Water level fluctuations in the reservoir deteriorate soils and rocks on the bank landslides by drying-wetting (D-W) cycles, which results in a significant decrease in mechanical properties. A comprehensive understanding of deterioration mechanism of sliding-zone soils is of great significance for interpreting the deformation behavior of landslides. However, quantitative investigation on the deterioration characteristics of soils considering the structural evolution under D-W cycles is still limited. Here, we carry out a series of laboratory tests to characterize the multi-scale deterioration of sliding-zone soils and reveal the mechanism of shear strength decay under D-W cycles. Firstly, we describe the micropores into five grades by scanning electron microscope and observe a critical change in porosity after the first three cycles. We categorize the mesoscale cracks into five classes using digital photography and observe a stepwise increase in crack area ratio. Secondly, we propose a shear strength decay model based on fractal theory which is verified by the results of consolidated undrained triaxial tests. Cohesion and friction angle of sliding-zone soils are found to show different decay patterns resulting from the staged evolution of structure. Then, structural deterioration processes including cementation destruction, pores expansion, aggregations decomposition, and clusters assembly are considered to occur to decay the shear strength differently. Finally, a three-stage deterioration mechanism associated with four structural deterioration processes is revealed, which helps to better interpret the intrinsic mechanism of shear strength decay. These findings provide the theoretical basis for the further accurate evaluation of reservoir landslides stability under water level fluctuations.

This study presents a fully coupled thermo-hydro-mechanical (THM) constitutive model for clay rocks. The model is formulated within the elastic-viscoplasticity framework, which considers nonlinearity and softening after peak strength, anisotropy of stiffness and strength, as well as permeability variation due to damage. In addition, the mechanical properties are coupled with thermal phenomena and accumulated plastic strains. The adopted nonlocal and viscoplastic approaches enhance numerical efficiency and provide the possibility to simulate localization phenomena. The model is validated against experimental data from laboratory tests conducted on Callovo-Oxfordian (COx) claystone samples that are initially unsaturated and under suction. The tests include a thermal phase where the COx specimens are subjected to different temperature increases. A good agreement with experimental data is obtained. In addition, parametric analyses are carried out to investigate the influence of the hydraulic boundary conditions (B.C.) and post-failure behavior models on the THM behavior evolution. It is shown that different drainage conditions affect the thermally induced pore pressures that, in turn, influence the onset of softening. The constitutive model presented constitutes a promising approach for simulating the most important features of the THM behavior of clay rocks. It is a tool with a high potential for application to several relevant case studies, such as thermal fracturing analysis of nuclear waste disposal systems.

When tunnelling through low-permeability saturated ground, the pore pressure decreases in the vicinity of the cavity. In certain instances of deep tunnels crossing weak rocks, the pore pressure may even become negative. All existing analytical solutions for the undrained ground response curve (GRC) in the literature assume that the ground fully retains its saturation, in which case the development of negative pore pressures has a stabilising effect – it results in increased effective stresses, and thus shearing resistance, which in turn leads to reduced deformations and plastification. In practice, however, negative pore pressures can induce partial or complete ground desaturation, which may even invalidate the premise of undrained conditions and lead to considerably increased deformations and plastification. In such cases, existing solutions are unsafe for design. The present paper aims to address this shortcoming, by presenting a novel analytical solution for the undrained GRC which incorporates the effect of the excavation-induced desaturation. The solution is derived under the assumption that the ground desaturates completely and immediately under negative pore pressures, which provides the upper bound of deformations and plastification for cases of partial desaturation. The rock is considered to be a linear elastic, brittle-plastic material, obeying a non-associated Mohr-Coulomb (MC) yield criterion. Nevertheless, the solution is also applicable to perfectly plastic rocks via a simple modification of the input parameters. Although the solution is in general semi-analytical, simple closed-form expressions are obtained in the special case of non-dilatant rocks. These expressions are also applicable to rocks exhibiting limited dilatancy, which is usually the case. An application example, based on the planned deep geological repository for radioactive waste in Switzerland, demonstrates the significant practical value and usefulness of the novel solution and underscores its necessity in cases where existing solutions that disregard desaturation are rendered thoroughly unsafe for design.

It is of great importance to obtain precise trace data, as traces are frequently the sole visible and measurable parameter in most outcrops. The manual recognition and detection of traces on high-resolution three-dimensional (3D) models are relatively straightforward but time-consuming. One potential solution to enhance this process is to use machine learning algorithms to detect the 3D traces. In this study, a unique pixel-wise texture mapper algorithm generates a dense point cloud representation of an outcrop with the precise resolution of the original textured 3D model. A virtual digital image rendering was then employed to capture virtual images of selected regions. This technique helps to overcome limitations caused by the surface morphology of the rock mass, such as restricted access, lighting conditions, and shading effects. After AI-powered trace detection on two-dimensional (2D) images, a 3D data structuring technique was applied to the selected trace pixels. In the 3D data structuring, the trace data were structured through 2D thinning, 3D reprojection, clustering, segmentation, and segment linking. Finally, the linked segments were exported as 3D polylines, with each polyline in the output corresponding to a trace. The efficacy of the proposed method was assessed using a 3D model of a real-world case study, which was used to compare the results of artificial intelligence (AI)-aided and human intelligence trace detection. Rosette diagrams, which visualize the distribution of trace orientations, confirmed the high similarity between the automatically and manually generated trace maps. In conclusion, the proposed semi-automatic method was easy to use, fast, and accurate in detecting the dominant jointing system of the rock mass.

We study CO2 injection into a saline aquifer intersected by a tectonic fault using a coupled modeling approach to evaluate potential geomechanical risks. The simulation approach integrates the reservoir and mechanical simulators through a data transfer algorithm. MUFITS simulates non-isothermal multiphase flow in the reservoir, while FLAC3D calculates its mechanical equilibrium state. We accurately describe the tectonic fault, which consists of damage and core zones, and derive novel analytical closure relations governing the permeability alteration in the fault zone. We estimate the permeability of the activated fracture network in the damage zone and calculate the permeability of the main crack in the fault core, which opens on asperities due to slip. The coupled model is applied to simulate CO2 injection into synthetic and realistic reservoirs. In the synthetic reservoir model, we examine the impact of formation depth and initial tectonic stresses on geomechanical risks. Pronounced tectonic stresses lead to inelastic deformations in the fault zone. Regardless of the magnitude of tectonic stress, slip along the fault plane occurs, and the main crack in the fault core opens on asperities, causing CO2 leakage out of the storage aquifer. In the realistic reservoir model, we demonstrate that sufficiently high bottomhole pressure induces plastic deformations in the near-wellbore zone, interpreted as rock fracturing, without slippage along the fault plane. We perform a sensitivity analysis of the coupled model, varying the mechanical and flow properties of the storage layers and fault zone to assess fault stability and associated geomechanical risks.

Bedding parallel stepped rock slopes exist widely in nature and are used in slope engineering. They are characterized by complex topography and geological structure and are vulnerable to shattering under strong earthquakes. However, no previous studies have assessed the mechanisms underlying seismic failure in rock slopes. In this study, large-scale shaking table tests and numerical simulations were conducted to delineate the seismic failure mechanism in terms of acceleration, displacement, and earth pressure responses combined with shattering failure phenomena. The results reveal that acceleration response mutations usually occur within weak interlayers owing to their inferior performance, and these mutations may transform into potential sliding surfaces, thereby intensifying the nonlinear seismic response characteristics. Cumulative permanent displacements at the internal corners of the berms can induce quasi-rigid displacements at the external corners, leading to greater permanent displacements at the internal corners. Therefore, the internal corners are identified as the most susceptible parts of the slope. In addition, the concept of baseline offset was utilized to explain the mechanism of earth pressure responses, and the result indicates that residual earth pressures at the internal corners play a dominant role in causing deformation or shattering damage. Four evolutionary deformation phases characterize the processes of seismic responses and shattering failure of the bedding parallel stepped rock slope, i.e. the formation of tensile cracks at the internal corners of the berm, expansion of tensile cracks and bedding surface dislocation, development of vertical tensile cracks at the rear edge, and rock mass slipping leading to slope instability. Overall, this study provides a scientific basis for the seismic design of engineering slopes and offers valuable insights for further studies on preventing seismic disasters in bedding parallel stepped rock slopes.

The strength of the sliding zone soil determines the stability of reservoir landslides. Fluctuations in water levels cause a change in the seepage field, which serves as both the external hydrogeological environment and the internal component of a landslide. Therefore, considering the strength changes of the sliding zone with seepage effects, they correspond with the actual hydrogeological circumstances. To investigate the shear behavior of sliding zone soil under various seepage pressures, 24 samples were conducted by a self-developed apparatus to observe the shear strength and measure the permeability coefficients at different deformation stages. After seepage-shear tests, the composition of clay minerals and microscopic structure on the shear surface were analyzed through X-ray and scanning electron microscope (SEM) to understand the coupling effects of seepage on strength. The results revealed that the sliding zone soil exhibited strain-hardening without seepage pressure. However, the introduction of seepage caused a significant reduction in shear strength, resulting in strain-softening characterized by a three-stage process. Long-term seepage action softened clay particles and transported broken particles into effective seepage channels, causing continuous damage to the interior structure and reducing the permeability coefficient. Increased seepage pressure decreased the peak strength by disrupting occlusal and frictional forces between sliding zone soil particles, which carried away more clay particles, contributing to an overhead structure in the soil that raised the permeability coefficient and decreased residual strength. The internal friction angle was less sensitive to variations in seepage pressure than cohesion.

Numerical challenges, incorporating non-uniqueness, non-convexity, undefined gradients, and high curvature, of the positive level sets of yield function F>0 are encountered in stress integration when utilizing the return-mapping algorithm family. These phenomena are illustrated by an assessment of four typical yield functions: modified spatially mobilized plane criterion, Lade criterion, Bigoni-Piccolroaz criterion, and micromechanics-based upscaled Drucker-Prager criterion. One remedy to these issues, named the "Hop-to-Hug" (H2H) algorithm, is proposed via a convexification enhancement upon the classical cutting-plane algorithm (CPA). The improved robustness of the H2H algorithm is demonstrated through a series of integration tests in one single material point. Furthermore, a constitutive model is implemented with the H2H algorithm into the Abaqus/Standard finite-element platform. Element-level and structure-level analyses are carried out to validate the effectiveness of the H2H algorithm in convergence. All validation analyses manifest that the proposed H2H algorithm can offer enhanced stability over the classical CPA method while maintaining the ease of implementation, in which evaluations of the second-order derivatives of yield function and plastic potential function are circumvented.

The widespread adoption of tunnel boring machines (TBMs) has led to an increased focus on disc cutter wear, including both normal and abnormal types, for efficient and safe TBM excavation. However, abnormal wear has yet to be thoroughly investigated, primarily due to the complexity of considering mixed ground conditions and the imbalance in the number of instances between the two types of wear. This study developed a prediction model for abnormal TBM disc cutter wear, considering mixed ground conditions, by employing interpretable machine learning with data augmentation. An equivalent elastic modulus was used to consider the characteristics of mixed ground conditions, and wear data was obtained from 65 cutterhead intervention (CHI) reports covering both mixed ground and hard rock sections. With a balanced training dataset obtained by data augmentation, an extreme gradient boosting (XGB) model delivered acceptable results with an accuracy of 0.94, an F1-score of 0.808, and a recall of 0.8. In addition, the accuracy for each individual disc cutter exhibited low variability. When employing data augmentation, a significant improvement in recall was observed compared to when it was not used, although the difference in accuracy and F1-score was marginal. The subsequent model interpretation revealed the chamber pressure, cutter installation radius, and torque as significant contributors. Specifically, a threshold in chamber pressure was observed, which could induce abnormal wear. The study also explored how elevated values of these influential contributors correlate with abnormal wear. The proposed model offers a valuable tool for planning the replacement of abnormally worn disc cutters, enhancing the safety and efficiency of TBM operations.

The coupling effects of rainfall, earthquake, and complex topographic and geological conditions complicate the dynamic responses and disasters of slope-tunnel systems. For this, the large-scale shaking table tests were carried out to explore the dynamic responses of steep bedding slope-tunnel system under the coupling effect of rainfall and earthquake. Results show that the slope surface and elevation amplification effect exhibit pronounced nonlinear change caused by the tunnel and weak interlayers. When seismic wave propagates to tunnels, the weak interlayers and rock intersecting areas present complex wave field distribution characteristics. The dynamic responses of the slope are influenced by the frequency, amplitude, and direction of seismic waves. The acceleration amplification coefficient initially rises and then falls as increasing seismic frequency, peaking at 20 Hz. Additionally, the seismic damage process of slope is categorized into elastic (2–3 m/s2), elastoplastic (4–5 m/s2) and plastic damage stages (≥6.5 m/s2). In elastic stage, ΔMPGA (ratio of acceleration amplification factor) increases with increasing seismic intensity, without obvious strain distribution change. In plastic stage, ΔMPGA begins to gradually plummet, and the strain is mainly distributed in the damaged area. The modes of seismic damage in the slope-tunnel system are mainly of tensile failure of the weak interlayer, cracking failure of tunnel lining, formation of persistent cracks on the slope crest and waist, development and outward shearing of the sliding mass, and buckling failure at the slope foot under extrusion of the upper rock body. This study can serve as a reference for predicting the failure modes of tunnel-slope system in strong seismic regions.

Carbonaceous slate is one kind of metamorphic rocks with developed foliation, which is frequently encountered during tunnel construction in Western China. The foliation plays a crucial role in the stability of tunnels. For this, we conducted uniaxial compression tests, acoustic emission (AE) monitoring and scanning electron microscope (SEM) tests on carbonaceous slate. The results show that the strength, failure mode, and AE characteristics exhibit marked anisotropy with the angle between the axial and the foliation (β). As β increases, the ultrasonic wave velocity decreases monotonically, whereas the uniaxial compressive strength (UCS) displays a distinctive U-shaped trend. The elastic modulus initially decreases and then increases. The cumulative AE counts curve and energy curve show a stepped growth when β ≤ 45°. The AE events are active during the crack compaction phase and remain calm during the linear elastic deformation phase when β > 45°. Upon failure, the energy release accounts for the highest proportion (67%) when β = 45°, while the proportions in other cases are less than 37%. The maximum percentage (31%) of shear cracks is reported when β = 60°, which is six times greater than that at β = 0°. Moreover, Kernel density estimation analysis reveals that the high concentration area with low AF (AE counts/duration time) and high RA (rise time/amplitude) increases initially, and then decreases when β > 60°. In addition, nine types of cracks and seven modes of failure were identified. The foliation angle has a pronounced impact on shear failure modes in comparison with tensile failure modes. The supports could suffer larger deformation when β ≥ 60° compared to other cases. The failure behaviors correspond well with field observations.

Seismicity resulting from the near- or in-field fault activation significantly affects the stability of large-scale underground caverns that are operating under high-stress conditions. A comprehensive scientific assessment of the operational safety of such caverns requires an in-depth understanding of the response characteristics of the rock mass subjected to dynamic disturbances. To address this issue, we conducted true triaxial modeling tests and dynamic numerical simulations on large underground caverns to investigate the impact of static stress levels, dynamic load parameters, and input directions on the response characteristics of the surrounding rock mass. The findings reveal that: (1) When subjected to identical incident stress waves and static loads, the surrounding rock mass exhibits the greatest stress response during horizontal incidence. When the incident direction is fixed, the mechanical response is more pronounced at the cavern wall parallel to the direction of dynamic loading. (2) A high initial static stress level specifically enhances the impact of dynamic loading. (3) The response of the surrounding rock mass is directly linked to the amplitude of the incident stress wave. High amplitude results in tensile damage in regions experiencing tensile stress concentration under static loading and shear damage in regions experiencing compressive stress concentration. These results have significant implications for the evaluation and prevention of dynamic disasters in the surrounding rock of underground caverns experiencing dynamic disturbances.

This study aims to understand the effect of injection rate on injection-induced fracture activation in granite. We performed water injection-induced slip tests on samples containing either a smooth or a rough fracture at four different injection rates under undrained conditions and monitored the acoustic emission (AE) signals during the tests. Experimental results reveal that the critical activation fluid pressure is related to the injection rate, pressure diffusion rate, stress state, and fracture roughness. For the smooth fracture, as the injection rate increases, the critical activation fluid pressure increases significantly, while the injection rate has little effect on the critical activation fluid pressure of the rough fracture. The quasi-static slip distance of fractures decreases as the injection rate increases, with rough fractures exhibiting a greater overall slip distance compared to smooth fractures. The number of AE events per unit sliding distance increases with the injection rate, while the global b value decreases. These results indicate that higher injection rates produce more large-magnitude AE events and more severe slip instability and asperity damage. We established a linkage between fluid injection volume, injection rate, and AE events using the seismogenic index (Σ). The smooth fracture exhibits a steadily increasing Σ with the elapse of injection time, and the rate of increase is higher at higher injection rates; while the rough fracture is featured by a fluctuating Σ, signifying the intermittent occurrence of large-magnitude AE events associated with the damage of larger fracture asperities. Our results highlight the importance of fracture surface heterogeneity on injection-induced fracture activation and slip.

Rock discontinuities such as joints widely exist in natural rock masses, and wave attenuation through rock masses is mainly caused by discontinuities. The displacement discontinuity model (DDM) has been widely used in theoretical and numerical analysis of wave propagation across rock discontinuity. However, the circumstance under which the DDM is applicable to predict wave propagation across rock discontinuity remains poorly understood. In this study, theoretical analysis and ultrasonic laboratory tests were carried out to examine the theoretical applicability of the DDM for wave propagation, where specimens with rough joints comprising regular rectangular asperities of different spacings and heights were prepared by 3D printing technology. It is found that the theoretical applicability of the DDM to predict wave propagation across rock discontinuity is determined by three joint parameters, i.e. the dimensionless asperity spacing (L), the dimensionless asperity height (H) and the groove density (D). Through theoretical analysis and laboratory tests, the conditions under which the DDM is applicable are derived as follows: L≤−0.21D+0.27 and H≤−0.12D+0.17, D∈(0,1]. With increase in the groove density, the thresholds of the dimensionless asperity spacing and the dimensionless asperity height show a decreasing trend. In addition, the transmission coefficient in the frequency domain decreases with increasing groove density, dimensionless asperity spacing or dimensionless asperity height. The findings can facilitate our understanding of DDM for predicting wave propagation across rock discontinuity.

To investigate the effects of the maximum principal stress direction (θ) and cross-section shape on the failure characteristics of sandstone, true-triaxial compression experiments were conducted using cubic samples with rectangular, circular, and D-shaped holes. As θ increases from 0° to 60° in the rectangular hole, the left failure location shifts from the left corner to the left sidewall, the left corner, and then the floor, while the right failure location shifts from the right corner to the right sidewall, right roof corner, and then the roof. Furthermore, the initial failure vertical stress first decreases and then increases. In comparison, the failure severity in the rectangular hole decreases for various θ values as 30° > 45° > 60° > 0°. With increasing θ, the fractal dimension (D) of rock slices first increases and then decreases. For the rectangular and D-shaped holes, when θ = 0°, 30°, and 90°, D for the rectangular hole is less than that of the D-shaped hole. When θ = 45° and 60°, D for the rectangular hole is greater than that of the D-shaped hole. Theoretical analysis indicates that the stress concentration at the rectangular and D-shaped corners is greater than the other areas. The failure location rotates with the rotation of θ, and the failure occurs on the side with a high concentration of compressive stress, while the side with the tensile and compressive stresses remains relatively stable. Therefore, the fundamental reason for the rotation of failure location is the rotation of stress concentration, and the external influencing factor is the rotation of θ.

The overall heat transfer coefficient (OHTC) of rock fractures is a fundamental parameter for characterizing the heat transfer behavior of rock fractures in hot dry rock (HDR) geothermal mining. Although a number of practical formulae for heat transfer coefficients have been developed in the literature, there is still no widely accepted analytical solution. This paper constructs highly accurate analytical solutions for the temperatures of the inner fracture wall and the fluid. Then they are employed to develop new definition-based formulae (formula A and its simplification formula B) of the OHTC, which are well validated by the experimental and numerical simulation results. An empirical correlation formula of heat transfer coefficient is proposed based on the definition-based formulae which can be directly used in the numerical simulations of heat transfer in rock fractures. A site-scale application example of numerical simulation also demonstrates the effectiveness of the empirical correlation formula.

Despite the extensive use of distributed fiber optic sensing (DFOS) in monitoring underground structures, its potential in detecting structural anomalies, such as cracks and cavities, is still not fully understood. To contribute to the identification of defects in underground structures, this study conducted a four-point bending test of a reinforced concrete (RC) beam and uniaxial loading tests of an RC specimen with local cavities. The experimental results revealed the disparity in DFOS strain spike profiles between these two structural anomalies. The effectiveness of DFOS in the quantification of crack opening displacement (COD) was also demonstrated, even in cases where perfect bonding was not achievable between the cable and structures. In addition, DFOS strain spikes observed in two diaphragm wall panels of a twin circular shaft were also reported. The most probable cause of those spikes was identified as the mechanical behavior associated with local concrete contamination. With the utilization of the strain profiles obtained from laboratory tests and field monitoring, three types of multi-classifiers, based on support vector machine (SVM), random forest (RF), and backpropagation neural network (BP), were employed to classify strain profiles, including crack-induced spikes, non-crack-induced spikes, and non-spike strain profiles. Among these classifiers, the SVM-based classifier exhibited superior performance in terms of accuracy and model robustness. This finding suggests that the SVM-based classifier holds promise as a potential solution for the automatic detection and classification of defects in underground structures during long-term monitoring.

A series of true triaxial unloading tests are conducted on sandstone specimens with a single structural plane to investigate their mechanical behaviors and failure characteristics under different in situ stress states. The experimental results indicate that the dip angle of structural plane (θ) and the intermediate principal stress (σ2) have an important influence on the peak strength, cracking mode, and rockburst severity. The peak strength exhibits a first increase and then decrease as a function of σ2 for a constant θ. However, when σ2 is constant, the maximum peak strength is obtained at θ of 90°, and the minimum peak strength is obtained at θ of 30° or 45°. For the case of an inclined structural plane, the crack type at the tips of structural plane transforms from a mix of wing and anti-wing cracks to wing cracks with an increase in σ2, while the crack type around the tips of structural plane is always anti-wing cracks for the vertical structural plane, accompanied by a series of tensile cracks besides. The specimens with structural plane do not undergo slabbing failure regardless of θ, and always exhibit composite tensile-shear failure whatever the σ2 value is. With an increase in σ2 and θ, the intensity of the rockburst is consistent with the tendency of the peak strength. By analyzing the relationship between the cohesion (c), internal friction angle (φ), and θ in sandstone specimens, we incorporate θ into the true triaxial unloading strength criterion, and propose a modified linear Mogi-Coulomb criterion. Moreover, the crack propagation mechanism at the tips of structural plane, and closure degree of the structural plane under true triaxial unloading conditions are also discussed and summarized. This study provides theoretical guidance for stability assessment of surrounding rocks containing geological structures in deep complex stress environments.

A complex geological environment with faults can be encountered in the process of coal mining. Fault activation can cause instantaneous structure slipping, releasing a significant amount of elastic strain energy during underground coal mining. This would trigger strong rockburst disasters. To understand the occurrence of fault-slip induced rockbursts, we developed a physical model test system for fault-slip induced rockbursts in coal mine drifts. The boundary energy storage (BES) loading apparatus and bottom rapid retraction (BRR) apparatus are designed to realize energy compensation and continuous boundary stress transfer of the surrounding rocks for instantaneous fault slip, as well as to provide space for the potential fault slip. Taking the typical fault-slip induced rockburst in the Xinjulong Coal Mine, China, as the background, we conducted a model test using the test system. The deformation and stress in the rock surrounding the drift and the support unit force during fault slip are analyzed. The deformation and failure characteristics and dynamic responses of drifts under fault-slip induced rockbursts are obtained. The test results illustrate the rationality and effectiveness of the test system. Finally, corresponding recommendations and prospects are proposed based on our findings.

This paper investigates the mechanisms of rock failure related to axial splitting and shear failure due to hoop stresses in cylindrical specimens. The hoop stresses are caused by normal viscous stress. The rheological dynamics theory (RDT) is used, with the mechanical parameters being determined by P- and S-wave velocities. The angle of internal friction is determined by the ratio of Young's modulus and the dynamic modulus, while dynamic viscosity defines cohesion and normal viscous stress. The effect of frequency on cohesion is considered. The initial stress state is defined by the minimum cohesion at the elastic limit when axial splitting can occur. However, as radial cracks grow, the stress state becomes oblique and moves towards the shear plane. The maximum and nonlinear cohesions are defined by the rock parameters under compressive strength when the radial crack depth reaches a critical value. The efficacy and precision of RDT are validated through the presentation of ultrasonic measurements on sandstone and rock specimens sourced from the literature. The results presented in dimensionless diagrams can be utilized in microcrack zones in the absence of lateral pressure in rock masses that have undergone disintegration due to excavation.

In the concurrent extraction of coal and gas, the quantitative assessment of evolving characteristics in mining-induced fracture networks and mining-enhanced permeability within coal seams serves as the cornerstone for effective gas extraction. However, representing mining-induced fracture networks from a three-dimensional (3D) sight and developing a comprehensive model to evaluate the anisotropic mining-enhanced permeability characteristics still pose significant challenges. In this investigation, a field experiment was undertaken to systematically monitor the evolution of borehole fractures in the coal mass ahead of the mining face at the Pingdingshan Coal Mining Group in China. Using the testing data of borehole fracture, the mining-induced fracture network at varying distances from the mining face was reconstructed through a statistical reconstruction method. Additionally, utilizing fractal theory, a model for the permeability enhancement rate (PER) induced by mining was established. This model was employed to quantitatively depict the anisotropic evolution patterns of PER as the mining face advanced. The research conclusions are as follows: (1) The progression of the mining-induced fracture network can be classified into the stage of rapid growth, the stage of stable growth, and the stage of weak impact; (2) The PER of mining-induced fracture network exhibited a typical progression that can be characterized with slow growth, rapid growth and significant decline; (3) The anisotropic mining-enhanced permeability of the reconstructed mining-induced fracture networks were significant. The peak PER in the vertical direction of the coal seam is 6.86 times and 4446.38 times greater than the direction perpendicular to the vertical thickness and the direction parallel to the advancement of the mining face, respectively. This investigatione provides a viable approach and methodology for quantitatively assessing the anisotropic PER of fracture networks induced during mining, in the concurrent exploitation of coal and gas.

The use of foam, as the most economical soil conditioning technique, in earth pressure balance tunnel boring machine (EPB-TBM) tunneling projects has significant effects on operation efficiency, excavation cost, and operation time. This study mainly focuses on developing models to predict the foam (surfactant) consumption. For this purpose, five empirical models are developed based on a database containing 11048 datasets of real-time foam consumption from three EPB-TBM tunneling projects in Iran. This database includes the most effective machine operational parameters and soil geomechanical properties on the foam consumption. Multiple linear regression analysis, multiple non-linear regression analysis, M5Prime decision tree, artificial neural network, and least squares support vector machine techniques are used to construct the models. To evaluate the performance of developed models, three performance evaluation criteria (including normalized root mean square error, variance account for, and coefficient of determination) are used based on the training and testing datasets. The results show that the developed models have high performance and their validity is guaranteed according to the testing dataset. Furthermore, the M5Prime model, which demonstrates the best performance compared to other models, is applied to predict the foam consumption in 19 excavation rings of Kohandezh station in Isfahan metro, Iran. After conducting an excavation operation in this station and comparing the results, it was found that the M5Prime model accurately predicts foam consumption with an average error of less than 13%. Therefore, the developed models, particularly M5Prime model, can be confidently applied in EPB-TBM tunneling projects for predicting foam consumption with a low error rate.

Joint roughness coefficient (JRC) is the most commonly used parameter for quantifying surface roughness of rock discontinuities in practice. The system composed of multiple roughness statistical parameters to measure JRC is a nonlinear system with a lot of overlapping information. In this paper, a dataset of eight roughness statistical parameters covering 112 digital joints is established. Then, the principal component analysis method is introduced to extract the significant information, which solves the information overlap problem of roughness characterization. Based on the two principal components of extracted features, the white shark optimizer algorithm was introduced to optimize the extreme gradient boosting model, and a new machine learning (ML) prediction model was established. The prediction accuracy of the new model and the other 17 models was measured using statistical metrics. The results show that the prediction result of the new model is more consistent with the real JRC value, with higher recognition accuracy and generalization ability.

The stimulation of shale reservoirs frequently involves significant shear failure, which is crucial for creating fracture networks and enhancing permeability to boost production. As the depth of extraction increases, the impact of elevated temperatures on the anisotropic shear strength and failure mechanisms of shale becomes pronounced, yet there is a notable lack of relevant research. This study conducts, for the first time, direct shear experiment on shales at four different temperatures and seven bedding angles. By employing acoustic emission (AE) and digital image correlation (DIC) techniques, the evolution of damage and the mechanism of crack propagation under anisotropic direct shearing at varying temperatures is revealed. The results indicate that both shear displacement and strength of shale increase with temperature across different bedding angles. Additionally, shale demonstrates distinct brittle failure characteristics under various conditions during direct shearing tests. The types of anisotropic shear failure observed under the influence of temperature include central shearing fracture, central shearing with secondary fracture, and deflected slip along the bedding. Moreover, the temperature effect enhances shear-induced crack propagation along bedding planes. Shear failure in shale predominantly occurs during higher loading stages, which coincide with a substantial amount of AE signals. Finally, the introduction of the anisotropy index and temperature sensitivity coefficient further elucidates the interaction mechanism between thermal effects and anisotropy. This study offers a novel methodology to explore the anisotropic shear failure behavior of shale under elevated temperatures, and also provides crucial theoretical and experimental insights into shear failure behavior relevant to practical shale reservoir stimulation.

Accurately predicting the overlying pressure is crucial for determining an appropriate cover depth of underwater box tunnels to avoid the uplifting failure. Based on the project of box jacking crossing the Beijing-Hangzhou Grand Canal in Suzhou, the characteristics of overlying pressure variation during tunneling are investigated. The monitoring results reveal that the fluctuation of overlying pressure is weakened during the rapid tunneling process. A modified analytical model for vertical earth pressure is conceived, in which the active and passive limit states for multi-layered soils are both considered. The probable range of overlying pressure obtained by the proposed model is suitable to cover the actual values. The anti-floating behavior of underwater box tunnels for two different working conditions is discussed by calculating the minimum cover depth. Using the calibrated analytical models, a parametric study is conducted to explore the influence of injection pressure, hardened slurry unit weight, soil internal friction angle, soil cohesion, and tunnel geometry. It is found that the injection pressure during the construction process is crucial for determining the necessary cover depth, and the change of box tunnel height makes it easier to trigger the variation of minimum cover depth.

Understanding the hydromechanical behavior and permeability stress sensitivity of hydraulic fractures is fundamental for geotechnical applications associated with fluid injection. This paper presents a three-dimensional (3D) benchmark model of a laboratory experiment on graywacke to examine the dynamic hydraulic fracturing process under a polyaxial stress state. In the numerical model, injection pressures after breakdown (postbreakdown) are varied to study the impact on fracture growth. The fluid pressure front and crack front are identified in the numerical model to analyze the dynamic relationship between fluid diffusion and fracture propagation. Following the hydraulic fracturing test, the polyaxial stresses are rotated to investigate the influence of the stress field rotation on the fracture slip behavior and permeability. The results show that fracture propagation guides fluid diffusion under a high postbreakdown injection pressure. The crack front runs ahead of the fluid pressure front. Under a low postbreakdown injection pressure, the fluid pressure front gradually reaches the crack front, and fluid diffusion is the main driving factor of fracture propagation. Under polyaxial stress conditions, fluid injection not only opens tensile fractures but also induces hydroshearing. When the polyaxial stress is rotated, the fracture slip direction of a fully extended fracture is consistent with the shear stress direction. The fracture slip direction of a partly extended fracture is influenced by the increase in shear stress. Normal stress affects the permeability evolution by changing the average mechanical aperture. Shear stress can induce shearing and sliding on the fracture plane, thereby increasing permeability.

In underground mining, especially in entry-type excavations, the instability of surrounding rock structures can lead to incalculable losses. As a crucial tool for stability analysis in entry-type excavations, the critical span graph must be updated to meet more stringent engineering requirements. Given this, this study introduces the support vector machine (SVM), along with multiple ensemble (bagging, adaptive boosting, and stacking) and optimization (Harris hawks optimization (HHO), cuckoo search (CS)) techniques, to overcome the limitations of the traditional methods. The analysis indicates that the hybrid model combining SVM, bagging, and CS strategies has a good prediction performance, and its test accuracy reaches 0.86. Furthermore, the partition scheme of the critical span graph is adjusted based on the CS-BSVM model and 399 cases. Compared with previous empirical or semi-empirical methods, the new model overcomes the interference of subjective factors and possesses higher interpretability. Since relying solely on one technology cannot ensure prediction credibility, this study further introduces genetic programming (GP) and kriging interpolation techniques. The explicit expressions derived through GP can offer the stability probability value, and the kriging technique can provide interpolated definitions for two new subclasses. Finally, a prediction platform is developed based on the above three approaches, which can rapidly provide engineering feedback.

This study investigates the effect of different in situ conditions like flaw infill, heat-treatment temperatures, and sample porosities on the anisotropic compressive response of jointed samples with an impersistent flaw. Jointed samples of different porosities are prepared by mixing Plaster of Paris (POP) with different water contents, i.e. 60% (i.e. for lower porosity) and 80% (i.e. for higher porosity). These samples are grouted with different infill materials, i.e. un-grouted, cement and sand-cement (3:1)-bio-concrete (SCB) mix and subsequently subjected to different temperatures, i.e. 100 °C, 200 °C and 300 °C. The results reveal the distinct stages in the stress-strain responses of samples characterized by initial micro-cracks closure, elastic transition, and non-linear response till peak followed by a post-peak behaviour. The un-grouted samples exhibit their lowest strength at 30° joint orientation. The ratios of maximum to minimum strength are 3.11 and 3.22 with varying joint orientations for lower and higher porosity samples, respectively. Strengths of cement and SCB mix grouted samples are increased for all joint orientations ranging between 16.13%-69.83% and 18.04%–73% at low porosity and 22%–48.66% and 27.77%–51.57% at high porosity, respectively as compared to the un-grouted samples. However, the strength of the grouted samples is decreased by 66.94%–75.47% and 77.17%–81.05% at lower porosity, and 79.37%–82.86% and 81.29%–95.55% at higher porosity for cement and for SCB grouts with an increase in the heating temperature from 30 °C to 300 °C, respectively. These observations could be due to the suppression of favourable crack initiation locations, i.e. flaw tips along the samples due to the filling of the crack by grouting and generation of thermal cracks with temperature. The mechanism of strength behaviour is elucidated in detail based on fracture propagation analysis and the anisotropic response of with or, without grouted samples.

In this study, a high-confining pressure and real-time large-displacement shearing-flow setup was developed. The test setup can be used to analyze the injection pressure conditions that increase the hydro-shearing permeability and injection-induced seismicity during hot dry rock geothermal extraction. For optimizing injection strategies and improving engineering safety, real-time permeability, deformation, and energy release characteristics of fractured granite samples driven by injected water pressure under different critical sliding conditions were evaluated. The results indicated that: (1) A low injection water pressure induced intermittent small-deformation stick–slip behavior in fractures, and a high injection pressure primarily caused continuous high-speed large-deformation sliding in fractures. The optimal injection water pressure range was defined for enhancing hydraulic shear permeability and preventing large injection-induced earthquakes. (2) Under the same experimental conditions, fracture sliding was deemed as the major factor that enhanced the hydraulic shear–permeability enhancement and the maximum permeability increased by 36.54 and 41.59 times, respectively, in above two slip modes. (3) Based on the real-time transient evolution of water pressure during fracture sliding, the variation coefficients of slip rate, permeability, and water pressure were fitted, and the results were different from those measured under quasi-static conditions. (4) The maximum and minimum shear strength criteria for injection-induced fracture sliding were also determined (μ = 0.6665 and μ = 0.1645, respectively, μ is friction coefficient). Using the 3D (three-dimensional) fracture surface scanning technology, the weakening effect of injection pressure on fracture surface damage characteristics was determined, which provided evidence for the geological markers of fault sliding mode and sliding nature transitions under the fluid influence.

Loess is susceptible to loading effects such as significant changes in strength and volume variation caused by loading and wetting. In this study, considering the different connection states of pore water and gas in loess fabric, the gas phase closure case is incorporated into a unified form of the generalized effective stress framework, introducing a damage parameter considering the effects of closed pore gas. The loading effects of unsaturated loess under wide variations in saturation are described in a unified way, and the model performance is verified by corresponding stress and hydraulic path tests. The results indicated that the collapse response involves the initial void ratio of loess, and the coupled outwards motion of the loading-collapse (LC) yield surface under loading enhances its structural strength. Suction-enhanced yield stress requires a greater "tensile stress" to counteract its structural stability. The nucleation of bubbles at high saturation causes a decrease in yield stress. The loading effect exhibits a smaller collapse behavior when the influence of closed gas is considered, whereas the suction path does not cross the LC in the stress space under hydraulic action for the same parameters, which amplifies the influence of closed gas on loess deformation.

Compacted sand–bentonite mixtures are used as waste containment barriers (e.g. landfill liners and bentonite buffers for nuclear waste) to restrict contaminant transport. The potential for enhanced chemical containment of compacted sand–bentonite mixtures due to semipermeable membrane behavior has also been demonstrated. However, the extent to which membrane behavior persists in the presence of highly concentrated chemical solutions, which have been shown to degrade membrane behavior in bentonite-based barriers, remains largely unknown. Moreover, the limiting (threshold) salt concentrations at which membrane behavior of compacted sand–bentonite mixtures is effectively destroyed have not been evaluated. Accordingly, this study quantified the limiting membrane behavior of two duplicated specimens of compacted sand–bentonite mixture comprising 15% sodium bentonite (by dry mass) by determining the limiting salt concentrations at which measurable membrane behavior was eliminated. The specimens were exposed to increasingly higher source concentrations, Cot, of boundary monovalent salt solutions (KCl and NaCl) until measured values of the membrane efficiency coefficient, ω, were effectively zero. Overall, ω decreased from an average of 0.032 to zero as Cot increased from 160 mmol/L KCl to 3.27 mol/L NaCl, resulting in limiting threshold salt concentrations for the two tests between 1.63 mol/L and 3.27 mol/L NaCl, which are significantly higher than those at which measurable membrane behavior has previously been demonstrated.

Sudden temperature drops cause soils in natural environments to freeze unidirectionally, resulting in soil expansion and deformation that can lead to damage to engineering structures. The impact of temperature-induced freezing on deformation and solute migration in saline soils, especially under extended freezing, is not well understood due to the lack of knowledge regarding the microscopic mechanisms involved. This study investigated the expansion, deformation, and water–salt migration in chlorinated saline soils, materials commonly used for canal foundations in cold and arid regions, under different roof temperatures and soil compaction levels through unidirectional freezing experiments. The microscopic structures of saline soils were observed using scanning electron microscopy (SEM) and optical microscopy. A quantitative analysis of the microstructural data was conducted before and after freezing to elucidate the microscopic mechanisms of water–salt migration and deformation. The results indicate that soil swelling is enhanced by elevated roof temperatures approaching the soil's freezing point and soil compaction, which prolongs the duration and accelerates the rate of water–salt migration. The unidirectional freezing altered the microstructure of saline soils due to the continuous temperature gradients, leading to four distinct zones: natural frozen zone, peak frozen zone, gradual frozen zone, and unfrozen zone, each exhibiting significant changes in pore types and fractal dimensions. Vacuum suction at the colder end of the soil structure facilitates the upward migration of salt and water, which subsequently undergoes crystallization. This process expands the internal pore structure and causes swelling. The findings provide a theoretical basis for understanding the evolution of soil microstructure in cold and arid regions and for the management of saline soil engineering.

It has been well recognized that sand particles significantly affect the mechanical properties of reconstituted sandy clays, including the hosted clay and sand particles. However, interrelation between the permeability and compressibility of reconstituted sandy clays by considering the structural effects of sand particles is still rarely reported. For this, a series of consolidation-permeability coefficient tests were conducted on reconstituted sandy clays with different sand fractions (ψss), initial void ratio of hosted clays (ec0) and void ratio at liquid limit of hosted clays (ecL). The roles of ψss in both the relationships of permeability coefficient of hosted clay (kv−hostedclay) versus effective vertical stress (σv′) and void ratio of hosted clay (ec−hostedclay) versus σv′ were analyzed. The results show that the permeability coefficient of reconstituted sandy clays (kv) is dominated by hosted clay (kv=kv−hostedclay). Both ψss and ec0 affect the kv of sandy clays by changing the ec−hostedclay at any given σv′. Due to the partial contacts and densified clay bridges between the sand particles (i.e. structure effects), the ec−hostedclay in sandy clays is higher than that in clays at the same σ′v. The kv−ec−hostedclay relationship of sandy clays is independent of ec0 and ψss, but is a function of ecL. The types of hosted clays affect the kv of sandy clays by changing the ecL. Based on the relationship between permeability coefficient and void ratio for the reconstituted clays, an empirical method for determining the kv is proposed and validated for sandy clays. The predicted values are almost consistent with the measured values with kv−predicted/kv−measured = 0.6–2.5.

When assessing seismic liquefaction potential with data-driven models, addressing the uncertainties of establishing models, interpreting cone penetration tests (CPT) data and decision threshold is crucial for avoiding biased data selection, ameliorating overconfident models, and being flexible to varying practical objectives, especially when the training and testing data are not identically distributed. A workflow characterized by leveraging Bayesian methodology was proposed to address these issues. Employing a Multi-Layer Perceptron (MLP) as the foundational model, this approach was benchmarked against empirical methods and advanced algorithms for its efficacy in simplicity, accuracy, and resistance to overfitting. The analysis revealed that, while MLP models optimized via maximum a posteriori algorithm suffices for straightforward scenarios, Bayesian neural networks showed great potential for preventing overfitting. Additionally, integrating decision thresholds through various evaluative principles offers insights for challenging decisions. Two case studies demonstrate the framework's capacity for nuanced interpretation of in situ data, employing a model committee for a detailed evaluation of liquefaction potential via Monte Carlo simulations and basic statistics. Overall, the proposed step-by-step workflow for analyzing seismic liquefaction incorporates multifold testing and real-world data validation, showing improved robustness against overfitting and greater versatility in addressing practical challenges. This research contributes to the seismic liquefaction assessment field by providing a structured, adaptable methodology for accurate and reliable analysis.

In the long-term exploitation of natural gas hydrate, the stress change intensifies the creep effect and leads to the destruction of pore structures, which makes it difficult to predict the permeability of hydrate reservoir. Although permeability is crucial to optimize gas recovery for gas hydrate reservoirs, until now, accurately modeling the permeability of hydrate-bearing clayey-silty sediments during the creep process remains a significant challenge. In this study, by combining the nonlinear fractional-order constitutive model and the Kozeny-Carman (KC) equation, a novel creep model for predicting the permeability of hydrate-bearing clayey-silty sediments has been proposed. In addition, experimental tests have been conducted to validate the derived model. The proposed model is further validated against other available test data. When the yield function F < 0, the permeability decreases gradually due to the shrinkage of pore space. However, when the yield function F ≥ 0, the penetrating damage bands will be generated. Results show that, once the model parameters are determined appropriately by fitting the test data, the model can also be used to predict permeability under any other stress conditions. This study has a certain guiding significance for elucidating the permeability evolution mechanisms of hydrate-bearing clayey-silty sediments during the extraction of marine gas hydrates.

Suffusion is the process defined as the migration of relatively small soil particles through the pores of a soil matrix composed of relatively large particles, driven by substantial hydrodynamic forces and weak attraction energies. This study investigates the influence of flow direction (upward and downward) on suffusion induced by interaction energies in sand-clay mixtures under both saturated and unsaturated conditions. The impact of clay mineralogy (kaolinite, illite, and montmorillonite), sand-grain size, and ionic concentration (IC) gradient were discussed based on the observed breakthrough curves (BTCs) and relative saturation rate (Sr) during injection (particularly for unsaturated conditions). Under saturated conditions, higher susceptibility to suffusion was observed in sand-kaolinite and sand-illite mixtures under downward flow compared to upward flow, whereas the suffusion of montmorillonite was more significant under upward flow than under downward flow. In contrast, for unsaturated conditions, more substantial suffusion of kaolinite and illite particles occurred under upward flow compared to downward flow, whereas the opposite trend was observed in sand-montmorillonite mixtures. In addition, the impact of sand-grain size (or the size ratio between sand and clay) on the suffusion of kaolinite and illite under unsaturated conditions suggests a reduced size ratio that leads to relatively significant suffusion under downward flow compared to upward flow. The findings presented in this study contribute to a comprehensive understanding of the influence of flow direction on suffusion in sand-clay mixtures under both saturated and unsaturated conditions.

Seepage in coarse-grained soil exhibits distinct non-Darcy characteristics, and the transition from linear to nonlinear seepage significantly affects the hydraulic characteristics and geotechnical applications. Due to the complexity of pore structure in heterogeneous coarse-grained soil, identifying the critical threshold for the transition from Darcy to non-Darcy seepage is challenging. This paper introduces equivalent particle size (dep) and relative roughness (λt) as indirect indicators reflecting the pore characteristics, quantifying the complex pore structure of heterogeneous coarse-grained soil. The formulae for the derivation of Reynolds number and resistance coefficient for heterogeneous coarse-grained soil are presented. By conducting permeability tests on coarse-grained soils with different pore structures, the effect of particle composition heterogeneity on seepage characteristics was analyzed. The flow regime of heterogeneous coarse-grained soil is divided into laminar, transitional, and turbulent stages based on the relationship between Reynolds number and resistance coefficient. The energy loss patterns in each stage are closely related to pore structure. By setting the permeability ratio k∗ = 0.95 as the critical threshold for the transition from Darcy to non-Darcy seepage, a method for calculating the critical Reynolds number (Recr) for heterogeneous coarse-grained soil is proposed. Furthermore, we applied this method to other published laboratory data, analyzing the differences in the critical threshold for seepage transition between homogeneous and heterogeneous coarse-grained soil. This study aims to propose a more accurate and general criterion for the transition from Darcy to non-Darcy seepage in heterogeneous coarse-grained soil.

Soil-bentonite (SB) backfills in vertical cutoff walls are used extensively to contain contaminated groundwater. Previous studies show that the hydraulic conductivity of backfill can exceed the typically recommended maximum value (k = 1 × 10−9 m/s) if exposed to groundwater impacted by organic acids commonly released from uncontrolled landfills and municipal solid waste dumps. Polymer amended backfills exhibit excellent chemical compatibility to metal-laden groundwater. However, few studies to date have explored the effect of organic acid contaminated groundwater on hydraulic performance of polymer amended backfills. This study presents an experimental investigation on the hydraulic performance and microstructural properties of a composite polymer amended backfill used to contain flow of acetic acid-laden groundwater. A series of laboratory experiments were performed to evaluate free-swell indices of the composite polymer amended bentonites, liquid limits of the composite polymer amended and unamended bentonites, and slump heights and hydraulic conductivity (k) values of the amended backfills to acetic acid solutions with varying concentrations. The results were compared with those of the unamended bentonites and unamended backfills reported in a previous study. The results showed that the free-swell index and liquid limit of the amended bentonites were higher than those of the unamended bentonites. Permeation with acetic acid solutions with concentrations ranging from 40 mmol/L to 320 mmol/L conducted on the amended backfill only resulted in an increase in k of less than a factor of about 10 related to that based on permeation with tap water (4.41 × 10−11 -1.68 × 10−10 m/s to acetic acid solution versus 1.65 × 10−11 m/s to tap water). Mechanisms contributing to enhanced chemical compatibility of amended backfill were ascertained based on scanning electron microscopy, mercury intrusion porosimetry, and zeta potential analyses.

Due to severe mass transfer limitations, contaminated soils with low-permeability limit the accessibility of amendments, resulting in less effective or even ineffective remediation. Enhancing the mass transfer properties of low-permeability soils by hydraulic fracturing is a promising technique. A quasi-three-dimensional (quasi-3D) analytical model was presented that accounted for advection-diffusion-adsorption-degradation processes in the fracture-matrix system. The model combined the injection-extraction technique to investigate the enhanced transport of amendments in low-permeability contaminated soil by hydraulic fractures. Then, the injection strategy and controllable parameter optimization were comprehensively studied by analyzing the radial transport behavior of the amendment within hydraulic fractures. The results showed that higher injection volumetric rates accelerated the formation of a uniform line source within the fractures. Although the differences in the effective ratio of the amendment among different injection modes were not significant, considering the amendment utilization rate and cost reduction, the recommended injection strategy was the combination of continuous pulsing injection and periodic injection.

Deep excavations in silt strata can lead to large deformation problems, posing risks to both the excavation and adjacent structures. This study combines field monitoring with numerical simulation to investigate the underlying mechanisms and key aspects associated with large deformation problems induced by deep excavation in silt strata in Shenzhen, China. The monitoring results reveal that, due to the weak property and creep effect of the silt strata, the maximum wall deflection in the first excavated section (Section 1) exceeds its controlled value at more than 93% of measurement points, reaching a peak value of 137.46 mm. Notably, the deformation exhibits prolonged development characteristics, with the diaphragm wall deflections contributing to 39% of the overall deformation magnitude during the construction of the base slab. Subsequently, numerical simulations are carried out to analyze and assess the primary factors influencing excavation-induced deformations, following the observation of large deformations. The simulations indicate that the low strength of the silt soil is a pivotal factor that results in significant deformations. Furthermore, the flexural stiffness of the diaphragm walls exerts a notable influence on the development of deformations. To address these concerns, an optimization study of potential treatment measures was performed during the subsequent excavation of Section 2. The combined treatment approach, which comprises the reinforcement of the silt layer within the excavation and the increase in the thickness of the diaphragm walls, has been demonstrated to offer an economically superior solution for the handling of thick silt strata. This approach has the effect of reducing the lateral wall displacement by 83.1% and the ground settlement by 70.8%, thereby ensuring the safe construction of the deep excavation.

The soil-bentonite (SB) cut-off wall has been widely considered a vertical barrier to effectively control the migration of pollutants in contaminated sites. Recently, active porous materials have been used as a promising candidate amendment for mitigating chemical degradation and improving the retardation capacity of the SB cut-off wall. In this study, the silty clay from a typical Cr(VI) contaminated site was selected as the body material of the SB cut-off wall, and zeolite and active carbon were used as the modifiers of the silty clay-bentonite backfills, respectively. The impact of the two modifiers on the engineering properties of the backfills was investigated through a series of slump tests, consolidation tests, hydraulic conductivity tests, and microstructure tests. The experimental results demonstrated that the slump curves closely exhibited a linear relationship between standard slump and moisture content. Meanwhile, bentonite could improve the optimum moisture content of the backfills, while the addition of the two modifiers yielded the opposite outcome. As the bentonite content increased, the compression index of the backfills significantly increased, while the hydraulic conductivity decreased. At a given bentonite content of 5%, the addition of zeolite or active carbon resulted in a reduction in the compression index and initial void ratio, while exhibiting minimal impact on the hydraulic conductivity. Scanning electron microscope (SEM) observations indicated that the silty clay-bentonite backfills became increasingly loose with increasing bentonite content, owing to the filling and expansion of dispersed bentonite layers. The amendment of zeolite or active carbon was able to decrease the backfill volume by promoting the agglomeration of layered bentonite. The findings of this study will be useful for the optimal selection of backfill materials and the performance evaluation of the cut-off wall.

Utilizing the Discrete Element Method, this research studied the stiffness distribution of gap-graded soils by modifying the conventional static method. By acknowledging the inherent particle property disparity between coarser and finer particles, this research differentiates the stiffness distribution of gap-graded soils from the perspective of contact and particle types. Results indicate that particle property disparity significantly influence the small-strain stiffness characteristics, consequently altering the overall stiffness distribution in gap-graded soil specimens. Specifically, with the equivalent coarser particle property, an increase in particle Young's modulus of finer particles results in an augmentation of small-strain stiffness values, alongside an increased stiffness distribution contribution from finer particles. Nevertheless, this study reveals that even with a higher particle Young's modulus of finer particles, the proportion of small-strain stiffness transferred by finer particles remains consistently lower than their volume fraction. Furthermore, the proportion of stiffness transferred by finer particles may fall below their contribution to stress transmission. This investigation accentuates the subtle yet significant effects of particle property variations on small strain stiffness and its subsequent distribution, providing a foundation for advancing the significance of particle property disparities in evaluating soil responses.