Elasto-plastic Soil Research Articles

Physical modelling of pipe embedment and equalisation in clay

Pipelines laid on the seabed are subjected to loads that may cause unacceptable displacements. On fine-grained soils, the capacity of a pipeline to resist these loads is affected by the pipe embedment and any excess pore pressures remaining in the surrounding soil from the laying process. This paper presents results from model tests, performed at near to full scale, investigating the embedment response and the subsequent pore pressure equalisation of a pipeline on a high plasticity marine clay. Existing models for the penetration and dissipation processes are compared with the experimental data. Conventional undrained bearing capacity theory, making minor allowances for strain rate and softening effects, shows good agreement with the observed penetration response. Dissipation solutions based on elastic and elasto-plastic soil models capture the general shape of the pore pressure response. The operative coefficient of consolidation varies between tests, spanning the range between the compression and recompression values observed in oedometer tests. The observations validate the theoretical solutions for penetration resistance, and highlight the uncertainty that must be considered in estimating equalisation times.

Co-seismic and post-seismic behavior of a wall type breakwater on a natural ground composed of liquefiable layer

The tsunami disaster countermeasures such as breakwaters might be damaged or lose their capacity to resist tsunami after a strong earthquake. Therefore, not only the co-seismic behaviors of a breakwater and its foundation system during earthquake loading, but also the post-seismic behavior after the earthquake loading should be investigated and estimated for future tsunami preparation, especially when the foundation ground is composed of liquefiable layer, which may cause large amount of displacement to the breakwater. In this study, the co-seismic and post-seismic behavior of an existing wall type breakwater on a natural ground which is composed of nonuniform liquefiable layer and thick cohesive layer with low permeability is investigated using an effective stress-based soil–water coupling numerical method. In the calculation, the complicated nonlinear dynamic behavior of the foundation soil is described by an advanced elasto-plastic soil constitutive model. A real recorded seismic wave, which consists of a major shock and two aftershocks from the 2011 Great East Japan earthquake, is adopted as the input earthquake loading. The calculation results indicate that the used numerical method is capable of capturing the progressive liquefaction and consolidation process of the foundation soil, as well as the subsidence and differential settlement of the breakwater during and after earthquake loading. The influence of earthquake loading can significantly reduce the capacity of breakwater to resist tsunami which may arrive within 1 h after earthquake; therefore, anti-seismic design should be taken into consideration in future tsunami preparation.

Behavior of piled rafts overlying a tunnel in sandy soil

The present research presents experimental and finite element studies to investigate the behavior of piled raft-tunnel system in a sandy soil. In the experimental work, a small scale model was tested in a sand box with load applied vertically to the raft through a hydraulic jack. Five configurations of piles were tested in the laboratory. The effects of pile length (L), number of piles in the group and the clearance distance between pile tip and top of tunnel surface (H) on the load carrying capacity of the piled raft-tunnel system are investigated. The load sharing percent between piles and rafts are included in the load-settlement presentation. The experimental work on piled raft-tunnel system yielded that all piles in the group carry the same fraction of load. The load carrying capacity of the piled rafttunnel model was increased with increasing (L) for variable (H) distances and decreased with increasing (H) for constant pile lengths. The total load carrying capacity of the piled raft-tunnel model decreases with decreasing number of piles in the group. The total load carrying capacity of the piles relative to the total applied load (piles share) increases with increasing (L) and the number of piles in the group. The increase in (L/H) ratio for variable (H) distance and number of piles leads to an increase in piles share. ANSYS finite element program is used to model and analyze the piled raft-tunnel system. A three dimensional analysis with elastoplastic soil model is carried out. The obtained results revealed that the finite element method and the experimental modeling are rationally agreed.

Critical State for Anisotropic Granular Materials: A Discrete Element Perspective

AbstractCritical-state failure is a salient feature for particular materials, such as granular media and soils, whereas the critical-state theory (CST) is widely accepted for understanding and interpreting the behavior of a range of materials under complex loading conditions. In elastoplastic soil modeling, the critical state is also a favorable reference state for capturing and simulating the soils’ responses, and thus many constitutive models have been developed within the CST framework. Notwithstanding this, the uniqueness of the critical state has evoked increasing debate for many years and remains an unresolved issue. The central difficulty lies in how to integrate fabric anisotropy into the delineation of the critical-state failure. Motivated by the recent development of the anisotropic critical-state theory, this paper presents a discrete element investigation of the critical-state behavior of anisotropic granular materials. Samples with different degrees of initial fabric anisotropy are sheared to...

Thermoplastic Analysis of a Thermoactive Pile in a Normally Consolidated Clay

AbstractThe use of shallow geothermal systems for buildings and infrastructure conditioning has numerous recognized environmental and potential economic benefits and is expected to contribute to climate change mitigation. These systems are already used in many countries and are foreseen in many more. The study of their behavior is a subject of current research interest, including thermoactive or energy geostructures. In this work the mechanical effects of a thermal action, reflecting seasonal atmospheric conditions prevalent in Lisbon, Portugal, on a single 20-m-long thermoactive floating pile with different load levels, embedded in a normally consolidated saturated clay (speshwhite kaolin), are evaluated. For this purpose a critical state elastoplastic soil model with thermal hardening was formulated, implemented, and calibrated with experimental results for thermal expansion available in the literature. The numerical analyses performed revealed that the mechanical effects on the pile caused by thermal a...

Relationship between small and large strain solutions for general cavity expansion problems in elasto-plastic soils

This paper presents a closed-form relationship between small and finite strain cavity expansion solutions. Its derivation is based on the non-linearly elastic–perfectly plastic cylindrical (or spherical) problem considering a general Mohr’s criterion and constant plastic dilatancy. It is shown, however, that it is sufficiently accurate for general expansion problems not obeying plane-strain rotationally (or spherically) symmetric conditions and involving strain-hardening/softening constitutive behaviour. Therefore, this relationship quantifies the error stemming from the computational assumption of small deformations and provides a simple and efficient way of accounting for geometric non-linearity based entirely on conventional computational methods: ‘self-correction’ of small strain analyses results.

A case study on the seismic performance of earth dams

The seismic non-linear behaviour of earth dams is investigated by using a well-documented case study and employing advanced static and dynamic coupled-consolidation finite-element analysis. The static part of the analysis considers the layered construction, reservoir impoundment and consolidation, whereas the dynamic part considers the response of the dam to two earthquakes of different magnitude, duration and frequency content. The results of the analysis are compared with the recorded response of the dam and exhibit a generally good agreement. The effects of the narrow canyon geometry, the reservoir impoundment and the elasto-plastic soil behaviour on the seismic dam behaviour are investigated. Finally the implications of the adopted constitutive modelling assumptions on the predicted response are discussed.

Recent developments of the Material Point Method for the simulation of landslides

The paper describes first the theoretical framework of a “single layer” three phase material. The formulation is general and particular cases are dry and fully saturated soils. The formulation and discretization of the motion and balance equations is presented. Two constitutive equations are used in the applications described: A brittle model for saturated soils and a Mohr-Coulomb elastoplastic soil formulated in terms of net stress and suction. Three aspects of the behaviour of landslides are discussed: first time failures in over-consolidated clays; internal shearing in deep seated landslides and rain induced failures in unsaturated slopes. The discussion is supported by three real cases which are described and analyzed in detail.

Comparative analysis of methods for modeling the penetration and plane–parallel motion of conical projectiles in soil

This paper presents an analysis of the accuracy of known and new modeling methods using the hypothesis of local and plane sections for solution of problems of the impact and plane–parallel motion of conical bodies at an angle to the free surface of the half-space occupied by elastoplastic soil. The parameters of the local interaction model that is quadratic in velocity are determined by solving the one-dimensional problem of the expansion of a spherical cavity. Axisymmetric problems for each of the meridional section are solved simultaneously neglecting mass and momentum transfer in the circumferential direction and using an approach based on the hypothesis of plane sections. The dynamic and kinematic parameters of oblique penetration obtained using modified models are compared with the results of computer simulation in a three-dimensional formulation. The results obtained with regard to the contact stress distribution along the generator of the pointed cone are in satisfactory agreement.

Parametric study on performance of laterally loaded drilled shafts in an MSE wall

Drilled shafts are sometimes built in an MSE wall to support superstructures subjected to lateral loads. However, current design methodology isolates the interaction between drilled shafts and MSE walls, and the designs are independent. This design practice results in inappropriately designed drilled shafts and MSE walls. A three-dimensional numerical model of drilled shafts within an MSE wall was developed using FLAC3D and was calibrated with published data of a full-scale field study. Then the calibrated model was used for a parametric study to investigate the influence of various parameters on the synergistic performance of the drilled shafts and the MSE wall. The performance was assessed in terms of lateral displacement of the drilled shaft and MSE wall, the induced lateral earth pressure, and the induced strain in the geogrid. The investigated parameters in this study included the backfill material properties, the geogrid tensile stiffness and length, the distance between the drilled shaft and the MSE wall, and the length of the drilled shaft. An elastoplastic soil constitutive model, able to consider the compression and shear hardening, was used for the backfill material. The facing of the MSE wall was simulated as an assembly of discrete blocks which interacted with each other through interfaces. It was found that the properties of the backfill material, the distance between the drilled shaft and MSE wall, and the length of the drilled shaft had influence on the deflections, lateral earth pressure and strain in the geogrid. The extent of the influence varied and depended on the loads. The geogrid tensile stiffness and length did not show salient influence.

Dissipation consistent fabric tensor definition from DEM to continuum for granular media

In elastoplastic soil models aimed at capturing the impact of fabric anisotropy, a necessary ingredient is a measure of anisotropic fabric in the form of an evolving tensor. While it is possible to formulate such a fabric tensor based on indirect phenomenological observations at the continuum level, it is more effective and insightful to have the tensor defined first based on direct particle level microstructural observations and subsequently deduce a corresponding continuum definition. A practical means able to provide such observations, at least in the context of fabric evolution mechanisms, is the discrete element method (DEM). Some DEM defined fabric tensors such as the one based on the statistics of interparticle contact normals have already gained widespread acceptance as a quantitative measure of fabric anisotropy among researchers of granular material behavior. On the other hand, a fabric tensor in continuum elastoplastic modeling has been treated as a tensor-valued internal variable whose evolution must be properly linked to physical dissipation. Accordingly, the adaptation of a DEM fabric tensor definition to a continuum constitutive modeling theory must be thermodynamically consistent in regards to dissipation mechanisms. The present paper addresses this issue in detail, brings up possible pitfalls if such consistency is violated and proposes remedies and guidelines for such adaptation within a recently developed Anisotropic Critical State Theory (ACST) for granular materials.

Influence of the void ratio and the confining on the static liquefaction in slopes in shangi sand

Se presenta un estudio numérico del inicio de la licuación estática en taludes, bajo condiciones no drenadas de carga, basado en un criterio de inestabilidad general para suelos elastoplás-ticos, fundamentado en el concepto de pérdida de controlabilidad. Se aplica el criterio al caso de carga axisimétrica, para detectar el punto de inicio de licuación con un modelo elastoplástico para arenas. Se comparan los resultados numéricos con evidencia experimental, encontrando un buen nivel de concordancia. Se presenta un estudio cuantitativo de la influencia de la presión media, relación de vacíos y la anisotropía inicial de esfuerzos sobre el inicio de la licuación en la arena de Changi. Se concluye que: a) el ángulo de fricción movilizado al inicio de la licuación no es una propiedad del material, sino que es una variable de estado; b) a pesar de las múltiples variables involucradas en el proceso de generación de inestabilidad no drenada, el estado de esfuerzos en el inicio de la licuación estática se puede representar convenientemente por una relación lineal entre Aq/p0 y r|0. Esta representación gráfica se puede usar en la práctica de la ingeniería geotécnica para cuantificar un margen de seguridad contra licuación estática de un talud arenoso.

ВЛИЯНИЕ РАСПРЕДЕЛЕНИЯ НАПРЯЖЕНИЙ ВДОЛЬ ОБРАЗУЮЩЕЙ НА ПАРАМЕТРЫ ДВИЖЕНИЯ КОНИЧЕСКИХ УДАРНИКОВ В ГРУНТОВОЙ СРЕДЕ

Plane parallel motion of conical bodies in an elastoplastic soil is modelled using the locality hypothesis in the frame of the local interaction model. The parameters of the local interaction model with a quadratic velocity are determined based on the solution of a one-dimensional problem of the constant-velocity expansion of a spherical cavity from a point in an infinite medium. The model of the elastoplastic medium includes linear relationships «pressure-volume deformation» and «yield strength-pressure». Earlier, inertial motion of a conical striker normal and oblique to the free surface of the half-space occupied by the elastoplastic soil medium was studied. It was found that the local interaction model with a quadratic velocity adequately enough describing the axisymmetric motion of the striker is also applicable for representing the initial stage of oblique penetration into the soil. The present article demonstrates that the angular rotation velocity depends on the distribution along the generatrix of the cone in an oblique impact. The behavior of angular velocities in time is analyzed; satisfactory quantitative and qualitative agreement is noted for the results obtained in 3D computations and in the frame of the modal interaction model accounting for the inconstant distribution of contact pressures along the generatrix of an acute cone. Keywords: conical striker, impact, oblique penetration, elastiolastic medium, local interaction model, 3D modeling.

292 粒子法による弾塑性地盤上のフーチング支持力解析

Nonlinear FEM analysis is necessary for analyzing the concentrated shear strain at the end of a Footing, which is put on elasto-plastic soil. An application of Particle method is evaluated by the comparison with nonlinear FEM analysis under the Drucker-Prager model.

Modeling stone column installation in an elasto-viscoplastic soil

The majority of numerical studies investigating stone column performance have used “wished-in-place” columns (no installation effects) in conjunction with elastoplastic soil models (no viscous effects). In the first part of this paper, cylindrical cavity expansion (CCE) is used in conjunction with the PLAXIS 2D elasto-viscoplastic soft soil creep (SSC) model to evaluate the effect of creep on column installation. Creep leads to lower post-installation lateral earth pressure coefficients (K) than if primary consolidation was considered alone, although the values of K/K0, which decay with distance from the column center, are nevertheless greater than 1·0. In the second part of this paper, two sets of unit cell analyses have been carried out to investigate the effect of accounting for column installation on subsequent settlement performance. The results indicate that “primary” settlement improvement factors are larger than “total” settlement improvement factors and that the relative differences between them increase when installation is taken into account.

Stability investigation of the Generalised-α time integration method for dynamic coupled consolidation analysis

In this paper, the stability of the Generalised-α time integration method (the CH method) for a fully coupled solid-pore fluid formulation is analytically investigated for the first time and the corresponding theoretical stability conditions are proposed based on a rigorous mathematical derivation process. The proposed stability conditions simplify to the existing ones of the CH method for the one-phase formulation when the solid–fluid coupling is ignored. Furthermore, by degrading the CH method to the Newmark method, the stability conditions are in agreement with the ones proposed in previous stability investigations on coupled formulation for the Newmark method. The analytically derived stability conditions are validated with finite element (FE) analyses considering a range of loading conditions and for various soil permeability values, showing that the numerical results are in agreement with the theoretical investigation. Then, the stability characteristics of the CH method are explored beyond the limits of the theoretical investigation, assuming elasto-plastic soil behaviour which is prescribed with a bounding surface plasticity constitutive model. Since the CH method is a generalisation of a number of other time integration methods, the derived stability conditions are relevant for most of the commonly utilised time integration methods for the two-phase coupled formulation.

Analysis of Friction Induced Thermo-Mechanical Stresses on a Heat Exchanger Pile in Isothermal Soil

In most analytical and numerical models of heat exchanger piles, strain incompatibilities between the soil and the pile are neglected, and axial stresses imposed by temperature changes within the pile are attributed to the thermal elongation and shortening of the pile. These models incorporate thermo-hydro-mechanical couplings in the soil and within the pile foundation, but usually neglect thermo-mechanical couplings between the two media. Previous studies assume that the stress changes imposed by temperature variations in a heat exchanger pile are mainly due to the constrained thermal elongation and shortening of the pile. Also, several recent approaches utilize spring models that focus only on the soil-pile interface in modeling temperature-induced stresses in a heat exchanger pile and implicitly ignore the effect of the full displacement field on soil-pile interaction. By contrast, in this paper, interface elements are introduced in a numerical model of a heat exchanger pile, analyzed in axisymmetric and stationary conditions. The pile is subjected to a uniform temperature increase, with free top and fixed top conditions in elastic and elasto-plastic soil profiles. Simulation results show that the constrained vertical elongation is the most detrimental factor for pile foundation performance. However it is worth noticing that while mechanical constraints (e.g., fixed top and/or fixed bottom) impose maximum stress increases at the ends of the pile, interface effects result in maximum stresses around the mid-length of the pile. This preliminary study indicates that soil-pile friction does not increase pile internal stresses to the point where it would be necessary to over-dimension the foundation pile for heat exchanger use. Furthermore, one cannot expect a significant gain in foundation performance due to the improvement of soil-pile frictional resistance as a result of increased lateral stresses at soil-pile contact. Additional numerical analyses are ongoing, in order to investigate the role of the degree of fixity induced by the building on the heat exchanger pile, and to extend these preliminary analyses to transient operational modes and cyclic thermo-mechanical loading of the heat exchanger pile.

Influence of energy parameters of shock pulse generator on the pipe penetration velocity in soil

In the context of the problem of increase in efficiency of vibro-percussion driving of steel pipes in soil, a pilot model of shock generator with step speed adjustment for the bit and anvil impact is described. The authors report experimental modeling of pipe penetration depending on load of stalk. The influence of the stalk load parameters on the amplitude of generated shock pulses and the pipe penetration rate in elastoplastic soil is defined.

Numerical derivation of CPT-based p–y curves for piles in sand

The formulations for the lateral load–displacement (p–y) springs conventionally used for the analysis of laterally loaded piles have been based largely on the back-analysis of the performance of small-scale instrumented piles subjected to lateral load. Although such formulations have been employed with much success in industry, their applicability to large-diameter piles, such as those used to support offshore wind turbines, is uncertain and has necessitated further research in this area. Moreover, with the growth in popularity of in-situ cone penetration tests (CPTs), there are demands for a theoretically supported direct method that can enable the derivation of p–y curves from the CPT end resistance (qc). In this paper, a numerical derivation of CPT-based p–y curves applicable to both small- and large-diameter laterally loaded single piles in sand is presented. Three-dimensional finite-element analyses are performed using a non-linear elasto-plastic soil model to predict the response of single piles in sand subjected to lateral loads. The corresponding CPT qc profile is derived using the same soil constitutive model by way of the cavity expansion analogue. An extensive series of computations of the lateral pile response and CPT qc values is then employed to formulate a direct method of constructing p–y curves from CPT qc values. The proposed method is shown to be generally consistent with existing empirical correlations and to provide good predictions in relation to the measurements obtained during lateral load tests on instrumented piles in an independent case study.

A NUMERICAL STUDY ON SOIL–GROUP-PILE–BRIDGE-PIER INTERACTION UNDER THE EFFECT OF EARTHQUAKE LOADING

There have been many costly damages of group-pile foundation of bridge-pier in soft or saturated-medium dense sandy ground after strong earthquakes. This study investigated the dynamic behavior of a 2 × 2 bridge-pier–group-pile foundation installed in a two-layer ground commonly found in Japan. A series of three-dimensional (3D) effective stress analysis, which adopted advanced cyclic elasto-plastic soil model and nonlinear axial force dependence reinforced-concrete model, has been implemented to demonstrate the interaction between the soil–group-pile–bridge-pier during an earthquake of the magnitude of the 1995 Kobe earthquake. The damages of the 2 × 2 group-pile foundation have been quantitatively found to be either caused by the inertial loading of the superstructure or the kinematic interaction of the soils.