2024, 16(11): 4335-4353. doi:10.1016/j.jrmge.2023.09.007
This paper presents the development of a coupled modeling approach to simulate cryogenic thermo-hydro-mechanical (THM) processes associated with a freezing medium, which is then implemented in the combined finite-discrete element method code (FDEM) for multi-physics simulation. The governing equations are deduced based on energy and mass conservation, and static equilibrium equations, considering water/ice phase change, where the strong couplings between multi-fields are supplemented by critical coupling parameters (e.g. unfrozen water content, permeability, and thermal conductivity). The proposed model is validated against laboratory and field experiments. Results show that the cryogenic THM model can well predict the evolution of strongly coupled processes observed in frozen media (e.g. heat transfer, water migration, and frost heave deformation), while also capturing, as emergent properties of the model, important phenomena (e.g. latent heat, cryogenic suction, ice expansion and distinct three-zone distribution) caused by water/ice phase change at laboratory and field scales, which are difficult to be all revealed by existing THM models. The novel modeling framework presents a gateway to further understanding and predicting the multi-physical coupling behavior of frozen media in cold regions.
[...]Read more.2024, 16(11): 4354-4368. doi:10.1016/j.jrmge.2024.09.006
Stress changes due to changes in fluid pressure and temperature in a faulted formation may lead to the opening/shearing of the fault. This can be due to subsurface (geo)engineering activities such as fluid injections and geologic disposal of nuclear waste. Such activities are expected to rise in the future making it necessary to assess their short- and long-term safety. Here, a new machine learning (ML) approach to model pore pressure and fault displacements in response to high-pressure fluid injection cycles is developed. The focus is on fault behavior near the injection borehole. To capture the temporal dependencies in the data, long short-term memory (LSTM) networks are utilized. To prevent error accumulation within the forecast window, four critical measures to train a robust LSTM model for predicting fault response are highlighted: (i) setting an appropriate value of LSTM lag, (ii) calibrating the LSTM cell dimension, (iii) learning rate reduction during weight optimization, and (iv) not adopting an independent injection cycle as a validation set. Several numerical experiments were conducted, which demonstrated that the ML model can capture peaks in pressure and associated fault displacement that accompany an increase in fluid injection. The model also captured the decay in pressure and displacement during the injection shut-in period. Further, the ability of an ML model to highlight key changes in fault hydromechanical activation processes was investigated, which shows that ML can be used to monitor risk of fault activation and leakage during high pressure fluid injections.
[...]Read more.2024, 16(11): 4369-4385. doi:10.1016/j.jrmge.2023.07.018
This paper presents a finite element framework for imposing frictional contact conditions on embedded fracture faces, implemented by the constant-strain assumed enhanced strain (AES) method, where penalty method is used to impose both non-penetration constraint and Coulomb's law of friction. The proposed constant-strain AES method for modeling embedded frictional contact can be cast into an integration algorithm similar to those used in the classical plasticity theory, where displacement jump is calculated from the local traction equilibrium at Gauss point, so the method does not introduce any additional global degrees of freedom. Moreover, constant-strain elements are often desirable in practice because they can be easily created automatically for large-scale engineering applications with complicated geometries. As encountered in other enriched finite element methods for frictional contact, the problem of normal contact pressure oscillations is also observed in the constant-strain AES method. Therefore, we developed a strain-smoothing procedure to effectively mitigate the oscillations. We investigated and verified the proposed AES framework through several numerical examples, and illustrated the capability of this method in solving challenging nonlinear frictional contact problems.
[...]Read more.2024, 16(11): 4386-4398. doi:10.1016/j.jrmge.2023.08.023
In this study, twelve machine learning (ML) techniques are used to accurately estimate the safety factor of rock slopes (SFRS). The dataset used for developing these models consists of 344 rock slopes from various open-pit mines around Iran, evenly distributed between the training (80%) and testing (20%) datasets. The models are evaluated for accuracy using Janbu's limit equilibrium method (LEM) and commercial tool GeoStudio methods. Statistical assessment metrics show that the random forest model is the most accurate in estimating the SFRS (MSE = 0.0182, R2 = 0.8319) and shows high agreement with the results from the LEM method. The results from the long-short-term memory (LSTM) model are the least accurate (MSE = 0.037, R2 = 0.6618) of all the models tested. However, only the null space support vector regression (NuSVR) model performs accurately compared to the practice mode by altering the value of one parameter while maintaining the other parameters constant. It is suggested that this model would be the best one to use to calculate the SFRS. A graphical user interface for the proposed models is developed to further assist in the calculation of the SFRS for engineering difficulties. In this study, we attempt to bridge the gap between modern slope stability evaluation techniques and more conventional analysis methods.
[...]Read more.2024, 16(11): 4399-4415. doi:10.1016/j.jrmge.2024.01.005
The interaction between multiple fractures is important in the analysis of rock fracture propagation, fracture network evolution and stability and integrity of rocks under hydro-mechanical (HM) coupling conditions. At present, modeling the mechanical behavior of multiple fractures is still challenging. Under the condition of multiple fractures, the opening, closing, sliding, propagation and penetration of fractures become more complicated. In order to simulate the HM coupling behavior of multi-fracture system, the paper presents a novel numerical scheme, including mesh reconstruction and topology generation algorithm, to efficiently and accurately represent fractures and their propagation process, and a potential function-based algorithm to address contact problem. The fracture contact algorithm does not need to set contact pairs and thus is suitable for complex contact situations from small to large deformations induced by HM loading. The topology of fracture interfaces is constructed by the dynamic adding algorithm, which makes the mesh reconstruction more rapid in the modeling of fracturing process, especially in the case of multiple fractures intersections. The numerical scheme is implemented in CASRock, a self-developed numerical code, to simulate the propagation process of rock fractures and the interaction of multiple fractures under the condition of HM coupling. To verify the suitability of the code, a series of tests were performed. The code was then applied to simulate hydraulic fracture propagation and fracture interactions caused by fluid injection. The ability of this method to study fracture propagation, multi-fracture interaction and fracture network evolution under hydro-mechanical coupling conditions is demonstrated.
[...]Read more.2024, 16(11): 4416-4427. doi:10.1016/j.jrmge.2024.04.010
The injection of large volumes of natural gas into geological formations, as is required for underground gas storage, leads to alterations in the effective stress exerted on adjacent faults. This increases the potential for their reactivation and subsequent earthquake triggering. Most measurements of the frictional properties of rock fractures have been conducted under normal and shear stresses. However, faults in gas storage facilities exist within a true three-dimensional (3D) stress state. A double-direct shear experiment on rock fractures under both lateral and normal stresses was conducted using a true triaxial loading system. It was observed that the friction coefficient increases with increasing lateral stress, but decreases with increasing normal stress. The impact of lateral and normal stresses on the response is primarily mediated through their influence on the initial friction coefficient. This allows for an empirical modification of the rate-state friction model that considers the influence of lateral and normal stresses. The impact of lateral and normal stresses on observed friction coefficients is related to the propensity for the production of wear products on the fracture surfaces. Lateral stresses enhance the shear strength of rock (e.g. Mogi criterion). This reduces asperity breakage and the generation of wear products, and consequently augments the friction coefficient of the surface. Conversely, increased normal stresses inhibit dilatancy on the fracture surface, increasing the breakage of asperities and the concomitant production of wear products that promote rolling deformation. This ultimately reduces the friction coefficient.
[...]Read more.2024, 16(11): 4428-4439. doi:10.1016/j.jrmge.2024.08.023
Fluid flow in fractures controls subsurface heat and mass transport, which is essential for developing enhanced geothermal systems and radioactive waste disposal. Fracture permeability is controlled by fracture microstructure (e.g. aperture, roughness, and tortuosity), but in situ values and their anisotropy have not yet been estimated. Recent advances in geophysical techniques allow the detection of changes in electrical conductivity due to changes in crustal stress and these techniques can be used to predict subsurface fluid flow. However, the paucity of data on fractured rocks hinders the quantitative interpretation of geophysical monitoring data in the field. Therefore, considering different shear displacements and chemical erosions, an investigation was conducted into the hydraulic-electric relationship as an elevated stress change in fractures. The simulation of fracture flows was achieved using the lattice Boltzmann method, while the electrical properties were calculated through the finite element method, based on synthetic faults incorporating elastic-plastic deformation. Numerical results show that the hydraulic and electrical properties depend on the rock's geometric properties (i.e. fracture length, roughness, and shear displacement). The permeability anisotropy in the direction parallel or perpendicular to the shear displacement is also notable in high stress conditions. Conversely, the permeability–conductivity (i.e., formation factor) relationship is unique under all conditions and follows a linear trend in logarithmic coordinates. However, both matrix porosity and fracture spacing alter this relationship. Both increase the slope of the linear trend, thereby changing the sensitivity of electrical observations to permeability changes.
[...]Read more.2024, 16(11): 4440-4461. doi:10.1016/j.jrmge.2024.01.028
A rigorous analytical model is developed for simulating the vibration behaviors of large-diameter open-ended pipe piles (OEPPs) and surrounding soil undergoing high-strain impact loading. To describe the soil behavior, the soil along pile shaft is divided into slip and nonslip zones and the base soil is modeled as a fictitious-soil pile (FSP) to account for the wave propagation in the soil. True soil properties are adopted and slippage at the pile-soil interface is considered, allowing realistic representation of large-diameter OEPP mechanics. The developed model is validated by comparing with conventional models and finite element method (FEM). It is further used to successfully simulate and interpret the behaviors of a steel OEPP during the offshore field test. It is found that the variation in the vertical vibrations of shaft soil along radial direction is significant for large-diameter OEPPs, and the velocity amplitudes of the internal and external soil attenuate following different patterns. The shaft soil motion may not attenuate with depth due to the soil slippage, while the wave attenuation at base soil indicates an influence depth, with a faster attenuation rate than that in the pile. The findings from the current study should aid in simulating the vibration behaviors of large-diameter OEPP-soil system under high-strain dynamic loading.
[...]Read more.2024, 16(11): 4462-4479. doi:10.1016/j.jrmge.2024.01.006
A comprehensive understanding of shale’s bedding anisotropy is crucial for shale-related engineering activities, such as hydraulic fracturing, drilling and underground excavation. In this study, seven Brazilian tests were conducted on shale samples at different bedding orientations with respect to the loading direction (0°, 45° and 90°) and the disc end face (0°, 45° and 90°). An acoustic emission (AE) system was employed to capture the evolution of damage and the temporal-spatial distribution of microcracks under splitting-tensile stress. The results show that the Brazilian tensile strength decreases with increasing bedding inclination with respect to the disc end face, while it increases with the angle between bedding and loading directions. Increasing the bedding inclination with respect to the end face facilitates the reduction in b value and enhances the shale’s resistance to microcrack growth during the loading process. Misalignment between the bedding orientation and the end face suppresses the growth of mixed tensile-shear microcracks, while reducing the bedding angle relative to the loading direction is beneficial for creating mixed tensile-shear and tensile cracks. The observed microscopic failure characteristics are attributed to the competing effects of bedding activation and breakage of shale matrix at different bedding inclinations. The temporal-spatial distribution of microcracks, characterized by AE statistics including the correlation dimension and spatial correlation length, illustrates that the fractal evolution of microcracks is independent of bedding anisotropy, whereas the spatial distribution shows a stronger correlation. The evolution features of correlation dimension and spatial correlation length could be potentially used as precursors for shale splitting failure. These findings may be useful for predicting rock mass instability and analyzing the causes of catastrophic rupture.
[...]Read more.2024, 16(11): 4480-4490. doi:10.1016/j.jrmge.2024.02.017
Rock fracture mechanics and accurate characterization of rock fracture are crucial for understanding a variety of phenomena interested in geological engineering and geoscience. These phenomena range from very large-scale asymmetrical fault structures to the scale of engineering projects and laboratory-scale rock fracture tests. Comprehensive study can involve mechanical modeling, site or post-mortem investigations, and inspection on the point cloud of the source locations in the form of earthquake, micro-seismicity, or acoustic emission. This study presents a comprehensive data analysis on characterizing the forming of the asymmetrical damage zone around a laboratory mixed-mode rock fracture. We substantiate the presence of asymmetrical damage through qualitative analysis and demonstrate that measurement uncertainties cannot solely explain the observed asymmetry. The implications of this demonstration can be manifold. On a larger scale, it solidifies a mechanical model used for explaining the contribution of aseismic mechanisms to asymmetrical fault structures. On a laboratory scale, it exemplifies an alternative approach to understanding the observational difference between the source location and the in situ or post-mortem inspection on the rock fracture path. The mechanical model and the data analysis can be informative to the interpretations of other engineering practices as well, but may face different types of challenges.
[...]Read more.2024, 16(11): 4491-4503. doi:10.1016/j.jrmge.2023.12.027
Development and production from fractured reservoirs require extensive knowledge about the reservoir structures and in situ stress regimes. For this, this paper investigates fractures and the parameters (aperture and density) through a combination of wellbore data and geomechanical laboratory testing in three separate wells in the Asmari reservoir, Zagros Belt, Iran. The Asmari reservoir (Oligo-Miocene) consists mainly of calcitic and dolomitic rocks in depths of 2000–3000 m. Based on the observation of features in several wellbores, the orientation and magnitude of the in situ stresses along with their influence on reservoir-scale geological structures and neotectonics were determined. The study identifies two regional tectonic fracture settings in the reservoir: one set associated with longitudinal and diagonal wrinkling, and the other related to faulting. The former, which is mainly of open fractures with a large aperture, is dominant and generally oriented in the N45°-90°W direction while the latter is obliquely oriented relative to the bedding and characterized by N45°-90°E. The largest aperture is found in open fractures that are longitudinal and developed in the dolomitic zones within a complex stress regime. Moreover, analysis of drilling-induced fractures (DIFs) and borehole breakouts (BBs) from the image logs revealed that the maximum horizontal stress (SHmax) orientation in these three wells is consistent with the NE-SW regional trend of the SHmax (maximum principal horizontal stress) in the Zagros Belt. Likewise, the stress magnitude obtained from geomechanical testing and poroelastic equations confirmed a variation in stress regime from normal to reverse, which changes in regard to active faults in the study area. Finally, a relationship between the development degree of open fractures and in situ stress regime was found. This means that in areas where the stress regime is complex and reverse, fractures would exhibit higher density, dip angle, and larger apertures.
[...]Read more.2024, 16(11): 4504-4514. doi:10.1016/j.jrmge.2023.12.016
One of the most effective methods for sand control is the chemical consolidation of sandstone structures. In this paper, the impacts of crude oil and brine in the static state and the impact of the flow rates of the fluids in the dynamic state have been assessed at the reservoir conditions. The analyses in this research were Young's modulus, compressive strength, porosity, and permeability which were done on core samples after and before fluid contact. Samples made with two different resins showed good resistance to crude oil in both states. No considerable change was seen in the analyses even at high crude oil injection rates in the dynamic state. Conversely, brine caused a noticeable change in the analyses in both states. In the presence of brine at the static state, Young's modulus and compressive strength respectively decreased by 37.5% and 34.5% for epoxy cores, whereas these parameters respectively reduced by 30% and 41% for furan cores. In brine presence at the dynamic state, compressive strength reduction was 10.28 MPa for furan and 6.28 MPa for epoxy samples and their compressive strength reached 16.75 MPa and 26.54 MPa respectively which are higher than the critical point to be known as weak sandstone core. Moreover, Young's modulus decrease values for furan and epoxy samples were respectively 0.37 GPa and 0.44 GPa. Therefore, brine had a more destructive effect on the mechanical characteristics of samples in the static state than the dynamic one for two resins. In addition, brine injection increased permeability by about 13.6% for furan and 34.8% for epoxy. Also, porosity raised by about 21.8% for furan, and 19% for epoxy by brine injection. The results showed that the chemical sand consolidation weakens in the face of brine production along with crude oil which can lead to increasing cost of oil production and treating wellbore again.
[...]Read more.2024, 16(11): 4515-4531. doi:10.1016/j.jrmge.2024.01.023
Integrating liquid CO2 phase transition blasting (LCPTB) technology with hydraulic fracturing (HF) methods can help reduce wellbore damage, create multiple radial fractures, and establish a complex fracture network. This approach significantly increases the recovery efficiency of low-permeability oil and gas fields. Accurately calculating the number of fractures caused by LCPTB is necessary to predict production enhancement effects and optimize subsequent HF designs. However, few studies are reported on large-scale physical model experiments in terms of a method for calculating the fracture number. This study analyzed the initiation and propagation of cracks under LCPTB, derived a calculation formula for crack propagation radius under stress waves, and then proposed a new, fast, and accurate method for calculating the fracture number using the principle of mass conservation. Through ten rock-breaking tests using LCPTB, the study confirmed the effectiveness of the proposed calculation approach and elucidated the variation rule of explosion pressure, rock-breaking scenario, and the impact of varying parameters on fracture number. The results show that the new calculation method is suitable for fracturing technologies with high pressure rates. Recommendations include enlarging the diameter of the fracturing tube and increasing the liquid CO2 mass in the tube to enhance fracture effectiveness. Moreover, the method can be applied to other fracturing technologies, such as explosive fracturing (EF) within HF formations, indicating its broader applicability and potential impact on optimizing unconventional resource extraction technologies.
[...]Read more.2024, 16(11): 4532-4553. doi:10.1016/j.jrmge.2024.01.017
To optimize the excavation of rock using underground blasting techniques, a reliable and simplified approach for modeling rock fragmentation is desired. This paper presents a multistep experimental-numerical methodology for simplifying the three-dimensional (3D) to two-dimensional (2D) quasi-plane-strain problem and reducing computational costs by more than 100-fold. First, in situ tests were conducted involving single-hole and free-face blasting of a dolomite rock mass in a 1050-m-deep mine. The results were validated by laser scanning. The craters were then compared with four analytical models to calculate the radius of the crushing zone. Next, a full 3D model for single-hole blasting was prepared and validated by simulating the crack length and the radius of the crushing zone. Based on the stable crack propagation zones observed in the 3D model and experiments, a 2D model was prepared. The properties of the high explosive (HE) were slightly reduced to match the shape and number of radial cracks and crushing zone radius between the 3D and 2D models. The final methodology was used to reproduce various cut-hole blasting scenarios and observe the effects of residual cracks in the rock mass on further fragmentation. The presence of preexisting cracks was found to be crucial for fragmentation, particularly when the borehole was situated near a free rock face. Finally, an optimization study was performed to determine the possibility of losing rock continuity at different positions within the well in relation to the free rock face.
[...]Read more.2024, 16(11): 4554-4569. doi:10.1016/j.jrmge.2024.02.051
The polyurethane foam (PU) compressible layer is a viable solution to the problem of damage to the secondary lining in squeezing tunnels. Nevertheless, the mechanical behaviour of the multi-layer yielding supports has not been thoroughly investigated. To fill this gap, large-scale model tests were conducted in this study. The synergistic load-bearing mechanics were analyzed using the convergence-confinement method. Two types of multi-layer yielding supports with different thicknesses (2.5 cm, 3.75 cm and 5 cm) of PU compressible layers were investigated respectively. Digital image correlation (DIC) analysis and acoustic emission (AE) techniques were used for detecting the deformation fields and damage evolution of the multi-layer yielding supports in real-time. Results indicated that the load-displacement relationship of the multi-layer yielding supports could be divided into the crack initiation, crack propagation, strain-hardening, and failure stages. Compared with those of the stiff support, the toughness, deformability and ultimate load of the yielding supports were increased by an average of 225%, 61% and 32%, respectively. Additionally, the PU compressible layer is positioned between two primary linings to allow the yielding support to have greater mechanical properties. The analysis of the synergistic bearing effect suggested that the thickness of PU compressible layer and its location significantly affect the mechanical properties of the yielding supports. The use of yielding supports with a compressible layer positioned between the primary and secondary linings is recommended to mitigate the effects of high geo-stress in squeezing tunnels.
[...]Read more.2024, 16(11): 4570-4585. doi:10.1016/j.jrmge.2024.05.011
The problem of shield tunnel uplift is a common issue in tunnel construction. Due to the decrease in shear stiffness at the joints between the rings, uplift is typically observed as bending and dislocation deformation at these joints. Existing modeling methods typically rely on the Euler-Bernoulli beam theory, only considering the bending effect while disregarding shear deformation. Furthermore, the constraints on the shield tail are often neglected in existing models. In this study, an improved theoretical model of tunnel floating is proposed. The constraint effect of the shield machine shell on the tunnel structure is considered using the structural forms of two finite long beams and one semi-infinite long beam. Furthermore, the Timoshenko beam theory is adopted, providing a more accurate description of tunnel deformation, including both the bending effect and shear deformation, than existing models. Meanwhile, the buoyancy force and stratum resistance are calculated in a nonlinear manner. A reliable method for calculating the shear stiffness correction factor is proposed to better determination of the calculation parameters. The proposed theoretical model is validated through five cases using site-monitored data. Its applicability and effectiveness are demonstrated. Furthermore, the influences of soil type, buried depth, and buoyancy force on the three key indicators of tunnel floating (i.e. the maximum uplift magnitude, the ring position with the fastest uplift race, and the ring position with the maximum uplift magnitude) are analyzed. The results indicate that the proposed model can provide a better understanding of the floating characteristics of the tunnel structure during construction.
[...]Read more.2024, 16(11): 4586-4604. doi:10.1016/j.jrmge.2024.05.014
This study focused on the mechanical behavior of a deep-buried tunnel constructed in horizontally layered limestone, and investigated the effect of a new combined rockbolt–cable support system on the tunnel response. The Yujingshan Tunnel, excavated through a giant karst cave, was used as a case study. Firstly, a multi-objective optimization model for the rockbolt–cable support was proposed by using fuzzy mathematics and multi-objective comprehensive decision-making principles. Subsequently, the parameters of the surrounding rock were calibrated by comparing the simulation results obtained by the discrete element method (DEM) with the field monitoring data to obtain an optimized support scheme based on the optimization model. Finally, the optimization scheme was applied to the karst cave section, which was divided into the B- and C-shaped sections. The distribution range of the rockbolt–cable support in the C-shaped section was larger than that in the B-shaped section. The field monitoring results, including tunnel crown settlement, horizontal convergence, and axial force of the rockbolt–cable system, were analyzed to assess the effectiveness of the optimization scheme. The maximum crown settlement and horizontal convergence were measured to be 25.9 mm and 35 mm, accounting for 0.1% and 0.2% of the tunnel height and span, respectively. Although the C-shaped section had poorer rock properties than the B-shaped section, the crown settlement and horizontal convergence in the C-shaped section ranged from 46% to 97% of those observed in the B-shaped section. The cable axial force in the B-shaped section was approximately 60% of that in the C-shaped section. The axial force in the crown rockbolt was much smaller than that in the sidewall rockbolt. Field monitoring results demonstrated that the optimized scheme effectively controlled the deformation of the layered surrounding rock, ensuring that it remained within a safe range. These results provide valuable references for the design of support systems in deep-buried tunnels situated in layered rock masses.
[...]Read more.2024, 16(11): 4605-4624. doi:10.1016/j.jrmge.2024.05.016
The failure modes of rock after roadway excavation are diverse and complex. A comprehensive investigation of the internal stress field and the rotation behavior of the stress axis in roadways is essential for elucidating the mechanism of roadway failure. This study aimed to examine the spatial relationship between roadways and stress fields. The law of stress axis rotation under three-dimensional (3D) stress has been extensively studied. A stress model of roadways in the spatial stress field was established, and the far-field stress state at different spatial positions of the roadways was analyzed. A mechanical model of roadways under a 3D stress state was established using far-field stress solutions as boundary conditions. The distribution of principal stresses σ1, σ2 and σ3 around the roadways and the variation of the stress principal axis were solved. It was found that the stability boundary of the stress principal axis exhibits hysteresis when compared with that of the principal stress magnitudes. A numerical analysis model for spatial roadways was established to validate the distribution of principal stress and the mechanism of principal axis rotation. Research has demonstrated that the stress axis undergoes varying degrees of spatial rotation in different orientations and radial depths. Based on the distribution of principal stress and the rotation law of the stress principal axis, the entire evolution mechanism of the two stress adjustments to form the final failure form after roadway excavation has been revealed. The on-site detection results also corroborate the findings presented in this paper. The results provide a basis for the analysis of the failure mechanism under a 3D stress state.
[...]Read more.2024, 16(11): 4625-4637. doi:10.1016/j.jrmge.2024.04.013
Weak structural plane deformation is responsible for the non-uniform large deformation disasters in layered rock tunnels, resulting in steel arch distortion and secondary lining cracking. In this study, a servo biaxial testing system was employed to conduct physical modeling tests on layered rock tunnels with bedding planes of varying dip angles. The influence of structural anisotropy in layered rocks on the micro displacement and strain field of surrounding rocks was analyzed using digital image correlation (DIC) technology. The spatiotemporal evolution of non-uniform deformation of surrounding rocks was investigated, and numerical simulation was performed to verify the experimental results. The findings indicate that the displacement and strain field of the surrounding layered rocks are all maximized at the horizontal bedding planes and decrease linearly with the increasing dip angle. The failure of the layered surrounding rock with different dip angles occurs and extends along the bedding planes. Compressive strain failure occurs after excavation under high horizontal stress. This study provides significant theoretical support for the analysis, prediction, and control of non-uniform deformation of tunnel surrounding rocks.
[...]Read more.2024, 16(11): 4638-4653. doi:10.1016/j.jrmge.2023.11.046
This study investigated the effects of weathering depth and thickness on the failure mechanisms of rock samples through experimental and numerical methods. The first configuration involved conducting artificial weathering on limestone using the freezing and thawing (F-T) for 40 cycles. The mechanical parameters of the samples were measured at the end of the 40th cycle. In the second configuration, a series of specimens underwent salt crystallization (S-C) tests for 20 cycles. Experimental results were validated using discrete element method (DEM). Next, the weathered limestone model with dimensions of 108 mm × 54 mm were prepared. The weathering layers were tested at four different thicknesses (i.e. 2.5 mm, 5 mm, 7.5 mm, and 10 mm) and three different positions (at the surface, 5 mm under the rock surface, and 10 mm under the rock surface). According to the results, weathering depth and thickness have a considerable effect on the failure process. The results also showed a correlation between the values of compressive strength and failure mechanisms associated with the weathering layer. The numerical results revealed that the tension crack was the dominant factor. Additionally, with increasing weathering thickness, Young's modulus, crack initiation stress, and final strength decreased in constant weathering depth. The results also demonstrated that the failure progress of the numerical models was similar to that observed in the laboratory.
[...]Read more.2024, 16(11): 4654-4668. doi:10.1016/j.jrmge.2023.12.005
A new thermomechanical (TM) coupled finite-discrete element method (FDEM) model, incorporating heat conduction, thermal cracking, and contact heat transfer, has been proposed for both continuous and discontinuous geomaterials. This model incorporates a heat conduction model that can accurately calculate the thermal field in continuous–discontinuous transition processes within a finite element framework. A modified contact heat transfer model is also included, which accounts for the entire contact area of discrete bodies. To align with the finite strain theory utilized in the FDEM mechanics module, the TM coupling module in the model is based on the multiplicative decomposition of the deformation gradient. The proposed model has been applied to various scenarios, including heat conduction in both continuous and discontinuous media during transient states, thermal-induced strain and stress, and thermal cracking conditions. The thermal field calculation model and the TM coupling model have been validated by comparing the numerical results with experiment findings and analytical solutions. These numerical cases demonstrate the reliability of the proposed model convincingly, making it suitable for use across a wide range of continuous and discontinuous media.
[...]Read more.2024, 16(11): 4669-4682. doi:10.1016/j.jrmge.2023.12.037
Understanding unsaturated flow behaviors in fractured rocks is essential for various applications. A fundamental process in this regard is flow splitting at fracture intersections. However, the impact of geometrical properties of fracture intersections on flow splitting is still unclear. This work investigates the combined influence of geometry (intersection angle, fracture apertures, and inclination angle), liquid droplet length, inertia, and dynamic wetting properties on liquid splitting dynamics at fracture intersections. A theoretical model of liquid splitting is developed, considering the factors mentioned above, and numerically solved to predict the flow splitting behavior. The model is validated against carefully-controlled visualized experiments. Our results reveal two distinct splitting behaviors, separated by a critical droplet length. These behaviors shift from a monotonic to a non-monotonic trend with decreasing inclination angle. A comprehensive analysis further clarifies the impacts of the key factors on the splitting ratio, which is defined as the percentage of liquid volume entering the branch fracture. The splitting ratio decreases with increasing inclination angle, indicating a decrease in the gravitational effect on the branch fracture, which is directly proportional to the intersection angle. A non-monotonic relationship exists between the splitting ratio and the aperture ratio of the branch fracture to the main fracture. The results show that as the intersection angle decreases, the splitting ratio increases. Additionally, the influence of dynamic contact angles decreases with increasing intersection angle. These findings enhance our understanding of the impact of geometry on flow dynamics at fracture intersections. The proposed model provides a foundation for simulating and predicting unsaturated flow in complex fractured networks.
[...]Read more.2024, 16(11): 4683-4696. doi:10.1016/j.jrmge.2023.12.035
Tunnel heading stability in two dimensions (2D) has been extensively investigated by numerous scholars in the past decade. One significant limitation of 2D analysis is the absence of actual tunnel geometry modeling with a considerable degree of idealization. Nevertheless, it is possible to study the stability of tunnels in three dimensions (3D) with a rectangular shape using finite element limit analysis (FELA) and a nonlinear programming technique. This paper employs 3D FELA to generate rigorous solutions for stability numbers, failure mechanisms, and safety factors for rectangular-shaped tunnels. To further explore the usefulness of the produced results, multivariate adaptive regression spline (MARS) is used for machine learning of big dataset and development of design equations for practical design applications. The study should be of great benefit to tunnel design practices using the developed equations provided in the paper.
[...]Read more.2024, 16(11): 4697-4710. doi:10.1016/j.jrmge.2024.05.002
Artificially cemented soils have been widely used as filling materials in highway and railway construction. The shear strength evolution of filling materials upon moist variation can determine the stability of subgrade and embankments. This study conducted water retention tests, MIP tests, and multi-stage triaxial shear tests on cement-treated granite residual soil (GRS) to determine its water retention curve (WRC) upon free drying, pore structure, and peak shear strength qf, respectively. The water retention behavior and shear strength evolution upon free drying were modeled based on the dual-porosity structure of cement-treated GRS and the effective stress principle, respectively. Results show that the drying-WRC is bimodal and higher cement dosage yields a more severe decrease in the water retention capacity within a specific suction range. For a given confining pressure, the peak shear strength qf increased with increasing cement dosage or suction value s. The peak shear strength qf also solely depends on the suction value in the peak stress state. In addition, the cement-treated GRS has a bimodal pore size distribution curve, and its macro- and micro-void ratios remain almost unchanged after free drying. The bimodal drying-WRC of the cement-treated GRS can be modeled by differentiating the water retention mechanisms in macro- and micro-pores. Moreover, using the macro-pore degree of saturation as the effective stress parameter χ = SrM, the qf–pf′ relationship (where pf′ is the effective mean pressure at failure) under various suction and stress conditions can be unified, and the qf–s relationships at various net confining pressures σ3,net can be well reproduced. These findings can help design subgrade and embankments constructed by artificially cemented GRS and assess their safe operation upon climate change.
[...]Read more.2024, 16(11): 4711-4726. doi:10.1016/j.jrmge.2023.11.042
Diatomaceous soils, composed of diatom microfossils with biological origins, have geotechnical properties that are fundamentally different from those of conventional non-diatomaceous fine-grained soils. Despite their high fines content, diatomaceous soils typically exhibit remarkably high shear resistance, approaching that of sandy soils. However, the exact role that diatoms play in controlling the mechanical properties of fine-grained soils and the underlying mechanisms remain unclear. In light of this, the shear strength response of diatomaceous soils was systematically investigated using consolidated undrained triaxial compression tests on diatom–kaolin mixtures (DKMs) with various diatom contents and overconsolidation ratios. The micro- and nano-scale structures of the soil samples were characterized in detail using scanning electron microscope (SEM) and atomic force microscope (AFM) to interpret the abnormal shear strength parameters of diatomaceous soils. The results indicated that the presence of diatoms could contribute to significantly higher strength, e.g. the friction angle of DKMs was improved by 72.7% to 37° and the value of undrained shear strength tripled with diatom content increasing from 20% to 100%. Such significant improvement in soil strength with diatom inclusion could be attribute to the hard siliceous skeleton of diatoms and the interlocking between particles with rough surfaces, which were quantitatively analyzed by the surface roughness parameters with AFM. Furthermore, a conceptual model established based on the macro-mechanical tests and microscopic observations portrays a microstructural evolution of soils with increasing diatoms. The microstructure of soils was gradually transformed from the matrix-type to the skeletal one, resulting in a continual augmentation in shear strength through mutual interactions between diatom microfossils. This paper provides new insights into the multi-scale structural properties of diatoms and significantly advances our understanding of the mechanical behavior of diatomaceous soils.
[...]Read more.2024, 16(11): 4727-4741. doi:10.1016/j.jrmge.2023.11.006
The influence of curing temperature on the strength development of cement-stabilized mud has been well documented in terms of strength-increase rate and ultimate strength. However, the strength development model is not mature for the extremely early stages. In addition, there is a lack of studies on quality control methods based on early-stage strength development. This paper presents a strength model for cement-stabilized mud to address these gaps, considering various curing temperatures and early-stage behaviors. In this study, a series of laboratory experiments was conducted on two types of muds treated with Portland blast furnace cement and ordinary Portland cement under four different temperatures. The results indicate that elevated temperatures expedite strength development and lead to higher long-term strength. The proposed model, which combines a three-step conversion process and a hyperbolic model at the reference temperature, enables accurate estimate of the strength development for cement-treated mud with any proportions cured under various temperatures. With this model, a practical early quality control method is introduced for applying cement-stabilized mud in field projects. The back-analysis parameters obtained from a 36-h investigation at temperature of 60 °C demonstrated a sufficient accuracy in predicting strength levels in practical applications.
[...]Read more.2024, 16(11): 4742-4753. doi:10.1016/j.jrmge.2024.03.036
The requisite functions of a bentonite buffer in a deep geological repository depend on the sealing/healing of bentonite interfaces, with particular emphasis on the self-healing (automatic healing upon wetting) of assembled bentonite-bentonite interfaces. This study determined the shear resistance (including the peak shear strength and secant modulus) of densely compacted Gaomiaozi (GMZ) bentonite and its assembled interface after confined water saturation. The effect of bentonite dry density and saturation time on the shear resistance of saturated healed interfaces was elucidated, and the interfacial self-healing capacity was assessed. The results indicate that the shear resistance of the saturated healed interfaces increased with the bentonite dry density but had a non-monotonic correlation with the saturation time. For a given dry density of the bentonite, the saturated healed interface exhibits a lower peak shear strength than the saturated intact bentonite but a higher peak shear strength than the saturated separated interface. The saturated healed and separated interfaces have comparable shear moduli (secant moduli), which are lower than that of the saturated intact bentonite. The saturated healed interfaces display smooth shear failure planes, while the saturated assembled interfaces and intact bentonite exhibit comparable frictional angles. This indicates that interfacial self-healing plays a pivotal role in enhancing interfacial peak shear strength by facilitating microstructural bonding at the assembled interface. Finally, it can be stated that densely compacted GMZ bentonite has a robust interfacial self-healing capacity in terms of shear resistance. These findings contribute to the design of the bentonite buffer and facilitate the evaluation of its safe operation at specified disposal ages.
[...]Read more.2024, 16(11): 4754-4768. doi:10.1016/j.jrmge.2024.02.039
Establishment of a creep model is an important method to analyze the relationship between soil creep deformation and time, and the element model is widely used for studying soil creep. However, the element creep model is employed for fitting saturated soil, and the mechanical element model is generally linear, which cannot well fit the nonlinear deformation of the soil with time in practice. The creep process of the soil is not only time-dependent, but also related to the deviatoric stress level. Therefore, the fractional calculus theory and a parameter n reflecting the effect of deviatoric stress level on the creep properties of the soil were introduced into the element model, and the fractional qBurgers creep model was established by using the fractional Koeller dashpot and Caputo fractional calculus. The proposed model was used to fit the triaxial test data of reticulated red clay under different net confining pressures and matric suctions by unsaturated triaxial apparatus. The proposed model can well describe the nonlinearity of unsaturated reticulated red clay, has memory and global correlation to the creep development process of unsaturated reticulated red clay, and has clear physical meaning. The functional relationships of the model parameters with the matric suction, net confining pressure and deviatoric stress level were deduced, so that the creep curves of unsaturated reticulated red clay can be obtained for any conditions, which is of great value for the study of unsaturated soils.
[...]Read more.2024, 16(11): 4769-4781. doi:10.1016/j.jrmge.2024.02.005
To efficiently predict the mechanical parameters of granular soil based on its random micro-structure, this study proposed a novel approach combining numerical simulation and machine learning algorithms. Initially, 3500 simulations of one-dimensional compression tests on coarse-grained sand using the three-dimensional (3D) discrete element method (DEM) were conducted to construct a database. In this process, the positions of the particles were randomly altered, and the particle assemblages changed. Interestingly, besides confirming the influence of particle size distribution parameters, the stress-strain curves differed despite an identical gradation size statistic when the particle position varied. Subsequently, the obtained data were partitioned into training, validation, and testing datasets at a 7:2:1 ratio. To convert the DEM model into a multi-dimensional matrix that computers can recognize, the 3D DEM models were first sliced to extract multi-layer two-dimensional (2D) cross-sectional data. Redundant information was then eliminated via gray processing, and the data were stacked to form a new 3D matrix representing the granular soil's fabric. Subsequently, utilizing the Python language and Pytorch framework, a 3D convolutional neural networks (CNNs) model was developed to establish the relationship between the constrained modulus obtained from DEM simulations and the soil's fabric. The mean squared error (MSE) function was utilized to assess the loss value during the training process. When the learning rate (LR) fell within the range of 10-5–10-1, and the batch sizes (BSs) were 4, 8, 16, 32, and 64, the loss value stabilized after 100 training epochs in the training and validation dataset. For BS = 32 and LR = 10-3, the loss reached a minimum. In the testing set, a comparative evaluation of the predicted constrained modulus from the 3D CNNs versus the simulated modulus obtained via DEM reveals a minimum mean absolute percentage error (MAPE) of 4.43% under the optimized condition, demonstrating the accuracy of this approach. Thus, by combining DEM and CNNs, the variation of soil's mechanical characteristics related to its random fabric would be efficiently evaluated by directly tracking the particle assemblages.
[...]Read more.2024, 16(11): 4782-4797. doi:10.1016/j.jrmge.2023.12.032
The ultrasonic pulse velocity (UPV) correlates significantly with the density and pore size of subgrade filling materials. This research conducts numerous Proctor and UPV tests to examine how moisture and rock content affect compaction quality. The study measures the changes in UPV across dry density and compaction characteristics. The compacted specimens exhibit distinct microstructures and mechanical properties along the dry and wet sides of the compaction curve, primarily influenced by internal water molecules. The maximum dry density exhibits a positive correlation with the rock content, while the optimal moisture content demonstrates an inverse relationship. As the rock content increases, the relative error of UPV measurement rises. The UPV follows a hump-shaped pattern with the initial moisture content. Three intelligent models are established to forecast dry density. The measure of UPV and PSO-BP-NN model quickly assesses compaction quality.
[...]Read more.2024, 16(11): 4798-4813. doi:10.1016/j.jrmge.2024.02.016
The accurate prediction of the bearing capacity of ring footings, which is crucial for civil engineering projects, has historically posed significant challenges. Previous research in this area has been constrained by considering only a limited number of parameters or utilizing relatively small datasets. To overcome these limitations, a comprehensive finite element limit analysis (FELA) was conducted to predict the bearing capacity of ring footings. The study considered a range of effective parameters, including clay undrained shear strength, heterogeneity factor of clay, soil friction angle of the sand layer, radius ratio of the ring footing, sand layer thickness, and the interface between the ring footing and the soil. An extensive dataset comprising 80,000 samples was assembled, exceeding the limitations of previous research. The availability of this dataset enabled more robust and statistically significant analyses and predictions of ring footing bearing capacity. In light of the time-intensive nature of gathering a substantial dataset, a customized deep neural network (DNN) was developed specifically to predict the bearing capacity of the dataset rapidly. Both computational and comparative results indicate that the proposed DNN (i.e. DNN-4) can accurately predict the bearing capacity of a soil with an R2 value greater than 0.99 and a mean squared error (MSE) below 0.009 in a fraction of 1 s, reflecting the effectiveness and efficiency of the proposed method.
[...]Read more.