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Acoustic emission (AE) is an effective tool for revealing the rock failure mechanism. However, research has found that over 90% of test tensile failure sources cannot be captured. Furthermore, previous studies on thermal damage and AE characteristics have neglected the influence of mineral and grain boundary strength. Given this, this study establishes a Grain-Based Model that can accurately reflect the irregular shape of minerals. The model also considers the influence of minerals and intergranular strength. Firstly, an analysis is conducted to investigate thermal crack generation rules. Secondly, the quantitative investigation of temperature and grain size on AE properties is examined through moment tensor inversion. Finally, a comparative analysis explores the disparities in thermal crack generation and AE characteristics between different modelling methods. The results indicate that the phase transition of quartz causes significant structural damage, resulting in a sharp increase in microcracks. Additionally, the number of thermally induced cracks decreases as grain size increases. AE magnitudes exhibit a normal distribution with AE frequency and follow a power-law distribution with the cumulative number of AE events. The maximum magnitude gradually decreases with increasing temperature, while magnitude shows no significant correlation with grain size. The frequency of large-magnitude events decreases with rising temperature but increases with grain size. The formation pattern of thermally induced cracks and the distribution of AE failure sources strongly correlate with mineral and intergranular strength. Therefore, to accurately depict failure mechanisms in crystalline rocks, the models should consider the influence of mineral and intergranular strength.HighlightsThe Grain-Based Model was employed to investigate the influence of grain size and temperature on the mechanical properties and failure mechanism of crystalline rocks.The influence of grain size and temperature on the acoustic emission characteristics of crystalline rocks was quantitatively studied based on moment tensor inversion.The differences in the study of thermal damage and acoustic emission characteristics of crystalline rocks using different modeling methods were revealed, and the sources of the differences were analyzed.

Investigation of hydromechanical behavior of fluid-saturated tight rock is motivated by the need to quantify the effect of changes of fluid pressure p and mean stress P on rock deformation, hydrothermal fluid, and mass transport. In particular, hydromechanical properties of low porosity crystalline rock are required for analysis of geological processes including areal hydration or dehydration, mineral weathering, and fault mechanics. In this study, poroelastic parameters – drained bulk modulus K, and Biot coefficient α – governing the volumetric response of Westerly blue granite, a typical crystalline rock of low porosity are measured. Three additional hydromechanical properties, unjacketed bulk modulus Ks′, expansion modulus H, and permeability k, are also measured. For the Terzaghi effective mean stress of 1.0 < P′ = P-p < 25.0 MPa, the unjacketed bulk modulus Ks′ = 57.5 GPa is constant within the range of mean stresses investigated but other poroelastic coefficients exhibit effective mean stress dependency; the ranges are 13.2 < K < 32.3 GPa, 19.0 < H < 60.0 GPa, 0.83 > α > 0.38, and 20 > k > 5 nanodarcy. The agreement between poroelastic coefficients determined from various methods suggests that the underlying linear elastic assumption in Biot’s theory of poroelasticity is applicable to Westerly blue granite over small increments of effective mean stress.HighlightsA procedure for saturation of tight, low-porosity rock is illustrated.Biot coefficient, unjacketed, expansion, and drained bulk moduli are measured and verified using different approaches.

Most of the contaminant conceptual models in weathered crystalline bedrock aquifers in Brazil rely on data collected using conventional site investigation methods, at insufficient spatial resolution for associating contaminant distributions with observed heterogeneity. The DFN methodology was adapted for the characterization of a chlorinated solvent contaminated weathered crystalline bedrock aquifer, located in the region of the Jurubatuba district, Sao Paulo city, Brazil. The objective of this research is to establish a detailed 1-D conceptual model to show variable contaminant distribution within the bedrock matrix under different fracture and weathering conditions. The numerous depth-discrete rock sample analyses, assigned to specific geologic features, indicate the presence of PCE-Dense Non-Aqueous Phase Liquid (DNAPL) at discrete-depth intervals along the profile. These zones are mainly related to the lithological contacts and interfaces among the weathered bedrock. The soft and hard weathered-bedrock transition zone does not indicate a sharp decrease in the contaminant mass retention within the bedrock matrix for the studied area. This indicates that contaminant mass storage in the bedrock remains beyond this drilling method-defined interface, which can potentially sustain dissolved VOC concentrations over time and maintain the continuous vertical mass flux into deeper parts of the aquifer. There is a demonstrated need for an appropriate characterization methodology including high-resolution soil coring and sampling to delineate contaminant distribution over crystalline weathered bedrock aquifers. This is a key assumption for the definition of appropriate remedial and groundwater management strategies.

Microbial life in the nutrient-limited and low-permeability continental crystalline crust is abundant but remains relatively unexplored. Using high-throughput sequencing to assess the 16S rRNA gene diversity, we found diverse bacterial and archaeal communities along a 2516-m-deep drill hole in continental crystalline crust in Outokumpu, Finland. These communities varied at different sampling depths in response to prevailing lithology and hydrogeochemistry. Further analysis by shotgun metagenomic sequencing revealed variable carbon and nutrient utilization strategies as well as specific functional and physiological adaptations uniquely associated with specific environmental conditions. Altogether, our results show that predominant geological and hydrogeochemical conditions, including the existence and connectivity of fracture systems and the low amounts of available energy, have a key role in controlling microbial ecology and evolution in the nutrient and energy-poor deep crustal biosphere.

This study presents a detailed statistical optimization methodology for improving the performance of high-strength concrete through the strategic integration of weathered crystalline rock (WCR) fine aggregate. Utilizing Taguchi L5 orthogonal array design, the research assessed the impact of varying WCR replacement levels (0%, 5%, 10%, 15% and 20%) on essential concrete properties, encompassing mechanical strength, fresh properties, durability metrics and impact resistance. The experimental program included 175 specimens tested across seven response variables to find out how they worked together in detail. By using signal-to-noise ratio analysis and grey relational analysis together, the best replacement strategies were found. For example, adding 5% WCR resulted in better multi-objective performance with a grey relational grade of 0.953. Using mechanical property correlations, advanced regression modeling was able to predict compressive strength very well (R² = 0.997). ANOVA statistical validation showed that all response variables had significant factor contributions (> 94%). The optimized mix had a compressive strength of 74.5 MPa, improved workability (66.3 mm slump) and impact resistance (778 blows). The results show that using WCR strategically can help meet both structural performance goals and sustainability goals at the same time. This sets a strong foundation for developing sustainable high-strength concrete for use in infrastructure.

In this paper, the micro-cracking behaviour of pre-cracked Barre granite is investigated using a grain-based distinct element model (GBM). We investigated and demonstrated a cohesive model in a distinct element code, PFC2D, to mimic the elastic and softening response of the intra-grain contacts in the GBM. The study employed the smooth-joint model to simulate the micro-cracking behaviour of grain interfaces. The grain size distribution, as well as the mineral constituent of Barre granite, was incorporated in the numerical model. The model was calibrated against uniaxial compressive strength and Brazilian split-tensile-strength tests. We found that the GBM framework successfully reproduced the macroscopic physical properties obtained from the laboratory tests. When calibration was complete, the geometries of pre-existing cracks, which were considered in the experimental testing, were imported into the numerical model and used to generate synthetic, pre-cracked Barre granite. The macroscopic cracking process in the generated numerical models was observed by monitoring the evolution of intra- and inter-granular micro-cracks. The cracking and coalescence behaviour of numerical pre-cracked granite revealed that the proposed GBM approach can replicate the macroscopic fracturing pattern of pre-cracked Barre granite with close agreement to the experimental observations. The crack initiation, coalescence, and peak axial stresses were also recorded during numerical testings, and a good agreement was also achieved between these simulated results and the laboratory data. The proposed GBM framework is promising for research into micro-cracking behaviour of pre-cracked crystalline rocks under compressive loading.

Characterization of mechanical behaviors of thermal-damaged rocks holds significant implications for deep-ground engineering such as nuclear waste disposal. A thermal-damaged rock can present an abnormal negative Poisson's ratio (NPR) and compression-hardening memory (CHM). However, thus far, how the NPR and CHM influenced the loading/unloading behaviors of thermal-damaged rocks remains unclear from the macro- to micro-scale. This study first experimentally explored the macro-behaviors of Crystalline Rocks with varied Thermal-Damaged Degrees under cyclic unconfined compression. Subsequently, Grain-Based Models considering heterogeneity were established to reproduce experimental results and further quantitatively analyze macro- to micromechanical behaviors of thermal-damaged Crystalline Rocks. Results showed that the CHM and NPR had prominent influences on the Loading/Unloading Response of thermal-damaged Crystalline Rocks, such as non-linear stress–strain relations and tensile stress concentration characteristics. With enhancing the thermal-damaged degree, the grain contact had a more significant reduction of compressive contact stiffness than the shear one and an increment of maximum allowable closure, thereby enhancing promoted NPR and CHM effects. Moreover, the CHM could weaken the NPR owing to enhancement of the compressive contact stiffness under Cyclic Compression. The CHM and NPR affected the magnitude and proportion of the tensile stress and formation of intergranular and intragranular failure which was influenced by the maximum historic compressive stress. Due to the NPR effect, the failure was initially concentrated at both ends of the synthetic sample. Under Cyclic Compression and increasing compressive stress, the NPR effect was attenuated, resulting in a more uniform failure pattern. Highlights A thermal-damaged rock could present the negative Poisson's ratio (NPR) effect and enhanced compression-hardening memory (CHM). The CHM could weaken the NPR due to enhancement of the compressive contact stiffness under Cyclic Compression. The NPR and CHM synergistically affected the Loading/Unloading Response of thermal-damaged rock.

The energy sector is undergoing a fundamental transformation, with a significant investment in low-carbon technologies to replace fossil-based systems. In densely populated urban areas, deep boreholes offer an alternative over shallow geothermal systems, which demand extensive surface areas to attain large-scale heat production. This paper presents numerical calculations of the thermal energy that can be extracted from the medium-deep borehole heat exchangers in the low-enthalpy geothermal setting at depths ranging from 600 to 3000 m. We applied the thermogeological parameters of three locations across Finland and tested two types of coaxial borehole heat exchangers to understand better the variables that affect heat production in low-permeability crystalline rocks. For each depth, location, and heat collector type, we used a range of fluid flow rates to examine the correlation between thermal energy production and resulting outlet temperature. Our results indicate a trade-off between thermal energy production and outlet fluid temperature depending on the fluid flow rate, and that the vacuum-insulated tubing outperforms a high-density polyethylene pipe in energy and temperature production. In addition, the results suggest that the local thermogeological factors impact heat production. Maximum energy production from a 600-m-deep well achieved 170 MWh/a, increasing to 330 MWh/a from a 1000-m-deep well, 980 MWh/a from a 2-km-deep well, and up to 1880 MWh/a from a 3-km-deep well. We demonstrate that understanding the interplay of the local geology, heat exchanger materials, and fluid circulation rates is necessary to maximize the potential of medium-deep geothermal boreholes as a reliable long-term baseload energy source.

Grain-scale heterogeneity strongly influences hydraulic fracture initiation and trajectory in crystalline rocks, yet its contributions to non-planar growth and the interaction of multiple nearby cracks remain insufficiently quantified. To address this gap, we perform numerical experiments on a model containing two parallel pre-existing cracks using a hydro-mechanical phase-field framework, systematically quantifying how mineral distribution and axial compression govern non-planar hydraulic fracture growth and inter-fracture competition. The results demonstrate that mineral distribution is the primary driver of fracture complexity. Even within the same Voronoi tessellation, redistributing minerals alone yields markedly different trajectories, deflections, branching patterns, and final morphologies. Furthermore, non-planar growth follows a stepwise, energy-threshold-driven mechanism. When cracks penetrate strong grains or undergo large-angle deflections, propagation is impeded, and injection pressure builds up. Once a critical energy threshold is reached, accumulated energy is rapidly released along the path of minimum incremental energy, manifested as abrupt pressure drops and rapid crack advance. Additionally, the two nearby fractures exhibit strong mechanical competition. Despite negligible hydraulic interference in low-permeability granite, early growth of one fracture redistributes stresses and suppresses the driving force of the other, resulting in asymmetric development. Finally, axial compression primarily governs the overall propagation orientation and influences local failure modes but has a limited effect on peak pressure relative to mineral distribution.

In Ghana, crystalline rock aquifers with secondary hydraulic features and preferential flow pathways serve as very important aquifers. Protecting and managing these geological porous media require identifying preferential flow pathways and hydraulic characteristics. In this study, preferential flow pathways and hydraulic parameters were characterised for two boreholes (Hilly and Valley) located in Tarkwa, Ghana via the integrated use of borehole dilution testing (BDT), slug testing, and geological mapping. The geological fieldwork mapped a relatively sparsely fractured Sandstone Unit overlying a heavily foliated and fissile Phyllite Unit. Geology influenced groundwater flow in boreholes. The BDT confirmed the geology by showing a stratified flow system with preferential flow pathways. The Hilly/recharge area borehole shows relatively low downward flow, whereas the Valley/transition area borehole is dominated by the concentrated diffuse horizontal flow. These flow patterns are in agreement with the borehole locations and their dominant geologies. The estimated hydraulic conductivity, transmissivity, and storage coefficient ranged between 5.1 × 10−4–7.7 × 10−2 m/d, 4.0 × 10−2–3.2 m2/d, and 10−7–10−5, respectively. The cheap integrated hydrogeophysical methods used in this study are applicable for characterising, protecting, and managing other crystalline aquifers in the West African sub-region and other tropical terrains.