Search

This paper proposes a calculation method of displacement-dependent three-dimensional (3D) earth pressure on flexible walls of the foundation pits, further considering spatial effects, layered cohesive soil, and seepage effect on earth pressure. Based on improved Coulomb's earth pressure theory, the displacement-dependent 2D earth pressure model for flexible walls is established. By introducing the concepts of spatial influence factor and plane strain ratio (PSR), the calculation model of displacement-dependent earth pressure on flexible walls of foundation pit considering spatial effects is further proposed. And the displacement-controlled solutions of earth pressure under different boundary conditions are obtained. The proposed solution is verified by numerical simulations and reported test data of foundation pit and shows good agreement. The traditional 2D earth pressure theory underestimates and overestimates the active and passive earth pressure in the corner effect area of the excavation, respectively. Through parameter analysis and discussion, the parameter effects on 3D earth pressure are ranked as soil cohesion > soil friction angle > wall friction angles. The results of the study provide an important theoretical basis for the 3D design calculation of foundation pits. [ABSTRACT FROM AUTHOR]

The unstable mechanical behavior of soil particles during suffusion, including migration and clogging of fine particles in porous media, is prone to induce seepage catastrophes. Mechanical behavior of migratory fines and porous soils during suffusion is significantly influenced by the particle shape, which remains unclear. In this study, a coupled computational fluid dynamics and the irregular discrete element method (CFD-iDEM) framework is developed to investigate the migration and clogging mechanisms. A series of numerical simulations that consider spheres and irregular particles with different levels of aspect ratio are carried out to elucidate the microscopic origins of shape effects on clogging. Migratory fine particles are discharged from the grain inlet and enter the coarse particle skeleton by imposing a downward seepage flow. The subsequent migration and clogging phenomena and microscopic mechanisms are investigated. The results reveal that irregular particles present varying degrees of ability to develop clogging clusters, and spheres are more prone to traverse deeper into soil skeleton. The proposed CFD-iDEM method is able to reproduce macroscopic phenomena of saturated porous medium as well as to analyze microscopic origins of fluid–particle interactions, which contributes to practical guidance for engineering applications. [ABSTRACT FROM AUTHOR]

For the excavation of a shallow tunnel in soft soil, the controllable range of the support pressure is narrow due to the small area of overburdened earth pressure, and passive failure of the tunnel face can be easily induced; therefore, the soft soil must be reinforced to increase the stability of the tunnel face. In this work, numerical simulation and theoretical analysis were adopted to explore the passive failure mechanism of a tunnel face in soft ground reinforced with a soil–cement wall. First, finite element analyses were conducted to investigate the passive failure mechanism. The impact of L1 (the distance between the tunnel face and soil–cement wall) on the passive failure mode was disclosed. Three types of failure modes—the full-penetration, partial-penetration, and non-penetration modes—were identified based on the shear failure zone and variation in Pp (the limit support pressure of passive failure) with L1. Then, analytical models based on the limit equilibrium method for the three passive failure modes were established, and formulas for predicting Pp were derived. The predictions by the formulas were validated by the numerical results, and their differences were within 10%. Both indicate that Pp achieved a maximum value when L1 = 0.2D (D is the diameter of tunnel); therefore, it is reasonable to choose a distance of 0.2D between cement walls in this engineering project. [ABSTRACT FROM AUTHOR]

To enhance our understanding of soil behavior at critical states, considering that natural soil is composed of granular matter, a quasi-static inertia number taking soil compaction into account is proposed. In analyzing classical triaxial test data of soil, the scaling law of quasi-statically deforming grains at the critical state is explored; a simple linear relationship is found between the coefficient of friction and the proposed number. This scaling law describes quantitatively the influence of initial compaction, shear rate, confining pressure, and particle size on the frictional strength of granular soils when they reach the critical state. The number proposed is employed to describe the scaling of volumetric behavior of granular soils undergoing quasi-static deformation. The difference between the particle volume fraction at the critical state and that at the initial compacted state is also found to be linearly correlated with the quasi-static inertia number, for soil at the critical state. [ABSTRACT FROM AUTHOR]

Particle shape in granular materials exhibits both polydisperse and multi-level characteristics, which are intrinsically linked to the mechanical behaviours of granular materials. This study develops a particle generation framework to achieve independent control of polydisperse distributions of roundness (second level of particle shape) and roughness (third level) while preserving form (first level) distributions. In this framework, a spherical harmonic amplitude-stretching (SH amplitude-stretching) technique is proposed to precisely control the form distributions. Two decisive factors for particle roundness and roughness are introduced, and two control coefficients are designed to determine their distributions. The applicability of the proposed framework is verified by employing the generated granular assemblies with varying distributions of particle roundness and roughness into DEM simulations for granular packing and repose angle tests. The simulation results demonstrate the independent effects of polydisperse distributions of particle roundness and roughness on packing density and repose angle, providing evidence of the effectiveness of the proposed framework. [ABSTRACT FROM AUTHOR]

Flexural performance of joints is critical for prefabricated structures. This study presents a novel channel steel-bolt (CB) joint for prefabricated subway stations. Full-scale tests are carried out to investigate the flexural behavior of the CB joint under the design loads of the test-case station. In addition, a three dimensional (3D) finite element (FE) model of the CB joint is established, incorporating viscous contact to simulate the bonding and detachment behaviors of the interface between channel steel and concrete. Based on the 3D FE model, the study examines the flexural bearing mechanism and influencing factors for the flexural performance of the CB joint. The results indicate that the flexural behavior of the CB joint exhibits significant nonlinear characteristics, which can be divided into four stages. To illustrate the piecewise linearity of the bending moment-rotational angle curve, a four-stage simplified model is proposed, which is easily applicable in engineering practice. The study reveals that axial force can enhance the flexural capacity of the CB joint, while the preload of the bolt has a negligible effect. The flexural capacity of the CB joint is approximate twice the value of the designed bending moment, demonstrating that the joint is suitable for the test-case station. [ABSTRACT FROM AUTHOR]

Underground excavation is usually accompanied by complex soil-structure interaction problems in practical engineering. This paper develops a novel multi-scale approach for investigating the soil arching effect through trapdoor tests. This approach adopts the finite element method (FEM) and smoothed particle hydrodynamics (SPH) method to handle the particle-rigid body interaction in the trapdoor tests, incorporating a micromechanical 3D-H model to derive the nonlinear material response required by the SPH method. The variation of the earth pressure on the trapdoor in simulations exhibits good agreement with those of the experiments. Extensive parametric analyzes are performed to assess the effects of soil height and inter-particle friction angle on the evolution of load transfer and soil deformation. Three deformation patterns are observed under different buried conditions, including the trapezoid, the triangle, and the equal settlement pattern. Results indicate that the planes of equal settlement develop progressively with the trapdoor movement and then enter the range of experimentally observed values. Additionally, three failure mechanisms are identified that correspond to the three deformation patterns. Due to the advantages of the micromechanical model, mesoscale behavior is captured. The anisotropy of stress distribution in the plastic region is found during the arching process. [ABSTRACT FROM AUTHOR]

Slit-type barriers serve as active countermeasures against debris flow. However, the dynamic interaction between debris flows and slit-type barriers is extremely complicated. To model the fluid-solid behavior of debris flows, a coupled approach of smoothed particle hydrodynamics (SPH) and discrete element method (DEM) is developed. We use the model to investigate the dynamics of how debris flows impact slit-type barriers. The proposed approach is able to simulate the embedding of arbitrarily shaped particles in viscous debris flows. A GPU acceleration technique is employed to overcome large computational costs. The approach is first validated by comparing numerical results to experimental observations for some standard conditions. Subsequently, experiments of the impact of water-enriched debris on slit-type barriers are modeled and validated. Moreover, the effects of barrier arrangement, solid volume fraction and boulder shape on debris-barrier interactions are further explored. This study analyzes the interaction between debris flows and slit-type barriers by coupled SPH–DEM and provides insight into the optimal design of barriers. [ABSTRACT FROM AUTHOR]

The real-time control and prediction of attitude and moving trajectory in shield tunneling is challenging, especially in the small radius of curvature. The present study focuses on developing a method for predicting the attitude of a shield based on a new model that incorporates a wavelet transform (WT) into a random forest (RF) deep learning method. Daubechies (dbN) is selected among several wavelet bases to denoise the original tunneling parameters in the time and frequency domains, and optimal wavelet parameters for different input variables are obtained. The proposed WT–RF model was implemented and validated by a case study of the Shizimen Tunnel in Zhuhai, China. Sorted by the correlation coefficient between the input parameters and the output parameters, eight cases with different input parameter combinations were also selected for comparison. The results revealed that the developed wavelet signal denoiser-RF model can successfully predict the shield attitude parameters and that using a suitable combination of input parameters can greatly improve the prediction accuracy of the model. The number of input parameters is not a key factor for determining the prediction accuracy of the model, and adding less relevant input parameters will greatly reduce the prediction performance of the model. [ABSTRACT FROM AUTHOR]

The apparent influence of stress states on rheological deformation should be considered in calculating consolidation behavior for natural structured soft clays. A novel nonparallel isotache model with naturally structural characteristics was developed in this study, taking into account the variation of secondary compression coefficient with stress state and collapse state of soil structure under the elastic viscoplastic framework. The proposed rheological model was used to couple the consolidation of multi-layered soft clays under various drainage conditions. The settlement solutions, degree of consolidation, and excess pore water pressure in a short time interval were derived mathematically, and the rheological consolidation solutions were programmed and calculated using a semi-analytical method. Good agreement with experimental results indicated that the model was reasonable, and the calculation method was efficient and accurate. A series of cases were presented to investigate the effects of model parameters, soil structure, and loading mode on rheological consolidation behavior. The variation law of soil parameters and state was also discussed. [ABSTRACT FROM AUTHOR]

This paper presents a case study of the clogging of a slurry-shield tunnel-boring machine (TBM) experienced during tunnel operations in clay-rich argillaceous siltstones under the Ganjiang River, China. The clogging experienced during tunneling was due to special geological conditions, which had a considerably negative impact on the slurry-shield TBM tunneling performance. In this case study, the effect of clogging on the slurry-shield TBM tunneling performance (e.g., advance speed, thrust, torque, and penetration per revolution) was fully investigated. The potential for clogging during tunnel operations in argillaceous siltstone was estimated using an existing empirical classification chart. Many improvement measures have been proposed to mitigate the clogging potential of two slurry-shield TBMs during tunneling, such as the use of an optimum cutting wheel, a replacement cutting tool, improvements to the circulation flushing system and slurry properties, mixed support integrating slurry, and compressed air to support the excavation face. The mechanisms and potential causes of clogging are explained in detail, and the contributions of these mitigation measures to tunneling performance are discussed. By investigating the actual operational parameters of the slurry-shield TBMs, these mitigation measures were proven to be effective in mitigating the clogging potential of slurry-shield TBMs. This case study provides valuable information for slurry-shield TBMs involving tunneling in clay-rich sedimentary rocks. [ABSTRACT FROM AUTHOR]

Adjacent construction around metro tunnels is a common situation in urban cities, which usually leads to the disturbance of surrounding soils and existing tunnels. Based on the influence zone theory, construction risks can be described in a graded manner. However, the calculation of the influence zone in existing research is mostly based on a single indicator, which is difficult to adapt to the increasingly stringent requirements of real projects. To address this limitation, a normalized method for the influence function and a weighted combination method for multi-indicators were proposed. These methods can combine any number of indicators to obtain an integrated result of the influence zone, facilitating direct guidance of engineering. Based on a case study of the Metro construction in Qingdao, a series of numerical simulations and calculations of the integrated influence zone was carried out. The results show that the shape of the integrated influence zone in this case is a necked bottle, i.e., the new tunnel causes a greater influence when it is below the existing tunnel. The change of soil layer and the weight of indicators both play important roles on the shape of the influence zone. The relevant conclusions and methods provide a basis for the safety assessment of similar projects. [ABSTRACT FROM AUTHOR]

Based on a cross-river tunnel of Wuhan Metro Line 8, we present a two-dimensional discrete element model for shield attitude adjustment considering the effect of overbreak cutters. The shield shell mechanics under the influence of over-excavation rate, over-excavation orientations, and overburden load are simulated, and the tunneling mechanics law and the ultimate range during the adjustment of the shield attitude are investigated. The simulation results indicate the following: (1) The greater the over-excavation rate, the smaller the force exerted by the soil layer in the negative direction of the shield movement; therefore, increasing the over-excavation rate is helpful in expanding the range of shield attitude adjustment. (2) The shield is stressed symmetrically while conducting positive and negative horizontal adjustments in the soil layer, which has a symmetrical distribution, but vertical upward adjustment is more difficult than vertical downward adjustment. (3) With the increase in overburden load, the space of the shield attitude adjustment is gradually reduced at the same over-excavation rate. A good engineering application was achieved in this project using the simulation model. It is recommended to use the attitude adjustment method by controlling the tunneling parameters. In difficult situations such as high overlying loads, the over-excavation cutter can be used to assist in adjusting the shield attitude. [ABSTRACT FROM AUTHOR]

Experimental results provide strong evidence that the deformation and strength of rocks are closely related to the damage suffered during loading. The classic constitutive models such as Mohr–Coulomb criterion and the Drucker–Prager criterion are usually unable to describe the non-linear deformation behavior of rocks, including strain hardening and softening. Their applications in numerical analysis and practical engineering are limited. For this purpose, an elastoplastic damage constitutive model that takes into account the competition mechanism between damage and strain hardening or softening during rock compression is proposed herein. This model is used to investigate the deformation behaviors and damage evolution of rocks. More importantly, this model has been implemented by finite element programming code and verified by a series of triaxial compression tests. The comparison results between theoretical analysis and experimental data indicate that this model can well describe the stress–strain curves and damage–strain curves of the investigated rocks (sandstone and salt rock), especially the characteristics of softening, hardening and residual strength. Based on parametric analysis, the influence of confining pressure, scale parameter and shape parameter on rock damage is revealed. It is found that the rock scale parameter in this model has a power function relationship with confining pressure. The ultimate plastic deformation that rocks can withstand is related to the scale parameter. The shape parameter controls the residual strength and deformation of rocks. Model results demonstrate that the strength and deformation vary with rock properties, and are strongly dependent on the stress-induced damage and strain characteristics. [ABSTRACT FROM AUTHOR]

The slurry infiltration process contributes significantly to the stability of tunnelling face during the application of slurry pressure balanced tunnel boring machine (SPB TBM) in construction. A coupled computational fluid dynamics and discrete element method (CFD-DEM) model is developed to simulate the formation and long-term impermeable stability of filter cake considering different infiltration conditions, e.g., the cohesiveness of bentonite slurry and the permeability of sandy ground. The morphology of formed filter cakes and the pressure drops are analysed macroscopically. Then, the long-term impermeable stability of filter cakes under increasing slurry pressure is evaluated based on the evolution of the pressure gradient magnitude along the infiltration direction; additionally, the contact and force fabrics of filter cakes are examined to investigate their failure process from the micro-mechanical aspect. The numerical results indicate that the increase of size ratio of sand to slurry particle and the slurry cohesion restrain and enhance, respectively, the formation of filter cakes and their long-term permeable and mechanical stability subjected to dynamic hydraulic loads. This work contributes to the further development of the phenomenological model and the micromechanical constitutive model of slurry infiltration involving multifield and multiphase media. [ABSTRACT FROM AUTHOR]

Constructing salt cavern gas storage in the ultra-deep strata more than 2000 m in depth is an important strategic technologic development in China. A primary issue is that higher temperature affects the creep behaviors of the surrounding rock. In this paper, triaxial creep tests of rock salt under multi-stage temperatures were carried out to study the influence of temperature on the creep behaviors of rock salt. Synchronous acoustic emission monitoring was performed throughout the creep test to study the microcracks characteristics of rock salt during creep at different temperatures. The results show that the duration of transient creep decreases with the increase of temperature. The steady-state creep rate increases with the increase of temperature. Temperature rise prompts the creep of rock salt to enter the volume dilatation stage. By analyzing the acoustic emission energy/counts, it is found that the microcracks generated during the creep at high temperature are few, but there are more at low temperature. Microcracks are generated intensively during rising temperatures. By analyzing the characteristics of the acoustic emission spectrum, it is found that when the temperature is low, the cracks generated are mainly intergranular cracks with a larger size, and they are mainly intragranular cracks with a smaller size when the temperature is high. The research in this paper is helpful to further understand the creep properties of ultra-deep rock salt and provides an important basis for the design, construction, and operation of gas storage salt caverns. Highlights: The influence of temperature on the creep of rock salt was studied by performing multistage temperature creep tests. The microcracks characteristics of rock salt during creep at different temperatures were studied using acoustic emission technology. The duration of transient creep phase decreases with the increase of temperature. The steady-state creep rate increases exponentially with the increase of temperature. [ABSTRACT FROM AUTHOR]

When highly abrasive hard rock strata are encountered during the TBM boring process, severe problems, such as low TBM boring efficiency and severe tool wear, emerge. High-pressure water jet-assisted TBM rock breaking technique has been developed and applied in steps. In this study, the effect of confining pressure and precutting slits on the penetration damage in highly abrasive hard rock is studied through TBM cutter penetration tests. Some main conclusions are drawn as follows: (1) For intact rock samples, the increase in confining pressure leading to a reduction in the unloading effect after the initial crack emerges when the confining pressure is higher than 15 MPa. Meanwhile, the boreability of the highly abrasive hard rock also significantly decreases, making it more difficult for rock breaking. (2) When examining the penetration damage process of intact and pre-cutting rock samples, a similar contact compaction stage (stage I) and microcrack initiation stage (stage II) can be found, while the crack propagation-penetration damage stage (stage III) shows significant differences in rock fragmentation patterns and force-penetration depth curve characteristics. (3) Under high confining pressure, precutting slits can enhance the initial decrease of the force by increasing the unloading effect of the TBM cutter on the rock sample after the initial crack is produced. Meanwhile, the reduction in peak strength index (PSI) and mean load index (MLI) values indicates that the penetration difficulty is significantly reduced. Hence, this contributes to the improvement of the boreability while the TBM is digging in highly abrasive hard rock. [ABSTRACT FROM AUTHOR]

During the operation of a compressed air energy storage (CAES) salt cavern, the surrounding rock experiences creep damage during the stages of constant internal pressure and undergoes fatigue damage due to the periodical injection-production. To describe the damage evolution of salt rock under creep-fatigue loading, a novel damage accumulation model based on the ductility exhaustion concept is proposed by applying a nonlinear summation method to represent the synergistic effect of creep and fatigue damage. Low-cycle fatigue (LCF) and creep-fatigue tests of rock salt were conducted under stress-control mode for various cycle stress amplitudes and hold times. Results show that the deformation of rock salt under creep-fatigue loading consists of initial, steady and accelerated phases. The proposed model matches well with the test data and can accurately describe the damage evolution as the applied stress amplitudes and dwell times change. The introduction of the hold times at the upper limit stress causes a strain increment and life reduction, which become more evident as the duration periods prolong and can be understood by the dislocation theory of crystals. [ABSTRACT FROM AUTHOR]

The cutting efficiency and wear of tunnel boring machine (TBM) cutters are two important parameters commonly used to estimate the boring performance and lifetime of the TBM in the tunneling process. These problems have been investigated by several researchers, whereas the influence of jointed rock mass and mixed ground condition on the cutting efficiency and cutter wear remains insufficiently understood. This study aims to elucidate the influence of rock mass and ground parameters on the cutting efficiency and cutter wear of the TBM by conducting a series of newly developed rolling boring tests on prefabricated joint sandstone samples with different orientations, spacings, and interlayers. Moreover, a numerical simulation is implemented through ANSYS software to verify the rationality of experimental results. The experimental and numerical results indicate that the optimal joint orientation is 30° for the TBM to intrude into a jointed rock mass. As joint spacing increases, the cutting efficiency decreases and the cutter wear increases. Furthermore, specific energy (SE) and cutter wear simultaneously reach their maximum values when the rock abrasivity index (RAI) is 60. Both the specific energy and cutter wear decrease with increasing specific strength. For the rock mass with a soft interlayer in mixed ground, the optimal dip angle is 45°. Additionally, the fitting formulas of SE and cutter wear in mixed ground are proposed to quantitatively illustrate the effect of RAI and interlayer strength on the cutting efficiency and cutter wear. The present study provides an improved understanding of rock-breaking mechanics, thereby making it possible for designers to adjust their design to enhance the TBM cutting efficiency. [ABSTRACT FROM AUTHOR]

Damage and self-healing characteristics of rock salt are key factors for the evaluation of underground salt cavern safety. During the construction of a salt cavern, characteristics of the surrounding rock (formation temperature, pressure and brine saturation) can affect the self-healing of rock salt. To study the recovery effect, we carried out a study through uniaxial compression tests. The rock salt samples were divided into group S and group X: samples in group X were annealed after initial damage (damage point: 15 Mpa, 20 Mpa, 25 Mpa and peak stress) while the samples in group S were not. For group S, a peak damage was applied immediately after the initial damage. Group X samples, after initial damage, were placed in the repair environment for 7 days. The repair environment mentioned above is set as saturated brine with a temperature of 50 °C and pressure of 12 MPa. It turned out that (1) the rock salt has almost no compaction phase during uniaxial compression. Damaged rock salt experienced an apparent compaction stage after being repaired; (2) for all the samples in the two groups, the yield stress and elastic modulus increase after the initial damage, which is regarded as the hardening of rock salt; (3) the plasticity of damaged rock salt can significantly recover after having been repaired. Thus, the environment during the construction of a salt cavern has a good effect on the self-healing of rock salt, and this characteristic of rock salt is beneficial to the safe operation of a cavern for storage purposes. [ABSTRACT FROM AUTHOR]

The wellbore tightness of a salt cavern gas storage must be tested before solution mining. According to the test results, it will be evaluated whether the wellbore can meet the cementing requirements of gas storage. However, there are many complex reasons that may cause wellbore leakage; hence, how to comprehensively analyze the test results and accurately expose the causes, locations, and scale of the leakage pose many challenges. These mainly include the incomplete test method and lack of theoretical analysis model. A nitrogen leak test was carried out for five wellbores that have been completed in Jintan (Jiangsu, China). The results show that two of them had leakage risk. To clarify the leakage causes and leakage types, we carried out an investigation of engineering geological data of the wellbores and further conducted laboratorial tests and theoretical analysis. The studies of drilling design and engineering geology show that the wellbores have good integrity and initially reveal that a mudstone interlayer intersecting the open hole between the casing shoe and the top of the salt cavern is a potential leaking layer. Furthermore, the permeability experiments and CT scans confirm that this mudstone interlayer is a leaking stratum and that the internal cracks develop severely. They are the key reasons leading to wellbore tightness failure. The proposed seepage theoretical model determines that the leakage type is horizontal flow in the mudstone interlayer. Comparing the theoretical results with the field test data, we find that the leakage rate curves of the two are in good agreement, which completely confirms that the mudstone interlayer is the fundamental cause of the wellbores tightness failure. This research not only identifies the leakage causes and leakage types of wellbores, but also enriches the leakage rate analysis method of gas storage and provides a theoretical and experimental analysis method for tightness evaluation of bedded salt cavern storage. [ABSTRACT FROM AUTHOR]