Performance of a Geogrid-Reinforced and Pile-Supported Highway Embankment over Soft Clay: Case Study

This paper describes a case history of a geogrid-reinforced and pile-supported (GRPS) highway embankment with a low area improvement ratio of 8.7%. Field monitored data from contact pressures acting on the pile and soil surfaces, pore-water pressures, settlements and lateral displacements are reported and discussed. The case history is backanalyzed by carrying out three-dimensional (3D) fully coupled finite-element analysis. The measured and computed results are compared and discussed. Based on the field observations of contact stresses and pore-water pressures and the numerical simulations of the embankment construction, it is clear that there was a significant load transfer from the soil to the piles due to soil arching. The measured contact pressure acting on the pile was about 14 times higher than that acting on the soil located between the piles. This transfer greatly reduced excess positive pore water pressures induced in the soft silty clay. The measured excess pore water pressure ratio max in the soft silty clay was only about 0.3. For embankment higher than 2.5?m, predictions of stress reduction ratio based on two common existing design methods are consistent with the measured values and the 3D numerical simulations. During the construction of the piled embankment, the measured lateral displacement–settlement ratio was only about 0.2. This suggests that the use of GRPS system can reduce lateral displacements and enhance the stability of an embankment significantly.     

Numerical Parametric Study of Piezocone Penetration Test in Clays

This paper presents a numerical model for the simulation of the piezocone penetration test that is used to carry out a parametric study of the piezocone penetration test in cohesive soils. The piezocone penetration is numerically simulated using an axisymmetric elasto-plastic large deformation finite-element analysis code. The numerical simulation is accomplished in two stages. First, the piezocone is expanded radially from an initial small radius (0.1ro) to the piezocone radius, ro, at the specified depth. Second, the continuous penetration of the piezocone is simulated by applying incremental vertical displacements of the nodes representing the piezocone boundary. The constraint approach is used to model the soil-piezocone interface friction. The Mohr-Coulomb frictional criterion is used to define the sliding potential of the nodes. The main objective of this paper is to present the numerical model and to investigate the effect of the lateral and vertical stresses and the overconsolidation ratio on the cone tip resistance and the developed excess pore pressure around the piezocone. The variation of the horizontal and vertical hydraulic conductivity coefficients on the developed spatial excess pore pressure and its dissipation are also investigated. The results of the numerical study are also compared with the miniature piezocone penetration tests in cohesive soil specimens conducted at the Louisiana State University Calibration Chamber. Results of this study are in good agreement with the measured values.     

Displacements and Stresses Due to a Uniform Vertical Circular Load in an Inhomogeneous Cross-Anisotropic Half-Space

In this work, we present the solutions for displacements and stresses along the centerline of a uniform vertical circular load in an inhomogeneous cross-anisotropic half-space with its Young’s and shear moduli varying exponentially with depth. The planes of cross anisotropy are assumed to be parallel to the horizontal surface. The presented solutions can be directly integrated from the point load solution in a cylindrical coordinate system, which were derived by the writers. However, the resulting integrals of the circular solution for displacements and stresses cannot be given in closed form; hence, numerical integrations are required. For a homogeneous cross-anisotropic half-space, the numerical results agree very well with the exact solutions of Hanson and Puja, published in 1996. Two examples are given to elucidate the effect of inhomogeneity, and the type and degree of soil anisotropy on the vertical displacement and vertical normal stress in the inhomogeneous isotropic/cross-anisotropic soils subjected to a uniform vertical circular load acting on the surface. The proposed solutions can more realistically simulate the actual stratum of loading problem in many areas of engineering practice.     

Finite-Element Parametric Study of the Consolidation Behavior of a Trial Embankment on Soft Clay

The paper presents a case study for numerical analysis of the consolidation behavior of an instrumented trial embankment constructed on a soft soil foundation. Details are given to the geological profile, field instrumentation, laboratory test results, and determination of soil parameters for numerical modeling. Embankment settlement is estimated based on one-dimensional consolidation analysis and nonlinear finite-element analysis following Biot’s consolidation theory. Finite-element results are calibrated against the measured field data for a period of more than 3?years. Development and dissipation of excess pore pressure, long-term settlement, and horizontal displacement are predicted and discussed in light of sensitivity of embankment performance to some critical factors through a parametric study.     

Analytical and Numerical Modeling of Soft Soil Stabilized by Prefabricated Vertical Drains Incorporating Vacuum Preloading

This paper describes the analytical formulation of a modified consolidation theory incorporating vacuum pressure, and numerical modeling of soft clay stabilized by prefabricated vertical drains, with a linearly distributed (trapezoidal) vacuum pressure for both axisymmetric and plane strain conditions. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are examined through average time-dependent excess pore pressure and consolidation settlement analyses. The plane strain analysis was executed by transforming the actual vertical drains into a system of equivalent parallel drain walls by adjusting the coefficient of permeability of the soil and the applied vacuum pressure. The converted parameters are incorporated in the finite element code ABAQUS, employing the modified Cam-clay theory. Numerical analysis is conducted to study the performance of a full-scale test embankment constructed on soft Bangkok clay. The performance of this selected embankment is predicted on the basis of four different vacuum pressure distributions. The predictions are compared with the available field data. The assumption of distributing the vacuum pressure as a constant over the soil surface and varying it linearly along the drains seems justified in relation to the field data.     

Consolidation of a Finite Transversely Isotropic Soil Layer on a Rough Impervious Base

A semianalytical solution to axisymmetric consolidation of a transversely isotropic soil layer resting on a rough impervious base and subjected to a uniform circular pressure at the ground surface is presented. The analysis uses Biot’s fully coupled consolidation theory for a transversely isotropic soil. The general solutions for the governing consolidation equations are derived by applying the Hankel and Laplace transform techniques. These general solutions are then used to solve the corresponding boundary value problem for the consolidation of a transversely isotropic soil layer. Once solutions in the transformed domain have been found, the actual solutions in the physical domain for displacements and stress components of the solid matrix, pore-water pressure and fluid discharge can finally be obtained by direct numerical inversions of the integral transforms. The accuracy of the present numerical solutions is confirmed by comparison with an existing exact solution for an isotropic and saturated soil that is a special case of the more general problem addressed. Further, some numerical results are presented to show the influence of the nature of material anisotropy, the surface drainage condition, and the layer thickness on the consolidation settlement and the pore pressure dissipation.     

Earth Dam on Liquefiable Foundation and Remediation: Numerical Simulation of Centrifuge Experiments

A series of four dynamic centrifuge model tests was performed to investigate the effect of foundation densification on the seismic performance of a zoned earth dam with a saturated sand foundation. In these experiments, thickness of the densified foundation layer was systematically increased, resulting in a comprehensive set of dam-foundation response data. Herein, Class-A and Class-B numerical simulations of these experiments are conducted using a two-phase (solid and fluid) fully coupled finite element code. This code incorporates a plasticity-based soil stress–strain model with the modeling parameters partially calibrated based on earlier studies. The physical and numerical models both indicate reduced deformations and increased crest accelerations with the increase in densified layer thickness. Overall, the differences between the computed and recorded dam displacements are under 50%. At most locations, the computed excess pore pressure and acceleration match the recorded counterparts reasonably well. Based on this study, directions for further improvement of the numerical model are suggested.     

Coupled Mechanical and Hydraulic Modeling of Geosynthetic-Reinforced Column-Supported Embankments

Geosynthetic-reinforced column-supported (GRCS) embankments have increasingly been used in the recent years for accelerated construction. Numerical analyses have been conducted to improve understanding and knowledge of this complicated embankment system. However, most studies so far have been focused on its short-term or long-term behavior by assuming an undrained or drained condition, which does not consider water flow in saturated soft soil (i.e., consolidation). As a result, very limited attention has been paid to a settlement-time relationship especially postconstruction settlement, which is critical to performance of pavements on embankments or connection between approach embankments and bridge abutments. To investigate the time-dependent behavior, coupled two-dimensional mechanical and hydraulic numerical modeling was conducted in this study to analyze a well-instrumented geotextile-reinforced deep mixed column-supported embankment in Hertsby, Finland. In the mechanical modeling, soils and DM columns were modeled as elastic-plastic materials and a geotextile layer was modeled using cable elements. In the hydraulic modeling, water flow was modeled to simulate generation and dissipation of excess pore water pressures during and after the construction of the embankment. The numerical results with or without modeling water flow were compared with the field data. In addition, parametric studies were conducted to further examine the effects of geosynthetic stiffness, column modulus, and average staged construction rate on the postconstruction settlement and the tension in the geosynthetic reinforcement.     

Vertical Vibration of a Flexible Plate with Rigid Core on Saturated Ground

In this paper, the vertical vibration of a flexible plate with rigid core resting on a semi-infinite saturated soil is studied analytically. The behavior of the soil is assumed to follow Biot’s poroelastodynamic theory with compressible soil skeleton and pore water, and the response of the time-harmonic excited plate is governed by the classical thin-plate theory. By virtue of the Hankel transform technique, the fundamental solutions of the skeleton displacements, stresses, and pore pressure are derived, and a set of dual integral equations associated with the relaxed boundary and completely drained condition at the soil-foundation contact interface are also developed. These governing integral equations are further reduced to the standard Fredholm integral equations of the second kind and solved by numerical procedures. Comparison with existing solutions for a rigid permeable plate on saturated soil confirms the accuracy of the present solution. Selected numerical results are presented to show the influence of the permeability, the size of the rigid core, and the plate flexibility on the dynamic interaction between the elastic plate with rigid core and the underlying saturated soil.     

Effect of Anisotropy and Destructuration on the Behavior of Murro Test Embankment

This paper investigates the influence of anisotropy and destructuration on the behavior of a test embankment on soft clay. The test embankment at Murro, Finland, was commissioned in 1993 by the Finnish Road Administration and has been monitored for over 10?years. The construction and consolidation of Murro test embankment is analyzed with finite element method using three different constitutive models to represent the soft soil. The results are compared with field observations. The constitutive models used include two recently proposed constitutive models, namely S-CLAY1 that accounts for initial and plastic strain induced anisotropy and its extension, called S-CLAY1S. The S-CLAY1S model accounts, additionally, for interparticle bonding and degradation of bonds. For comparison, the test embankment is also analyzed using the isotropic Modified Cam Clay model. The simulations demonstrate that for this type of problem, it is important to account for the anisotropy, whereas destructuration appears to have less influence on predicted deformations. However, only a model incorporating destructuration can explain the decrease in undrained shear strength during consolidation that was measured in field.     

Analytical and Numerical Modeling of Consolidation by Vertical Drain beneath a Circular Embankment

In the analysis of axisymmetric problems, it is often imperative that aspects of geometry, material properties, and loading characteristics are either maintained as constants or represented by continuous functions in the circumferential direction. In the case of radial consolidation beneath a circular embankment by vertical drains (i.e., circular oil tanks or silos), the discrete system of vertical drains can be substituted by continuous concentric rings of equivalent drain walls. An equivalent value for the coefficient of permeability of the soil is obtained by matching the degree of consolidation of a unit cell model. A rigorous solution to the continuity equation of radial drainage towards cylindrical drain walls is presented and verified by comparing its results with the existing unit cell model. The proposed model is then adopted to analyze the consolidation process by vertical drains at the Sk?-Edeby circular test embankment (Area II). The calculated values of settlement, lateral displacement, and excess pore-water pressure indicate good agreement with the field measurements.     

2D and 3D Numerical Modeling of Combined Surcharge and Vacuum Preloading with Vertical Drains

This paper presents a three-dimensional (3D) and two-dimensional (2D) numerical analysis of a case study of a combined vacuum and surcharge preloading project for a storage yard at Tianjin Port, China. At this site, a vacuum pressure of 80?kPa and a fill surcharge of 50?kPa were applied on top of the 20-m-thick soft soil layer through prefabricated vertical drains (PVD) to achieve the desired settlements and to avoid embankment instability. In 3D analysis, the actual shape of PVDs and their installation pattern with the in situ soil parameters were simulated. In contrast, the validity of 2D plane strain analysis using equivalent permeability and transformed unit cell geometry was examined. In both cases, the vacuum pressure along the drain length was assumed to be constant as substantiated by the field observations. The finite-element code, ABAQUS, using the modified Cam-clay model was used in the numerical analysis. The predictions of settlement, pore-water pressure, and lateral displacement were compared with the available field data, and an acceptable agreement was achieved for both 2D and 3D numerical analyses. It is found that both 3D and equivalent 2D analyses give similar consolidation responses at the vertical cross section where the lateral strain along the longitudinal axis is zero. The influence of vacuum may extend more than 10?m from the embankment toe, where the lateral movement should be monitored carefully during the consolidation period to avoid any damage to adjacent structures.     

Load-Factor Stability Analysis of Embankments on Saturated Soil Deposits

A continuum-based finite-element methodology is established for quantifying the stability of earthen embankments built on saturated soil deposits. Within the methodology the soil is treated as a fluid-solid porous medium, in which the soil skeleton's constitutive behavior is modeled using a smooth elastoplastic cap model that features continuous coupling between deviatoric and volumetric plasticity. In the stability analysis procedure, self-weight of the embankment soils is monotonically increased at rates characteristic of the embankment construction time, until instability mechanisms develop. The transient effects of excess pore pressures and their impact on soil strength are explicitly modeled, allowing for computation of embankment safety factors against instability as a function of construction rate. Details on the proposed method are presented and discussed, including (1) how the construction rate of an embankment can be modeled; (2) how load-based safety factors can differ from resistance-based safety factors; and (3) solved example problems corresponding to a case history of an embankment failure.     

Vertical Excitation of Stochastic Soil-Structure Interaction Systems

This paper considers two stochastic models for a soil-structure interaction problem with vertical propagation of P waves during strong earthquake motion. These models include the horizontal and vertical spatial variability of stiffness of the soil medium. The first model involves a two-dimensional stochastic Winkler foundation, which takes into account the horizontal variability of the soil. This model elucidates some experimental results obtained on a nuclear power station physical model built in Hualien (Taiwan). The second model is developed as a continuum system of random columns involving, this time, horizontal and vertical random characteristics of the soil medium. For both models a statistical analysis was performed with respect to determining probabilistic properties resonance frequencies and amplitudes of the corresponding transfer functions. The theoretical development and numerical results demonstrate the importance of considering soil variability for geotechnical design applications.     

Numerical Study of Reinforced Soil Segmental Walls Using Three Different Constitutive Soil Models

A numerical finite-difference method (FLAC) model was used to investigate the influence of constitutive soil model on predicted response of two full-scale reinforced soil walls during construction and surcharge loading. One wall was reinforced with a relatively extensible polymeric geogrid and the other with a relatively stiff welded wire mesh. The backfill sand was modeled using three different constitutive soil models varying as follows with respect to increasing complexity: linear elastic-plastic Mohr-Coulomb, modified Duncan-Chang hyperbolic model, and Lade’s single hardening model. Calculated results were compared against toe footing loads, foundation pressures, facing displacements, connection loads, and reinforcement strains. In general, predictions were within measurement accuracy for the end-of-construction and surcharge load levels corresponding to working stress conditions. However, the modified Duncan-Chang model which explicitly considers plane strain boundary conditions is a good compromise between prediction accuracy and availability of parameters from conventional triaxial compression testing. The results of this investigation give confidence that numerical FLAC models using this simple soil constitutive model are adequate to predict the performance of reinforced soil walls under typical operational conditions provided that the soil reinforcement, interfaces, boundaries, construction sequence, and soil compaction are modeled correctly. Further improvement of predictions using more sophisticated soil models is not guaranteed.     

Deformation of Pores in Viscoplastic Soil Material

Changes in soil pore volume and shape in response to internal and external mechanical stresses alter key soil hydrologic and transport properties. The extent of these changes is dependent on details of pore shape and size evolution. We present a model for quantifying rates of deformation and shape evolution of idealized spheroidal pores as functions of macroscopic stresses and soil rheological properties. Previous solutions for shrinkage of spherical pores embedded in a viscoplastic matrix under isotropic stress were extended to spheroidal pore shapes and biaxial stresses using Eshelby’s classical theory. Bulk soil behavior was obtained from upscaling of detailed single pore deformation. Results show that pore closure rates increase with decreasing initial aspect ratio (i.e., oblate pores close faster than spherical pores), and with higher deviatoric stress. Incomplete pore closure is attributed to soil hardening due to pore shape accommodation under biaxial stresses. The model provides a means for approximating pore deformation as input to predictive models for soil hydraulic properties.     

悬臂桩支护基坑开挖对邻近埋地管线的影响

采用MIDAS GTS软件建立了三维有限元模型,分析开挖深度、桩径、桩心距、管线与桩的距离、管线埋深、土体弹性模量等因素对地埋管线的影响.结果表明：管线水平和竖向位移在基坑角点处约为基坑中部的1/2;管线埋深在基坑深度1/3位置时,水平位移最大,而竖向位移随埋深的增大而减小;当桩径由0.6 m增大到1.2 m时,水平位移变化较小,管线中央竖向位移则减少为原来的1/2;土体弹性模量增大时,管线中间位移明显减小,水平位移约为竖向位移的4倍.     

Poromechanics of Anisotropic Hollow Cylinders

It has been known that inherent material anisotropy influences the mechanics of geoengineering applications. Aiming at the experimental studies associated with geoengineering applications in anisotropic materials, this paper proposes a poromechanics analysis of a fully saturated transversely isotropic hollow cylinder. Closed-form analytical solutions for the pore pressure and stress fields were derived. These solutions are obtained under various loading conditions that are encountered in laboratory testing procedures. Numerical analyses were carried out to demonstrate the material anisotropy effect on stress, displacement, and pore pressure distributions in the cylinder. It is also shown that uncertainties in the estimation or measurements of the poromechanical parameters have proven effects on the time-dependent responses of the hollow cylinder geometry during laboratory testing.     

Automatically Computing the Displacements and Stresses Induced by Three-Dimensional Arbitrarily Shaped Loads in a Transversely Isotropic Medium

Since the planes of foundations are not usually regularly shaped, and the loads are often applied on the anisotropic materials, such as transversely isotropic soils or rocks, calculating the induced displacements and stresses by an arbitrarily shaped load for a transversely isotropic medium is rather tedious and time consuming. Hence, how to estimate those values correctly and quickly by computer was the major objective in constructing the fast anisotropic displacements and stresses (FADAS). FADAS is based on the solutions of displacements and stresses in a transversely isotropic half space subjected to three-dimensional buried right-angled triangular loads, which were derived by the first writer. Utilizing these solutions, the displacements and stresses for a general triangular region at any point can be obtained by superposition. An illustrative example is given to demonstrate a few features of FADAS, and to elucidate how to compute the vertical displacement induced by a uniform vertical circular load in an equivalent medium. Results from FADAS reveal that the usage of it is correct, easy, and very fast to offer a good tool for practitioners.     

Response of Inhomogeneous Seabed around Buried Pipeline under Ocean Waves

Recently, considerable efforts have been devoted to the phenomenon of wave-seabed-pipeline interaction. However, conventional investigations for this problem have been concerned with a uniform seabed, despite the strong evidences of variable permeability and shear modulus. In this paper, a finite-element model is proposed to investigate the wave-induced pore pressure, effective stresses, and soil displacements in the vicinity of a buried pipeline in a porous seabed with variable permeability and shear modulus. The numerical results indicate that the inclusion of variable permeability and shear modulus significantly affects the wave-induced soil response around the pipeline. The influence of the variable permeability and shear modulus and geometry of the pipe on the wave-induced soil response around a buried pipeline are also detailed.