International Journal of Geomechanics

Engineering practice has shown that shield machines may become clogged in coarse-grained strata. The criteria for determining the clogging potential in coarse-grained strata were proposed. With the surface area ratio of fine to coarse particles in a soil specimen being > 30:1, the interfacial shear adhesion showed a trend of increasing first and then decreasing with an elevation in water content. However, the interfacial shear adhesion remained negligible for the soil specimen with a surface area ratio of fine to coarse particles < 25:1. Interfacial shear adhesion could be adopted for characterizing the risk of shield clogging. Thus, a critical surface area ratio of 25:1 was suggested to evaluate the possibility of clogging in coarse-grained soils. The particle gradation had no effect on the critical surface area ratio with clogging potential. The bound water between the fine particles was released after the injection of the dispersant, resulting in a reduction in the interfacial shear adhesion. The clogging potential of the soil also decreased, while the critical surface area ratio of fine to coarse particles increased.

Risk assessment of seismically loaded slopes is often a prerequisite for guiding decision-making and mitigation, which could be more difficult to quantify when taking the spatial variability of material properties into account. In this study, a random finite-element limit analysis (RFELA) is developed to assess the risk of a seismically loaded slope where the undrained shear strength of soils is spatially variable with depth. The nonstationary random field is first introduced to model the linearly increasing undrained shear strength. Then, finite-element limit analysis (FELA) is employed to evaluate the seismic stability and consequence, in which the seismic loading is characterized by the pseudostatic approach with a range of horizontal seismic coefficients. Finally, the quantitative risk assessment is conducted based on Monte Carlo simulations. The results show that using the nonstationary random field to model the soil spatial variability could significantly reduce the risk compared with the stationary random field, which matches better with the site-specific data. The risk of the slope failure increases with the increase in the seismic coefficient. In addition, the effects of the correlation structure of the undrained shear strength, which includes the coefficient of variation (COV) and autocorrelation distance, on the risk assessment are studied by parametric analyses.

A series of model tests about vibroflotation were conducted to investigate soil property change on silica sand foundations, especially for earthquake-resistant behavior. First, the process of double-point vibroflotation was investigated with a simulative vibrator system. Thereafter, shaking table tests were conducted to evaluate seismic responses. The results illustrated that the relative density was significantly enhanced after the first vibratory period, with the highest enhancement appearing at the vibropoint in the plane and the middle layer along the depth. As for the seismic tests, the excess pore pressure ratios show that the vibroflotation can apparently improve the liquefaction-resistance capacity of the silica sand foundation. Moreover, the shear stiffness of the unconsolidated foundation decreases distinctly under 0.2g earthquake motion, while the consolidated foundation remains capable of transmitting acceleration. Inflicting white noise before the any earthquake motion and after 0.2g earthquake, the result shows that the predominant frequency of unconsolidated foundation decreases apparently. The dynamic shear stress–strain curves were compared, while the development law of the time history was also investigated. These results demonstrated that this method can obviously prevent the shear stiffness attenuation. To sum up, the vibroflotation compaction method can make a great densification and increase the earthquake-resistant capacity.

The coronavirus pandemic (COVID-19) has led to a surge in disposable mask waste, posing an urgent environmental challenge. This study explores a sustainable solution by using mask chips as reinforcement material in geotechnical engineering, simultaneously improving ground strength and recycling waste. The mechanical behavior of Fujian sand reinforced with mask chips under plane strain biaxial shear condition was examined. Biaxial shear tests were performed on dense pure sand and sand samples containing three mask chip sizes (20 mm × 5 mm, 10 mm × 10 mm, and 12 mm × 3 mm) at a 0.5% mass ratio, under effective lateral pressures of 50 and 100 kPa. The test results show that incorporating mask chips improved strength and delayed stress peak, with the 20 mm × 5 mm chips demonstrating the best performance. Within the tested lateral pressure range, the sample dilates along the minor principal strain direction during shearing. The volumetric strain of mask–sand mixtures exhibits more significant contraction before the dilation compared with pure sand. Moreover, two distinct shear band shapes were observed through imaging technique where the thickness of the shear band decreases as the lateral pressure rises. Additionally, the Roscoe solution was found to provide the closest approximation of the inclination angle of the shear band in mask-reinforced sand.

Remnant coal pillars in longwall goaf are vulnerable to creep deformation, damage, and instability under sustained pressure from the overlying strata. To study the creep damage status of coal pillars, we first built a mechanical model of the pillar–roof (P–R) structure. The stress and deformation analytical equations in the viscoelastic and viscoplastic zones of the coal pillar were then constructed following the fractional creep damage constitutive model and the elastic–viscoelastic correspondence theory. The coal pillar creep stress, deformation, and damage development laws were analyzed. The findings indicated that the abutment stress decreases as the creep time increases, and the peak stress gradually shifts to the coal pillar. Finally, a mechanical instability model of the P–R structure based on the coal pillar bearing capacity was established. The instability index and critical elastic kernel ratio for P–R structures considering the creep effect were presented. Moreover, the effects of coal seam depth, coal pillar width, and roof cracking angle on creep damage and coal pillar stability were studied. The findings of this study can be used to optimize the settings for coal pillars and analyze the stability of the remnant coal pillars in goafs.

The adverse effects of the frequent reservoir earthquakes (REs) and the severe degradation of the hydrofluctuation belt (HFB) on the long-term stability of bedding bank slope since the Three Gorges Reservoir Area (TGRA) impoundment were considered fully in this paper. Under such complex conditions, the dynamic response pattern and instability mechanism of typical bank slopes were investigated systematically by using a method of combining field survey, laboratory shaking table (ST) model test, and Universal Distinct Element Code (UDEC) numerical exploration. The research shows that the degradation forms of the HFB of typical bank slopes in the key field survey of the TGRA can be arranged into four types: erosion (denudation), corrosion (suffosion), avalanche (collapse), and slip (flow). The dynamic response of slope shows obvious characteristics of “surface effect” and “elevation effect,” and the cumulative displacement (C_D), damping ratio (D_R) and natural frequency (N_F) of slopes show a trend of gradually increasing, and increasing and decreasing, respectively. Meanwhile, the “S–type” cubic function and “Steep rise type” exponential function can be used to characterize the cumulative damage degree (C_DG) of slope at the excitation stages of microseisms—small seisms and strong seisms, respectively. Subsequently, the dynamic evolvement process for cumulative damage and unstable failure of slope considering the impact of degradation of rock mass in the HFB was obtained, and there are good agreement between the model test and numerical exploration results. Furthermore, fully considering the influence of slope height (SH), slope angle (SA), thickness of bedding plane (TBP), dip angle of bedding plane (DABP), dynamic load amplitude (DLA), dynamic load frequency (DLF), height of water level of the HFB (HWHB), degradation range of the HFB (DRHB), and degradation shape of the HFB (DSHB), the C_D of slope subjected to continuous excitation of multiple seismic loads increases in the form of exponential function, the dynamic stability coefficient (D_SC) of slope decreases slowly first and then drops sharply, and the response degree of slope deformation and sliding velocity is obviously different. Especially, the sensitivity level of every preceding factor to the dynamic stability of slope based on the orthogonal analysis method (OAM) is in the order of DLA > DRHB > SA > SH > DLF > HWHB > DSHB > DABP > TBP.

Worldwide, an increasingly huge number of end-of-life tires (ELTs) are disposed of in landfills, illegally dumped, or otherwise unaccounted for, which causes significant environmental and socioeconomic issues. Finding sustainable engineering solutions to recycle and reuse ELTs, which transform them from unwanted waste into useful resources, has become a priority. In geotechnical engineering, researchers have performed laboratory and field tests to determine the mechanical properties of innovative geomaterials that consist of soil–rubber mixtures (SRMs) [i.e., gravel–rubber mixtures (GRMs)] that are obtained using recycled ELT-derived granulated rubber aggregates. Suitable engineering properties and low installation cost encourage the use of GRMs and SRMs in many applications, such as in free-draining energy-adsorption backfill material for retaining walls, underground layers for liquefaction mitigation and geotechnical seismic isolation systems for structures and infrastructures. However, due to the heterogeneity of SRMs, their ultimate adoption as geomaterials must be supported by constitutive relationships that can accurately describe their mechanical behavior under typical field loading conditions. The aim of the paper is to evaluate the effectiveness and limits of the hardening soil model with small strain stiffness (HS-small), which is present in many finite-element (FE) codes, to model the behavior of GRMs in geotechnical engineering applications. An extensive finite-element method simulation of drained triaxial tests was performed.

Rainfall can cause seepage erosion of the fine particles in soil (FPS) within the wide grading of particles present in a deposit slope, reducing its strength and eventually leading to a landslide. Unfortunately, there is no effective method for simulating this phenomenon; therefore, using the finite-element method (FEM), the whole process for establishing the slope stability analysis method is described considering the effect of the erosion of FPS, which includes equations of the FPS erosion process, introducing the real seepage parameters obtained by large-scale soil-water characteristic curve (SWCC) tests, and establishing correlations among water content, fine particle content, and slope strength. The findings of the application of this method to a real project reveal that FPS erosion occurs most readily on the first and second levels of a subslope, causing shallow instability of the slope and a reduction in the slope factor of safety (FOS); however, the higher the rainfall intensity, the lower the slope FOS. The method in this paper is suitable for the deposit slope with a certain content of FPS.

This study used a two-stage method to derive an analytical solution for assessing the vertical displacement of an existing pipeline caused by the undercrossing construction of a shield tunnel. The aim was to evaluate the impact of shield excavation on adjacent pipelines. A Timoshenko beam lying on a Vlasov foundation was adopted to model the existing pipeline subjected to soil disturbance induced by underneath tunneling. The model considered the shear stiffness of the pipeline and the vertical nonuniformity of the foundation parameters. The lateral soil action was considered in the force analysis to develop a prediction model for pipeline settlement. The proposed analytical solution was tested against project examples and field measurements collected from a construction site. The prediction results of our new model, the Vlasov–Timoshenko beam model, exhibited greater consistency with field monitoring data than those of the existing models, such as the Euler–Bernoulli beam model. The model with lateral soil action exhibited higher prediction accuracy than that without lateral soil action. Furthermore, parametric analyses evaluated the influences of different factors; for instance, when the pipe and tunnel angles decrease and pipe shear stiffness reduces, the tunnel radius increases, the pipe settlement increases, and the growth rate gradually increases. When the tunnel depth increases, the pipe settlement decreases. For the same calculation example, the maximum value of the pipe settlement and the range of the settlement trough increase after considering the lateral soil action on the pipeline. In a further parametric comparison analysis, when Poisson’s ratio increases, the settlement deformation of the pipeline decreases, and when the volume loss and elasticity modulus of the soil increase, the maximum settlement value of the pipeline increases and the growth rate gradually decreases. Hence, the reasonableness of the proposed method for calculating the settlement of adjacent pipelines by considering both the shear stiffness of the pipeline and the lateral soil action is demonstrated.

Coal mining generally induces stress concentration around the goaf range, and the hard roof undergoes fracture and instability when the stress exceeds the tensile strength. It is crucial to investigate the mechanical behavior of the hard roof and determine the corresponding influencing factors of fracture. However, the roof load is treated as the normal distribution function or simple piecewise linear function in the existing research, which makes the model applicable only to specific load forms. For this purpose, the eight-segment linear function is used in this study to subdivide the supercharged load acting on the roof based on different foundation conditions, which can help one to avoid complex calculations of mechanical behavior and ensure proximity to the actual load. The coal seam in front of the coal wall is regarded as an elastic foundation, and the deflection curve equation is established by analyzing the force on the roof. The internal force and deformation of the hard roof are obtained before the initial and periodic fractures. The accuracy of this model is verified by comparing it with other models in the literature. The results of parameter sensitivity analysis indicate that with the increase of foundation stiffness, roof thickness, and support resistance, the deflection and bending moment significantly decrease. The deflection, bending moment, and shear force are generally positively correlated with goaf length. When the roof is thin and the coal seam is soft, the initial fracture position occurs at the midspan, while the periodic fracture always occurs inside the coal wall. The fracture position gradually moves toward the coal wall with the increase of coal seam stiffness for both the initial and the periodic fractures. The research results are helpful to understand the internal force and deformation mechanism of the hard roof and provide a valuable theoretical basis for predicting the fracture mode of the roof.

Abstract

Disregarding the partial drainage of saturated clay foundations during staged construction of a building would underestimate the calculated short-term bearing capacity of foundations, resulting in imposing an extra burden on clients due to prescribing soil improvement. This paper aims to investigate the effect of partial drainage of saturated clay foundations subjected to different loading patterns on the mechanical response. Therefore, two main stages, including determination of shear strength parameters from triaxial simulations and foundation analysis based on the acquired shear parameters, were considered. In triaxial models, a change in the consolidation phase was proposed to determine more realistic shear strength parameters in a way that reflects the partial consolidation through the staged construction. It was found that drainage during construction could significantly increase the short-term bearing capacity up to 70% in comparison with the unconsolidated undrained triaxial test condition. Moreover, a well-justified equation and stepwise method were proposed to estimate the short-term bearing capacity of clay foundations with respect to the partial drainage during construction.

Practical Applications

In civil engineering, there are several cases in which superstructures should be constructed over saturated clay foundations. Therefore, the bearing capacity of these foundations should be determined and controlled in short-term (end of construction) and long-term (during service life of the structure) conditions. Owing to the fact that during staged construction of superstructures, a sort of partial drainage occurs, estimating the short-term bearing capacity of saturated clay foundations using the parameters derived from the undrained unconsolidated triaxial test could be very conservative, resulting in imposing an extra burden on clients due to prescribing unnecessary soil improvement. This paper by a combination of triaxial and foundation analysis investigated the effect of partial drainage of saturated clay foundations subjected to different loading patterns on the mechanical response. An attempt was made to provide engineers and designers with an appropriate relationship and a stepwise method so that they could determine the short-term bearing capacity of foundations considering the partial drainage in the common geotechnical practices. The proposed approach is applicable for clayey foundations in which taking undisturbed samples is possible.

Reinforcement of soils with fibers generally increases the mechanical properties of the fiber-reinforced soil (FRS) system. However, published literature is limited to investigating the undrained response of clay and synthetic fibers, with few studies targeting natural clay and natural fibers under drained conditions. There is a need to study the response of fiber-reinforced clay systems under drained conditions to assess long-term stability. This paper investigated the drained shear strength and durability of clays reinforced with natural hemp fibers using isotropically consolidated drained triaxial tests, in which the fiber content, confining pressure, and compaction water content were varied. Results showed that the incorporation of hemp fibers improved the deviatoric stress at failure by up to 60%, which increased the drained cohesion and friction angle of the FRS by 7–10 kPa and 3–7°, respectively. The increase in cohesive intercept was not affected by the compaction water content, while the increase in friction angle was pronounced in specimens compacted at optimum water content (w = 18%). Durability tests showed that the improvement in strength due to hemp fibers diminishes after 3 weeks of curing prior to drained testing, indicating the dramatic negative impact of degradation of natural fibers on the mechanical performance of fiber-reinforced clay and the need for industrial treatment of the fiber.

A series of strain-controlled cyclic triaxial tests were performed on Firoozkuh sand‒silt mixtures with wide ranges of fines content (FC), relative density (D_r), and effective confining stress ( ${\sigma }_c^{\mathrm{\prime }}$) to investigate their liquefaction resistance in terms of capacity energy (W_liq). Also, several cyclic test results from previous studies were collected and reanalyzed. The results showed that D_r could be used as a proper parameter to define soil density state for predicting W_liq of clean sands and sand‒silt mixtures when FC is greater than dispersing fines content (FC_dis). However, due to the complicated role of FC in coarse‒fine assemblies, no universal relationship between W_liq and FC has been reported for the soil mixtures when FC is less than the threshold fines content (FC_th). Therefore, the concept of equivalent granular void ratio ( $e^{\ast }$) was used to capture the coarse‒fine interactions in such mixtures. It was also found that the fines contribution factor (b), which is the fraction of fines participating in load transfer, is dependent on D_r, as well as particle size disparity ratio (χ) and FC, neglected in previous studies. Finally, a new model was proposed for the prediction of the b value and also a unique relationship between $e^{\ast }$ and W_liq was obtained for all mixtures of specified sand and silt where FC ≤ FC_th.

Nonlocal regularization is frequently used to resolve the mesh dependency issue that is caused by strain softening in finite-element (FE) simulations. Some or all variables that affect strain softening are assumed to depend on the local, neighboring, or both in this method. The weight function is the main component of a regularization method. There are three widely used weight functions, which include the Gaussian distribution (GD), Galavi and Schweiger (GS), and over-nonlocal (ON) functions. All of them could alleviate or eliminate the mesh dependency in simple boundary value problems (BVPs), such as plane strain compression; the evaluation of their performance in real-world BVPs is rare. A detailed comparison of these functions has been carried out based on an anisotropic sand model that accounts for the evolution of anisotropy. The increment of void ratio is assumed nonlocal. All functions give mesh-independent force–displacement relationships in drained and undrained plane strain compression tests. The shear band thickness shows a small variation when the mesh size is smaller than the internal length. None could eliminate the mesh dependency of shear band orientation. The GS method is the most efficient in eliminating the mesh dependency in the strip footing problem. The ON method could give excessive overpredictions of the volume expansion around strip footings, which leads to unrealistic low reaction forces on strip footings at large deformations. All three weight functions give mesh-independent results for the earth pressure that acts on a retaining wall.

The secant shear modulus is an important parameter in several geotechnical engineering models and calculation approaches. The characteristics of this parameter are very complex due to the hysteretic behavior of cyclically loaded soils. Its characteristics primarily depend on the mean effective confining stress and the void ratio. In this paper, the characteristics of the secant shear modulus of sand under large shear strain amplitude are investigated in detail based on the results of several cyclic triaxial and simple shear tests under drained and undrained conditions. The evaluation of the secant shear modulus based on the shape of the open stress–strain hysteresis loop is then discussed. Under certain loading conditions, the secant shear modulus defined from the reloading and unloading branches of the loop obviously differ. The evolution of the stiffness index, which measures the change in the secant shear modulus due to cyclic loading, shows that the drainage conditions play an essential role in the behavior of the secant shear modulus.

Conducting soil stability assessments around tunnels has always been a concern. However, most existing studies have regarded soil as an isotropic and homogeneous material. To overcome this limitation, within the framework of upper bound theory, this paper proposes a novel rotational–translational failure mechanism where the velocity discontinuity surfaces are derived numerically. This theoretical mechanism includes two cases according to the positions of the velocity discontinuity surfaces. An analytical solution for pore water pressure is obtained using the conformal mapping method, which involves solving the two-dimensional (2D) Laplace equation and considering the soil and shotcrete permeability. Then, upper bound expressions for the limit supporting pressure are derived by computing work equations with and without pore water pressure. Comparisons with previous work and numerical results illustrate that the presented approach offers improvements and could be applicable for stability analyses of shallow rectangular tunnels in anisotropic and nonhomogeneous soils. Finally, this paper discusses the effects of the anisotropy and nonhomogeneity of soil properties on the normalized limit supporting pressure and the collapsing domains of rectangular tunnels with different geometric shapes. In addition, the impact of pore water pressure on the changed water levels is assessed. The results demonstrate that for rectangular tunnels that are excavated in water-bearing zones, the width-to-height ratio plays a significant role in the stability of the surrounding soils.

Aeolian and kaolin deposits contribute greatly to infrastructural damage during the rainy seasons because of massive collapse settlements. The prediction of wetting-induced collapse potential and compressibility behavior under partly saturated states is essential for the development of infrastructure on these deposits. In this study, a general constitutive model was developed from the suction-controlled compression and wetting-induced collapse tests. The effect of particle orientation resulting from various initial compaction conditions, drying paths, and wetting paths on the yielding behavior of soil was investigated. A new collapsible soil model (CoSM) was presented by considering the wetting-induced changes to the clay fabric associations in the collapsible soils. The proposed CoSM requires eight parameters for evaluating the mechanical behavior. These model parameters can be readily estimated from simple compression tests, which is the major advantage of the model. The derived equations were capable of predicting three crucial mechanical characteristics, namely, loading-collapse yield, compression, and collapse behavior from the basic compression data. The model shows an excellent agreement with the measured data for two kaolin soils from the present work and several collapsible soils from the literature. The generalized model is capable of predicting mechanical behavior of collapsible soils with various initial compaction states and loading stress histories.

Red clay is commonly used as filler for roadbed or subgrade, but it always leads to engineering problems of uneven settlement and strength decay under cyclic dynamic loading because of its engineering characteristics such as high water content, high plasticity, and poor compaction. It is essential to assess the dynamic characteristics of undamaged red clay in the small-strain range, but few relevant researches are reported. In this study, the small-strain dynamic property of undisturbed red clay from Guiyang, Southwest China, was investigated. Multistage dynamic triaxial shear tests were performed to analyze the development of the skeleton curve (σ_d–ɛ_d), the maximum dynamic modulus of elasticity, the maximum dynamic shear modulus, and the maximum dynamic damping ratio in the small-strain range of undisturbed red clay with different confining pressures, consolidation stress ratios, loading frequencies, and water content ratios. Moreover, under the small-strain conditions, the small-strain dynamic deformation mechanism of the specimens with isobaric consolidation and anisotropic consolidation was analyzed. In addition, some empirical prediction models for the dynamic constitutive relationship, dynamic elastic modulus, and dynamic damping ratio of undisturbed red clay under small-strain range test conditions were proposed. Results indicate that the skeleton curve of undisturbed red clay showed softening behavior when the dynamic axial strain ɛ_d > 1.0%, the center of the hysteresis curve, shifted positively along the coordinate axis with the increasing confining pressure and the loading number. The undisturbed specimens exhibited mainly two types of deformation under small-strain test conditions, including axial compression deformation when the consolidation ratio K_c was 1.0 and bulging deformation when the K_c was 1.5 and 2.0. The proposed models could be used to predict the skeleton curve, dynamic elastic modulus, and dynamic damping ratio of the undisturbed red clay in the small-strain range.

Abstract

There has been limited research conducted to date on the role and significance of soil parameters in an unsaturated state on the amount of deformation and safety of reinforced deep urban excavations. This study investigates stress–deformations and static/pseudostatic safety factors against the general failure of an anchored deep excavated wall in an unsaturated soil deposit through two-dimensional finite-element modeling and limit equilibrium analysis, respectively. A suction-dependent elastic–plastic Mohr-Coulomb model is used in the analyses considering the effective stress approach in unsaturated soils. The results obtained from numerical modeling are validated against field monitoring data, including wall deformations and tensile forces in the anchors. A series of parametric studies are then performed assuming different groundwater levels, surcharge loads, and surcharge load distances from the wall crest to investigate their effects on the stability and deformation of the unsaturated soil excavation. Results are compared with those obtained from the corresponding routine analyses, in which the unsaturated state of the soil is not considered. The parametric study shows that the depth of the groundwater table is more influential on the results compared with the intensity and location of the surcharge load. The study demonstrates that unsaturated soil conditions result in a reduction of up to 37% in the maximum horizontal deformations of the excavation and increase the static and pseudostatic safety factors against general failure by 28% and 19%, respectively. In addition, taking unsaturated soil conditions into account during analysis leads to a decrease of up to 31% in the estimated tensile forces in the anchors. More importantly, it is shown that a more cost-effective stabilization plan can be developed for deep urban excavations by considering soil unsaturation effects, as demonstrated by comparing the results of the numerical analyses with the field data.

Practical Applications

Presenting a safe and economically feasible plan for stabilizing deep urban excavations is a significant challenge for geotechnical engineers. Traditionally, engineers have relied on classical methods that consider soil parameters under dry or saturated conditions. However, in practice, the soil above the water table is unsaturated, and its mechanical properties are greatly influenced by the saturation level. Contrary to common belief, rainfall or pipe leakage near an excavation site does not fully saturate the soil and eliminate suction effects. Previous studies have shown that percolation from rainfall or other factors does not completely infiltrate clay-rich soils, mainly affecting moisture content and suction at shallow depths. This study evaluates the stress–strain behavior and the factor of safety against sliding in a deep excavated wall in Tehran, Iran, by considering unsaturated soil parameters. Comparing the outcomes with conventional methodologies, the results demonstrate that incorporating unsaturated soil parameters provides more accurate results aligned with field observations. This approach also results in smaller displacements of the excavation wall and higher safety factors against wall sliding. Accordingly, incorporating unsaturated soil parameters ensures accurate safety assessments for urban excavation walls and enables the creation of cost-effective and optimized design suggestions.

Obtaining higher-resolution and more realistic characteristics of tectonic stress fields is the goal of research into tectonic stress field simulations. In this study, a regional heterogeneous rock mechanics model was established on the basis of seismic, logging, and sample experimental data. The simulation results show that the minimum principal stress values in the lower Niutitang formation are between 25.0 and 120.0 MPa. The quantitative distribution prediction results for shale reservoir fractures based on the high-resolution tectonic stress field numerical model show that the fracture development zones in the Sangzhi block are mainly distributed near the fault, fold axis turning, and fold wing in an NE–SW orientation. There are two types of fractures developed in this region: shear fractures and tensile fractures. The shear fractures mainly developed near the NE–SW-oriented faults, with shear fracture rates ranging from 1 to 3, and the tensile fractures mainly developed in the southeast wing of the Wudaoshui anticline, with tensile fracture rates ranging from 1 to 2. The low fracture development zone is mainly located in the western slope of the block, with shear fracture rates and tensile fracture rates that are both < 1.

Excavations of foundation pits around tunnels can induce alterations in stress and deformation within the tunnel, thereby influencing the safety of pre-existing tunnels, especially those with curved tunnel segments. This study focuses on investigating the effects of excavation processes and diverse construction methods on underlying curved tunnels, including excavation sequencing, reinforcement techniques, and support strategies. The efficacy of different construction methods in controlling deformation was assessed by a numerical analysis using finite difference software. The results show that adopting a skip excavation approach, positioned at a distance three times that of the excavation width, effectively diminishes heaving at the foundation pit bottom. In situations where tunnels intersect foundation pits at small angles, the generation of the hoop effect through intermittent reinforcement minimizes lateral extrusion forces, thereby enhancing control over both vertical and horizontal deformation of the curved tunnel. Deformation control within the tunnel, ranked from the least to the most effective, includes configurations of discontinuous reinforcement without a bottom plate, discontinuous main reinforcement, no reinforcement, damaged lining with discontinuous reinforcement, discontinuous reinforcement, and portal reinforcement. Moreover, the incorporation of a bottom plate in conjunction with discontinuous reinforcement methods proves efficient in restraining tunnel heave within safety limits. This study provides insights into safeguarding curved tunnels during irregular foundation pit excavation, offering potential time and cost savings through improved protective measures.

The seismic response of seabed is fundamental for researching the seismic response of marine structures. The one-dimensional transient response of an unsaturated seabed was studied in this paper. An analytical solution for a special case with infinite permeability coefficients and a semianalytical solution for a general case with arbitrary permeability coefficients were developed. The advantage of this semianalytical solution is that it can compute the behavior of unsaturated soils with very low permeability coefficients. During the derivation, the boundary conditions were transformed into homogeneous boundary conditions first, and then the eigenvalues and eigenfunctions were obtained. To obtain solutions in the time domain, the eigenfunction expansion method and the precise time-step integration method were employed. Through numerical examples, the reflection and transmission characteristics of three types of compression waves at the fluid–solid interface were elucidated. The impact of permeability coefficients and water saturation on the seismic response of the seabed was investigated through parametric analysis. When the soil is nearing saturation, even a slight change in saturation can lead to a drastic alteration in the seismic response of the nearly saturated seabed.

A soil–water–air fully coupled numerical framework is proposed to predict the deformation and hydraulic behavior of soils under different saturation states in response to dynamic loading. The proposed framework is developed based on the mixture theory of porous media and the governing equations are discretized with the finite-element method. The soil behavior is modeled by a sophisticated constitutive model. The hysteresis characteristic and the mutual dependence between volume change and the degree of saturation in the soil–water characteristic curve (SWCC) are considered. A consistent description for both unsaturated and saturated soils is achieved by taking the effective stress and the degree of saturation as the two independent variables, with only one set of unified mechanical/hydraulic parameters. The numerical framework is first validated at the element level against undrained and unvented strain-controlled cyclic triaxial test results of unsaturated Toyoura sand. The reliability of the proposed framework in addressing boundary value problems is further validated by the simulation of a dynamic centrifuge model test. The numerical framework is further applied to the dynamic stability analysis of unsaturated embankments with different initial water contents. The results show that initially unsaturated embankments with relatively high water content are susceptible to liquefaction during seismic loading. There is a significant correlation between the liquefaction and the deformation-induced soil saturation. Deformation-induced saturation initially occurs at the toe of the embankment and then gradually extends to the areas near the lateral surfaces of the embankment, which triggers the development of shear bands.

Abstract

Earth-fill dams should be monitored during the first filling of the reservoir to avoid several potential risks. Following the completion of dam construction, the primary safety check is monitoring the dam behavior during the first interaction of the earth material with water. The presented work fills the gap in the literature on the determination of intermediary hold durations of multistage filling schedule and provides insights into the first-filling process. The study investigated the behavior of a 20 m high earth-fill dam model subjected to staged reservoir filling. The durations of the intermediary holds were determined based on coupled stress and pore water pressure analyses using the finite-element method, with a particular emphasis on the transient behavior of the filling process. Hydraulic fracturing and internal erosion possibilities were evaluated based on minimum principal stress and seepage quantities at critical sections. It was observed that the minimum principal stress reaches a local minimum and tends to increase, leading to significant convergence for longer hold alternatives. In addition, seepage quantities exhibited similar behavior of decreasing with longer hold durations. A specific hold duration was recommended for the low-pool level, which resulted in a hold-to-height ratio of 2.35. At the high-pool level, it was found that further waiting has a positive effect in mitigating the risk of hydraulic fracturing and internal erosion initiation. The presented practical methodology is proposed to be used for the design of filling schedules of new dams.

Practical Applications

This study provides practical insights for design engineers to determine the first-filling schedule of earth-fill dams. The stress–pore pressure coupled finite-element model information obtained from this study can be helpful for onsite engineers who have access to instrumentation data during the first filling. These data can be used to predict the convergence of key parameters. For intermediary holds at low-pool levels, monitoring minimum principal stress and effective stress behavior over time is crucial to prevent cracking which can lead to hydraulic fracturing if filling continues. Specifically, the initially decreasing principal stress at the upstream face of the core should be closely monitored until an increasing trend is observed, to ensure that sufficient time is allowed before the dam is subjected to higher loads.

Pile-supported embankments, with advantages of high bearing capacity and effectiveness, have been widely used for constructing embankments on soft ground. The long-term load transfer and cumulative deformation mechanism caused by traffic loading are crucial to pile-supported embankment design and maintenance. A three-dimensional (3D) finite-element model of a pile-supported embankment to investigate its long-term load transfer and cumulative deformation mechanism under traffic loading is presented in this paper. The dynamic nonlinear behavior of the soft ground is considered by a simplified dynamic constitutive model that that has been effectively incorporated into the Abaqus program. In comparison with the results of published literatures, the applicability and accuracy of the proposed finite-element model are well verified at first. Then, the long-term load transfer and deformation mechanism are revealed by investing the soil arching and the cumulative deformation with a series of 3D numerical modeling. Furthermore, the influences of traffic loading type, speed of the vehicle, pile spacing, and total number of layers of grid on the long-term load transfer and deformation mechanism of the pile-supported embankment are investigated.

Abstract

The conventional Mohr–Coulomb (M-C) failure criterion is commonly used in existing studies to determine the ultimate pullout capacity of plate anchors buried in cohesive-frictional soils. However, this criterion overestimates the tensile strength of these soils by assuming a linear failure envelope (in both compression and tensile regimes) within the normal-shear stress space. In this study, the ultimate pullout capacity of horizontally buried plate anchors in cohesive-frictional soils was determined, accounting for the effect of tensile strength cutoff. The study proposed a series of semianalytical solutions utilizing the kinematic horizontal slice approach and investigated the ultimate pullout capacity of four different anchor shapes, including circular, square, rectangular, and strip anchors. The impact of nonassociated flow rule on the results was also examined. It has been observed that the results obtained using the M-C failure criterion with a tension cutoff are significantly lower than those obtained using the conventional M-C failure criterion. The difference between both results increases as soil cohesive strength increases and anchor burial depth decreases. These findings clearly demonstrate that eliminating tensile strength from the strength envelope leads to a more conservative estimate of plate anchor pullout capacity. The study emphasizes changes in anchor failure surfaces for various parameter combinations and provides results that are consistent with specific solutions reported in the literature.

Practical Applications

The study investigates using plate anchors to enhance the stability of both onshore and offshore structures, including transmission towers, guyed masts, chimneys, and mooring platforms. These structures often encounter significant vertical pullout forces. The research establishes a theoretical framework for calculating the ultimate vertical pullout capacity of plate anchors with various shapes (strip, rectangular, and circular) when buried in cohesive-frictional soils. A crucial aspect of this analysis involves incorporating the specified/limited value of tensile strength in cohesive-frictional soils when assessing the ultimate pullout capacity of plate anchors. This consideration is vital for ensuring a safe and conservative design, especially for structures subject to uplift forces. The findings provide valuable insights for engineers and practitioners, offering a reliable method to optimize the usage of plate anchors in the design of pullout-resistant structures.

To predict strain localization behaviors of granular soils, the modified Cam-Clay (MCC) model is incorporated into the second-order cone programming optimized micropolar continuum finite-element method (mpcFEM-SOCP). Based on a cylindrical cavity expansion problem, a biaxial compression problem, and a rigid strip footing problem, the numerical analyses reveal that the nonphysical strain localization behaviors including mesh-dependency of shear band, rumpling, or bifurcation can be alleviated or even removed if mpcFEM-SOCP is implemented appropriately. Furthermore, the internal characteristic length in mpcFEM-SOCP is a macroscopic physical parameter that characterizes the microscopic response of soil particles and is utilized to model the shear band width. A comparison between mpcFEM-SOCP and discrete element method (DEM) is performed, and the analysis results disclose that the internal characteristic length is closely related to the median particle size, and the evolution trend of the local void ratio in the specimen predicted by mpcFEM-SOCP agrees well with that by DEM. A larger shear dilatancy, however, is generally simulated by the latter. Finally, in the undrained analysis of the rigid footing problem, the evolution curves of excess pore-water pressure predicted by standard finite-element method and mpcFEM-SOCP may differ to some extent, as they enable the observations on the interesting evolution behaviors of excess pore-water pressure.

Shanghai clay exhibits overconsolidation, soil structural, and small-strain characteristics. While there have been several studies on the effects of small-strain characteristics on the deformation caused by deep excavation, the effects of overconsolidation and structural behaviors on this deformation remain unexplored. Therefore, in this study, a constitutive model considering the aforementioned three characteristics is developed and then implemented in the commercial software program FLAC3D (version 7.00) to investigate the effects of overconsolidation and structural behaviors. Initially, validation and parametric study of the constitutive model are conducted based on a cylindrical numerical model. Subsequently, the constitutive model is validated by a well-documented case history of a deep excavation in Shanghai. Thereafter, the effects of overconsolidation and structural behaviors on wall deflection and ground settlement induced by deep excavation are investigated. It is observed that overconsolidation has a significant effect on the deformation induced by deep excavation, while the effect of soil structure is negligible. With an increase in excavation depth, the effect of overconsolidation on the deformation increases, and its effect on ground settlement is more significant than that on wall deflection. According to the results of the parametric study, it is revealed that by the time the shear strain induced by deep excavation reaches a maximum value, overconsolidation has partially degraded while soil structure has not yet collapsed, which accounts for the magnitude of the aforementioned effects induced by deep excavation.

A simplified theoretical approach for predicting the longitudinal behaviors of the existing shield tunnel related to over-crossing tunneling is developed by considering the circumferential joint effect. A simplified beam–spring model (SBSM) is established to describe the shield tunnel longitudinal behaviors, in which the segmental linings are simplified into Euler–Bernoulli short beams, and the circumferential joints are represented using the linear rotation and shearing springs. The SBSM can reflect the opening and dislocation of shield tunnels simultaneously. The existing shield tunnel is then regarded as the SBSM lying on the Pasternak foundation, and the displacement and joint deformations of the existing tunnel are obtained by the finite-difference method (FDM). By virtue of the FDM, the complex tunnel–soil interaction and ring-to-ring interaction can be easily transformed into the numerical solutions. The feasibility of the present approach is examined by two published cases and comparison with previous solutions. Parametric analyses are also conducted to explore the impacts of several dominant variables. The findings showed that the predicted tunnel displacements from the present solution are almost the same as those from the Timoshenko beam model and the measurements, but the present method predicts a slightly higher joint opening and lower dislocation between rings than the Timoshenko beam model. In addition, the present method takes the stiffness reduction at circumferential joints into consideration, which leads to the obtained tunnel displacement curves being neither smooth nor continuous. The rigid motion primarily appears at the lining rings, whereas the rotation and dislocation appear at the circumferential joints. The existing solutions always give continuous tunnel heave curves, which fails to truly reflect the discontinuous deformations at circumferential joints.

The combining of electroosmotic, vacuum, and surcharge preloading is an emerging technique for soft foundation treatment. Considering smear effects and free strain, an analytical solution for the radial consolidation of combined electroosmotic, vacuum, and surcharge preloading was derived based on the characteristic function method and Bessel function. The correctness of the proposed solution was verified by comparing with existing solutions and numerical results. On this basis, the influence of smear effects, vacuum pressure, surcharge load, and applied voltage on the consolidation characteristics of soil was further analyzed. The results showed that when the electroosmosis permeability coefficient of the undisturbed zone was greater than that of the smear zone, the excess pore-water pressure at the interface between the smear zone and the undisturbed zone increased in the early stage of consolidation owing to the electroosmotic effect. Vacuum pressure had a great influence on soil consolidation in the smear zone, while applied voltage had a great influence on the consolidation of soil in the undisturbed zone.

In undrained triaxial tests, membrane penetration varies with the change of effective confining pressure. Membrane compliance correction is a necessary step to accurately obtain the dynamic characteristics of soils in geotechnical tests. In this paper, a real-time instrument compensation equipment was adopted to eliminate membrane compliance The influence of membrane compliance on pore pressure was explained from experimental perspective, and a two-stage formula was proposed to establish the relationship between the optimized membrane correction coefficient C_m and the normalized pore pressure $U/{\sigma }_0^{\mathrm{\prime }}$. C, which is the constant value of C_m in the stationary segment, is analyzed under different confining pressures, relative densities, consolidation ratios, and gravel contents, and the power function of C with the characteristic particle size d₅₀ is proposed. Based on C_m, a series of predictions of pore pressure development without membrane compliance are implemented, which are basically consistent with the actual experimental results. The method to predict the pore pressure development without membrane compliance based on C_m is accurate and effective, and a large amount of experimental workload can be reduced for practical engineering applications.

An important characteristic of some clays is their abundance of fissures. In the case study reported here, to investigate how the fissure inclination angle affects the deformation and strength of fissured clay, samples of undisturbed fissured clay with different inclination angles of its inherent fissures (0°, 45°, and 90°) were subjected to consolidated undrained plane-strain shear tests using a true triaxial apparatus. Moreover, consolidated undrained triaxial tests were carried out on samples with the same inclination angles for comparison. The results showed that compared with the triaxial state, the degree of fissure influence on samples with different fissure angles is different under plane strain, which weakens the influence of the fissure inclination angle on the soil’s mechanical behavior. Under the designed consolidation pressures, the peak stress of the 45° fissured soil samples was the smallest, with a stress–strain curve that exhibits strain softening. The 0° fissured soil samples exhibited the highest peak stress, with a stress–strain curve that exhibits strain hardening. The 90° fissured soil samples fell in between, with a stress–strain curve that exhibits a relatively stable trend. The intermediate principal stress coefficient b-value showed different trends at different fissure angles, which also reflects the influence of fissure dip angle. According to the von Mises and Lade–Duncan strength criteria, the generalized plane-strain criterion for fissured soil was obtained. The dip angle of the shear band was calculated from Mohr–Coulomb theory, and the difference between the calculated and measured dip angles was found to be small.

Due to the unique conditions of the Wudong subvertical coal seam, damage to the underground roadway occurs frequently during the coal mining process. With the increase in mining depth, the energy and incidence of microseismic events increase significantly, showing characteristics of zoning and grading along the strike. Therefore, the damage to the underground roadway of the Wudong coal seam was studied with a focus on dynamic load disturbance. The results showed that the underground roadway damage exhibits asymmetric characteristics, with significant sinking on the north shoulder and various degrees of bulging on both sides of the roadway. The existing support scheme in the mine could not effectively control the deformation and damage to the rock surrounding the underground roadway under the action of dynamic loads. Therefore, the asymmetric support scheme with the high prestressed anchoring long and short anchor cable network was proposed to provide targeted reinforcement for the underground roadway, which has been shown to effectively mitigate the deformation of the surrounding rock. Eventually, considering the characteristics of microseismic event distribution and the observed increase in the rock damage with the increase of the seismic source energy, a segmented and asymmetric support scheme was suggested to tactically support the underground roadway. The findings of this study could potentially improve the ground safety of underground roadways under similar mining conditions.

The dynamic behaviors of subgrade soil are usually investigated by use of continuous loading mode in most studies; however, the dynamic loading induced by traffic loading is composed of cyclic loading and intermittent periods. Moreover, the existence of cyclic deviator stress, cyclic confining pressure, and shear stress has been already observed in the stress field induced by traffic loading. Recognizing this, intermittent cyclic loading was applied to saturated soft clay for this study. The impacts of cyclic deviator stress, cyclic confining pressure, and drained condition during intermittent periods on the deformation behaviors of soft soil were analyzed. The variations in strain increment were similar in all cases: as the number of loading stages increased, the strain increment decreased, and the difference in strain increment was more significant in the first loading stage although it could be ignored in subsequent loading stages. Furthermore, the strain increment increased with increasing cyclic stress ratio (CSR) and decreased with increasing cyclic confining pressure. Moreover, the dissipation of excess pore-water pressure induced during the cyclic loading period resulted in the increase of accumulated axial strain under intermittent partially drained conditions, while the recovery of specimen deformation during intermittent period led to the decreasing of accumulated axial strain under undrained conditions. In addition, an empirical formula of accumulated axial strain under intermittent cyclic loading was established, and the calculated results were consistent with the measured data.

Sandstone is an important and common engineering rock, and understanding the meso/macroscale mechanical properties of sandstones under different confinements are crucial for engineering safety. It is still difficult to correlate the mesoscale bonding parameters and macroscale mechanical behavior. In this paper, the Hoek–Brown parallel bond correlation model (HB-PBCM) is proposed based on the mesoscale analysis of the parallel bond model (PBM). The mathematical analytical expression of HB-PBCM is derived from the sensitivity analysis of the PBM parameters to the Hoek–Brown criterion strength parameters. The model is able to reproduce both the PBM and HB parameters obtained from laboratory tests. The parameters were validated via numerical tests, where the final strength and failure patterns of sandstones are shown to be consistent with those via real experiments. Combined with the numerical and experimental results, we find that the macroscale brittle‒ductile transition of sandstone is characterized by the alteration of the tensile-to-shear crack ratio, different mesoscale crack distributions, and mesoscale crack accumulation rates.