the failure surface under cyclic loading. Thus, the relationship of shear stress level with temperature was established. Furthermore, the cyclic time FN of dynamic creep instability at 60 °C was obtained by the triaxial dynamic creep test, and the effects of confining pressure and shear stress level were considered. Results showed that FN decreases exponentially with the increase in stress levels under the same confining pressure and increases with the increase in confining pressure. The ratio between shear stress level and corresponding shear strength under the same confining pressure was introduced; thus, the relationship curve of FN with shear stress level can eliminate the effect of different confining pressures. The instability prediction model of FN for asphalt mixture was established using exponential model fitting analysis, and the rationality of the model was verified. Finally, the change rule of the parameters in the instability prediction model was investigated by further changing the temperature, and the instability forecast model in the range of high temperature for the same gradation mixture was established by the interpolation calculation.

Determination of Mechanical Properties of Altered Dacite by Laboratory Methods

This paper presents a methodology for determining the uniaxial and triaxial compressive strength of heterogeneous material composed of dacite (D) and altered dacite (AD). A zone of gradual transition from altered dacite to dacite was observed in the rock mass. The mechanical properties of the rock material in that zone were determined by laboratory tests of composite samples that consisted of rock material discs. However, the functional dependence on the strength parameter alteration of the rock material (UCS, intact UCS of the rock material, and mi) with an increase in the participation of “weaker” rock material was determined based on the test results of uniaxial and triaxial compressive strength. The participation of altered dacite directly affects the mode and mechanism of failure during testing. Uniaxial compressive strength (σciUCS) and intact uniaxial compressive strength (σciTX) decrease exponentially with increased AD volumetric participation. The critical ratio at which the uniaxial compressive strength of the composite sample equals the strength of the uniform AD sample was at a percentage of 30% AD. Comparison of the obtained exponential equation with practical suggestions shows a good correspondence. The suggested methodology for determining heterogeneous rock mass strength parameters allows us to determine the influence of rock material heterogeneity on the values σciUCS, σciTX, and constant mi. Obtained σciTX and constant mi dependences define more reliable rock material strength parameter values, which can be used, along with rock mass classification systems, as a basis for assessing rock mass parameters. Therefore, it is possible to predict the strength parameters of the heterogeneous rock mass at the transition of hard (D) and weak rock (AD) based on all calculated strength parameters for different participation of AD.

Experimental Study of Effect of Glass Fibres on Properties of Concrete Containing Micro-silica and Limestone Powder

The purpose of this work is to investigate the possibility of using mine wastes in the improvement of concrete properties. This research work investigates the physical and mechanical properties of the concrete specimens. These concrete specimens include concrete-lacking fibres, micro-silica and limestone powder (C), concrete-containing glass fibres without micro-silica and limestone powder (GC), concrete-containing micro-silica and limestone powder without fibres (CML), and concrete-containing glass fibres, micro-silica, and limestone powder (CGML). The physical and mechanical properties including the effective porosity, longitudinal wave velocity, water absorption, unit weight, tensile strength, uniaxial compressive strength, triaxial compressive strength, cohesion, and internal friction angle are investigated. The results obtained show that adding glass fibres to the concrete (GC) improve its properties compared to the fibre-less concrete (C). However, the properties of GC are improved significantly less than CGML. The Brazilian tensile strength and uniaxial compressive strength of GC increase by 13.6% and 10.95% relative to C. The Brazilian tensile strength and uniaxial compressive strength of CGML increase by 21.8% and 45.94% relative to C. Finally, it can be concluded that adding the micro-silica and limestone powder to the glass fibre concrete as well as the use of mine wastes also significantly improves the properties of the concrete.

Intelligent Prediction Model of the Triaxial Compressive Strength of Rock Subjected to Freeze-Thaw Cycles Based on a Genetic Algorithm and Artificial Neural Network

Rock compressive strength is an important mechanical parameter for the design, excavation, and stability analysis of rock mass engineering in cold regions. Accurate and rapid prediction of rock compressive strength has great engineering value in guiding the efficient construction of rock mass engineering in a cold regions. In this study, the prediction of triaxial compressive strength (TCS) for sandstone subjected to freeze-thaw cycles was proposed using a genetic algorithm (GA) and an artificial neural network (ANN). For this purpose, a database including four model inputs, namely, the longitudinal wave velocity, porosity, confining pressure, and number of freeze-thaw cycles, and one output, the TCS of the rock, was established. The structure, initial connection weights, and biases of the ANN were optimized progressively based on GA. After obtaining the optimal GA-ANN model, the performance of the GA-ANN model was compared with that of a simple ANN model. The results revealed that the proposed hybrid GA-ANN model had a higher accuracy in predicting the testing datasets than the simple ANN model: the root mean square error (RMSE), mean absolute error (MAE), and R squared ( R 2 ) were equal to 1.083, 0.893, and 0.993, respectively, for the hybrid GA-ANN model, while the corresponding values were 2.676, 2.153, and 0.952 for the simple ANN model.

Dynamic triaxial compressive response and failure mechanism of basalt fibre-reinforced coral concrete

The dynamic triaxial mechanical properties of basalt fibre-reinforced coral aggregate concrete (BFRCAC) were studied using the split Hopkinson pressure bar device with an active confining. The results revealed that the dynamic triaxial compressive strength and dynamic triaxial elastic modulus of BFRCAC increase with an increase in strain rate and confining pressure. The increase in confining pressure reduces the strain rate effect of the dynamic triaxial compressive strength and dynamic triaxial elastic modulus, and the increase in strain rate reduces the confining pressure effect of the dynamic triaxial compressive strength and dynamic triaxial elastic modulus. Furthermore, the critical strain of BFRCAC increases with an increase in the strain rate but decreases with an increase in the confining pressure. The confining pressure reduces the damage degree of BFRCAC; as a result, the failure mode of BFRCAC changes from longitudinal splitting failure to shear failure. The increase in the confining pressure and strain rate reduces the pull-out length of basalt fibre (BF). The addition of BF improves the strain rate effect of the dynamic mechanical properties of coral aggregate concrete (CAC), reduces the failure degree of CAC, and increases the shear failure characteristics of CAC.

Effects of Long-Term Freeze-Thaw Cycles on the Properties of Stabilized/Solidified Lead-Zinc-Cadmium Composite-Contaminated Soil.

Lead, zinc, and cadmium were used to prepare a composite-contaminated soil to replicate common situations, in which soil is usually simultaneously contaminated by multiple metals. To examine the long-term durability of stabilized/solidified (S/S) contaminated soil, specimens were subjected to a series of freeze–thaw (F-T) cycles, up to ninety times (one day per cycle), prior to testing. Triaxial compression tests, soil column leaching tests, and X-ray diffraction analysis were then employed to study the mechanical properties, environmental influences, and micro-mechanisms of the S/S lead-zinc-cadmium composite-contaminated soils after long-term F-T. The results showed that triaxial compressive strength increases within three F-T cycles, then decreases before slightly increasing or stabilizing after thirty F-T cycles. The stage of decreased cohesion thus occurs between three and fourteen F-T cycles, with variation in other factors similar to that of the triaxial compressive strength. The cohesion mainly increases between three and seven cycles. The soil column leaching test showed that the permeability of soil is more than four times higher than that of soil not subject to freeze–thaw cycles after ninety F-T cycles. XRD tests further revealed that the chemical composition of S/S contaminated soil and the occurrence of each heavy metal (HM) remained unchanged under F-T treatment.

Evaluation Method of Production Pressure Differential in Deep Carbonate Reservoirs: A Case Study in Tarim Basin, Northwest China

Deep and even ultra-deep petroleum resources play a gradually increasing and important role with the worldwide continuous advancement of oil and gas exploration and development. In China, the deep carbonate reservoirs in the Tarim Basin are regarded as the key development areas due to their huge reserves. However, due to the unreasonable design of production pressure differential, some production wells suffered from severe borehole collapse and tubing blockage. Therefore, the main purpose of this paper is to optimize a more practical method for predicting the critical production pressure differential. The commonly used analytical methods with different failure criteria for predicting production pressure differential were summarized. Furthermore, their advantages and disadvantages were analyzed. A new numerical model is established based on the finite element theory in order to make the prediction of production pressure differential more accurate. Additionally, both analytical and numerical methods were applied to evaluating the production pressure differential of deep carbonate reservoirs in the Tarim Basin, and the results were discussed compared with field data. In addition, a series of laboratory tests, including porosity and permeability measurements, electron microscope scanning, XRD for mineral analysis, uniaxial and triaxial compressive strength test, etc., were carried out by using the collected carbonate cores from formations deeper than 7000 m to obtain the input parameters of the simulation such as the rock properties. The experimental results showed that the carbonate rocks exhibited a remarkable brittleness and post-peak strain softening. The calculation results revealed that the Mogi-Coulomb criterion is slightly conservative; however, it is more suitable than other criteria to evaluate pressure differential. Furthermore, it has been confirmed by the field data that the finite element numerical method can not only reveal the instability mechanism of the wellbore but also predict the critical production pressure differential accurately. Unfortunately, the on-site operators sometimes require a more convenient way, such as an analytical method, to figure out the pressure differential, even though the evaluation of the numerical method is more accurate. Therefore, the discussion in this paper can provide a basis for the operators to determine the production pressure differential flexibly.

Creep Properties of Altered Rock Subjected to Water Influx-Lost Cycles

Creep properties of altered granite were changed due to water level rise and fall in the tailings ponds. In this paper, the effects of water influx-lost cycles on the creep properties were tested. The axial load was applied with the 30%, 40%, 60%, 75% and 100% of the triaxial compressive strength, the rock creep test results showed that: (1) Significant creep effects of altered granite subjected to water influx-lost cycles, the creep stages were included decay creep stage, steady creep stage, and accelerated creep stage. It was found that the creep strain gradually increases with of the number of water influx-lost cycles increases, and the ratio of the maximum creep strain to total strain can reach 10.42%. (2) The degradation of long-term strength of altered granite subjected to water influx-lost cycles was studied, the long-term strength was decreased from 32.13 to 5.46 MPa. The relationship between the long-term strength and number of cycles was indicated by an exponential function. It was found that water influx-lost cycles have adversely effects on the long-term stability of the tailing pond slope, and the research results are of great significance to the prevention of slope engineering disasters.

Experimental Study on Triaxial Unloading Failure of Deep Composite Coal-Rock

With the deep mining of coal, the phenomenon of high ground stress is more likely to cause dynamic disaster. In view of the above problems, this paper takes the unloading process of coal mining as the background to study the effects of mining rates under different conditions on the mechanical properties and triaxial failure criterion of composite coal-rock, so as to provide a theoretical basis for the prevention and control of dynamic disasters in coal mines. The composite coal-rock models with a composite ratio of 1 : 1 : 1 were tested under different unloading rates or confining pressures. The results show that the triaxial unloading process of coal-rock can be divided into five stages: compaction, single elasticity, elastic-plastic unloading, partial fracture, and complete fracture. In this paper, the failure criterion of composite coal-rock triaxial unloading is derived. The unloading rate has an exponential relationship with the triaxial compressive strength, and the relationship between initial confining pressure and compressive strength is linear. The triaxial compressive strength is determined by both. The peak strains ε of all samples under different unloading conditions were around 0.01. And initial confining pressure had an influence on the strain variation trend during the unloading of composite coal-rock. The higher the initial confining pressure, the greater the elastic modulus. In addition, an increase of initial confining pressure led to the increase of the total energy converted into dissipated part in the process of fracture and caused the obvious increase of the rebound characteristics of the curve. However, the unloading rate had no influence on the strain trend.

Experimental investigation on the mechanical behavior of hybrid steel-polypropylene fiber reinforced concrete under conventional triaxial cyclic compression

Hybrid fiber reinforcing technology has gained a wide recognition in concrete built infrastructure. The cyclic stress–strain relation of hybrid fiber reinforced concrete (HFRC) plays a crucial role in the elastoplastic analysis of HFRC that further determines the structural damage progression and hysteretic behaviors. This paper investigates the mechanical behavior of HFRC subjected to conventional triaxial cyclic compression. A total of 48 HFRC cylindrical specimens are tested for different fiber volume fraction, fiber aspect ratio and confining pressure. The acoustic emission behavior of HFRC is analyzed with an elaboration of failure pattern transition as well as the mechanism analysis of fiber effects, and the macroscopic mechanical properties evolutions in terms of triaxial compressive strength, plastic behavior and stiffness deterioration were also investigated. It is clearly indicated from the test results that the incorporation of hybrid fibers together with the applying confinement can significantly enhance the mechanical property of concrete with the failure pattern transformed from a tensile failure to a shear failure mode. An increment in the strength of HFRC is observed as the steel fiber dosage increases. The polypropylene fiber contributes largely on improving the plastic behavior of HFRC at an expense of a slight decrease in the compressive strength. It is noted that the effect of confinement outweigh the fiber inclusion in restriction of damage degradation whilst the fiber effect is more pronounced as the confinement is low. Thereafter, the stiffness deterioration in terms of Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio are investigated. The results show that the Young’s modulus and shear modulus of HFRC monotonously reduce during the damage process, whilst the Poisson’s ratio would monotonously increase and the bulk modulus exhibits a hyperbolic evolution form. Applying confinement and adding fibers can restrain the deterioration process of the stiffness, and the influences of fibers on stiffness deterioration are more significant under a low confinement level than that under a high level.

The impact of geomechanical units (GMUs) classification on reducing the uncertainty of wellbore stability analysis and safe mud window design

The current study proposed an improved approach for application of Geomechanical Units (GMUs) classification in reducing the uncertainty of wellbore stability models. For this purpose, the optimal drilling direction and safe mud window were modeled in the Iranian Offshore oilfield, Persian Gulf basin. Two main normal faults created a graben with three geomechanical sectors in the study area including northern, middle and southern sectors. The middle sector, which is under high compression, is the main target of this study. A set of well logs and Fullbore Formation Micro Imager (FMI) data associated with the rock mechanical tests including Uniaxial Compressive Strength (UCS), Triaxial Compressive Strength (TCS) and ultrasonic results were used to establish and tune the geomechanical models. The dynamic rock elastic moduli including UCS, Poisson's ratio, Young's modulus, shear modulus and bulk modulus were estimated from well logs data. Through appropriate correlations between well logs and laboratory results, the dynamic modules were converted to static data. Five geomechanical units were obtained based on the classification of rock elastic moduli by using multi-resolution graph-based clustering.The interpretation of FMI logs and their incorporation in the geomechanical models showed that most safe drilling direction is in the 120° azimuth which is parallel to the direction of minimum horizontal stress, σh (NW-SE direction). Higher mud weight is expected at N030E azimuth to overcome collapse pressure, which is the worst drilling direction, σH (NE-SW direction). Wellbore inclination more than 60° angle is not recommended for formations comprising of GMU E and partially GMU D (mainly shales and clayey limestones). The results of this study showed that GMUs classification is an effective way of updating the wellbore stability models and reducing the uncertainty of safe mud window design.

Experimental Study on the Strength and Deformation Characteristics of Concrete under True Triaxial Compression after Sulfate Attack

To explore the mechanical properties of concrete under true triaxial static compressive load after sulfate attack, uniaxial static compression test and true triaxial static compression test at four stress ratios were carried out on concrete specimens immersed in 15% sulfate solution for 0–120 days by the integrated true triaxial static and dynamic load testing system, and the variation of performance indicators such as the strength and deformation of concrete under the coupling action of sulfate attack and complex stress state was analyzed. The results show that the uniaxial compressive strength of concrete increases at the beginning and then decreases with the increase of sulfate attack time and reaches the peak on the 30th day, with an increase rate of 16.57%; the strength of concrete under triaxial compression increases significantly, and the maximum triaxial compressive strength is 3.18 times of uniaxial compressive strength under the combination of 0-day sulfate attack and 0.2 : 0.8 stress ratio; and the deterioration of concrete under sulfate attack is more prominent at high confining pressure, and as the sulfate attack worsens, the sensitivity of triaxial compressive strength of concrete to lateral compressive stress is reduced. In conclusion, triaxial compression can significantly enhance the ductility of concrete by playing a role in restraining the deformation and cracking of concrete after sulfate attack.

Subsurface Characterization for Secure CO2 Storage in the Pennsylvanian-permian Storage Complex in the Midcontinent Region, U.S.A.

Subsurface characterization of target injection and confining zones is essential for developing a safe and successful Carbon Capture Utilization and Storage (CCUS) project. The Integrated Midcontinent Stacked Carbon Storage Hub (IMSCS-HUB) Project is assessing the feasibility of commercial-scale CCUS implementation in the Midcontinent Region of the United States of America (USA) as part of the U.S. Department of Energy-National Energy Technology Laboratory’s (DOE-NETL) Carbon Storage Assurance Facility Enterprise (CarbonSAFE) initiative. The IMSCS-HUB project area is comprised of a carbon dioxide (CO2) source corridor that runs from eastern Iowa to western Nebraska and a stacked CO2 storage corridor that spans across western Nebraska and Kansas, representing the first large-scale CCUS project for the Midcontinent Region. The deep saline reservoirs and caprocks of the IMSCS-HUB stacked storage corridor subsurface make up a Pennsylvanian-Permian Storage Complex which is characterized by thick stratigraphic successions of marine and non-marine sedimentary rocks, (i.e. cyclothems). These stratigraphic successions provide alternating sequences of deep saline formations, oil-bearing reservoirs, shale, and evaporite units that are conducive to vertically stacked CO2 injection for geologic storage and enhanced oil recovery (EOR). Selected areas within the IMSCS-HUB storage corridor have been identified for detailed CO2 storage site characterization in southwest-central Nebraska (Sleepy Hollow Site), in western Nebraska (Madrid Site), and in western Kansas (Patterson Site). As part of the IMSCS-HUB project, characterization wells were drilled, whole and sidewall core were acquired, advanced geophysical well logs were collected, and well testing was conducted the knowledge gaps at the selected storage sites. Regional and site-specific subsurface analyses of the Pennsylvanian-Permian Storage Complex in the Midcontinent Region are conducted to validate caprock integrity through stacked-injection simulations, and to inform the post-injection monitoring plan. The lateral and vertical extents of the formations of interest are defined through well log analysis and geologic mapping. The stress regime is defined using image log data, mini-fracture tests, and drill stem tests. Petrophysical and geomechanical properties of reservoir and caprock intervals are determined through core laboratory testing, advanced well log analysis (sonic and image logs), and well testing (mini-fracture and drill stem tests). The well testing data is also used to determine the fracture pressure gradient/injection pressure constraints. The core laboratory testing includes routine core analysis (porosity, permeability, net confining stress, and grain density), mercury injection capillary pressure, effective permeability, threshold entry pressure, and triaxial compressive strength. The results of the subsurface characterization are used to construct and populate the static earth model and are key inputs for stacked-injection simulations. Three-dimensional simulations assess the baseline stresses in the subsurface and provide information on the fracture gradient to estimate the maximum permissible injection pressure, simulate the mechanical integrity of the caprocks, and evaluate the risk of induced seismic events using an injection scenario of 50 million metric tons, per the CarbonSAFE objective. The IMSCS-HUB project is supported by the U.S. Department of Energy-National Energy Technology Laboratory Agreement No. DE-FOA-0031623.

Stability analysis of complex behavior of salt cavern subjected to cyclic loading by laboratory measurement and numerical modeling using LOCAS (case study: Nasrabad gas storage salt cavern)

Stability analysis of salt caverns is a very complicated subject due to the coupled time-dependent thermo-mechanical behavior of salt during leaching and operational phase of the gas storage subjected to the cyclic loading. Because of purely plastic behavior of salt and the relevant convergence during injection and withdrawal, investigation of the salt cavern stability becomes more challenging. The objective of this study is stability analysis of Nasrabad salt cavern by numerical method using a comprehensive software entitled LOCAS having capability to model the complex time-dependent thermo-mechanical behavior of salt under cyclic loading of natural gas pressure. Measurement of geomechanical properties of salt is also the important requirement for modelling. First, geomechanical properties of Nasrabad salt including uniaxial and tri-axial compressive strength, tensile strength, uniaxial and tri-axial creep under different temperatures were measured. Thereafter, time-dependent behaviors and parameters of dilatancy criterion of the test results were analyzed by the advanced constitutive models for rock salt to obtain accurate parameters for modeling. Then, long-term stability was analyzed for Nasrabad salt cavern having different shapes, sizes, and depths under cyclic loading 3–8 MPa as minimum and maximum gas pressures. The results showed that an ellipsoidal cavern having initial volume of 100,000 m3 at 450 m depth by 0.3% creep closure rate per year and volume loss of 0.8% of the initial volume per year as ideal conditions can store 8,000,000 m3 natural gas with working capacity of about 6,000,000 m3.

A Method to Correct Indirect Strain Measurements in Laboratory Uniaxial and Triaxial Compressive Strength Tests

When carrying out uniaxial, standard triaxial, or true triaxial compressive tests on rock, the strain response of the specimens can be measured directly or indirectly. The most commonly used devices are strain gauges (local direct measurements) and displacement sensors (global indirect measurements). Strain gauges are glued to specimens and directly measure electrical resistance. Displacement sensors, typically linear variable differential transformers (LVDTs), indirectly measure displacements. The former is more precise at its smaller measurement scale, but may not work under high temperatures or when a strain gauge detaches due cracking under stresses close to the peak or in the post-peak portion of the stress–strain curve. It was observed that the strain responses recorded using strain gauges and displacement sensors tend to differ, which is why a corrective approach was proposed to recover stress–strain curves based on indirect measurements. This study describes an approach to correct platen-to-platen displacement measurements based on energy calculations to derive axial strain and to obtain radial strain using modified computations based on the volume fluid change within the Hoek’s cell in triaxial testing. The study concludes that part of the displacement recorded by LVDTs can be associated with displacement in the plate sample contacts, but the component of energy transmitted to the samples can be estimated at different levels of load, so that these indirect measurements can be corrected. For the case of triaxial tests, the volumetric or radial strain can be computed based on the volume change in the Hoek’s cell. This approach is useful for correcting indirect strain measurements. It can also serve as a starting point for the development of corrections for application to data from other test types, such as for true triaxial tests. The study aims to ultimately contribute to a better understanding of strain measurements while conducting compressive tests on rock.

Experimental Investigation of the Mechanical Properties of Sandstone–Coal–Bolt Specimens with Different Angles under Conventional Triaxial Compression

The complex nonlinear mechanical behavior of the combined system of rock and coal surrounding mine openings is a major obstacle for excavation, stability, and support design in mining thin coal seams. In this paper, the triaxial compression failure characteristics and mechanical response of sandstone–coal (SC) specimens with different combination angles are determined. In addition, the influence of the anchor bolt on the mechanical properties of SC specimens is also considered. The results show that the mechanical parameters of the SC and sandstone–coal–bolt (SCB) specimens are different from those of other intact rocks and are substantially affected by the angle. The peak axial strain and triaxial compressive strength (TCS) of the specimens decreased with increasing angles. To comprehensively analyze the phase mechanical characteristics of the specimens and the differences between them, we also established the crack initiation stress index (CI) and crack damage stress index (CD) to evaluate the specimen fracture process. The established stress indices CI and CD can accurately describe the roles of the bolt and angle of the SC and SCB specimens. The bolt can enhance the mechanical properties of the SC specimen after reaching the peak load, restrain the development of cracks, and ensure that the specimen maintains integrity. It should be noted that this type of SCB system is in line with the long-term stability of roadway support engineering applications.

Design and application of hydro-mechanical coupling test system for simulating rock masses in high dam reservoir operations

A meter-level direct shear test system and a true triaxial test system were designed by placing the traditional test apparatus into sealed cabins subjected to high water pressure. The influences of three-dimensional seepage water pressure on the shear and compression deformation of rock mass in Xiluodu Hydropower Station were studied. The test results showed that the changes in water pressure caused obvious shear deformation of the interlayer dislocation zone and tensile deformation and reduction in the triaxial compression strength of the fractured rock mass. The effect of water pressure on shear displacement and tensile displacement had a hysteresis effect. This was consistent with deformation data collected through field monitoring. The deformation mechanism in the reservoir valley was the coupling of the stress and seepage fields caused by reservoir impoundment. The effective stress was reduced, the mechanical parameters were weakened, and the change of the initial stress field led to the slightly overall shear slip and tensile deformation of the bank slope.

Experimental Study on the Mechanical Properties of Triaxial Compression of White Sandstone under the Coupling Action of Chemical Corrosion and Temperature

In order to study the chemical corrosion and temperature coupling effect on the triaxial compression mechanical properties of white sandstone, first, the white sandstone is soaked in acidic, neutral, and alkaline solutions respectively in different temperatures, after that, triaxial compression tests were carried out on the treated white sandstone, then, the strength and deformation of white sandstone after coupling action are analyzed, finally, the chemical corrosion and temperature coupling mechanism is studied. The results show that: At the same temperature (below 100°C),the triaxial compressive strength of the specimens in acidic and alkaline environments is lower than that in neutral environments. In the same chemical (acid or alkali) environment, the triaxial compressive strength of the specimen decreases with the increase of temperature (below 100°C); In a neutral environment, with the increase of temperature (below 100°C),the triaxial compressive strength of the specimen decreases slowly and basically remains unchanged. Below 100°C, the damage of rock caused by temperature is small and can be ignored; Different chemical (acid or alkali) environment can degrade rock damage and reduce its triaxial compressive strength. Temperature mainly affects the rate of chemical reaction, which damages the rock and reduces its strength.