Showing papers in "International Journal of Geomechanics in 2016"

Influence of Particle Breakage on Critical State Line of Rockfill Material

Abstract: The influences of particle breakage on the position of the critical state line (CSL) were systematically investigated in this paper through a series of large-scale triaxial compression tests on Tacheng rockfill material (TRM). It was found that the critical-state stress ratio of TRM (i.e., the gradient of the CSL in the p−q space) was approximately regarded as a constant. In the e−logp space, the CSL of TRM descended with a decrease in the initial void ratio, whereas the gradient of the CSL was constant. A procedure was established for evaluating the critical state point at a same particle breakage, which comprised the breakage critical state line (BCSL). An increase of the particle breakage led to not only a vertical translation but also a rotation on the BCSL of TRM in the e−logp space, which was similar to the observation of Dog’s Bay sand. Consequently, the initial gradation (or the corresponding initial void ratio) was the dominant factor that affected the position of the CSL of TRM in the e−...

Pipelines Subjected to Fault Movement in Dry and Unsaturated Soils

Abstract: Because pipelines traverse large geographical areas, they frequently must cross active faults when constructed in locations vulnerable to earthquakes. In this study, the authors performed three-dimensional (3D) finite-element analyses to investigate the behavior of buried pipe subject to strike-slip fault movement in dry sand and, more realistically, in partially saturated sand. The performance of the finite-element model was first validated by comparing the computed results with the data from the full-scale experiments at Cornell University. The analysis was then extended by varying the initial conditions of the sand (e.g., sand type, density, moisture content), pipe material, pipe burial depth, and pipeline–fault-rupture inclination to assess the effect of these parameters on the soil loads applied to the pipe and the corresponding deformations. On the basis of the simulation results, the authors propose a soil–structure interaction mechanism for pipelines crossing active faults. The authors also propose design recommendations for the mitigation of ground-deformation effects at buried pipeline crossings of strike-slip faults.

Hydromechanical Modeling of Unsaturated Flow in Double Porosity Media

Abstract: Geomaterials with aggregated structure or containing fissures often exhibit a bimodal pore size distribution that can be viewed as two coexisting pore regions of different scales. The double-porosity concept enables continuum modeling of such materials by considering two interacting pore scales satisfying relevant conservation laws. This paper develops a thermodynamically consistent framework for hydromechanical modeling of unsaturated flow in double-porosity media. With an explicit treatment of the two pore scales, conservation laws are formulated incorporating an effective stress tensor that is energy-conjugate to the rate of deformation tensor of the solid matrix. A constitutive framework is developed on the basis of energy-conjugate pairs identified in the first law of thermodynamics, which is then incorporated into a three-field mixed finite-element formulation for double-porosity media. Numerical simulations of laboratory- and field-scale problems are presented to demonstrate the impact of d...

Face Stability Analysis for a Shield-Driven Tunnel in Anisotropic and Nonhomogeneous Soils by the Kinematical Approach

Abstract: The stability on a tunnel face is of great practical significance for safe construction in tunnel engineering. In this work, an advanced three-dimensional (3D) failure mechanism is extended to the face stability analysis of a tunnel driven in anisotropic and nonhomogeneous soils using the kinematic approach of limit analysis. The results obtained from the presented approach are in good agreement with the existing ones, showing that this method can be further applied to the proposed situation in this paper. Two common examples of cohesion variation, a linear variation with depth and a layered soil, are then analyzed. The numerical results show that both anisotropy and nonhomogeneity have a significant impact on the critical face pressure, especially when the cohesion is relatively large or when there is a weaker layer. Therefore, these two factors should be taken into account in tunnel design.

Effective Stress-Based Limit-Equilibrium Analysis for Homogeneous Unsaturated Slopes

Abstract: With a suction stress–based effective stress representation, stability analysis of unsaturated engineered and natural slopes can be performed effectively in the same manner as the classical limit-equilibrium (LE) methodologies. This paper presents an analytical framework for effective stress LE analysis of unsaturated homogeneous slopes under steady one-directional (vertical) flow. The proposed log spiral failure surface–based LE method involves only two additional hydromechanical parameters for unsaturated soil, approximating the inverse of the air-entry pressure and pore-size distribution. Both parameters are used to describe seepage and effective stress variations in unsaturated soils. Unlike most other LE formulations, the method is statically determinate. A parametric study was performed, and stability charts for general use are presented. The impact of infiltration and evaporation on the stability of slopes for four hypothetical soil types was studied. It is shown that the apparent cohesion ...

Numerical Study of the Mechanical Behavior of Nonpersistent Jointed Rock Masses

Abstract: Estimating the mechanical properties of nonpersistent jointed rock masses is one of the most challenging problems in practical rock engineering due to the complex interaction of rock joints and intact-rock bridges. In this paper, the effect of joint geometrical parameters of nonpersistent rock mass on uniaxial compressive strength (UCS) and the deformation modulus was studied by using the discrete-element particle flow code PFC3D. In this numerical approach, the intact material is represented by an assembly of spherical particles bonded together at their contact points, and the joint interface is explicitly simulated by slip surfaces that are applied at contacts between the particles lying on the opposite sides of the joint interface. The failure process is simulated by the breakage of bonds between particles. A previous study of the authors has shown that this approach is capable of reproducing the mechanical behavior of nonpersistent jointed rock masses by a comparative study against physical ex...

Effect of Earthquake on Combined Pile–Raft Foundation

Abstract: The combined pile–raft foundation (CPRF) has been widely recognized as economic and rational foundation for high-rise buildings when subjected to vertical loading because of its effectiveness in load sharing by both raft and pile components. This results in smaller total and differential settlements with a reduced number of piles as compared with group piles. Until recently, the behavior of CPRF when subjected to lateral and real earthquake loading conditions in addition to vertical loads has not been well understood as a result of the complexities involved in the interaction of the pile, soil, and raft under such loading considerations. In the present study, an attempt has been made to investigate the behavior of CPRF with the use of centrifuge testing and a numerical model under pseudostatic and dynamic loading conditions carried out with finite-element software. After successful validation of the present CPRF model with both centrifuge and numerical model results, the same model was used for fu...

Numerical Analysis of Large-Diameter Monopiles in Dense Sand Supporting Offshore Wind Turbines

Abstract: Large-diameter monopiles are widely used foundations for offshore wind turbines. In the present study, three-dimensional finite-element (FE) analyses are performed to estimate the static lateral load-carrying capacity of monopiles in dense sand subjected to eccentric loading. A modified Mohr–Coulomb (MMC) model that considers prepeak hardening, postpeak softening, and the effects of mean effective stress and relative density on stress–strain behavior of dense sand is adopted in the FE analysis. FE analyses are also performed with the Mohr–Coulomb (MC) model. The load–displacement behavior observed in model tests can be simulated better with the MMC model than with the MC model. On the basis of a parametric study for different length-to-diameter ratios of the pile, load–moment capacity interaction diagrams were developed for different degrees of rotation. A simplified model, based on the concept of lateral pressure distribution on the pile, is also proposed for the estimation of its capacity.

Experimental Analysis of Embankment on Ordinary and Encased Stone Columns

Abstract: This work investigated the behavior of embankment models resting on soft soil reinforced with ordinary and encased stone columns (ESCs). Model tests were performed with different spacing distances between stone columns and two length-to-diameter ratios (L/d) of the stone columns, in addition to different embankment heights. A total of 39 model tests were performed on soil with an undrained shear strength of ≈10 kPa. The system consisted of a stone column–supported embankment at different spacing-to-diameter ratios (s/d) of stone columns. Earth pressure cells were used to measure directly the vertical stress on the column for all models, and another cell was placed at the base of the embankment between two columns to measure directly the vertical stress in reinforced soft soil. For embankment models constructed on soft clay reinforced with ESCs, it was found that whether a column was floating or end bearing (resting on a rigid stratum), encasement of the column by a geogrid was most effective in im...

Observed Effects of Interparticle Friction and Particle Size on Shear Behavior of Granular Materials

Abstract: This paper presents an experimental study on the shear behavior of granular materials, focusing on the effects of interparticle friction and particle size, which are of fundamental importance but are not yet well understood. The experimental program consisted of a large number of direct shear tests on glass beads of varying sizes and interparticle friction conditions, performed under a range of packing densities and normal stress levels. Test data were interpreted in terms of the stress–dilatancy relationship and shear strength parameters. The study indicates that under otherwise similar testing conditions, oil-lubricated glass beads tend to have substantially lower shear strength as compared with water-lubricated, water-flooded, and dry glass beads. It has also been found that at similar particle size uniformity, increasing mean particle size (d50) leads to more dilatant shear response and higher shear strength. A generalized stress–dilatancy relation is proposed, which introduces a variable dila...

State-of-the-Art: Prediction of Resilient Modulus of Unsaturated Subgrade Soils

Abstract: In this paper, equations that were proposed in the literature over the past four decades to estimate or predict the variation of the resilient modulus with respect to soil suction for pavement base-course materials and subgrade soils are summarized into three groups: (1) empirical relationships, (2) constitutive models incorporating the soil suction into applied shearing or confining stresses, and (3) constitutive models extending the independent stress state variable approach. Two equations selected from each of the groups (a total of six equations) are used to predict the resilient modulus–soil suction correlations for three compacted subgrade soils. Strengths and limitations of these widely used equations are discussed based on the comparisons between the measurements and predictions. The key objective of the state-of-the-art research summarized in this paper is for assisting practicing engineers to choose suitable equations for the rational prediction of the resilient modulus taking into accou...

Fully Coupled Thermo-Hydro-Mechanical Double-Porosity Formulation for Unsaturated Soils

Abstract: This work presents a fully coupled formulation developed to handle engineering problems in unsaturated (and saturated) soils that present two dominant void levels. The proposed framework assumes the presence of two porous media linked through a mass-transfer term between them. In its more general form, the proposed approach allows the consideration of nonisothermal multiphase flow coupled with the mechanical problem. The double-porosity formulation was implemented in a finite-element code and has been used to analyze a variety of engineering problems. The approach is especially suitable for cases in which the material exhibits a strong coupling between the mechanical and the hydraulic problems in both media, such as with swelling clays. For those types of problems, the proposed formulation is used in conjunction with the mechanical double-structure model already proposed by the authors. This paper presents the coupled formulation and the application of the proposed approach to problems involving e...

Pitfalls in Interpretation of Gravimetric Water Content–Based Soil-Water Characteristic Curve for Deformable Porous Media

Abstract: Pitfalls in the common approaches to obtaining material parameters from soil-water characteristic curves (SWCCs) are discussed, and the typical mistakes made in the literature are highlight...

Unified size-effect law for intact rock

Abstract: A suite of laboratory testing was performed on Gosford sandstone samples having a range of sizes, including point-load and uniaxial compressive tests A unified size-effect law (USEL), based on the work by Zdenek Bazant, involving fracture energy as well as fractal theories, was introduced It was shown that USEL correlates well with the ascending and descending uniaxial compressive strength trends obtained from Gosford sandstone as well as five other rock types reported by Brian Hawkins Fractal characteristics found to be the primary mechanism for ascending strength trends and surface flaws could be considered as a secondary mechanism The influence of the contact area on the size-effect behavior of point-load results was investigated using a new approach This approach was novel in the way it incorporated the load contact area Determination of the point-load strength index using this new approach led to opposite size-effect trends compared with those observed using a conventional point-load st

One-Dimensional Consolidation Analysis of Unsaturated Soils Subjected to Time-Dependent Loading

Abstract: During consolidation, excess pore-air and pore-water pressures are forced to dissipate through permeable boundaries. This dissipation process inevitably results in the reduction of the soil volume, and thus settlement. Such a phenomenon can be mathematically described by inhomogeneous governing equations of flow according to Fick’s (with respect to air phase) and Darcy’s (with respect to water phase) laws. This paper discusses the dissipation of excess pore-air and pore-water pressures and settlement of an unsaturated soil layer subjected to various time-dependent external loadings. An analytical solution is derived from the governing flow equations with respect to air and water using eigenfunction expansion and Laplace-transform techniques. Eigenfunctions and eigenvalues are parts of the general solution and can be obtained using one-way or two-way drainage boundary conditions. On the other hand, four types of external loadings, namely ramping, asymptotic, sinusoid, and damped sine wave, are math...

Analytical Prediction of Ground Movements due to a Nonuniform Deforming Tunnel

Abstract: An analytical solution is presented to predict ground movement induced by a nonuniformly deforming circular tunnel in a heavy elastic half-plane. Simple expression is proposed to describe the nonuniform soil deformation around the tunnel. The tunnel-deforming components (i.e., ground loss, ovalization, vertical translation) can be determined by the solution-inherent mechanism and by fitting from measured field data. The effects of tunnel-deforming components on the resulting displacement field are discussed. Case studies from various ground conditions were used to check the capacity and applicability of the proposed solution. Although elasticity is a rough representation of the soil behavior, the proposed analytical solution can serve as a simple tool for predicting a reasonable ground settlement profile in the preliminary design of tunnels.

Deformation Response of an Existing Tunnel to Upper Excavation of Foundation Pit and Associated Dewatering

Abstract: Deep excavation clearly impacts existing underlying subway tunnels and threatens their operational safety. The prediction of existing tunnel deformation induced by nearby excavation is a major concern for urban construction. This research presents an analytical calculation method for predicting tunnel deformation induced by upside excavation and also discusses the role of dewatering in the deformation mechanism. First, the existing tunnel is assumed to be nonexistent in the soil, and the vertical unloading stress at the location of the existing tunnel caused by the upper excavation and associated dewatering is calculated. Second, the existing underlying tunnel is simplified as an elastic beam on a Pasternak foundation to calculate its vertical deformation. The proposed method was verified by the good agreement found between the predictions and the field measurements of the construction in the Shenzhen Chegongmiao hub project. Such an analytical method can provide fast and accurate evaluation resul...

Fully Coupled XFEM Model for Flow and Deformation in Fractured Porous Media with Explicit Fracture Flow

Abstract: A hydromechanical model with explicit fracture flow is presented for the fully coupled analysis of flow and deformation in fractured porous media. Extended finite-element method (XFEM) was utilized to model the fracture discontinuity in the two-dimensional plane-strain mechanical model. Two flow models, a one-dimensional laminar flow within the fracture and a two-dimensional Darcy flow through porous media, were considered. The flow domains were coupled through a mass exchange term (leak-off) accounting for discontinuous Darcy flow velocity across the fracture. Particular attention was given to the coupling of the flow domains with the mechanical model. Spatial and temporal discretization was achieved using the standard Galerkin method and the finite-difference technique, respectively. Unlike the successive coupled models in which the results of the mechanical model are used to update the fracture flow model and vice versa, the fully coupled hydromechanical formulation is solved simultaneously. Th...

Modeling the Mechanical Behavior of Carbonate Sands Using Artificial Neural Networks and Support Vector Machines

Abstract: Carbonate sands that are specific soils have some unusual characteristics, such as particle crushability and compressibility, that distinguish their behavior from other types of soil. Because of their large diversity, they have a wide range of mechanical behavior. Recently, there have been many attempts to predict the mechanical behavior of carbonate sands, but all these attempts have been focused on experimental and case studies of some specific soils, and there is still no unique method that can consider all types of carbonate sands behavior and describe their various aspects. In the present study, two artificial intelligence-based models, namely artificial neural networks and support vector machines are used together and comparatively to predict the mechanical behavior of different carbonate sands. The models were trained and tested using a database that included results from a comprehensive set of triaxial tests on three carbonate sands. The predictions of the proposed models were compared wit...

Behavior of Geosynthetic Reinforced Soil Foundation Systems Supported on Stiff Clay Subgrade

Abstract: A series of laboratory model tests was performed to investigate the behavior of geosynthetic reinforced stiff clay foundation systems under circular loading. The footing consisted of a rigid circular steel plate with a diameter of 150 mm. Five different series of tests were performed in both homogeneous (clay or sand) and layered configurations. The tests used planar geogrid and three-dimensional geocell reinforcements. Test results indicate that both types of reinforcements substantially improve the performance of the stiff clay foundation bed. A maximum threefold improvement was observed in bearing pressure, depending on the reinforcement type. However, geocell was found to be the most advantageous soil reinforcement technique, giving maximum performance improvement.

Stress-Dilatancy Relationship of Zipingpu Gravel under Cyclic Loading in Triaxial Stress States

Abstract: In this study, cyclic triaxial tests were performed on Zipingpu gravel, and they were followed by a discrete element study that was designed to investigate the stress-dilatancy relationship of gravelly soils under cyclic loading in triaxial stress states. Several conclusions emerged from the results. (1) A nearly linear relationship was found between the stress ratio η=q/p and the dilatancy ratio Dp=devp/desp under both conventional compression and extension monotonic loading. The slope parameter α, which relates η and Dp, was smaller during triaxial compression; (2) The stress-dilatancy relationship was different during the virgin and cyclic loading. The dilatancy line under cyclic loading was located inside the virgin/monotonic loading dilatancy lines, and the dilatancy relationship was related to the location of the most recent load reversal point; and (3) A nearly parallel linear relationship was found between η and Dp in the dη>0 and dη<0 paths under cyclic loading, with α smaller under cycli...

Particle Flow Modeling of Rock Blocks with Nonpersistent Open Joints under Uniaxial Compression

Abstract: In this study, numerical simulation of rock blocks with nonpersistent open joints under uniaxial compression was undertaken using the particle flow modeling method. First, the micromechanical parameter values of intact material were calibrated through a trial-and-error process using macromechanical laboratory test results. Then, a back-analysis procedure was used to calibrate the joint gap and joint micromechanical parameter values using laboratory test results conducted on jointed rock blocks. Afterward, the effects of joint dip angle, joint persistency, and joint gap on the mechanical behavior of block models having nonpersistent open joints was investigated using the calibrated micromechanical parameter values. The joint dip angle and joint persistency were found to play significant roles in the failure mode, strength, and stress–strain relationship of jointed blocks. The joint gap played a significant to negligible role in the mechanical behavior of jointed block models gradually when the join...

Model development and experimental verification for permeability coefficient of soil-rock mixture

Abstract: A soil–rock mixture consists of soil and broken rocks mixed proportionally, and it has more complicated seepage characteristics than pure soil or broken rocks. Current research on the seepage characteristics of soil–rock mixtures is still limited to empirical equations. This paper presents an improved theoretical compound seepage model for the permeability coefficient of soil–rock mixtures and relative concentrations of individual constituents (i.e., pure soil and pure broken rocks). This model considers the gap filled by particle grains in a soil–rock mixture under the same compaction level, as well as the reduced porosity of the mixture and the decreased permeability caused by the fine particles serving as a filler. A revised version of the seepage model of soil–rock mixtures consisting of a combination of the proposed model and the Kozeny-Carman seepage model is also suggested, and the precision of the model is verified against experimental results. The model provides a straightforward and effe...

Calibration and Verification of Two Bonded-Particle Models for Simulation of Intact Rock Behavior

Abstract: In this study, bonded-particle models were used to simulate the intact rock behavior of Westerly granite. Two three-dimensional models were tested: the enhanced bonded-particle model (EBPM) and the flat-joint contact model (FJCM). To calibrate both models, a new standard procedure was developed. The results of the simulations of the models were compared with the intact rock behavior determined by laboratory tests, including elastic parameters, peak and threshold envelopes, and postpeak behavior. From this study, it can be concluded that the FJCM better represents the mechanical behavior of Westerly granite rock, especially at the low to intermediate confining pressures. Based on this work, it is recommended to use the FJCM to model the behavior of brittle rock at the low to intermediate confining pressures.

Assessment of the Elastic-Viscoplastic Behavior of Soft Soils Improved with Vertical Drains Capturing Reduced Shear Strength of a Disturbed Zone

Abstract: Soil disturbance induced by the installation of vertical drains reduces the horizontal soil permeability and shear strength in the disturbed zone. Thus, the soil disturbance contributes to the reduced overconsolidation ratio (OCR) of the soil in the vicinity of drains, influencing soil deformation. Although a significant amount of research has been conducted on the effect of permeability variations in the smear zone, the influence of the reduced shear strength in the smear zone on the ground behavior has not been investigated. In this study, a numerical solution adopting an elastic-viscoplastic model with nonlinear creep function in combination with the consolidation equations has been developed. Moreover, the effects of shear strength variation in the disturbed zone on the time-dependent behavior of soft soil deposits improved with vertical drains and preloading have been studied. The applied elastic-viscoplastic model is based on the framework of the modified Cam-clay model, capturing soil creep...

Vertical Vibration of a Pipe Pile in Viscoelastic Soil Considering the Three-Dimensional Wave Effect of Soil

Abstract: An analytical solution is developed in this paper to investigate the vertical vibration of a pipe pile in viscoelastic soil. The soil is assumed to be a homogeneous and isotropic layer. The pipe pile is considered as a one-dimensional Euler rod. Considering both the vertical and radial displacements of the outer and inner soils and the soil pile–coupled vibration, the dynamic equilibrium equations of the soil and pile are established. The dilatations of the outer and inner soil are obtained by differential transformation on the equations of soil and the variable separation method. Then, the vertical and radial displacements of the outer and inner soils are obtained. The displacement response and impedance function of the pipe pile are derived using the continuity assumption of the displacement and stress between the piles and the soils. Numerical examples are presented to analyze the vibration characteristics of the pile.

Thermomechanical Response of Geothermal Energy Pile Groups in Sand

Abstract: The thermomechanical response of geothermal energy pile groups in sand was studied in the present work by using a three-dimensional nonlinear finite-element analysis procedure. A raft was considered on the piles. The stress-strain responses of the piles and raft were considered linear elastic. The stress-strain response of sand was reproduced by using the state parameter-based constitutive clay and sand model (CASM). The CASM was implemented in finite-element software through a user-defined material subroutine. The geothermal energy pile group was analyzed for different combinations of thermal and nonthermal piles in the group in a single layer of sand. Analysis results were studied for the displacement at the base of the pile and the axial stresses in the pile. Parametric sensitivity studies were performed by varying the spacing between the piles. It can be observed from the results that the geothermal piles, when subjected to heating, exhibit higher displacement and axial stress than the piles u...

Determination of active earth pressure on rigid retaining wall considering arching effect in cohesive backfill soil

Abstract: A new simplified method is proposed to compute the active earth pressure acting on the backface of a rigid retaining wall undergoing horizontal translation. The effect of soil arching for cohesive backfill soil and friction mobilized along the wall–soil interface was considered. Analytical expressions to determine the slip surface angle and the coefficient of active earth pressure were obtained using the limit-equilibrium approach. These expressions were used for the horizontal flat-element method to obtain theoretical formulas for the active earth pressure, active thrust, and its point of application. Additionally, an implicit solution was derived for the depth of the tension crack in the backfill. A parametric study was undertaken to assess the effects of cohesion, unit weight, friction angle, surcharge pressure, and wall–soil friction angle on the active earth pressure, as well as the effects of the friction angle of backfill soil and wall–soil friction angle on the slip surface angle. The resu...

Bounding Surface Constitutive Model for Cemented Sand under Monotonic Loading

Abstract: This paper presents a critical state constitutive model for cemented sand. The model uses a single capped yield surface as a function of the void ratio, confining pressure, preconsolidation pressure, and stress ratio at the peak of the undrained effective stress path. To model the cemented materials, the formulation of the yield function, elastic moduli, plastic modulus, flow rule, and other components of the model have been modified. Having incorporated the tensile strength and cohesion, the radial-mapping formulation of the bounding surface plasticity is incorporated in the model. The modified model has been calibrated and verified based on experimental results.

Multiple-Liquefaction Behavior of Sand in Cyclic Simple Stacked-Ring Shear Tests

Abstract: Following major earthquakes that occurred in New Zealand (2010–2011) and Japan (2011), soil multiple liquefaction, or reliquefaction, regained major attention in the field of geotechnical earthquake engineering. Not only can liquefaction occur multiple times at the same site, but the devastation caused by reliquefaction is often more severe than that triggered by the first liquefaction. In this study, to address this issue and provide new insights into reliquefaction mechanisms, a series of cyclic simple shear tests was conducted with the use of a newly developed stacked-ring shear apparatus. In the multiliquefaction tests, subsequent liquefaction stages were applied to a single Toyoura sand specimen sheared at different levels of maximum shear strain double amplitude (γDAmax), from 2% to 10%. Tests results showed that: (1) the increase in soil density during the postliquefaction reconsolidation stages had only a minor effect on sand resistance against multiple liquefaction; (2) the extent of γDAm...