Fracture network is an important factor in the utilization of geothermal energy, its composition and structure have an important influence on the development of geothermal energy. As a component of EGS system, fractures have various forms. Therefore, this paper studies the influence of fracture morphology on reservoir heat extraction. The Voronoi is used to describe the unique network structure and is applied to geothermal development system as a form of fracture network. Therefore, a THM coupled EGS model with Voronoi fractures is established to observe its thermal extraction performance. In present work, the effects of fracture block numbers, well length, well spacing, injection mass flow rate and injection temperature on the thermal extraction performance of EGS system are numerically investigated. The results show that the model has higher production temperature when the number of Voronoi blocks is 77, and the production temperature in the 30th year is 494.01 K. The influence of the change of well spacing on the production temperature of the model is more obvious than that of the change of well length. At the same time, the longer the well length, the smaller the injection pressure, the stronger the connectivity between the injection and production wells, and the narrower the end of the injection well in the temperature diagram. Furthermore, the increase of injection mass flow rate reduces the production temperature of the model, when the injection mass flow rate increases from 80 kg/s to 140 kg/s, the production temperature decreases from 494.01 K to 450.99 K. However, because the increase of the total injection mass flow rate, the large mass flow rate still has a high heat extraction ratio. The higher the injection temperature is, the smaller the pressure difference between injection wells and production wells is. The temperature changes the permeability and liquid viscosity of the reservoir, thereby reducing the pressure difference. In addition, with the increase of injection wells, the production temperature of the model increases, and the injection pressure decreases. However, the pressure difference between injection wells and production wells increases, indicating that the pressure of production wells is large, so the production cost increases. Therefore, in the actual production situation, reasonable selection of geothermal operation parameters should consider the operation cost and the difficulty of development.

Hydraulic fracturing is an important technique to enhance the production rate of oil and gas. We extend the functions of Permeability-based Hydraulic Fracture, Level Set Method (PHF-LSM) by including homogenization approaches to take into account the influence of the heterogeneities of rock media at both mesoscale and macroscale during hydraulic fracture propagation. The center position pattern of mesoscale heterogeneities is generated by introducing the Fast Poisson Disk (FPD) approach with an inherent parameter controlling the randomly distributed inclusions, and we developed an algorithm to detect inclusion collision by binary image matrix calculation. The level set function is used to describe the distributed joints at the macroscale. Both numerical and theoretical homogenization approaches are applied to study the characteristics of the material properties of rock matrices with randomly distributed hard inclusions at the mesoscale. Based on these results, we adopt the Mori–Tanaka (MT) and the Halpin–Tsai (HT) methods to homogenize the elasticity parameters (elastic modulus and Poisson’s ratio) and the hydraulic conductivity of the rock material, respectively. The Voigt upper bound is applied to estimate the tensile strength of the rock material at the integration points. A series of numerical simulations first indicated that the proposed method can model multi-scale heterogeneous rock that contains both the influences of macro-heterogeneities, e.g., distributed joints, and meso-heterogeneities, e.g., randomly distributed inclusions. Second, the development of pore pressure and stress path varied at the injection point, on the fracture path and on a joint. Finally, both the macro- and meso- heterogeneities influenced the propagation of multiple hydraulic fractures with joints significantly influenced the propagation path of the fractures, and increased injection number Np enhanced the height of the equivalent fracture zone when Np<5.

One of the most important parameters affecting shale gas extraction is the gas permeability of shale. Because there are many influencing factors and the mechanism of interaction is complex, it is difficult to accurately predict the gas permeability of shale. In this paper, a new machine learning model is proposed by combining Mind Evolutionary Algorithm (MEA) and Adaptive Boosting Algorithm-Back Propagation Artificial Neural Network (ADA-BPANN), which predicted the gas permeability of cement mortar with different moisture contents under different stress conditions based on the results of 616 laboratory gas permeability experiments. This is the first time that a combination of MEA and ADA-BPANN algorithms has been used to predict shale gas permeability. Compared to the traditional machine learning algorithms such as Particle Swarm Optimization Algorithm (PSO) and Genetic Algorithm (GA) optimized ADA-BPANN. The excellent performance of MEA optimized ADA-BPANN has been verified. This novel algorithm has higher prediction accuracy, shorter training time, and can avoid problems such as local optimization and overfitting. Secondly, the sensitivity of the parameters is analysed by using the novel model, and the results show that the parameter with the greatest influence on gas permeability is relative moisture content, followed by confining pressure, seepage pressure and confining pressure loading/unloading stage. The present research shows that the MEA optimized ADA-BPANN model has great potential for estimating the stress-dependent gas permeability of shale with different moisture contents. It is very helpful for the shale gas exploitation.

Various uncertainties are frequently encountered in the assessment of landslide stability. However, the uncertainty of parameters in the Mohr–Coulomb strength criterion is a major concern in the studies, neglecting the uncertainty related to the strength criterion itself that is used to describe the strength of geotechnical materials. To assess the reliability of landslides, a new probabilistic analysis method is proposed in this study. According to the traditional limit equilibrium framework, an improved method based on a nonlinear strength criterion is deduced to determine the safety factor of landslides. The local mean value of the random variables on the failure surface is estimated by the analytical method, avoiding random field simulation. The first-order reliability method (FORM) is applied to estimate the failure probability of landslides. An example is analyzed using the proposed probability analysis method, and its effectiveness is primarily verified. The results show that the traditional limit equilibrium method based on the linear Mohr–Coulomb strength criterion estimates the stability and failure probability of landslides inaccurately.

Prediction of thermal crack propagation in desiccated soils is imperfect due to the obscure field measurements, modeling approximations, and the underlying soil uncertainties. To address these issues, a probabilistic framework is developed to quantify the uncertainties and enhance the crack estimation reliability. Hence, the uncertainties associated with the soil properties, including the tensile strength, matric suction, Poisson’s ratio, elastic modulus and suction ratio are associated in a probabilistic model. Then, the probabilistic model is utilized with the improved Monte Carlo simulation to calculate the failure probability due to the exceedance of the crack propagation. Furthermore, failure probability optimization is exemplified by evaluating the spatial correlation between variables. The Bayesian optimization is employed to optimize the covariance kernels via the Gaussian process regression. Then, the covariance matrices are decomposed to generate random fields to further enhance the calculated failure probabilities. The results indicate that the cracking probability in near-surface layers is imminent. However, with the increase of the tensile strength and decrease in the matric suction, a sharp drop in crack propagation is highly expected. The findings also suggest that the importance of variable uncertainties is in the order of Matric suction> Elastic modulus and Suction ratio > Tensile strength> Poisson’s ratio. Also, the operation of the probabilistic model with different random field sampling outperforms the improved and directed Monte Carlo simulations because of the comprehension of the spatial correlation. However, the Gaussian fields are restricted to the second-order statistics, while lognormal fields show relatively lesser constraints. Hence, the random field sampling with a larger length scale as well the Karhunen–Loève expansion followed by the approximated eigenvalue decomposition offer viable options for the probabilistic estimation of the crack depth.

Water–rock reactions between shale and fracturing fluids can change the brittleness of shale, which affects the efficiency of hydraulic fracturing. In order to understand the effect of different fracturing fluids imbibition on shale brittleness, soaking experiments with different fracturing fluids (pH = 6, 7, 8) and different soaking times (15 days, 30 days, 90 days, 180 days) are conducted on shale samples. A new constitutive model combining a quadratic function and the Weibull distribution is applied to investigate the variations of shale brittleness. The results show that the model can well describe the stress–strain relationship of shale samples under uniaxial compression. The model parameter a is related to the initial elastic modulus E0 of shale samples. The significant decrease of a values indicates the decrease of initial elastic modulus after imbibition. The model parameter m is positively correlated with shale brittleness and can be used to compare shale brittleness preliminarily.Water–rock reactions and hydration expansion cause the deterioration of shale brittleness. According to the calculated values of the energy-based brittleness index (BI), the brittleness of shale shows decreasing–increasing–decreasing trends during the process of imbibition. The acidic fracturing fluid has the weakest effect on shale brittleness compared with the other two fracturing fluids used in this study.

This study explores the application of roughness pretreatment on plant-based natural fiber surfaces to enhance the sealing effectiveness of the sand-fiber interfacial. The technique roughens the reinforced material by presetting CaCO3 seeds on the fiber surface, thereby immobilizing more bacteria as nucleation sites, further increasing the interfacial bonding area by promoting bio-mineralization and achieving efficient sealing of preferential flow channels. Experimental investigation was conducted to compare the promoting effects on bio-mineralization of reinforced sand with three pretreated fibers. Results showed that the fibers pretreated with CaCO3 could fill the pores to bridge sand particles more effectively by aggregating bio-mineralized precipitates around them. The bacterial immobilization rate and calcium carbonate content of bio-cemented sands with fiber pretreatment remarkably increased with increasing fiber surface roughness characterized by fractal dimension (Fd) when compared to the samples without pretreatment, and the maximum increase was 35.66% (coir, Fd=1.390) and 152.3% (jute, Fd=1.330). The profile water-conducting rate and average hydraulic conductivity have been reduced by 46% and 86%, Among the three fibers, ramie has the lowest hydraulic conductivity of 5.396 × 10−8m/s, which is lower than that of jute and coir (1.041 × 10−7m/s and 3.885 × 10−7m/s). Furthermore, the promotion of bacteria on bio-mineralization and more even distribution (upper, middle, and lower parts are 36%, 34%, and 30%, respectively) of CaCO3 were confirmed through bacteria staining testing and dye tracer testing. Finally, the effects of average flow velocity and shear rate on bacterial immobilization and sealing of preferential flow channels are discussed.

To develop a novel 3D Hoek–Brown strength criterion and an associated constitutive model based on the finite deformation behavior of rock under the disturbance stress paths, a series of mechanical tests are initially conducted under designed disturbance stress paths (hydrostatic stress stage (HSS), initial loading–unloading stage (ILUS), and disturbance stage (DS)). Additionally, low disturbance (LD), medium disturbance (MD), and high disturbance (HD) modes are taken into consideration. Then, the finite deformation theory is employed to study the nonlinear behavior. This nonlinear behavior can be regarded as a nonlinear relationship between the stress and strain of rock under loading, especially the initial compression stage and plastic stage, as well as a complex deformation field, which is described by the finite deformation theory and featured by the mean rotation angle (Θ). Afterwards, the damage variable (DV) regarding Θ is proposed and used to establish a novel strength criterion (NSC) based on the current Hoek–Brown strength criterion. Finally, by considering the NSC, an associated elasto-plastic constitutive model is developed. The results show that the initial confining pressure (ICP) and disturbance mode have a significant effect on the elastic modulus (E). Meanwhile, the ICP has a higher effect on E than the disturbance mode. Then, the evolution of Θ is closely associated with the nonlinear deformation of rock masses because the σ1−Θ curves can be divided into three stages. Those stages can match well with the initial compression stage, elastic stage, and plastic stage, respectively. Regarding Θ as a parameter of the damage variable (DV), the NSC is established. Additionally, it can be observed that the failure envelopes of the NSC scale down gradually with increasing Θ on the deviatoric plane. Meanwhile, as the value of the coefficient of determination (DC) is higher than that of other strength criteria, the NSC may exhibit better agreement with the experimental data. Furthermore, a constitutive model based on the aforementioned strength criterion and the hardening function that takes Θ and the disturbance of rock masses (D) into account is proposed and validated as a solution to predict the nonlinear behavior of rock specimens under the disturbance stress paths.

The deformation-to-failure process of rocks is accompanied by energy dissipation and release, indicating that it is an energy conversion process. To investigate the influence of lithology as well as the loading and unloading rates on the evolution and distribution of rock energy, the MTS 816 rock mechanics testing system was used for performing uniaxial compression and uniaxial cyclic loading–unloading tests on the 165 samples of four rock types. The total energy density, elastic energy density, and dissipated energy density absorbed by rocks of different lithology were obtained, and the evolution and distribution laws of lithology and loading and unloading rates on the accumulation and dissipation of rock energy were investigated. The results revealed that the energy density of all rock samples increased nonlinearly with the increase in axial stress, and the elastic energy density increased gradually first and subsequently rapidly with the increase in the axial stress. The evolution curves were not affected by the loading and unloading rates. The dissipated energy density increased gradually with the increase in axial stress, and the discrepancy of the evolution curve was large. The proportion of the elastic energy density varied nonlinearly with the increase in axial stress or the cycle index, exhibiting the evolutionary process of increasing, then stabilizing, and finally decreasing. However, the proportion of the dissipated energy density exhibited an opposite trend. The elastic energy accumulated in the rock sample at the pre-peak stage was considerably higher than the dissipated energy, and the proportion of the elastic energy density was large. With the increase in loading and unloading rates, the proportion of elastic energy exhibited an approximate growth trend, and the proportion of dissipated energy exhibited an approximate decreasing trend.

Microbially induced carbonate precipitation (MICP) with additives is a promising approach for seepage control in the field of geotechnical engineering. This study designed and fabricated four cross-fracture models using three-dimensional printing. One of the models did not include additives and was treated with a mixed injection strategy. The other three models were prepared using different types of additives such as calcium carbonate, jute fiber, and gravel particles, which were treated with a staged injection strategy to improve our understanding of the optimized MICP process. The increased mass of calcium carbonate in the model was determined by the difference in the model’s weight before and after four-cycle intervals to evaluate the effect of the three additives and two injection strategies on seepage control and precipitation patterns. The results indicated that the masses of the precipitates increased the most for the model without additive owing to gravity during the MICP process; however, the clogging effect was weakest during the seepage experiment. This may be because the smooth inner surfaces of the model and a higher flow rate prevented the floc and cementation solution from settling. The models that included plant fibers and gravel particles provided more surfaces that facilitated calcium carbonate precipitation and contributed to seepage control. The model with calcium carbonate produced a considerable amount of calcium carbonate and interacted positively with the fracture surfaces, leading to the largest decrease in seepage. In the case of calcium-carbonate additives, a layer of precipitates was formed, which increased the inner surface roughness and enhanced the boundary-layer effect. This promoted the precipitation and settling of floc and the formation of more reliable bridges within the cross-fracture model compared with those of other additives.

To investigate the mechanical behavior and crack evolution properties of the anisotropic bedded rocks, a series of uniaxial compression tests were conducted using bedded limestone, phyllite, and shale specimens, and the bedding angle α of specimens was designed as 0°, 15°, 30°, 45°, 60°, and 90°. The acoustic emission (AE) and digital image correlation (DIC) techniques were applied to monitor the crack evolution mechanism. The cementation types of bedding planes were also different, which greatly affected the mechanical properties of bedded rocks. In this study, the cementation types of bedding planes for limestone, phyllite, and shale were defined as embedded cementation, oriented cementation, and cracked cementation, respectively. The experimental results showed that with the increase of the α, the peak strength σucs, elastic modulus E, AE hit rate, cumulative AE hit and cumulative AE energy all exhibited a decreasing firstly and then increasing trend, and all showed obvious anisotropy. A novel trigonometric strength criterion for bedded rocks was developed. The fluctuation of the b-value indicated that the propagation of the internal micro-cracks was unstable, and showed an overall downward trend. Meanwhile, a tensile and shear crack classification criterion combining the AE parameters, clustering idea and genetic algorithm was proposed, and then the rational dividing line of tensile and shear cracks was determined in AF-RA maps. The α and cementation types both affected the failure mode of bedded rocks. For limestone with α = 0°– 90°, the longitudinal splitting, forming approximate vertical macro tensile cracks, was the primary failure mode, and local shear slipping along the bedding plane occurred in the specimens with α = 30°– 60°. For phyllite and shale, when α≤ 30°, the inclined macro-cracks were not along the bedding planes but showed a shear-dominated failure mode. When 45° ≤α≤90°, the main macro-cracks were along the bedding planes, and some macro-sub cracks penetrated the bedding planes in shale, showing shear-dominated failure for specimens with 45°≤α<90° and tensile-dominated failure for specimens with α=90°. The method of rating scores corresponding to the uniaxial compressive strength in RMR has been improved and an optimization method of the revised rating scores of bedded rocks in different situations considering α and σucs has also been proposed.

The stability control of surrounding rock is always one of the important problems in tunnel construction. As a representative of tunnel pre-construction technology, advance grouting pipe roof support method is widely used in underground space engineering under various complex geological conditions, especially in strata with many fractured rock masses and rich groundwater content. Due to the complexity of the application of advance grouting pipe roof method, few studies have been conducted on the whole process of pipe roof application in actual projects. Taking the shield departure project of Guangzhou Metro Line 13 as the background, this paper uses the modified BQ method to grade the surrounding rock environment of the tunnel from the perspective of the stability evaluation of the surrounding rock of the tunnel. The result shows that the revised surrounding rock is grade v. further combining with the geological exploration report, it is found that the rock mass joints and fissures are large and may further develop under the change of the surrounding rock mass stress environment, and the formation water content is large. The surrounding rock is prone to large deformation under the action of groundwater, and even lead to surface collapse in serious cases. Therefore, it is necessary to adopt the advance grouting pipe roof support method. Subsequently, the influence of different pipe roof design parameters such as pipe roof diameter, number of pipe roofs, reinforcement thickness and layout range on its support effect is analyzed by using finite element software, and appropriate parameter values are determined and applied to the background project. Finally, considering the measurement error existing in actual monitoring, a dynamic safety evaluation method of tunnel structure based on reliability theory is proposed, and the background project is analyzed and evaluated by this method. The analysis shows that the tunnel structure with advance grouting pipe roof support meets the safety requirements, confirms the effectiveness of the design parameters, and can provide reference for tunnel construction under the same geological conditions.

Marine natural gas hydrate currently attracts increasing research attention as a potential resource of alternative clean energy. However, exploitation disturbance threatens the stability of the wellbore wall as the physical parameters and mechanical properties of hydrate-bearing reservoirs change due to hydrate decomposition, which causes the futility of conventional analysis. This study determines the influence of production disturbance on wellbore wall stability by solving the fluid–solid coupling model during the hydrate decomposition based on a self-developed solver. Through this model, the conditions of depressurized hydrate production are simulated to obtain the horizontal deformation and settlement laws of hydrate-bearing reservoirs and overburden layers. These deformations and settlements cause uneven horizontal in-situ stress on hydrate-bearing reservoirs, which will cause wellbore wall instability during hydrate production. Results show that the middle of the hydrate-bearing reservoir is the best depressurization position for production. In addition, production time, production pressure, and reservoir permeability are the key factors that affect the hydrate decomposition and cause inhomogeneous in-situ stress. The borehole wall first destabilizes at the minimum effective horizontal stress direction. Results can provide a reference for the safe and efficient exploitation of marine NGH.

Over the past two decades, the thermo-hydro-mechanical behavior of earth-contact structures such as piles, walls, slabs, and tunnels, which simultaneously provide structural support and energy supply, has been studied through various field experiments and numerical simulations. These endeavors have built an understanding of the behavior of so-called energy piles, energy walls, energy slabs, and energy tunnels. This paper explores the thermo-hydro-mechanical behavior of a novel class of earth-contact structures that has the specificity of providing structural support and renewable energy to tall buildings: energy barrettes. This study particularly presents the first field experiments and 3-D time-dependent numerical simulations of energy barrettes. The work investigates the thermo-hydro-mechanical behavior of energy barrettes for a comprehensive set of variables: geometric variables (i.e., pipe configuration, barrette aspect ratio, and barrette section ratio), site variables (i.e., barrette–soil stiffness ratio and ground effective thermal conductivity), and operational variables (i.e., flow rate of the fluid circulating in the pipes). The obtained results expand the current knowledge base to analyze the behavior of energy barrettes in multiple situations that are likely to be encountered in practice and highlight markedly non-uniform temperature, displacement, and stress fields within and around such foundations.

The sealing failure of cement sheaths caused by temperature variations in wellbores seriously threatens the safe production of oil and gas wells. The finite element method (FEM) is usually employed to analyze this complex engineering problem. However, the current FEM models solved this problem are based on the constant temperature change value assumption, which ignore the damage of cement sheaths during the evolution of temperature field and the temperature change rate. This work established a FEM model based on the transient heat conduction theory and concrete damage plastic (CDP) model. The proposed numerical model is implemented in the commercial finite element software Abaqus and its accuracy has been verified through experiments. The damage states of the cement sheath during temperature changes were analyzed. The results indicate the variety of failure types of the cement sheath caused by temperature variation in the wellbore. An increase in temperature by 70 °C in the wellbore leads to both compression and tensile damages of the cement sheath, while a decrease in temperature by more than 46 °C will lead to the interface failure between casing and cement sheath. The variation rate of temperature in the wellbore has a significant influence on the sealing integrity of the cement sheath, and damage to the cement sheath increases with the temperature change rate increasing. Therefore, the cement sheath will be protected to reduce its damage by lowering the variation rate of temperature, such as by controlling the production rate of oil and gas wells. These research results provide theoretical supports for production allocation measures in oil and gas exploitation.

The physical and mechanical properties of shale exhibit significant anisotropy, impacting the design and construction of high geothermal tunnels and deep resource mining. Laboratory studies on the physical and anisotropic mechanical properties of shale after various thermal treatments showed that rising temperatures caused an exponential decline in shale’s bulk and density, and the uneven expansion of the mineral particles within the rock caused a decrease in its uniaxial compression strength (UCS) and elastic modulus while increasing their anisotropy. The UCS is U-shaped with a change in bedding dip angle, whereas the elastic modulus is U-shaped or shoulder-shaped. Under the assumption that the rock microelement obeyed the Weibull distribution, the microstructure tensor improved Hoek–Brown strength criterion was introduced as the damage basis. Then a thermal–mechanical coupling damage constitutive model considering bedding dip angle and temperature was established. The model was found to be valid via the test results, especially in capturing the compaction and yield softening stages of shale. These findings have reference value for both underground structural engineering and deep resource mining.

Uniaxial compression tests were conducted on rocks under different loading rates to analyze their effect on rock fragmentation. The fractal dimension of the macroscopic fragmentations of the rocks were then calculated by combining the fractal theory. Based on scanning electron microscopy (SEM), the macroscopic failure characteristics and mesoscopic fracture morphology characteristics of the rocks under different loading rates were compared and analyzed; the mesoscopic fractal dimension of the fracture was calculated using the box dimension method. The results demonstrate that when the loading rate is increased from 0.001 to 0.05 mm/s, the average fragmentation distribution coefficient decreases from 18.99 to 16.37 mm. As the loading rate increases, the fragmentation distribution coefficient gradually decreases, and the rock sample breaks; the degree of failure gradually increases, and the failure mode of the rock transforms from tension to shear failure. The SEM analysis reveals that under low loading rates, an intergranular fracture indicates the primary failure mode. The macroscopic performance primary presents tensile failure; as the loading rate increases, the range of the transgranular fracture gradually increases, and the failure mode evolves from an intergranular to transgranular fracture. The fractal dimension of the mesoscopic structure of the rock fracture tends to increase with an increase in the loading rate, and demonstrates a good positive relationship by fitting with the macrostructure fractal dimension. A larger fractal dimension of the microstructure of a rock fracture indicates more complex structure of the fractured surface. This study provides a better understanding of the meso-rock failure mechanism, and presents significant findings regarding ore rock fragmentation and rockburst prevention.

It is clear that the efficient use of recycled construction waste materials can reduce waste generation and preserve natural resources. Although the use of recycled concrete aggregates (RCA) as a CCBE subjected to small intensity rainfall has been reported, no physical model test data and numerical simulation to investigate the use of RCA as a landfill cover subjected to high intensity of rainfall under humid climatic conditions. In this study, one-dimensional (1D) soil column and two-dimensional (2D) flume model tests simulating humid climatic conditions were carried out to evaluate the hydrological performance of a sustainable three-layer landfill cover system using recycled concrete aggregates (RCA) but without a geomembrane. This three-layer system consists of a layer of fine-grained and a layer of coarse-grained recycled aggregates (i.e., FRC and CRC, respectively) overlying the bottom fine-grained completely decomposed volcanic soil (CDV). The effects of vegetation on the three-layer cover system were also considered in the 1D column tests. In addition, numerical simulations were carried out to back analyse the model tests by using the modified Darcy-Richards equation in COMSOL Multiphysics to verify test results and to reveal the water flow mechanisms. Consistent results were obtained between the measured data and numerical predications. Even after the simulated extreme rainfall of Hong Kong with a return period more than 1,000-year, a higher matric suction was well-retained in vegetated cover system than that in bare cover due to evapotranspiration of plants. As revealed in the 2D flume model test and numerical simulation, the middle CRC layer can switch from a capillary barrier layer to a drainage layer to reduce infiltration into the bottom layer even under the heavy rainfall. Most of infiltrated rainfall water (i.e., more than 95% of total precipitation) can be diverted as surface runoff and lateral drainage in the two upper RCA layers. The rest of infiltrated water is stored in the cover system. No percolation is observed through the cover system using RCA during the flume test and predicted by numerical analysis. The physical model tests and numerical simulations consistently verified the effectiveness of the proposed sustainable three-layer landfill cover system using RCA without geomembrane under heavy rainfall in humid climatic conditions.

Ice lenses and their unique patterns formed by the amount and distribution of ice within sediments and rock within various geologic and climate environments, or cryostructures, are ubiquitous in permafrost. The mechanical characteristics and deformation properties of the individual components (i.e., ice lenses, frozen soil) and the interaction between them control the overall behavior of permafrost. Unlike ice lenses and frozen soil, the mechanical behavior of the ice lenses–frozen soil interface is less studied. This article presents the results from a series of cryogenic direct shear tests and an elastoplastic model based on the damage at the ice–frozen soil interface. The tests were conducted under different temperatures (i.e., -3, -2, and -1 °C), normal stresses (i.e., 50, 100, and 200 kPa), and initial saturations (i.e., 55, 67, 77, 96, and 100%), respectively. The test results show that under the same normal stress and initial saturation, the shear stress curves transit from strain-softening to strain-hardening as the temperature increases. The volumetric deformation initially contracts and then dilates as the shear displacement increases. Further, rheological elements are introduced to explain the various components of the interface’s mechanical behavior. An elastoplastic damage constitutive model is established to describe the overall shear behavior of the ice–frozen soil interface during the entire shearing process. The parameters in the model are clearly defined with physical meanings and are easy to determine. Finally, the proposed model is applied to predict the experimental data. Comparison of the model results with experimental data demonstrates that the proposed model can well capture the shear behavior of the ice–frozen soil interface.

Severe formation damage easily occurs in ultradeep fractured carbonate reservoirs during drilling, yet the formation damage mechanisms have not been studied extensively. This study focused on typical fractured carbonate reservoirs in the Shunbei field, China. The multiscale reservoir space characteristics and core wettability of ultradeep fractured carbonate reservoirs were investigated. The results showed that the reservoirs mainly comprise a fourth-order fracture network. The high-temperature and high-pore-pressure steady-state method also identified fluid sensitivity and phase trapping damage. The fluid sensitivity damage at the formation temperature (170 °C) indicated that this reservoir is affected by water sensitivity, salt sensitivity (critical value: 3/4 formation water salinity (FWS)), alkali sensitivity (critical value: pH 7.5), and liquid phase trapping damage. Particle movement and deposition and liquid phase trapping damage are thought to be the primary damage mechanisms. Furthermore, abnormally low water saturation, mixed wettability, abundant clay minerals, and the fourth-order fracture network contribute to fluid sensitivity and liquid phase trapping damage. Finally, a formation damage control strategy was proposed.