Assessment of Dyke-Induced Strength Variations in Coal and Its Surroundings Using a Non-Destructive In Situ Testing Approach

Uniaxial compressive strength (UCS) and peak strain (PS) are essential indices for studying the mechanical properties of coal and rock masses, and they are closely related to mechanical parameters such as the elastic modulus (E), Poisson’s ratio (υ), cohesion (C) and internal friction angle (Φ) of coal and rock masses. This study took the No. 2-1 coal seam of Zhaogu No. 2 Mine, in Henan Province, China, as the research object. An RMT-150B servo testing machine was used to test all mechanical parameters, including the E, υ, C and Φ of coal and rock masses. Based on the principle of orthogonal testing, Three Dimensions Fast Lagrangian Analysis of Continua (FLAC3D) was used to select E, υ, C, Φ, tensile strength (Rm) and dilation angle (Ψ) as initial participation factors. Using these six parameters and a five-level combination scheme (L25 (56)), the influence of coal mechanical parameters on UCS and PS was investigated, using the software SPSS for stepwise regression analysis, and a uniaxial pressure-resistant regression prediction equation was established. The research showed that, under uniaxial compression conditions, the main parameters controlling UCS of coal masses are C and Φ; conversely, the main parameters controlling PS are E and C. UCS and PS exhibit significant linear relationships with these main controlling parameters. Here, a stepwise regression prediction equation was established through reliability verification analysis using the main controlling parameters. This prediction method produces very small errors and a good degree of fit, thus allowing the rapid prediction of UCS. The precision of the stepwise regression model depends on the number of test samples, which can be increased in the later stages of a design project to further improve the precision of the projection model.

Scale effects and strength anisotropy in coal

Wood–plastic composites (WPCs) are environmentally friendly materials with good weather resistance and low cost. To investigate the feasibility of their use in different environments, a WPC was designed and subjected to a uniaxial compression test at seven temperatures to obtain the failure mode, uniaxial compressive strength, elastic modulus, proportional limit stress, peak stress, and ultimate strain. The results showed the following. The failure modes of the WPC specimens at various temperatures were mainly shear compression failure, double shear failure, and end compression failure. The uniaxial compressive strength and elastic modulus decreased with increasing temperature. Specifically, at temperatures of −60°C, 20°C (normal temperature), and 60°C, the WPC had an average compressive strength of 73.55, 33.7, and 13.51 MPa, respectively, and an average elastic modulus of 7819.11, 6141.71, and 2650.17 MPa, respectively. In terms of the WPC's stress–strain relationship, at a temperature greater than the normal temperature, the WPC had a small peak stress but good ductility; at the normal temperature and below, the WPC had a large peak stress but poor ductility. Based on these findings, the experimental phenomena and characteristic constants were analyzed to establish models of factors that reduce the uniaxial compressive elastic modulus and compressive strength of WPC at different temperatures, to provide a theoretical basis for the mechanical calculations for the application of WPC in various extreme environments.

The uniaxial compression tests and Brazil tests in non-atmospheric environments were conducted on Kumamoto andesite to investigate the environmental dependence on strength of rock. The environments used in the experiment were organic vapor environments as methanol, ethanol and acetone, inorganic gas environments as argon, nitrogen and oxygen and water vapor environment.The obtained results are as follows:1. From the uniaxial compression test, it was clear that the uniaxial compressive strength of Kumamoto andesite decreases in order of the environment of acetone, ethanol, methanol and water vapor, that the strength in inorganic environments was independent on environments and that the average uniaxial compressive strength in inorganic environments was 1.7 times of that in water vapor environment.2. From the Brazilian test, the tensile strength of Kumamoto andesite in environments except water vapor was almost constant, although that in methanol is slightly low.3. Both uniaxial compressive strength and tensile strength of Kumamoto andesite were the lowest in water vapor environment and highest in inorganic environments and these strengths in alcohol environments existed between those in water vapor environment and inorganic environments.4. Water is the most effective agent that promotes stress corrosion of rock among the materials used in this research and the strength of rocks is dependent on the subcritical crack growth due to stress corrosion.

The study investigated geotechnical parameters of sandstone in Banda, Kogi State, Nigeria using regression model. To achieve the set objectives, in‐ situ tests were carried out by Schmidt hammer and laboratory tests were conducted on the sandstone samples collected from the study area. Five rock samples were prepared for the determination of density and porosity. The results obtained from the Schmidt hammer tests and those of density determined from laboratory tests were used to estimate the uniaxial compressive strength of the investigated rock. Point load index and tensile strength were determined from the laboratory tests and the relationship established by Brook, (1993) respectively. The results of the analyses reveal that the average density of sandstones is 2.08 g/cm3, the average porosity of the tested sandstones is 13.0% while the average uniaxial compressive strength value is 26.3 MPa. The average point load index and that of the tensile strength value are 1.17 MPa and 1.76 MPa respectively. The determined density, porosity, uniaxial compressive strength, point load index and tensile strength were analyzed statistically. From the statistical analysis mathematical models were established. The regression coefficients obtained from the analysis show that there are strong relationships between the physical and mechanical properties of sandstone.

This research examines the correlation between physico-mechanical properties of some selected granite rocks in Ondo State, Nigeria. Samples were collected at Zibo FM Quarry, Aaye and Samchase Quarry, Itaogbolu and tested in the laboratory for the determination of rock density, hardness, and strength. An average density of between 2.57g/cm3 and 2.67g/cm3 was recorded. Schmidt hammer and Rockwell hardness machine wereused for the determination of rock samples hardness. An average uniaxial compressive strength of rock sample was estimated from the values obtained from Schmidt hammer rebound value. The Hardness test varies from 115.90MPa to 185.00MPa (classified to have high strength) while the average Rockwell hardness varied from 45.1HRA - 57.32HRA. The average point load index ranges from 7.402MPa - 7.505MPa classified to have very high strength. Multiple regression analysis using computer-aided solution (Statistical Package for Social Sciences - SPSS) was used to analyze data obtained from the laboratory and field tests. Five parameters were input into the multiple regression analysis to generate the models. Two parameters which are Schmidt hammer rebound value and Rockwell hardness value out of five parameters are dependent variables, while point load index, uniaxial compressive strength, and density are independent variables. The result of the models developed shows positive correlations – an indication that the hardness of rock gives a significant contribution in drilling bit selection. Keywords : Rock density, Schmidt hardness, Rockwell hardness, uniaxial compressive strength, point load strength, and SPSS

Rock mass is a volume of rock consisting of rock material in the form of minerals, texture, composition and also consisting of discontinuous planes, forming a material and interconnected with all elements as a unit. The rock mass itself is composed of several intact rocks which basically have isotropic, continuous and homogeneous properties. However, the conditions found in the field are different, namely anisotropic, discontinuous and heterogeneous. These properties will certainly influence the test results in the uniaxial test. There are several factors that influence the results of uniaxial rock tests, one of which is the scale effect. The purpose of this test is to determine and analyze the effect of rock sample size on the uniaxial compressive strength value of claystone. This rock testing was carried out at the Mineral and Coal Technology Laboratory and the rock sampling locations were in Palaran District, Samarinda City and in North Samarinda District, Samarinda City. In this uniaxial compressive strength test, 3 side widths with different lengths will be used. After carrying out the uniaxial compressive strength test, the average uniaxial compressive strength test value was obtained in each formation, such as the Balang Island formation, the average rock compressive strength test value was 1.68 Mpa and the Balang formation, the average rock compressive strength test value was 3.10. Mpa. Based on these results, it can be concluded that the larger the sample size, the smaller the rock compressive strength test value tends to be.

This paper investigates the variation of mechanical properties of granite during temperature and stress cycling, which is an important part of evaluating the long-term thermal and mechanical stability of thermal energy storage. Cyclic temperature and loading tests were conducted where the upper limit of cyclic temperature was 100–600 °C, and the upper stress limits were 70% and 85% of the average uniaxial compressive strength (UCS) at the corresponding temperature. The response of stress–strain characteristics of the granite samples to changes in temperature, and cyclic load upper limit, while the number of temperature and loading cycles was comprehensively analyzed. The results show that the temperature and stress cycles have significant effects on the mechanical properties of granite (i.e., stress–strain curve, strength, elastic modulus, and deformation). The elastic modulus of the sample during loading increases gradually. The strain corresponding to the upper loads of the granite samples decreases with an increasing number of cycles. Additionally, the UCS of samples after 10 cycles at 70% loading stress is greater than that at 85% loading stress. The mechanical properties of samples change dramatically during the first and second cycles at 85% loading stress, whereas at 70% loading stress, the mechanical properties change gradually in the first few cycles, and then tend to stabilize. Cyclic hardening is observed at temperatures below 500 °C, where post cyclic UCS is greater than the uncycled average UCS. This phenomenon requires further research.

Volcanoes are dynamic and complex natural systems, constantly changing through eruptions, alteration, and erosion. Hydrothermal systems are ubiquitous on volcanoes, causing physical and mechanical change to rock properties via hydrothermal alteration. The most commonly measured rock physical properties are porosity and uniaxial compressive strength (UCS) as they provide insight as to their history and potential mechanical behavior. Porosity and UCS are affected by the primary properties and emplacement history (e.g., volatile content, crystallinity, cooling rate, composition, fragmentation type) and the post-emplacement conditions (e.g., surface weathering and hydrothermal alteration). This creates highly heterogenous rock masses, with variation occurring across mm to m scales. Currently, destructive testing is required to measure UCS and porosity (destructive of the wider sample) accurately and needs a large volume of samples to capture the heterogeneity of volcanic rock masses. This testing is cost and time prohibitive, requiring large sample volume, in terms of sample size and number, which often require shipping to specialist labs. Schmidt hammers can be used to estimate UCS non-destructively, however, they produce inaccurate results on soft rocks such as hydrothermally altered rocks. Here, we present a new non-destructive method for predicting porosity and UCS across a range of volcanic rocks, from non-altered to highly altered.This study uses visible-near infrared (VNIR) to shortwave infrared (SWIR) wavelengths (350-2500 nm) reflectance spectroscopy to predict porosity and UCS via Partial Least Squares Regression (PLSR). Reflectance spectroscopy is a non-destructive method that is sensitive to both physical (surface roughness and crystal/particle size) and chemical (mineral species and abundance) properties of volcanic rocks. Because these rock attributes also influence the physical and mechanical properties of rock, reflectance spectroscopy could be used to quantitatively predict porosity and UCS. This study used experimentally deformed volcanic rocks from Ruapehu, Ohakuri, Whakaari, and Banks Peninsula (New Zealand), Merapi (Indonesia), Chaos Crags (USA), Styrian Basin (Austria), La Soufrière de Guadeloupe (Eastern Caribbean), Volvic (France), and Cracked Mountain (Canada) to evaluate the accuracy of PLSR-based predictions for porosity and UCS. The training samples encompass a wide range of volcanoes, alteration degree (non-altered, silicic, argillic, and phyllic alteration), mineralogical differences (initial composition from basalt to rhyolite and alteration products), and textural differences (original textures such as lava and pyroclastic, and alteration textures including veins). Model sensitivity is evaluated by adding randomly individual samples to the training database or performing leave one group out cross validation based on characteristics (e.g., alteration mineral types, textural features, or volcano location). From this analysis, specific alteration mineralogy can be evaluated for its effect on porosity and UCS predictions such as the role of phyllosilicate formation causing a reduction in UCS. The proposed non-destructive method via VNIR-SWIR spectroscopy can complement existing rock mechanical testing methods to better quantify highly heterogenous volcanic and hydrothermal systems and their rock successions.

Understanding the behavior of the top-coal caving mining face and immediate roof can be used to enhance buffering effects. The mechanical properties of the coal-rock combined body (CRCB) play a vital role in the performance of overburden load transmittance and support resistance design. We define and derive the relative physical and mechanical parameters of CRCB to illustrate and analyze the influence of coal-rock height ratio (CRHR), coal and rock mass behavior, and interface parameters on CRCB mechanical properties. We conducted uniaxial compression tests to obtain uniaxial compressive strength (UCS), elastic modulus (EM), and the full range of stress–strain curves. Our results show that UCS is positively correlated with EM. However, CRCB EM and UCS decrease with increasing CRHR or effective coal-rock height ratio (ECRHR) and the slope of the curves gradually decreases. CRCB mechanical parameters increase linearly with EM of the coal or rock mass. Although increased coal-rock interface angles (IA) lead to increased CRCB mechanical parameters, the incremental value can be ignored. Sensitive analysis shows that the rank of influential factors on CRCB properties is CRHR/ECRHR > coal strength > rock strength > IA.

A sound understanding of the water permeability evolution in fractured shale is essential to the optimal hydraulic fracturing (reservoir stimulation) strategies. We have measured the water permeability of six fractured shale samples from Qiongzhusi Formation in southwest China at various pressure and stress conditions. Results showed that the average uniaxial compressive strength (UCS) and average tensile strength of the Qiongzhusi shale samples were 106.3 and 10.131 MPa, respectively. The nanometre-sized (tiny) pore structure is the dominant characteristic of the Qiongzhusi shale. Following this, we proposed a pre-stressing strategy for creating fractures in shale for permeability measurement and its validity was evaluated by CT scanning. Shale water permeability increased with pressure differential. While shale water permeability declined with increasing effective stress, such effect dropped significantly as the effective stress continues to increase. Interestingly, shale permeability increased with pressure when the pressure is relatively low (less than 4 MPa), which is inconsistent with the classic Darcy's theory. This is caused by the Bingham flow that often occurs in tiny pores. Most importantly, the proposed permeability model would fully capture the experimental data with reasonable accuracy in a wide range of stresses.

The mechanical properties of frozen rocks vary significantly from the properties of the same lithology under ambient temperature. The goal of this paper is to investigate these changes in the physical and mechanical properties of rocks due to saturation and freezing. Besides, the attention was paid on discovering new correlations between the mechanical characteristics. To fulfill these objectives, 36 uniaxial compressive strength tests, 36 Brazilian splitting tests, and 48 point load tests were carried out. The samples were tested in air dry, water saturated, and frozen (− 20 °C) conditions. The measured physical and mechanical parameters were analyzed by using regression analyses. It was found that the average uniaxial compressive strength of frozen samples (21.93 MPa) is 86.4% more than saturated ones (11.76 MPa) but 25.9% less than dry specimens (29.62 MPa). Additionally, high correlations were established between uniaxial compressive strength and IS(50) under air-dry, saturated, and frozen conditions for the investigated marl samples. Furthermore, it is of particular interest to observe a high correlation with the determination coefficient (R2 = 0.95) between the constants of previously published linear regressions of UCS- Is(50) under dry status.

Like any other coal, the highly heterogeneous nature of brown coal can sometimes make it difficult to interpret the results of laboratory experiments. More homogeneous samples with properties reproducible in the laboratory would provide significant advantage, especially in understanding the effects of various factors in the properties of coal. An attempt was made to develop reconstituted coal (RC) samples in the laboratory through an extensive material development and laboratory testing programme. The latter consisted of mainly uniaxial compression tests. The main objective in developing the RC material is to use it in future research on CO2 sequestration in unmineable coal seams. A highly homogeneous coal sample would make it much easier to identify, for example, the effect of CO2 sorption on the mechanical, flow and transport properties of coal. Uniaxial compression tests were conducted on some brown coal samples to determine the approximate mechanical properties. The results revealed an average uniaxial compressive strength of 1.46 MPa, an average elastic modulus of 77.43 MPa and a Poisson’s ratio of 0.16. The measured properties were used as a reference for the development of RC samples. An extensive laboratory experimental programme was conducted to develop RC samples with the desirable mechanical properties. Portland cement was used as the cementing agent for the RC. Different variables such as percentage of cement, water content, compaction load and curing time were taken into account when developing RC samples. Uniaxial compression tests were carried out to ensure that the RC samples were reasonably homogeneous and the properties were similar to those of natural coal. Percentages of cement by weight of coal such as 4, 6 and 8% were attempted and a 4% cement mix with 50% water was considered most suitable for the RC samples. Average compressive strength of 0.8 MPa (28-day strength) and an average elastic modulus of 34 MPa were achieved for the RC samples. Further efforts at improvement would involve better matching of the uniaxial compressive strength and elastic modulus of RC samples with the natural coal samples.

Exploration of petrographic, elemental, and material properties of dynamic failure-prone coals

Microbial-induced calcium carbonate precipitation (MICP) technology mainly uses carbonates produced by the reaction of microbial activities to repair rocks and soils. Temperature influences microbial metabolism and the kinetics of chemical reactions. In this study, microbial repair experiments on fractured sandstone under different temperatures are carried out. The repair effects are tested with nuclear magnetic resonance (NMR), an X-ray automatic diffractometer (XRD), uniaxial compressive strength (UCS), and a scanning electron microscope (SEM) test. The influence of the temperature on the restorative effects of MICP was discussed. The results show that the repair effect of the Sporosarcina pasteurii is significantly better as the temperature increases. When the temperature reaches 33 °C, the porosity and permeability of fractured sandstone can be reduced by 55.174% and 98.761%, respectively. The average uniaxial compressive strength can be restored to 6.24 MPa. The repair effect gradually weakens with the increase in temperature. However, the Sporosarcina pasteurii can still maintain relatively good biological activity at temperatures from 33 °C to 39 °C. The main form of CaCO3 produced in the process of MICP is calcite. It can fill in the rock pores, and result in reducing the size and number of large pores and improving the impermeability and strength of fractured yellow sandstone.