Plastic Soil Model Research Articles

Settlement performance of pad footings on soft clay supported by stone columns: A numerical study

The settlement behaviour of small loaded areas (such as pad and strip footings) on soft soil supported by stone columns is poorly understood. The lack of confinement associated with peripheral columns and the sharp stress decay with depth are features which are incompatible with the widely-used unit cell method for infinite column groups thereby rendering small group behaviour particularly difficult to analyse. Useful field data is virtually non-existent and although innovative laboratory modelling has been insightful, the findings cannot easily be extrapolated to field scale. In this study, a 3-D finite element analysis in conjunction with an elastic–plastic soil model is used to identify the effect of variables in the design process and interactions between them: these include column arrangement, spacing, length, and Young׳s modulus of the column material. A simplified method is proposed to relate the settlement of small groups to a reference unit cell settlement predicted by current analytical approaches.

Modeling the behavior of natural gas pipeline impacted by falling objects

A numerical approach, based on the finite-element method was established in this paper to simulate impact behavior of buried gas pipeline. A three-dimensional elastic perfectly plastic soil model with plane strain conditions was employed in this analysis. The influence of relative stiffness of pipeline–soil, buried depth of pipeline and impact energy on the stress distribution of buried pipelines was discussed. The analysis results showed that the maximum pipeline stress decreased with the increase of the relative stiffness of pipeline–soil; however, the penetration depth was independent of the relative stiffness of pipeline–soil. Pipeline stress decreased with the increase of buried depths and began to stabilize when the buried depth was more than 3m. The results of impact load and penetration depth considering the plasticity of soil were much larger than those not considering it. To minimize the discrepancy of results between the current approaches and the actual situation, a new method was put forward to improve the accuracy of the proposed formula computing impact load and penetration depth. The results from the new theoretical method were closer to those from the finite element analysis and tests.

A parkable piezoprobe for measuring cv at shallow depths for offshore design

The coefficient of consolidation (cv) at shallow depth is an important parameter for the design of many offshore facilities, particularly pipelines. This paper sets out the concept, operation and interpretation of a new simple tool for measuring cv in situ at shallow depths. This tool is a large cylindrical penetrometer with a hemispherical tip, which is embedded statically under self-weight, for example from a winch. After embedment, the tool is ‘parked' – not requiring support from a drill-string – and the pore pressure dissipation is monitored. This ‘parkable' feature allows the tool to be added to conventional drilling or penetrometer systems and used in parallel. The operation of the tool has been simulated using large-deformation finite-element analysis with the Cam Clay plasticity soil model. Results in the form of the normalised undrained penetration response and pore water pressure dissipation–time history are presented. The penetration response indicates the weight requirement of the device. The dissipation curves allow cv to be determined from field measurements of pore pressure.

On the influence of elastic–plastic coupling on sands response

A sand constitutive model accounting for elastic–plastic coupling is presented. To this aim, general constitutive equations describing an elastic–plastic coupling effect are developed first. Afterwards, a modified critical state plasticity model for granular soils is introduced accordingly. Several examples are presented to show the achieved improvements compared to the existing approaches. Comparing directly with experimental data, it is shown that the proposed model provides realistic simulations for pore pressure built-up under undrained cyclic loadings.

Uniqueness and existence in plasticity models for unsaturated soils

The paper investigates the mathematical structure of plasticity models for unsaturated soils and provides a strategy to capture the loss of uniqueness of the incremental solution upon loading and/or wetting paths. To derive bifurcation conditions in simple analytical form, the analysis is restricted to isotropic stress states. This choice has allowed the inspection of the most common classes of constitutive models through a unified notation, as well as the study of different forms of coupling between plasticity and state of saturation. It is shown that, similar to saturated soil plasticity, the loss of admissibility of the plastic solution is governed by critical values of the hardening modulus. At variance with the classical case, however, these moduli can be positive even if the plastic flow rule is associated (bifurcation in the hardening regime). The paper shows that such non-trivial features derive from hydro-mechanical coupling, i.e. they depend on the approach used to reproduce suction effects and evolving retention properties. In other words, although the problem of loss of uniqueness affects all classes of plasticity models for unsaturated soils, different constitutive assumptions may not have the same outcome in terms of bifurcation potential. As a result, new concepts are introduced to compare the mathematical robustness of the different constitutive approaches, as well as to interpret their predictions in the light of precise bifurcation criteria.

FEM Analysis of Dynamic Soil-Pile-Structure Interaction in Liquefied and Laterally Spreading Ground

A two-dimensional nonlinear dynamic finite element (FE) model was developed and calibrated against dynamic centrifuge tests to study the behavior of soil-pile-structure systems in liquefied and laterally spreading ground during earthquakes. The centrifuge models included a simple structure supported on pile group. The soil profiles consisted of a gently sloping clay crust over liquefiable sand over dense sand. The FE model used an effective stress pressure dependent plasticity model for liquefiable soil and a total stress pressure independent plasticity model for clay, beam column elements for piles and structure, and interface springs that couple with the soil mesh for soil-structure interaction. The FE model was evaluated against recorded data for eight cases with same set of baseline parameters. Comparisons between analyses and experiments showed that the FE model was able to approximate the soil and structural responses and reproduce the lateral loads and bending moments on the piles reasonably well.

Coupled consolidation analysis of pipe–soil interactions

Current design practice for pipe–seabed interaction in soft soils is generally based on the assumption of undrained behaviour throughout laying and subsequent operation. In reality, drainage and consolidation around a partially embedded pipe can have a marked effect on the vertical penetration and horizontal breakout resistance. In this paper, a large-deformation finite element methodology coupled with the “modified Cam clay” plasticity soil model has been developed to study the coupled consolidation behaviour of soil around partially embedded seabed pipelines. Simulations of penetration show that after laying, subsequent consolidation leads to further embedment by an amount dependent on the level of drainage that occurred during laying. Also, if the pipe is embedded under undrained conditions, the waiting period between laying and operation allows the soil around the pipe to consolidate under the pipe self-weight. The consolidation process results in an increase in the strength of the soil. The lateral breakout resistance and the direction of pipe movement on breakout thus depend on the consolidated strength of the soil around the pipe, as well as the applied loading. The envelopes of vertical–lateral combined loading bearing capacity differ markedly from those predicted assuming undrained behaviour throughout.

Direct differentiation method for response sensitivity analysis of a bounding surface plasticity soil model

Finite element (FE) response sensitivity analysis is an important component in gradient-based structural optimization, reliability analysis, system identification, and FE model updating. In this paper, the FE response sensitivity analysis methodology based on the direct differentiation method (DDM) is applied to a bounding surface plasticity material model that has been widely used to simulate nonlinear soil behavior under static and dynamic loading conditions. The DDM-based algorithm is derived and implemented in the general-purpose nonlinear finite element analysis program OpenSees. The algorithm is validated through simulation of the nonlinear cyclic response of a soil element and a liquefiable soil site at Port Island, Japan, under earthquake loading. The response sensitivity results are compared and validated with those obtained from Forward Finite Difference (FFD) analysis. Furthermore, the results are used to determine the relative importance of various soil constitutive parameters to the dynamic response of the system. The DDM-based algorithm is demonstrated to be accurate and efficient in computing the FE response sensitivities, and has great potential in the sensitivity analysis of nonlinear dynamic soil-structure systems.

Finite Element Analysis of the Movement of the Tie-Back Wall in Alluvial-Silty Soils

Excavations in urban–developed areas were commonly supported by diaphragm walls with internal braces or tieback anchors. There are still required the necessary civil works to serve the demands and also to solve the environmental problems. Wales and cross-lot struts are by far the predominant method for wall support, the large working space inside the excavation provided by a tieback anchor system has a significant construction advantage. This paper aims to evaluate the soil stiffness parameters of the alluvial Taipei soil for tie-back diaphragm wall based on back analysis of case study. The case histories of the Taipei County Administration Centre (TCAC) and NTU Hospital (NTUH) were studied in this paper. The deformation analysis was carried out by available finite element analysis tool using PLAXIS. In the analysis, beam element method together with an elastic perfect plastic soil model was used to design the diaphragm wall and the tieback anchor system. The soil was modelled as Mohr-Coulomb model. According to the basic design, the clay deposits are modelled as undrained behaviour while silty-sand behaves in drained condition. The simulation results show that the maximum horizontal movement occurred at around the bottom of wall. At the final stage of construction, the root mean square deviation (RSMD) between measured and calculated the wall movement was 4% and 7% respectively at TCAC and NTUH sites. It was concluded that tied-back diaphragm wall can be satisfactorily modelled using elasto-plastic Mohr-Coulomb soil model. The predicted soil's modulus of elasticity can be obtained from the SPT-N relationship, for clay deposit layer and for silty-sand deposit.

Finite element analysis of laterally loaded fin piles

A three-dimensional analysis of laterally loaded fin piles is presented. The behaviour of fin piles is difficult to explain using simple pile-soil theories or two dimensional numerical analyses because of the complicated geometry of the piles. In this study, a nonlinear 3D analysis with an elastic plastic soil model, an elastic pile material and interface elements are used to model the pile-soil interaction. Experimental data confirmed that the predicted behaviour of a monopile was acceptable leading to a more detailed study of the impact of fins upon the lateral resistance. The stress distribution within the pile and the deformation of the fin pile are presented. The increase in resistance gained by placing fins on a pile is illustrated by plotting the lateral resistance against displacement of the pile head for various geometries of the pile. This is expressed as the efficiency of the fins which is the ratio of the lateral resistance of a fin pile to that of a monopile with the same core diameter where the lateral resistance is defined as the resistance at a displacement of 10% of the pile diameter. The efficiency of the piles increases as the length of the fins; the increase is independent of the loading direction relative to the fins.

A State Parameter Based Generalized Plasticity Model for Unsaturated Soils

A State Parameter Based Generalized Plasticity Model for Unsaturated Soils

Large Deformation Finite-Element Analysis of Submarine Landslide Interaction with Embedded Pipelines

Submarine landslides represent one of the most significant geohazards on the continental slope in respect of the risk they pose to infrastructure such as deep water pipelines. A numerical approach, based on the finite-element method but using remeshing, was established in this paper to simulate large flow deformation of debris from a landslide and to quantify the loads and displacements imposed on pipelines embedded in the seabed. A simple two-dimensional elastic perfectly plastic soil model with plane strain conditions was employed in this analysis. The pipeline was restrained by a set of springs so that the load on the pipeline built up to a stable value, representing the limiting load at which the debris flowed over the pipeline. A parametric study was undertaken by varying the pipeline embedment and the relative strengths of the debris and seabed. The analysis results show that the various combinations of soil strength and embedment depth lead to different debris-pipeline movement patterns and consequ...

Flow rule effects in the Tresca model

The Tresca failure criterion is used regularly in geotechnical engineering to compute the failure loads of clay soils deforming under undrained conditions. When this criterion is used together with the finite element method a plastic flow rule must also be incorporated in the elasto-plastic soil model. The effects of the flow rule on the performance of a non-linear analysis using an elastic perfectly plastic soil model obeying the Tresca failure criteria are discussed in this note. Application of this model in a three-dimensional analysis causes computational difficulties, due to the gradient discontinuities that exist at the corner of the Tresca yield surface. Such discontinuities can be removed from the yield (or failure) surface using different methods. Two of the most widely used methods in removing singularities from the yield surface and their overall performances in a three-dimensional finite element analysis are discussed. The motivation for this study comes from a concern raised by Randolph and Puzrin [Randolph MF and Puzrin AM Upper bound limit analysis of circular foundations on clay under general loading. Geotechnique, (2003);53(9):785–796, [5]] about reported instances of under predictions of the collapse loads by finite element analysis [Taiebat HA and Carter JP Numerical studies of the bearing capacity of shallow foundations on cohesive soil subjected to combined loading. Geotechnique, (2000);50(4):409–418, [7]] and [Taiebat HA and Carter JP Bearing capacity of strip and circular foundations on undrained clay subjected to eccentric loads. Geotechnique, (2002);52(1):61–64, [8]], [Gourvenec S and Randolph M Effect of strength non-homogeneity on the shape of failure envelopes for combined loading of strip and circular foundations on clay, Geotechnique, (2003);53(6):575–586, [4]], when it is usually expected that finite element results should overestimate the true collapse loads. The intent of this study is to demonstrate and reiterate that although the finite element method is an extremely powerful analytical tool for solution of engineering problems, it is nevertheless subjected to approximation errors due to simplifications that are necessarily made to prevent other numerical difficulties.

Computational Techniques and Shear Band Development for Cylindrical and Spherical Penetrometers in Strain-Softening Clay

This paper addresses numerical simulation of deep penetration of full-flow penetrometers in strain-softening, rate-dependent, cohesive soil, and the observed phenomenon of periodic shear bands. The analysis was conducted using a large deformation finite element approach, modifying the simple elastic–perfectly plastic Tresca soil model to allow strain-softening, with strain-rate dependency being incorporated in order to avoid spurious mesh dependency. Parametric analyses were carried out varying the strain-softening parameters (hence the relative brittleness of the soil), the rigidity index of the soil, and the strain-rate parameter. Increased brittleness of the soil led to reduction in the penetration resistance, but also to increasingly significant oscillations in the resistance–penetration responses. The oscillation was found to result from periodic shear bands evolving cyclically ahead of the advancing cylindrical and spherical penetrometers. Analyses with different values of rigidity index confirmed further that the periodic shear bands were a real material phenomenon, rather than due to errors in numerical simulation. Similar phenomena have been observed for continuous flow problems in granular materials. However, rising strain-rate dependency tended to suppress the oscillations.

Cyclic Suction Loading Plasticity Model for Granular Soils

Cyclic Suction Loading Plasticity Model for Granular Soils

Dynamic analysis of a water–soil–pore water coupling system

This paper explores the possible integration of the dynamic analyses of interaction of pore fluid flow and porous structure. The generalized fluid–structure interaction (FSI) algorithm in ADINA is applied for seismic analysis of a reservoir–earth dam–foundation system. In the given numerical example, water in the reservoir interacts with the earth dam on the upstream slope of the dam, and the foundation at the reservoir bottom. Pore water couples with soil particles throughout the earth dam and foundation. The reservoir water is modeled using both the Navier–Stokes equation based fluid element and the subsonic potential based fluid element. In the coupled analysis, hydrodynamic pressures and velocities are presented in the reservoir zone, while displacements, pore pressures and stresses are given for the earth dam and foundation domains at any earthquake time. The generalized FSI model is of great significance in performing soil liquefaction analysis if an appropriate soil plasticity model is employed in accounting for cyclic behavior of soils.

A unified bounding surface plasticity model for unsaturated soils

A unified constitutive model for unsaturated soils is presented in a critical state framework using the concepts of effective stress and bounding surface plasticity theory. Consideration is given to the effects of unsaturation and particle crushing in the definition of the critical state. A simple isotropic elastic rule is adopted. A loading surface and a bounding surface of the same shape are defined using simple and versatile functions. The bounding surface and elastic rules lead to the existence of a limiting isotropic compression line, towards which the stress trajectories of all isotropic compression load paths approach. A non-associated flow rule of the same general form is assumed for all soil types. Isotropic hardening/softening occurs due to changes in plastic volumetric strains as well as suction for some unsaturated soils, enabling the phenomenon of volumetric collapse upon wetting to be accounted for. The model is used to simulate the stress–strain behaviour observed in unsaturated speswhite kaolin subjected to three triaxial test load paths. The fit between simulation and experiment is improved compared to that of other constitutive models developed using conventional Cam-Clay-based plasticity theory and calibrated using the same set of data. Also, the model is used to simulate to a high degree of accuracy the stress–strain behaviour observed in unsaturated Kurnell sand subjected to two triaxial test load paths and the oedometric compression load path. For oedometric compression theoretical simulations indicate that the suction was not sufficiently large to cause samples to separate from the confining ring. Copyright © 2005 John Wiley & Sons, Ltd.

On a class of non-coaxial plasticity models for granular soils

The non-coaxiality of the directions of principal stresses and principal plastic strain rates in granular soils under stress rotations has long been observed and recognized in soil tests using both simple shear and hollow cylinder apparatuses. A few constitutive theories have also been proposed in the literature to account for the effect of stress rotations and the subsequent non-coaxial soil behaviour, particularly in the context of shear band analysis. However, the lack of corresponding general numerical methods makes it difficult to investigate the influence of non-coaxial stress–strain behaviour on the results of geotechnical boundary value problems. This paper presents a numerical evaluation of a class of non-coaxial, elastic–plastic models that are developed by combining the conventional plastic potential theory and the double shearing theory. The general non-coaxial constitutive theories are first formulated and then a finite element implementation of the theories is carried out. To evaluate the non-coaxial theories, the problem of simple shear of soils is chosen to investigate the predicted behaviour of soils under simple shear loading conditions where the axes of principal stresses rotate. In particular, the influence of initial stress states and the degree of non-coaxiality are examined. It is found that the numerical results predicted using the non-coaxial model are in general agreement with the experimental observations reported in the literature.

Limit loads for multilayered half-space using the linear matching method

This paper describes a technique for computing optimal upper bound limit load solution, based on the linear matching method, for the bearing capacity of surface strip footings on multilayer soils. The method assumes a perfectly plastic soil model with a Drucker–Prager yield criterion and involves the sequential matching of an appropriate linear material to the yield function. In this paper, results are presented in the form of bearing capacity factors based on various layer properties and geometries. Solutions for both an associated and a non-associated flow rules are discussed. Comparisons with existing analytic and finite element solutions demonstrate the efficiency of the current method.

Parametric Studies on the Behavior of Reinforced Soil Retaining Walls under Earthquake Loading

The finite element procedures are extremely useful in gaining insights into the behavior of reinforced soil retaining walls. In this study, a validated finite element procedure was used for conducting a series of parametric studies on the behavior of reinforced soil walls under construction and subject to earthquake loading. The procedure utilized a nonlinear numerical algorithms that incorporated a generalized plasticity soil model and a bounding surface geosynthetic model. The reinforcement layouts, soil properties under monotonic and cyclic loadings, block interaction properties, and earthquake motions were among major variables of investigation. The performance of the wall was presented for the facing deformation and crest surface settlement, lateral earth pressure, tensile force in the reinforcement layers, and acceleration amplification. The effects of soil properties, earthquake motions, and reinforcement layouts are issues of major design concern under earthquake loading. The deformation, reinforcement force, and earth pressure increased drastically under earthquake loading compared to end of construction.