The failure phenomenon of thin-layered rock tunnels not only exhibits asymmetric spatial characteristics, but also significant time-dependent characteristics under high in-situ stress, which is attributed to the time-dependent fracture of thin-layered rocks. This paper conducted a series of true triaxial creep compression tests on typical thin-layered rock siliceous slate with acoustic emission technique to reveal its anisotropic time-dependent fracture characteristics. The anisotropic long-term strength, creep fracturing process, and fracture orientation characteristics of thin-layered rocks under different loading angles (β, ω) and intermediate principal stress were summarized. A three-dimensional (3D) non-linear visco-plastic creep model for thin-layered rock was developed to simulate its anisotropic creep behavior. The time-dependent fracturing of rocks during true triaxial creep loading is reflected through the change of equivalent strain based on an improved Euler iteration method. By constructing the plastic potential function and overstress index related to loading angles and stress state, the anisotropic time-dependent fracturing process and propagation of thin-layered rocks under different loading angles and intermediate principal stress are expounded. The model was validated experimentally to show it can reflect the long-term strength and creep deformation characteristics of thin-layered rocks under true triaxial compression.

Ground control in underground mines employs rock reinforcement and surface support to maintain the integrity of excavations for their anticipated working life. The performance of a ground support system, however, is more complex and relies on effective load distribution between reinforcement and surface support elements. Typically, failure of the ground support system is along its weakest link, often the surface support. Consequently, the degradation of any ground support element over time will compromise the structural integrity of the ground support system. Degradation of ground support can be due to multiple factors. This paper focuses on the role of corrosion in the long-term performance of mesh. It presents a comprehensive methodology for quantifying the impact of degradation on bolted welded wire mesh over time. This is an important aspect, as the mesh is often the first element that fails in a ground support system. This paper combines information from extensive field and laboratory studies on mesh corrosion with calibrated numerical models to capture the long-term performance of different bolting patterns under a range of corrosion environments. A series of three-dimensional (3D) distinct element models (DEM) were constructed to quantify the impact of different corrosion rate scenarios on the loading capacity, displacement, and failure mechanisms of bolted welded wire mesh in diamond and square bolting patterns. This work can contribute to the management of long-term hazards associated with corrosion of mesh in corrosive environments under non-seismic mining conditions.

This study presents the classification and prediction of severity for brittle rock failure, focusing on failure behaviors and excessive determination based on damage depth. The research utilizes extensive field survey data from the Shuangjiangkou Hydropower Station and previous research findings. Based on field surveys and previous studies, four types of brittle rock failure with different failure mechanisms are classified, and then a prediction method is proposed. This method incorporates two variables, i.e. Kv (modified rock mass integrity coefficient) and GSI (geological strength index). The prediction method is applied to the first layer excavation of the powerhouse cavern of Shuangjiangkou Hydropower Station. The results show that the predicted brittle rock failure area agrees with the actual failure area, demonstrating the method's applicability. Next, it extends to investigate brittle rock failure in two locations. The first is the k0+890 m section of the traffic cavern, and the second one is at K0-64 m of the main powerhouse. The criterion-based prediction indicates a severity brittle rock failure in the K0+890 m section, and a moderate brittle rock failure in the K0-64 m section, which agrees with the actual occurrence of brittle rock failure in the field. The understanding and application of the prediction method using Kv and GSI are vital for implementing a comprehensive brittle rock failure prediction process in geological engineering. To validate the adaptability of this criterion across diverse tunnel projects, a rigorous verification process using statistical findings was conducted. The assessment outcomes demonstrate high accuracy for various tunnel projects, allowing establishment of the correlations that enable valuable conclusions regarding brittle rock failure occurrence. Further validation and refinement through field and laboratory testing, as well as simulations, can broaden the contribution of this method to safer and more resilient underground construction.

Interest in soundless chemical demolition agents (SCDAs), also known as expansive cements, as potentially viable alternatives to explosives for rock fragmentation, has been growing in recent years. Consequently, there is an increasing amount of literature on the use of SCDA for the breakage of rock blocks and boulders. Limited research has been conducted so far on the breakage of excavation fronts, such as tunnel or drift faces, using SCDA. This is due to the perception that the planar compressive in-situ stresses in the face would inhibit the creation and propagation of fracturing due to expansive pressure. This study proposes a novel V-cut method for demolishing rock panels under biaxial stress using SCDA. This method was examined through large-scale tests and numerical modelling. The rock panels were subjected to high biaxial confinements of 26 MPa and 40 MPa. Such a level of confinement corresponds to an in-situ stress state 1000 m below the surface in the Canadian shield. The V-cut drillhole pattern employs two sets of three SCDA holes angled at 45° from the face of a Stanstead granite panel. The drillhole arrangement aims to create a V-shaped wedge in the plane of major principal stress. When angled drillholes are subjected to expansive pressure, they tend to cast out of the panel face, causing fragmentation. Two panels of 1 m × 1 m × 0.25 m were successfully demolished using the proposed method. The three-dimensional fast Lagrangian analysis code FLAC3D modelling was used to reconstruct the panel failure mechanism owing to the V-cut. This study demonstrates the feasibility of fragmenting an excavation front, such as a rock excavation face, with SCDA using a V-cut drill hole pattern while subjected to high biaxial confinement.

The control and management of mining-induced seismic hazards have attracted ever-rising attention, especially in underground longwall coal mines, where continuous mining activities dynamically alter the stress states and induce seismic events. In this work, the Epidemic Type Aftershock Sequence (ETAS) model was applied to formulate the aftershock catalogue of mining-induced seismicity and investigate the formation of event triggering associated with longwall mining. The conventional Baiesi and Paczuski method (2004) was used to separate longwall mining-induced seismic events into triggered and non-triggered catalogues. The latter catalogue contains both non-triggering (NT)-isolated events that do not trigger subsequent events and NT-parent events of the former catalogue. Statistical properties of triggered events were analysed spatially and temporally. The temporal triggering sequence follows the Omori-Utsu law, where the temporal decay of aftershocks is influenced by the magnitude of NT-parent events in mining-induced seismicity. The spatial distribution of aftershocks follows an inverted U-shaped relationship with distance to their corresponding NT-parent events. The quantitative forecasting of triggered events was performed based on the nonhomogeneous Poisson distribution, which achieved a good consistency with their NT-parent events. Amongst the non-triggered catalogue, NT-isolated events are concentrated ahead of NT-parent events, potentially acting as foreshocks for the latter.

Injection-induced seismicity has been a focus of industry for decades as it poses great challenges to the associated risk mitigation and hazard assessment. The response surface methodology is integrated into the geo-mechanical model to analyze the effects of multiple factors on induced seismicity during the post shut-in period. We investigate the roles of poroelastic stress and pore pressure diffusion and examine the differences in the controlling mechanism between fault damage zones and the fault core. A sensitivity analysis is conducted to rank the selected factors, followed by a Box‒Behnken design to form response surfaces and formulate prediction models for the Coulomb stress and its components. Reservoir properties significantly affect the potentials of induced seismicity in the fault by changing pore pressure diffusion, which can be influenced by other factors to varying degrees. Coulomb stress is greater in pressurized damage zones than in fault cores, and the seismicity rate exhibits a consistent variation. Poroelastic stress plays a similar role to pore pressure diffusion in the stability of the fault within the pressurized damage zones. However, pore pressure diffusion dominates in the fault core due to the low rigidity, which limits the accumulation of elastic energy caused by poroelastic coupling. The slip along the fault core is a critical issue to consider. The potential for induced seismicity is reduced in the right damage zones as the pore pressure diffusion is blocked by the low-permeability fault core. However, poroelastic stressing still occurs, and in deep basements, the poroelastic effect is dominant even without a direct increase in pore pressure. The findings in this study reveal the fundamental mechanisms behind injection-induced seismicity and provide guidance for optimizing injection schemes in specific situations.

This study investigated the sidewall rockburst characteristics of highly stressed circular tunnel subjected to impact loads resulting from rock blasting or other mining-related dynamic disturbances, aiming at exploring the influence of vertical prestress and dynamic load on sidewall rockburst. Using a biaxial Hopkinson pressure bar (BHPB) system, we studied the sidewall rockburst of a circular tunnel by applying various prestresses (horizontal and vertical static stresses) to a sand prefabricated circular hole specimen, followed by impact loads. The real-time process and strain field of the sidewall rockburst around the specimen were tracked by the high-speed camera and digital image correlation (DIC). The tests reveal that the sidewall rockburst process can be summarized as: calm stage, slab buckling and spalling stage, rock slabs ejection stage, and V-shaped notch formation stage. Furthermore, the sidewall rockbursts exhibit typical dynamic tensile failure. The mechanism of sidewall rockburst under the coupled static-impact loads was summarized, i.e. the static prestress determines the initial stress and strain distribution, and the vertical prestress influences the affected range and strain values of the strain concentration zone; the impact load disrupts the original static stress equilibrium, inducing alterations in the stress and strain of the surrounding rock and triggering sidewall rockburst.

The effects of cyclic heat treatments on the fracture shear behaviors are rarely reported. To enhance our understanding, granite fractures having almost the same roughness were first exposed to cyclic heating at 400 °C and air-cooling treatments, and then direct shear tests were performed under four levels of normal loading. The influences of thermal cycles on roughness degradation and shear properties are analyzed. The roughness degradation in the joint roughness coefficient and the three-dimensional (3D) roughness metric exhibit linear increasing tendency with increasing thermal cycles. Typical fracture shear properties, including cohesion and friction angle, peak and residual shear strength, peak and residual shear displacement, and initial and secant shear stiffness, fluctuate generally within the first 10 thermal cycles, followed by gradual decreasing tendencies. The thermal effect on the shear properties become weaker as the number of heat treatments increases from 10 to 80. Nonuniform expansion and shrinkage of mineral grains after thermal treatments produce micro-cracks within the rock matrix and on the rock surface, suggesting that asperities are easier to be sheared-off. Thermal alteration in fracture peak-shear strength could be attributed to the deterioration in rock strengths and the mismatch in opposing fracture walls. The observations would provide better insights into rock friction after high temperatures in geothermal energy exploitation.

The stable protection of the walls of high-temperature geothermal wells is a challenging issue for sustainably exploiting geothermal resources. However, the cement stone filling layer of the cemented portion of the well deteriorates gradually during geothermal mining due to the dry-wet cycles of the saline geothermal water, reducing the service life of the geothermal well. For this, this paper presented five groups of cement stone cylinders with salt contents of 0%, 1%, 6%, and 11%, which were subjected to heating to 300 °C and 1–5 dry-wet cycles. Nuclear magnetic resonance (NMR) and nonmetallic detection were used to test and analyze the porosity and wave velocity. Additionally, the damage evolution induced by dry-wet cycles was captured based on acoustic emission (AE) data. The experimental results indicated that the heating process primarily resulted in mineral and salt crystal expansion, which in turn caused damage. The damage threshold due to the salt content was found to be 6%. The sudden increase in the thermal stress caused by cooling and deterioration of the tensile strength of the cement column were the key factors in the damage during the cooling process. As the number of cycles increased, the accumulated AE energy moved forward and backward, with decreasing and increasing temperature, respectively. The threshold of signal mutation in the heating process is 200 °C, and the accumulated AE energy decreases by 11.7%. When the salt content was 0%, 1%, 6% and 11%, the wave velocity decreased by 19%, 27.3%, 35.5% and 35.9%, respectively. This study also proposed a damage model, which could provide theoretical support for long-term health monitoring and safety protection of geothermal wells.

Understanding the mechanical response of Q2 loess subjected to dry-wet cycles (DWCs) is the premise for the rational design of a hydraulic tunnel. Taking the Hanjiang-to-Weihe south line project in China as the research background, the microstructure evolution, strength degradation and compression characteristics of Q2 loess under different DWCs were investigated, and the fluid-solid coupling analysis of the hydraulic tunnel was carried out using the FLAC3D software. The amplification effect of tunnel surrounding soil pressure (SSP) and its influence on the long-term stability of the tunnel under different DWCs were obtained. The results showed that the pore microstructure parameters of the undisturbed and remolded loess basically tend to be stable after the number of DWCs exceeds 3. The porosity of Q2 loess is increased by 26%. The internal friction angle and cohesion of Q2 loess are decreased by 35% and 31%, respectively. The vertical strain of Q2 loess is increased by 55% after considering the DWCs. After the DWCs stabilized, the SSP ratio is increased between 10% and 25%. With the increase in buried depth of the tunnel, the SSP ratio is increased by 8%–10%. The SSP is reduced from 8% to 16% by the rise in groundwater level. As the number of DWCs increases and the burial depth of the tunnel decreases, the distribution of SSP becomes progressively more non-uniform. Based on the amplification factor and the modified compressive arch theory, the SSP distribution model of loess tunnel was proposed, which can be preliminarily applied to the design of supporting structures considering DWCs.

Rock masses in high-elevation or cold regions are vulnerable to the combined effects of freeze-thaw (F-T) weathering and dynamic mixed-mode loading, posing a serious threaten to the safety and stability of geotechnical engineering. In this study, a series of dynamic fracture tests were conducted on notched semi-circular bend (NSCB) granite specimens subjected to different mixed-mode loading and F-T cycles using a split Hopkinson pressure bar (SHPB) test system. The effects of F-T treatment and dynamic mixed-mode loading on the fracture properties of granite, including effective fracture toughness, progressive fracture process, and macroscopic morphology of fracture surface, were comprehensively revealed. The experimental results suggest that the dynamic effective fracture toughness of NSCB specimens is dependent on the loading rate, particularly when the mode I loading is dominant. Additionally, the fracture toughness decreases as the number of F-T cycles increases, with an inflection point at 30 F-T cycles. All granite specimens subjected to mixed-mode loading exhibit a curved fracture path, with faster crack propagation speed and more fine cracks in specimens exposed to higher F-T cycles. Macroscopic morphology of fracture surface obtained using three-dimensional (3D) scanning indicates that the fractal dimension of the fracture surface increases with increasing F-T cycles, and the increment is more pronounced for specimens subjected to a higher mode II loading component. Moreover, this study compared the fracture resistance of F-T treated granite subjected to dynamic mixed loading using the maximum tangential stress (MTS) criterion and the generalized maximum tangential stress-based semi-analytical (SA-GMTS) criterion. Compared with the MTS criterion, the SA-GMTS criterion shows a more reasonable consistency with the experimental results, with a root mean square error within ±7%.

Understanding the mechanical and transport behavior of thin (i.e. small aperture) cracks slipping under supercritical carbon dioxide (sc-CO2) conditions is essential to evaluate the integrity of sealing formations with buoyant sc-CO2 below and the success of waterless fracturing. The two major items of interest in this work are frictional strength and permeability change of the crack. We used a triaxial cell that permits in situ visualization to conduct and monitor slippage along the faces of narrow cracks subjected to triaxial stresses. Such cracks are analogs to small geological faults. We tested carbonate-rich, 1-inch diameter Wolfcamp shale samples that are saw cut 30° to vertical to create a thin crack. Friction coefficients ranged from about 0.6 to 0.8 consistent with expectations for brittle rocks. The sc-CO2 generally did not alter friction coefficient over the time scale of experiments. From a transport perspective, saturating cracks with sc-CO2 substantially decreased permeability of the crack by 26%–52%, while slip resulted in a variety of permeability responses. Overall, the combined impact of sc-CO2 saturation and slip reduced fault permeability for all tests. Our observations support the notion that the sealing capacity of some caprocks improves when saturated with sc-CO2 and that some slip of small fractures is not necessarily detrimental to caprock integrity.

The natural property of rock material, whether impact occurs, is the key influencing factor of the occurrence of rock burst disaster. To accurately assess rock burst proneness, this study focuses on typical sandstone as the research object. Uniaxial cyclic loading and unloading tests were conducted to measure the elastic strain energy accumulated in sandstone under different stress levels and a relationship between elastic strain energy and stress level was established. The results show that: (1) The peak stress under cyclic loading and unloading conditions is slightly lower than the uniaxial compressive strength. With an increase in the number of cycles, the internal damage of sandstone continues to accumulate, and the mechanical properties such as compressive strength continue to deteriorate; (2) With an increase in stress, the input strain energy, elastic strain energy, and dissipated strain energy also increase; (3) When the stress is low, the increase in elastic strain energy is large and shows a steady growth; with an increase in stress, the increase of elastic strain energy decreases; (4) The square of stress at any time has a good linear relationship with elastic strain energy. According to the relationship obtained from the test, the elastic strain energy at the peak stress time can be obtained; (5) A new criterion for assessing rock burst proneness is proposed: residual energy release rate index WT, which characterizes the energy release per unit time when the rock burst occurs. The intervals for evaluating the rock burst proneness of the residual energy release rate index WT are as follows: WT <0.025, indicating no rock burst proneness; 0.025≤ WT <0.15, indicating weak rock burst proneness; 0.15≤ WT <2, indicating medium rock burst proneness; WT >2, indicating strong rock burst proneness; and (6) The rationality of the proposed residual energy release rate index WT is verified by the multi-index method and the multi-sample method, and the proposed residual energy release rate index is used to determine the rock burst proneness of 10 kinds of rock samples. The evaluation accuracy is shown to be high, and it can reflect the actual rock burst proneness

Cases of simultaneous inrush of high-temperature water and harmful gases are infrequently reported in areas without geothermal anomalies, hydrocarbon source rock, or coal measures. For this, we investigated the origin, development, and formation of the high-temperature water and harmful gases that rushed into Bangfu tunnel, Southwest China. During excavation of the Bangfu tunnel through the F1-2 fault in sandstone, a significant incident occurred involving a sudden influx of high-temperature water (45.4 °C) of Na–HCO3 type and harmful gases (CO2, H2S). An extensive geological examination uncovered a fault network extending from the crust to the mantle in the tunnel site area. The site features a substantial presence of both surface water and groundwater. Furthermore, within the middle crust at depths ranging from 19 km to 23 km, there are high-temperature ductile melts enriched with fluids and gases. Monitoring and experiments conducted on the harmful gases reveal that the primary source is identified in the crust, with the mantle source being secondary, followed by the atmospheric source being a minimal contribution. The hydrochemical and isotopic composition characteristics of the high-temperature rushed water indicate its evolution was formed through the infiltration of atmospheric precipitation from cold groundwater of the Ca–HCO3 type. The mechanism underlying the formation of the inrush high-temperature water and harmful gases can be outlined as follows. The fault network, spanning from the crust to the mantle, serves as a migration pathway for the inflow substances. Mantle-derived volatiles and high-temperature melts make heat energy facilitate the inrush activity, while groundwater contributes to heat transfer and acts as a medium for gas transport. As mantle-derived volatiles migrate towards the surface through the fault network, they mix with high-temperature melts and crust-derived gases, forming a crust-mantle mixed gas. Through processes such as deep hydrothermal circulation, shallow hydrothermal circulation, water/rock reaction, near-surface mixing, and dilution, Ca–HCO3 type cold groundwater transforms into high-temperature water of Na–HCO3 type. The methodologies and findings of our research offer insights into the route selection, investigation, and construction of mountain tunneling projects under similar geological conditions.

Brittle fracture and its relationship to deformation and strength have been a fundamental area of research in rock mechanics. This paper presents an expanded pore-emanated cracking model to better understand the fracture behaviors and predict the compressive strength of sedimentary rocks. This proposed model is developed to account for a triaxial compression condition using the triaxial compression test results on sandstone, limestone and siltstone samples from Wyoming, USA and experimental data on sedimentary rocks collected from published literature. The normalized critical crack length is determined from the proposed model through which the peak compressive strength is estimated when the stress intensity at the crack tip reaches a critical value called the fracture toughness. Results indicate that the rock porosity and pore radius have an inverse relationship with the compressive strength. Adopting the porosity-permeability relationship, the pore radius is calculated in terms of porosity and grain size. Next, the effect of grain size is implicitly included in the model and negatively correlated with the compressive strength. Moreover, a new approach is proposed for the estimation of fracture toughness based on the pore radius and confining pressure. The predicted compressive strengths from the proposed model show a good agreement with the measured strengths with a mean bias (i.e. average ratio of the measured to predicted strengths) of 1.014. The influence of ϕ and KIC on σ1 was thoroughly studied using parametric study. The study concludes that the effect of ϕ is more prominent than KIC on σ1. At a constant porosity of 0.1, the stress ratio decreases from 0.0082 to 0.0078 when KIC increases from 0.1 to 0.2, indicating a 5% decrease in stress ratio. Whereas, at a constant KIC of 0.1, the stress ratio increases from 0.0082 to 0.014 when the porosity increases from 0.1 to 0.2, indicating 71% increases in stress ratio and therefore compressive strength.

The method for precursor information acquisition based on acoustic emission (AE) data for jointed rock masses is of significant importance for the early warning of dynamic disasters in underground engineering. A clustering-convolutional neural network (CNN) method is proposed, which comprises a clustering component and a CNN component. A series of uniaxial compression tests were conducted on granite specimens containing a persistent sawtooth joint, with different strain rates (10−5–10−2 s−1) and joint inclination angles (0°–50°). The results demonstrate that traditional precursory indicators based on full waveforms are effective for obtaining precursor information of the intact rock failure. However, these indicators are not universally applicable to the failure of rock masses with a single joint. The clustering-CNN method has the potential to be applied to obtain precursor information for all three failure modes (Modes I, II and III). Following the waveform clustering analysis, the effective waveforms exhibit a low main frequency, as well as high energy, ringing count, and rise time. Furthermore, the clustering method and the precursory indicators influence the acquisition of final precursor information. The Birch hierarchical clustering method and the S value precursory indicator can help to obtain more accurate results. The findings of this study may contribute to the development of warning methods for underground engineering across faults.

Fiber-reinforced polymer (FRP) wrapping is a potential technique for coal pillar reinforcement. In this study, an acoustic emission (AE) technique was employed to monitor coal specimens with carbon FRP (CFRP) jackets during uniaxial compression, which addressed the inability to observe the cracks inside the FRP-reinforced coal pillars by conventional field inspection techniques. The spatiotemporal fractal evolution of the cumulated AE events during loading was investigated based on fractal theory. The results indicated that the AE response and fractal features of the coal specimens were closely related to their damage evolution, with CFRP exerting a significant influence. In particular, during the unstable crack development stage, the evolutionary patterns of the AE count and energy curves of the CFRP-confined specimens underwent a transformation from the slight shock–major shock type to the slight shock–sub-major shock–slight shock–major shock type, in contrast to the unconfined coal specimens. The AE b-values decreased to a minimum and then increased marginally. The AE spatial fractal dimension increased rapidly, whereas the AE temporal fractal dimension fluctuated significantly during the accumulation and release of strain energy. Ultimately, based on the AE count and AE energy evolution, a damage factor was proposed for the coal samples with CFRP jackets. Furthermore, a damage constitutive model was established, considering the CFRP jacket and the compaction characteristics of the coal. This model provides an effective description of the stress–strain relationship of coal specimens with CFRP jackets.

Enzyme-induced carbonate precipitation (EICP) has emerged as an innovative soil stabilization technology to precipitate CaCO3 by catalyzing urea decomposition. Although extensive efforts have been made to increase the calcium carbonate content (CCC) formed in the EICP process for the better bio-cementation effect, the cementability and micromechanical properties of CaCO3 are rarely known. A study of the cementitious characteristics and micromechanical properties of CaCO3 precipitates with different mixing percentages of crystal morphology is essential for soil improvement. In the present study, ultrasonic oscillation tests and nanoindentation tests were performed to investigate the cementability and micromechanical properties of CaCO3 precipitate. The results show that the cementability and micromechanical properties of CaCO3 precipitate are related to the composition of the crystal morphology. A high content of calcite is beneficial to improve the adhesion of calcium carbonate precipitate. Calcite has better mechanical properties (elastic modulus, hardness and ductility) than vaterite, and the presence of vaterite can significantly affect the measured value of mechanical properties in nanoindentation tests. The ductility of CaCO3 precipitate induced by crude soybean urease (CSU) is higher than that of CaCO3 precipitate induced by commercially available pure enzyme, suggesting that commercially available pure enzyme can be replaced by CSU for cost-effective field-scale engineering applications. This work can provide insight into optimizing the properties of CaCO3 precipitate from the micro-scale.

Bacterial suspension is an essential component of microbially induced carbonate precipitation (MICP)-based biocement and a large-scale production is required for field applications. In this study, a new bacterial concentration method is proposed to enable high concentration bacterial suspension to be produced to facilitate field work. By adding low concentration calcium to bacterial suspension, flocs are formed and bacterial cells are adsorbed on the flocs to achieve bacterial concentration. Compared to the traditional bacterial concentration method using centrifugation and freezing-drying method, the proposed method can concentrate a large volume of bacterial suspension without using special equipment. The feasibility of this method is verified by bacterial concentration tests, solution tests and sand column treatment tests. The results of both the solution test and the sand column treatment test show that the bacterial suspension concentrated by the proposed method can be effectively used for soil biocementation. There is a threshold calcium concentration that allows a complete bacterial concentration for the proposed method, and this threshold calcium concentration tends to increase linearly with the optical density of the cell suspension at a wavelength of 600 nm (OD600).

The application of microbially induced carbonate precipitation (MICP) in clayey soils has attracted much attention, and many studies used clay as an additive to enhance microbial mineralization efficiency in sandy soils. Within the sand-clay-bacteria-calcite system, the property and content of clay play crucial roles in affecting bacterial growth and calcite formation. More important, bentonite is particularly sensitive to changes in the geochemical environment. In this study, the sand-bentonite mixtures were treated by biostimulated MICP, aiming to provide insights into the behavior of this system. The bacterial activity and cementation pattern at different bentonite contents were evaluated through a series of tests such as enrichment tests, unconfined compressive strength (UCS) tests, cementation content measurements, mercury intrusion porosimetry (MIP) tests, scanning electron microscopy (SEM) observations, and energy dispersive X-ray spectroscopy (EDS) analyses. The findings showed that the bentonite presence promoted the enrichment of indigenous ureolytic bacteria, with lower bentonite levels enhancing ureolytic activity. Macroscopic and microscopic characterization indicated that the bentonite-coating sand structure was more conducive to the formation of large-sized calcite crystals capable of cementing soil particles compared to sand-supported and bentonite-supported structures. Additionally, excessive calcium ions (Ca2+) concentrations in the cementitious solution would lead to predominant calcite deposition on soil particle surfaces, contributing minimally to strength improvement.

The one-phase-low-pH method is a simple, efficient, and user-friendly biogrouting technique that can effectively improve the biomineralization of enzyme-induced carbonate precipitation (EICP) using free urease enzyme. One of the most significant advantages of this method is its capacity to effectively delay calcium carbonate (CaCO3) precipitation by reducing the pH of the solution through the addition of acid. This prevents bioclogging during the biogrouting process and improves the biomineralization effect. However, the biomineralization of the one-phase-low-pH based EICP method may be influenced by the specific acid used. To investigate the impact of acid type on the one-phase-low-pH EICP method using crude soybean urease solution (CSUS), four types of acids, including hydrochloric acid (HCl), nitric acid (HNO3), acetic acid (CH3COOH), and lactic acid (C3H6O3), were used to adjust the pH of CSUS. A series of macroscopic and microscopic experiments were conducted to evaluate the effect of acid type on the one-phase-low-pH EICP method. The results indicate that the acid has an inhibition on the urease activity (UA) of CSUS. Among the acids tested, HNO3 exhibits the most pronounced inhibitory effect on the UA of CSUS, followed by HCl, and the least pronounced inhibitory effect for CH3COOH and C3H6O3 under the same pH conditions. Meanwhile, CH3COOH and C3H6O3 could provide a longer delay duration of CaCO3 precipitation than HNO3 and HCl. Therefore, the one-phase-low-pH EICP method based on CH3COOH and C3H6O3 can significantly improve the effective biocementation depth compared to that based on HNO3 and HCl. Nevertheless, the different types of acids appear to have no obvious effect on the polymorph and crystalline of the precipitated CaCO3 crystals.

Sporosarcina pasteurii (S. pasteurii) is widely used in microbial-induced carbonate precipitation (MICP) due to its high urease activity. In this paper, the effects of nutrients on the metabolic mechanism and mineralization ability of S. pasteurii were studied by comparing the bacteria's growth, gene expression, and mineralized sand column under different nitrogen sources. The results showed that urea and soy peptone replacing the inorganic and organic nitrogen sources in ammonium sulfate-yeast extract (NH4-YE) medium can increase the urease activity of S. pasteurii by 11.43% and 17.10%, respectively. In the composite nitrogen source medium composed of urea and soy peptone, the urease activity of S. pasteurii increased by 25.30%. The transcriptome sequencing results showed that the modified medium of urea and soy peptone could promote the basic life activity and metabolism of S. pasteurii and is beneficial to urease expression. Among them, the gene difference between the modified urea medium and the primary medium was more obvious, and the urea medium could promote the ATP synthase related to urease expression and urea hydrolysis. The unconfined compressive strength (UCS) of sand columns reinforced with S. pasteurii cultured in Urea-YE, NH4-Soy peptone and Urea-Soy peptone increased by 27.6%, 36% and 58.1% respectively. The permeability decreased by 14.8%, 20.1% and 81.3% respectively compared with that of sand columns reinforced with S. pasteurii cultured in NH4-YE. The higher the urease activity of cultured bacteria, the more calcium carbonate produced after mineralization reaction. In addition, the urease activity of bacteria has an influence on the morphology of calcite crystals. This study can facilitate our understanding of optimizing the culture medium of S. pasteurii and the artificial regulation of urease activity in the process of MICP.

The deep geological repository for radioactive waste in Switzerland will be embedded in an approximately 100 m thick layer of Opalinus Clay. The emplacement drifts for high-level waste (approximately 3.5 m diameter) are planned to be excavated with a shielded tunnel boring machine (TBM) and supported by a segmental lining. At the repository depth of 900 m in the designated siting region Nördlich Lägern, squeezing conditions may be encountered due to the rock strength and the high hydrostatic pressure (90 bar). This paper presents a detailed assessment of the shield jamming and lining overstressing hazards, considering a stiff lining (resistance principle) and a deformable lining (yielding principle), and proposes conceptual design solutions. The assessment is based on three-dimensional transient hydromechanical simulations, which additionally consider the effects of ground anisotropy and the desaturation that may occur under negative pore pressures generated during the drift excavation. By addressing these design issues, the paper takes the opportunity to analyse some more fundamental aspects related to the influences of anisotropy and desaturation on the development of rock convergences and pressures over time, and their markedly different effects on the two lining systems. The results demonstrate that, regardless of these effects, shield jamming can be avoided with a moderate TBM overcut, however overstressing of a stiff lining may be critical depending on whether the ground desaturates. This uncertainty is eliminated using a deformable system with reasonable dimensions of yielding elements, which can also accommodate thermal strains generated due to the high temperature of the disposal canisters.

The interpretation of the cone penetration test (CPT) still relies largely on empirical correlations that have been predominantly developed in resource-intensive and time-consuming calibration chambers. This paper presents a CPT virtual calibration chamber using deep learning (DL) approaches, which allow for the consideration of depth-dependent cone resistance profiles through the implementation of two proposed strategies: (1) depth-resistance mapping using a multilayer perceptron (MLP) and (2) sequence-to-sequence training using a long short-term memory (LSTM) neural network. Two DL models are developed to predict cone resistance profiles (qc) under various soil states and testing conditions, where Bayesian optimization (BO) is adopted to identify the optimal hyperparameters. Subsequently, the BO-MLP and BO-LSTM networks are trained using the available data from published datasets. The results show that the models with BO can effectively improve the prediction accuracy and efficiency of neural networks compared to those without BO. The two training strategies yielded comparable results in the testing set, and both can be used to reproduce the whole cone resistance profile. An extended comparison and validation of the prediction results are carried out against numerical results obtained from a coupled Eulerian-Lagrangian (CEL) model, demonstrating a high degree of agreement between the DL and CEL models. Ultimately, to demonstrate the usability of this new virtual calibration chamber, the predicted qc is used to enhance the preceding correlations with the relative density (Dr) of the sand. The improved correlation with superior generalization has an R2 of 82% when considering all data, and 89.6% when examining the pure experimental data.

Previous studies on the hollow cylinder torsional shear test (HCTST) have mainly focused on the macroscopic behavior, while the micromechanical responses in soil specimens with shaped particles have rarely been investigated. This paper develops a numerical model of the HCTST using the discrete element method (DEM). The method of bonded spheres in a hexagonal arrangement is proposed to generate flexible boundaries that can achieve real-time adjustment of the internal and external cell pressures and capture the inhomogeneous deformation in the radial direction during shearing. Representative angular particles are selected from Toyoura sand and reproduced in this model to approximate real sand particles. The model is then validated by comparing numerical and experimental results of HCTSTs on Toyoura sand with different major principal stress directions. Next, a series of HCTSTs with different combinations of major principal stress direction (α) and intermediate principal stress ratio (b) is simulated to quantitatively characterize the sand behavior under different shear conditions. The results show that the shaped particles are horizontally distributed before shearing, and the initial anisotropic packing structure further results in different stress–strain curves in cases with different α and b values. The distribution of force chains is affected by both α and b during the shear process, together with the formation of the shear bands in different patterns. The contact normal anisotropy and contact force anisotropy show different evolution patterns when either α or b varies, resulting in the differences in the non-coaxiality and other macroscopic responses. This study improves the understanding of the macroscopic response of sand from a microscopic perspective and provides valuable insights for the constitutive modeling of sand.

Various industrial waste binders (IWBs) are being recycled in soil stabilization to save cement consumption. However, the coupled effects brought out by combined IWBs on stabilized soils are still unclear. IWBs are categorized into two typical categories (IWB-A and IWB-B) referring to their chemical role in this study. The alkali-source effect, pore-filling effect and cementation damage effect by IWBs in soil stabilization are explored. A series of mechanical and microscopic tests is performed on stabilized clay with different proportions of IWB-A and IWB-B. Moreover, initial water contents and cement contents of cement-stabilized clay are varied to examine the evolution of coupled effect with void ratio and cementation level. The results indicate that the alkali-source effect strengthens the cementation bonds and increases the early strength by 0.5–1.3 times, whereas the pore-filling effect improves the microfabric especially for the specimen with a large void ratio. The alkali-source effect increases soil cohesion cu at the pre-yield stage, and the pore-filling effect increases frictional angle φu at the post-yield stage. The cementation damage effect is remarkable at a low void ratio, which may result in many extruded pores among soil aggregates. The strength evolution with IWB proportions can be well stimulated by considering the coupled alkali-source effect, pore-filling effect and cementation damage effect. The optimal proportion of IWBs corresponds to an optimal combination of coupled effect.

Despite the fact that mixing uniformity (i.e. the consistency of binder distribution) significantly influence the quality of ground improvement during in situ soil mixing projects, its quantitative evaluation was rarely concerned due to the difficulty of measurement from an engineering perspective. A methodology was proposed to quantitatively evaluate the mixing uniformity of stabilized soil using handheld X-fluorescence spectrometry (XRF), which is helpful to elucidate the significance of mixing uniformity on strength. In other words, the calcium content was monitored to ascertain the distribution of cement within the matrix, and a quantitative index was subsequently established. It was observed that an increase in mixing uniformity resulted in a transition in the behavior of the stabilized clay from a plastic to a brittle failure mode, and from a localized failure to a global shear failure under unconfined compression. Subsequent observation of the destruction process revealed that cracks were more readily formed in the low cement zones and then bypass the high cement zones. Furthermore, the effect of mixing uniformity on strength is likely to be amplified with prolonged curing periods. The enhancement of uniformity would increase the volume of the high binder zones, thereby enhancing the overall high-strength performance. The proposed methodology is capable of characterizing the discreteness between the tracked element's measured and theoretical contents, thusing avoiding the uncertainty associated with other indirect indicators. The convenience of the portable handheld XRF apparatus was confirmed, as it can be readily deployed in situ or ex situ to track calcium content within the stabilized mass after borehole sampling.

The vacuum-assisted prefabricated horizontal drain offers a promising method for strengthening soil slurry, allowing simultaneous filling and vacuum-dewatering via staged construction. However, there is limited research on the unique characteristics of staged filling. This study aims to investigate the vacuum consolidation process of staged-filled soil slurry through laboratory model tests and numerical simulations, also assessing the impact of anionic polyacrylamide. Comparative analyses are conducted between vacuum consolidation with and without anionic polyacrylamide, as well as self-weight consolidation without anionic polyacrylamide. Results reveal contour lines of excess pore pressure, water content, and soil strength forming an ellipse around the prefabricated horizontal drain board. During the consolidation process, a higher degree of consolidation, lower water content, and higher soil strength were observed closer to the prefabricated horizontal drain board. After treatment, the uppermost filling layer exhibits an average water content that was approximately 40% higher than the lower filling layer, and its average strength was about 60% lower. This discrepancy is primarily due to the absence of sealing on the top surface and the relatively short vacuum consolidation time caused by staged filling. The introduction of anionic polyacrylamide-induced flocculation significantly improves the initial consolidation rate but minimally affects the dewatering capacity of vacuum preloading. Using flocculant can enhance both the staged filling rate and soil strength (by 1–2 times). Additionally, employing a staggered arrangement between different prefabricated horizontal drain layers is advisable to prevent top-down penetration in areas with low soil strength.

The influence of strain rate on the mechanics of particles is well documented. However, a comprehensive understanding of the strain rate effect on calcareous particles, particularly in the transition from static to dynamic loading, is still lacking in current literature. This study conducted 720 quasi-static and impact tests on irregular calcareous particles to investigate the macroscopic strain rate effect, and performed numerical simulations on spherical particles to explore the underlying microscopic mechanisms. The strain rate effect on the characteristic particle strength was found to exhibit three regimes: in Regime 1, the particle strength gradually improves when the strain rate is lower than approximately 102 s−1; in Regime 2, the particle strength sharply enhances when the strain rate increases from 102 s−1 to 104 s−1; and in Regime 3, the particle strength remains almost constant when the strain rate is higher than 104 s−1. The three-regime strain rate effect is an inherent property of the material and independent of particle shape. The asynchrony between loading and deformation plays a dominant role in these behaviors, leading to a thermoactivation-dominated effect in Regime 1, a macroscopic viscosity-dominated effect in Regime 2, and a combined thermoactivation and macroscopic viscosity-dominated effect in Regime 3. These mechanisms induce a transition in the failure mode from splitting to exploding and then smashing, which increases the energy required to rupture a single bond and, consequently, enhances the particle strength.

This study aims to evaluate the possibility of reusing treated marine clayey soils by stabilization/solidification (S/S) technology as geomaterial in reclamation projects from the aspects of engineering strength, chemical modification and environmental risk assessment. The lime-activated incinerated sewage sludge ash (ISSA) together with ground granulated blast furnace slag (GGBS) was employed as the binder. The multi-controlling factors including water content, curing time, salinity, and chemical compositions of mixing solution were taken into account to identify the S/S treated Hong Kong marine deposit (HKMD) slurry based on the strength tests, pH measurement, thermo-gravimetric (TG) analysis, X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS) and toxicity characteristic leaching procedure (TCLP) tests, etc. The results show that the S/S treatment using lime-activated ISSA-GGBS can effectively enhance the strength of marine soil at the initial water content of 110% and 200%. The water content and curing time have a significant impact on the S/S treated HKMD. The pH of treated soils is higher than 11.1, which proves an alkaline environment for the reactions in the treated soil. A special case is the treated HKMD at 200% water content hydrated by MgCl2 solution, which has a low pH of 10.23 and maintains a slurry state. Based on the TCLP results, the leaching concentration of heavy metals from S/S treated HKMD is environmentally safe and meets Hong Kong standard for reusing treated soil with a low level of <0.2 mg/L. The content of main products such as calcium/magnesium silicate hydrate, ettringite or Friedel's salt depends on the chemical additions (e.g. distilled water, seawater, NaCl and Na2SO4). The products in the specimens mixed with MgCl2 solutions are mainly composed of Mg(OH)2, M-S-H and MgCO3, which is distinct with the neoformations in the other cases. Therefore, this study proves that the S/S treated soil slurry could be reused as geomaterials in reclamation projects, and the S/S process is greatly affected by water content, curing time and solution compositions, etc.

The demand for sustainable ground improvement methods is rising as urban development expands into areas with challenging soil conditions. Traditional approaches, mostly reliant on cement and lime, contribute significantly to anthropogenic greenhouse gas emissions. Researchers, therefore, are constantly searching for new environmentally friendly stabilization methods to improve the engineering properties of soils. One alternative material used for this purpose is gypsum in its hydrated and dehydrated (hemihydrate/anhydrate) states. Not only can natural gypsum be used for ground improvement but also industrial waste and by-products (e.g. used or waste plasterboard, phosphogypsum, flue gas desulfurization gypsum, titanium dioxide production gypsum by-product) can be recycled, and used. Successful application of these materials could lower the carbon footprint of the construction industries (by reducing the consumption of cement and lime) as well as other industries (by recycling their waste and by-products). However, using gypsum presents challenges due to its moderate water solubility, the formation of swelling clay minerals under certain conditions, and the tendency of dehydrated gypsum to swell upon exposure to water, to name a few. Furthermore, the mechanisms leading to the improved behavior of the gypsum-treated soils are complicated, which has resulted in some seemingly contradictory results reported in the literature. This study presents a systematic and extensive review of the observed behavior of gypsum-treated soils and the different mechanisms causing the observed behavior. The research gaps and the required future steps to address these gaps have been identified and reported. A summary of the effect of gypsum treatment on the mechanical and engineering properties of soils, including unconfined compressive strength (UCS), California Bearing Ratio (CBR), swell potential, Atterberg limits, optimum moisture content (OMC), maximum dry density (MDD), durability, and environmental effects has also been presented.

Rock fracture toughness is a critical parameter for optimizing reservoir stimulation during deep resource extraction. This index characterizes the in situ resistance of rocks to fracture and is affected by high temperature, in situ stress, thermal shock, and chemical corrosion, etc. This review comprehensively examines research on rock fracture properties in situ environments over the past 20 years, analyses the influences of various environmental factors on rock fracture, and draws the following conclusions: (i) Environmental factors can significantly affect rock fracture toughness through changing the internal microstructure and grain composition of rocks; (ii) While high temperature is believed to reduce the rock strength, several studies have observed an increase in rock fracture toughness with increasing temperature, particularly in the range between room temperature and 200 °C; (iii) In addition to a synergistic increase in fracture toughness induced by both high temperature and high in situ stress, there is still a competing effect between the increase induced by high in situ stress and the decrease induced by high temperature; (iv) Thermal shock from liquid nitrogen cooling, producing high temperature gradients, can surprisingly increase the fracture toughness of some rocks, especially at initial temperatures between room temperature and 200 °C; and (v) Deterioration of rock fracture toughness occurs more rapidly in acidic environments than that in alkaline environments. In addition, this review identified current research trends and suggested some potential directions to provide suggestions for deep subsurface resource extraction.