Sandstone is mostly distributed worldwide, and jointed rock masses often experience high temperatures in tunnels, mines, geothermal systems, and fire-exposed structures. To replicate such environments, cylindrical sandstone samples with fully persistent joints at 30° and 60° inclinations are grouted with cement paste and exposed to controlled temperatures ranging from 30 °C to 900 °C. Mechanical performance is evaluated using uniaxial compressive strength (UCS) and Brazilian tensile strength tests, while fracture mechanisms are analyzed through digital image correlation (DIC), quantification of fracture patterns (FracPaQ). The microstructural analysis is carried by Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), and Mercury Intrusion Porosimetry (MIP) techniques. Uniaxial compression tests reveal temperature-induced ductile transition, reduced strength and stiffness, and increased peak strain. The 60° joints exhibit maximum weakening, whereas 30° joints retain higher strength at lower temperatures. Intact specimens exhibit higher strength and elastic behaviour, while 30° jointed specimens show maximum ductility and energy absorption (1.92 MJ/m3 at 300 °C). In contrast, the 60° jointed specimens show the weakest thermo-mechanical behaviour, with absorbed energy decreasing from 0.86 MJ/m3 at 30 °C to 0.028 MJ/m3 at 900 °C. Fracture evolves from matrix cracking to interface failure and, ultimately, thermal disintegration with increasing temperature. Microstructural deterioration is driven by dehydration, mineral decomposition, pore coalescence, and quartz phase transformation. Porosity remains stable up to 500 °C but rises sharply beyond 700 °C, causing severe strength loss in steeply inclined joints. The results identify a critical damage threshold near 500 °C accelerates chemical degradation and strength loss in rock and grout.
This paper reports the findings of a preliminary study that develops a real-time vibration-based cutter wear identification method for pipe jacking shield machine (PJSM) tunnelling. Vibration signals were collected during the excavation of a tunnel through a mountain in Changsha, China, using a custom-designed system of accelerometers deployed on the PJSM. Time-domain analysis revealed a key finding: vibration signals exhibited significant differences across various cutter wear conditions, providing a basis for real-time identification of cutter wear. Considering the advantages of multi-source data fusion and the distinct characteristics of operational parameters and vibration signals, a hybrid encoder-decoder model based on multi-source data fusion, termed CNN-FCNN, was proposed to accurately identify the cutter wear index. The comparative analysis shows that fusing vibration and operational parameters effectively improved model performance compared with single-source models, achieving a test-set R2 of 0.971 using field data collected throughout the pipeline project.
To investigate the influence of moisture on displacement efficiency during CO2-enhanced CH4 diffusion, CO2 was injected under isobaric conditions into coal samples with varying moisture contents to displace adsorbed CH4. In this study, the effect of moisture on CO2-enhanced coalbed methane recovery was systematically investigated through competitive adsorption experiments on anthracite with varying moisture contents (0-3%) at 25 degrees C under equilibrium pressures of 0.8-2.4 MPa. Results show that moisture suppresses gas adsorption more strongly for CO2 than for CH4, as evidenced by the moisture-induced replacement amount of CO2 being consistently greater than that of CH4 under identical conditions. The CH4 desorption rate increases with equilibrium pressure but decreases with moisture content; within 0.8-2.4 MPa, moisture causes a reduction of 6.39% to 9.10%. The CO2 injection ratio declines with increasing moisture, with decreasing gradients ranging from 0.263 at 0.8 MPa to 0.192 at 2.4 MPa, indicating a diminishing marginal effect. Adsorption capacity is enhanced by pressure but suppressed by moisture, showing opposite regulatory trends. Empirical exponential functions were established to quantitatively describe the dependence of CH4 desorption rate and CO2 injection ratio on moisture content, with negative k values reflecting the exponential attenuation of moisture effects. These findings provide quantitative insights into moisture-governed displacement behavior and offer practical guidance for CO2 injection in water-bearing coal seams.
Anthropogenic activities, especially large-scale engineering, can rapidly alter key subsurface conditions (e.g., confined pressure and thermal regime) in deep-buried confined limestone aquifers, affecting safety and long-term performance. Quantifying responses of hydrogeological parameters - porosity (φ), permeability (k), compressibility coefficient (α), and storativity coefficient (µs) - under evolving stress is fundamental for reserve assessment, exploitation optimization, and geotechnical risk mitigation. This study examines stress-porosity-permeability coupling in such aquifers, using the Xiluodu Reservoir (285.5 m arch dam, 4th globally) as a representative field-scale validation case. Core samples from the confined limestone aquifer were systematically tested under effective stress increments from 0 to 29 MPa in seven steps to observe porosity and permeability evolution. Their stress-dependent responses were quantitatively characterized, emphasizing anthropogenic stress perturbations. Comparative analysis of limestones in groundwater aquifers and hydrocarbon reservoirs reveals distinct hydrogeological and petrophysical patterns, particularly effective-stress sensitivity. To improve empirical fitting of measured parameter-response curves, an optimized exponential method (R2 > 0.98) incorporating dead-end pores was proposed. Monte-Carlo simulations established probabilistic stress - parameter relationships, strengthening the utility of limited experimental data. Notably, water compressibility is non-negligible in storage-capacity calculations for deep-buried, low-porosity reservoirs under high effective stress. This study improves aquifer storage prediction, reveals microscopic response mechanisms, supports modeling, and advances understanding of geotechnical stability, over-exploitation remediation, and deep groundwater circulation. Integrating Monte Carlo simulations with experimental data enhances field-scale predictions from limited core measurements. Differential response dynamics of reservoir parameters to changing environments are elucidated. A modified fitting method suitable for various reservoirs is proposed by incorporating dead-end pores. A response expression for Groundwater reservoir reserve to changing effective stress is derived. Water compressibility is indispensable for the reserve calculation in deep-buried dense rocks.
High temperatures not only alter the geotechnical behavior of fine-grained soils but also influence their near-surface geological evolution. Using short-term heating (100–300 °C, 0–96 h) on soils from Changchun and Funing, this study reveals thermally induced regressive evolution, shifting soils from a mature state (rich in organics and clays, fine particles, strong cohesion, and well-cemented microstructure) to an incipient stage with opposite traits. Termed “Retro-Weathering”, this phenomenon counteracts natural weathering processes characterized by clay enrichment and cementation. Combined analyses of Loss on Ignition, X-ray Fluorescence, and hydrometer tests identify three driving mechanisms of “Retro-Weathering”: organic matter volatilization, clay mineral decomposition, and particle coarsening. Liquid-plastic limit, direct shear, and consolidation tests demonstrate that these mechanisms reduce soil plasticity, shift shear strength from a cohesive-frictional to friction-dominated regime, and decrease compressibility toward sandy soil behavior. Scanning Electron Microscopy reveals a dual microstructural evolution: simplified and homogenized particle size distribution contrasts with increasingly complex pore structures, indicating “Retro-Weathering” follows nonlinear multi-stage dynamics rather than a straightforward reversal of weathering processes. The degradation magnitude D (e.g., Δc/c₀) and regressive evolution rates R (e.g., ΔLOI/Δt) increase with heating temperature, duration, and soil developmental maturity (e.g., stronger effects in Changchun soils than Funing soils). This study systematically elucidates heat-induced “Retro-Weathering” in fine-grained soils, advancing geological understanding and providing parameter references for assessing post-heating property changes in fine-grained soils.
