The recirculation of concentrated leachate (CL) in municipal solid waste landfills has been shown to effectively reduce organic biochemical indicators in the effluent; however, it may also increase salinity within the landfill, raising concerns about its long-term sustainability. In this study, a laboratory-scale bioreactor was established to periodically inject nanofiltration-treated CL into waste with high and low organic matter content. Variations in waste composition, leachate characteristics, and landfill gas production were monitored. Results indicated that CL recirculation inhibited waste degradation and methanogenesis, leading to a 47.5% and 77.0% reduction in methane potential compared to the control groups. At the end of the experiment, 34.0% and 33.7% of the biodegradable material remained in the solid phase of the high and low organic waste, respectively. CL recirculation facilitated the release of NH3 and H2S, with total emissions being one to two times higher than those of the control group. It also promoted salinity accumulation; influenced by the temperature of the waste core, the final leachate salinity exceeded that of the recirculated CL, reaching 33,800-36,000 mg/L. Low organic matter waste was more vulnerable to the adverse effects of CL recirculation, with degradation and methanogenesis processes nearly halted. These observations were consistent with salinity-related inhibition and may also have been influenced by free ammonia toxicity and substrate competition involving sulfate-reducing bacteria. Considering methane's economic value, the microbial environment in the waste, and the emission of odorous gases, the sustainability of CL recirculation appears to be relatively low.

Groundwater seepage is one of the primary drivers for the migration of contaminants at a site. Currently, the long-term direction of groundwater seepage is critical to the service life of vertical pollution-barrier walls in engineering design. Based on the convection–diffusion equation, we created a numerical model in three dimensions for this investigation. The site’s horizontal hydraulic gradient was 0.02 and the barrier’s insertion depth was fixed at 12 m. This was done to examine how the angle (α) between the suspended vertical barrier’s length direction and seepage direction affected the barrier’s resistance to breakthrough from flow around the bottom for contamination containment. The numerical model is then validated against experimental and analytical results in order to demonstrate its accuracy. According to simulation results, changing α alters the distribution of maximum bypass flow velocities and resulting in an uneven flow-field distribution inside the barrier-controlled area. The position of the breakthrough failure gradually moves from the middle of the downstream bottom of the barrier to the end of the barrier when the value of α falls between 45° and 90°. The service life against breakthrough can be increased by up to 10.7

The long-term safety of geological disposal for high-level radioactive waste relies critically on the effective retardation of radionuclide migration by geological barriers.Given the characteristics of radionuclide migration processes at large spatiotemporal scales,hypergravity experiments and multi-scale numerical simulations have become essential tools for investigating long-term seepage and solute transport behaviors in fractured media.Research progress in hypergravity testing techniques for seepage and solute migration in fractured rock masses,similarity theories for hypergravity tests of fractured rock masses,and long-term safety evaluations of geological barriers was reviewed.To address current challenges such as the integrated preparation of micron-scale matrix and fracture systems,similarity theories for complex rough fractures,and coupled thermal-hydrological-mechanical-chemical(THMC)processes,future research should focus on the integrated printing of micro-fracture and pore structures,similarity theories for natural rough fracture structures in hypergravity experiments,multi-process response mechanisms,and full-process simulations spanning ten-thousand-year timescales.

A novel stress-based hybrid phase-field model is developed to describe mixed-mode cracking in frictional rock-like materials, with particular attention to gravitational stress-gradient effects. To capture tensile-shear competition, the formulation introduces two physically motivated driving forces: the tensile stress-related elastic strain energy governing tensile cracking, and an equivalent shear stress-related energy driving shear cracking while incorporating internal friction. Their interaction is embedded in a mixed damage evolution criterion. In addition, a damage-hardening law is proposed to describe the evolution of shear fracture toughness with accumulated local compressive strain, enabling transition between tensile and shear dominated cracking modes. Implemented within a finite element framework, the model is first validated against four representative benchmarks. It is then applied to plate specimens with multiple pre-existing flaws under uniaxial compression subjected to both normal gravity and hypergravity conditions. The results indicate that, under normal gravity, cracks may initiate at different locations and a transition from shear to tensile dominated cracking can occur, whereas under hypergravity cracks preferentially initiate in the lower region of the specimen and propagate upward as tensile wing cracks. Moreover, the crack initiation angle and peak strength are influenced by the gravitational stress gradient. Hypergravity exhibits a clear suppressing effect on shear cracking.

Geological carbon sequestration (GCS) mitigates climate change by storing anthropogenic carbon dioxide (CO2) in geological formations. CO2 undergoes complex physical and chemical transformations in deep geological formations, governed by various interacting trapping mechanisms. Because the trapping mechanisms operate over a wide range of different timescales, their long-term interplay remains unclear. We develop an integrated numerical modeling framework to analyze and track the plume footprint and phase transition processes that occur throughout the entire cycle of the injected CO2 in saline aquifers. The key novelty of the modeling framework lies in its capability to describe multiple hydrodynamic processes and their interactions, including injection, dissolution-driven convection, reactive transport, and gravity-induced Ostwald ripening. The results suggest that dissolution reduces the lateral migration of free-state CO2, while geochemical reactions generate preferential pathways for CO2-rich flow. For the scenarios we analyze, after 500 years of mass transfer, dissolved CO2 accounts for 42.80 % of total trapped CO2 mass, while reactive CO2 contributes less than 1 %. The results also illustrate that low vertical permeability is unfavorable for the long-term transition of CO2 from physical trapping to dissolution trapping. When the permeability anisotropy index γ increases from 0.5 to 10, the total dissolution storage amount within the domain is reduced to one-third over the 500-year simulation period. This integrated modeling framework provides critical insights into the long-term evolution of CO2 plume migration and phase transition behavior, thereby offering a practical tool to quantitatively assess the long-term fate of the injected CO2 in saline aquifers.

This study develops a two-dimensional numerical model for seepage and pollutant transport in sites with suspended cutoff walls, grounded in the advection-dispersion equation. The impacts of key factors, including the pressure head of the pollution source, the site's horizontal hydraulic gradient, and the horizontal distance between the downstream wall and the pollution source, on the non-linear seepage distribution and the wall's anti-pollution performance were investigated. Centrifuge model tests were conducted to confirm the numerical model's dependability. Furthermore, the concentration at the downstream wall bottom reaching 10% C0 was established as the threshold for identifying the suspended containment system's failure. The research findings demonstrate that modifications to the site flow field due to parameter changes are a significant factor contributing to the macroscopic variations in pollutant migration. As the pressure head of the pollution source diminishes from 1.2 m to 0.3 m, the peak flow velocity at the mid-point of the liner bottom reduces by 25.6%, while the service life of the wall extends by 51.1%. The site flow-field distribution is significantly impacted, with the wall's service life increasing by 209.1% and 22.8%, respectively, as the site's horizontal hydraulic gradient decreases from 0.04 to 0.01 and the horizontal distance between the downstream wall and the pollution source increases from 4 m to 16 m. Therefore, the design of suspended cutoff walls in risk control of contaminated sites with deeply buried aquitards should focus on aspects related to these three factors.

Dissolution trapping is one of the most promising mechanisms for safe geological carbon storage. Density-driven convection substantially accelerates the conversion of free-phase CO2 to the dissolved state, enhancing the sequestration safety. Since this process occurs on time scales of hundreds to thousands of years, reproducing it through conventional laboratory physical model tests is challenging. The hypergravity experiment reduces the model size and shortens the experimental time, enabling the modeling of gravity-driven flow processes at the field scale. However, it is uncertain whether the preferential flow effect caused by fractures can be reproduced in a hypergravity experiment. In this study, a three-dimensional discrete fracture-matrix model (3D-DFM) was used to evaluate the feasibility of hypergravity experiment of the transport of dissolved CO2 in fractured reservoirs. Numerical hypergravity tests were performed to examine the feasibility of modeling density-driven convection in homogeneous and heterogeneous media at different centrifuge accelerations. The hypergravity experiment can be used to study density-driven convection of dissolved CO2 at the field scale in homogeneous system. The numerical results show that the hypergravity experiment enables a faster breakthrough of plume and overestimates CO2 migration in the matrix surrounding the fractures.

The stability and integrity of salt rock caverns for Underground Energy Storage (UES) are commonly assessed through finite element calculations. The geometrical model is based on sonar-mapping of the cavern contour. However, this technique misses the large space occupied by insoluble sediments at the bottom of the cavern called the sump, which leads to a misrepresentation of the geometry in particular in bedded salt formations with a high content of insolubles. In this article, solution mining simulation is employed to reconstruct the sediment-occupied space, which is then integrated with sonar-mapped data to form three comprehensive “full-geometry” models of caverns JT52, JT86 and JT103 in Jintan, China. Finite element analyses are conducted on these models and compared with “sonar-scanned” models that neglected the sump. The simulation results show that, compared with the ”sonar-scanned” models, the volume shrinkage value of the full-geometry model increases by 18.1%, 30.5% and 13.7% for JT52, JT86 and JT103, respectively. The total volume of the damage zone increases by 15.7%, 19.3% and -22.0%, respectively. Consequently, neglecting the sump can make stability and integrity assessment results either more or less conservative. Therefore, it is recommended that stability and integrity analysis of storage salt caverns consider the sediment-occupied sump below the sonar-mapped cavern floor in bedded salt.

The leakage of leachate from landfills, tailing ponds, and other underground facilities for storing hazardous materials poses severe contamination risks to groundwater and soil. This paper presented a self-developed 1.4 mm-diameter optical fiber probe, which is based on the principle of fluorescence quenching and to monitor the chloride ion concentration in the pore solution of sand in centrifuge tests for the first time. The measurement accuracy of this optical fiber probe is 4%-6% under a centrifugal acceleration of 80 g. The results demonstrate that this optical fiber probe can effectively monitor the in-situ chloride ion concentration in the pore solution of saturated soil in real-time and continuously under the extreme centrifugal force conditions. It has good compressive performance and reversibility, indicating a promising application in monitoring for deep-buried sites in the future.

The India-Asia collision resulted in the formation of Qinghai-Tibet Plateau. Lower crustal flow model was proposed to explain the mechanism of Cenozoic tectonic deformation of Qinghai-Tibet Plateau. In this study, we propose a new approach by combining centrifugal analog modeling with numerical simulation to simulate the tectonic uplift history of the plateau based on the lower crustal flow model, and to investigate the material migration characteristics and the influence of crustal motion velocity and ductile layer viscosity on the plateau tectonic geomorphology. The models reproduce steep-sided flat-topped geomorphic features and clockwise rotation of the material at eastern Himalayan Syntaxis, verifying the rationality of the models. The results show that the greater the crustal motion velocity and the greater the ductile layer viscosity, the steeper the terrain change; and conversely, the smaller the crustal motion velocity and the smaller the ductile layer viscosity, the gentler the terrain change. This study further indicates that the weak lower crust plays an important role in the formation of geomorphic features and material migration characteristics of Qinghai-Tibet Plateau, and provides a new insight for the study of the uplift mechanism of the Tibetan Plateau.

In fractured rock masses, solute transport process in rough-walled fractures is directly related to the construction and long-term maintenance of major underground infrastructure. However, the complex geometric features of fractures and the coupling of physical fields often lead to high computational costs. Therefore, this paper proposes a deep-learning-based surrogate model framework for efficiently predicting the concentration evolution in rough-walled fractures, a research area where the application of deep learning remains largely unexplored. A large number of rough-walled fracture models with self-affine characteristics and normal/shear deformations are first generated using the modified successive-random addition (SRA) method, after which data augmentation is applied to expand and diversify the dataset. The “Deactivated Subdomain” method is further incorporated to ensure stable numerical convergence under extreme geometric conditions. Next, high-fidelity datasets are obtained through parallel numerical simulations, and finally, a deep-learning-based surrogate model is utilized for efficient prediction. Compared to autoregressive U-Net and ED-convLSTM, the surrogate model in this paper demonstrates superior accuracy and efficiency in spatiotemporal predictions. Furthermore, it exhibits excellent extrapolation and interpolation capabilities within a certain range of fracture geometric variations, but its prediction accuracy deteriorates markedly under extreme conditions (e.g., the presence of contact regions). Similarly, the performance of the fine-tuned method varies significantly across scenarios. This paper fills a technological gap in microscale fracture transport modeling, provding a novel approach for predicting solute transport in rough-walled fractures.

Suspended cutoff walls hold wide application prospects in controlling pollutant migration in contaminated-site aquifers. This study established a three-dimensional numerical coupling calculation model at the field scale to simulate the site seepage and pollutant migration caused by suspended cutoff walls. The effectiveness of this calculation model was verified through a centrifuge model test (80 g), and we focused on evaluating the influence of different insertion depths on the service life against pollutant breakthrough at the wall bottom. Results show the suspended cutoff walls cause local flow-around, stronger downstream than upstream, verified by flow velocity, flow net, pore pressure, and pollution plume transient distribution. As the wall insertion depth increases from 12 m to 48 m, the service life against breakthrough (reached 10

Physical modeling is an important technical means and research method for simulating the construction of salt cavern energy storage projects, but it is limited by the lack of effective model salt rock materials. Therefore, in order to obtain the sedimentation and diagenesis process of natural salt rock at high temperature, high temperature and high pressure were applied to granular salt to prepare 20 groups of artificial salt rock specimens. Physical, mechanical, and microscopic tests were conducted on the artificial salt rock. X-ray Diffraction (XRD) showed that the mineral composition before and after specimen preparation was consistent. The uniaxial compressive strength (UCS), density, porosity, and P-wave velocity covered the range of mechanical properties of natural salt rock. Density-UCS relationship prediction formula was established. Under the combined effects of thermo-mechanical conditions, the failure mode of the artificial salt rock specimens shifted from brittle shear to ductile cleavage. Scanning electron microscopy (SEM) results showed that crystal recrystallization and particle densification significantly improved density and mechanical properties. The recrystallization of salt particles can be divided into three stages under different thermal-mechanical combinations: low-performance, transitional, and high-performance. Based on an analysis of the recrystallization evolution stages and energy consumption, safety, and economics, 600 degrees C and 40-80 MPa were determined as the optimal specimen preparation conditions. This research provides a theoretical basis for the preparation of large artificial salt rock and model testing of salt caverns.

The present work is devoted to numerical modeling of fracture initiation and propagation in saturated porous rocks. The emphasis is put on hydromechanical coupling and gravitational stress gradient effect. Compared with previous studies, several novel features are introduced. Based on the formulation of total potential energy and free poroelastic energy, the poroelastic relations of damaged porous media are established and the driving force of fracture propagation is identified. The effect of pore fluid pressure on fracturing is explored in greater depth by incorporating both effective stress and fluid enthalpy. Inspired by experimental results, the interaction between energy dissipation related to viscous fluid flow and fracture evolution is further investigated. Considering the strong pressure-dependency of mechanical behavior of rocks, the evolution of both tensile and shear fractures is influenced by compressive effective stresses. In order to better describe mixed fracturing in rocks under compression-dominating stresses, a hybrid phase-field model is proposed by considering two distinct driving forces respectively for tensile and shear fractures and their interaction is taken into account. Implemented in a finite element framework for fully coupled hydromechanical problems incorporating gravitational fluid flow and stress gradient, a wide series of representative cases are investigated. The proposed model is able to capture all main features involved in hydromechanical fracturing process. In particular, the kinetics of fracturing is influenced by fluid enthalpy contribution and pressure gradient. The proposed model is applied to investigate fracturing processes in laboratory tests under hypergravity conditions. The mechanistic mechanisms of hypergravity effect on fracturing pattern are elucidated.

Coal thermal spalling poses significant risks to cavity stability and gasification efficiency during underground coal gasification (UCG) and other high-temperature coal engineering processes, yet its underlying mechanisms remain poorly understood due to strong thermo-hydro-chemo-mechanical coupling. In this study, anthracite from the Jincheng mining area (Shanxi, China) was subjected to staged thermal treatments from 25 to 600 °C. A multi-scale experimental approach integrating Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), permeability measurements, and mechanical testing was employed to systematically characterize the coupled evolution of physicochemical properties, pore-fracture structures, and mechanical behavior. Results reveal a pronounced stage-dependent thermal response: below 200 °C, moisture loss and gas desorption induce pore contraction, matrix densification, and temporary strength enhancement; between 200 and 500 °C, organic pyrolysis and mineral dehydration or decomposition dominate, promoting volatile release, pore expansion, fracture development, permeability increase, and mechanical degradation; above 500 °C, the combined effects of gas-pressure buildup, mineral transformations, and heterogeneous thermal expansion promote fracture coalescence, coal skeleton collapse, and layered thermal spalling. Thermal stress analysis further demonstrates that thermal spalling is controlled by coupled microcrack initiation and instability driven by four primary mechanisms: (i) surface thermal stress induced by temperature gradients, (ii) differential thermal expansion between minerals and the coal matrix, (iii) lamination-induced stress mismatch, and (iv) internal pore pressure from volatile release. These mechanisms collectively govern fracture networks propagation, interconnection, and ultimate material detachment. Based on these findings, a multi-mechanism coupled evolution framework is proposed to link thermal stress-fracture mechanics, physicochemical transformations, pore-fracture structure evolution, and mechanical degradation, providing a mechanistic basis for evaluating thermal-spalling behavior in UCG and other high-temperature coal-related engineering environments.

Accurate identification and three-dimensional(3D) reconstruction of laboratory-scale rock fractures form a crucial foundation for studying rock failure mechanisms. This study proposes a 3D reconstruction method for multi-configuration rock fractures based on the Optuna optimization framework. It achieves end-to-end prediction from 1D ultrasonic signals to 3D voxel-level defect distributions, validated on a jointly constructed numerical-experimental red sandstone dataset. Key research contributions and conclusions are as follows: A numerical-experimental fusion dataset encompassing 19 configurations was constructed. A systematic comparison of three typical network architectures revealed that the 1D Convolutional Neural Network (1D CNN) demonstrated optimal performance in the signal-to-space mapping task, achieving an average test set loss reduction of 7.6% and 11.3% compared to ResNet and DenseNet, respectively. The Optuna framework driven by the TPE algorithm achieves joint optimization of network architecture and hyperparameters. After 10-fold crossvalidation, the Mean Squared Error (MSE) on simulated and experimental data reached 0.0056 and 0.0154, respectively, with determination coefficients (R2) of 0.9727 and 0.9248. The established six micro-fracture evaluation metrics achieved average accuracies of 95.5% and 89.0% on simulated and experimental data, respectively. Within the tested laboratory conditions, both single and intersecting fracture configurations were reconstructed reliably at voxel level.

Borehole thermal energy storage (BTES) is widely used in ground source heat pump systems (GSHP) to support building heating and cooling through seasonal thermal storage. However, geological stratification and groundwater flow can strongly affect thermal plume migration, inter-borehole thermal interference, and heat pump operation. This study develops a composite analytical framework to predict BTES thermal performance in stratified ground with groundwater advection. The framework is validated against four independent experimental and numerical benchmarks, yielding RMSE, MAE, and MAPE values spanning 0.163–0.68 °C, 0.003–0.50 °C, and 0.012%–2.02%, respectively. It is then applied to a 90-day heat injection process in a 63-m-deep, three-layered ground system, under L-shaped, linear, and rectangular layouts, each with nine boreholes. Under the baseline condition of 4-m spacing and a groundwater velocity of 5 × 10−7 m/s, the rectangular array shows the least variation in field-averaged outlet temperature with groundwater flow direction (0.06 °C), versus 0.09 °C for the L-shaped and 0.40 °C for the linear arrays. Extended analyses of spacing and velocity show that this robustness is not universal. At 8-m spacing, the L-shaped array becomes comparable to, or slightly more stable than, the rectangular array, while the linear array remains the most sensitive to flow direction. The COP analysis further indicates a trade-off between efficiency and stability, with COP fluctuations of 0.009, 0.014, and 0.070 for the rectangular, L-shaped, and linear arrays, respectively. The vertical thermal discontinuity is quantitatively explained by Peclet numbers, which range from 3.96 to 7.00 in groundwater-bearing layers but fall to zero above the groundwater table. These findings enable more reliable design of BTES for GSHP systems in stratified ground with groundwater advection.

Gas transport in porous media has been widely investigated in the geo-environmental, geotechnical, and chemical engineering fields. However, the mechanism for the transition of the gas flow pattern under the influence of the gravity field is not clear. This study gives some insights into this issue by using a hypergravity microfluidic experiment and pore-scale simulations based on the phase-field method. As the centrifugal acceleration increases, the gas transport pattern undergoes a gradual transition from a continuous finger flow to a discontinuous bubbly flow. An increase in pore size results in a weakening of the capillary effect, allowing the buoyancy force to gradually become the dominant force. The numerical simulation method reproduces experimental results of hypergravity microfluidics for chips with different pore sizes. Enhanced buoyancy effect leads to a significant reduction in sweep efficiency. Hypergravity field enhances separation of gas and liquid phases, thereby facilitating the generation of discontinuous bubbles. The dividing line between continuous and discontinuous flow is defined based on dimensionless numbers, which are supplemented by the numerical simulation results.

This study presents an investigation of seepage and pollutant migration in a site with suspended cutoff walls through centrifuge model tests with the advantages of reduced scale in time and size at 80 g, employing an independently designed control system. The experimental setup simulated and reproduced the long-term effects of site seepage and continuous nonpoint source pollution release under a hyper-gravity environment for the first time. It enables in situ and real-time monitoring of variations in pollutant concentration at the base of the downstream cutoff wall. The distribution of seepage and pollutant migration patterns in two types of suspended cutoff wall sites, namely shallow wall (12 m) and deep wall (24 m), was investigated by means of numerical simulations, with the goal of complementing and comparing to the testing data. Results indicate that the suspended cutoff wall prolongs the contaminated seepage path and delays the downstream wall's failure, extending its service life by causing diversion effects of the site and ongoing variations in seepage direction. Although the containment impact on pollutants is greatly improved by deeper walls, the efficiency does not rise in a linear fashion with depth. Numerical simulations were also used to assess and validate the dispersion pattern of the contaminated plume and consequences of site diversion. This study provides a valuable reference for the design of suspended vertical cutoff walls at contaminated sites.