Elasto-plastic Soil Behaviour Research Articles

Hazard Curves for Reliability Assessment of Strip Footings on Spatially Varying Cohesive Soils

The present study aimed to create a series of hazard curves against maximum total settlement and angular rotation of strip footings for probabilistic shallow foundation design on clays. Random field finite element method (RFEM) was adopted with elasto-plastic clay-like soil behavior, deformation modulus (Ed) and shear strength parameters (c and ) were employed as random field inputs. Parameters were defined and assigned to the analysis models with varying correlation lengths (h, v). Models have been iteratively solved one thousand times, and output distributions of maximum settlement and angular rotations were recorded. Probability density functions (PDF) were fitted to the outputs, and probability of failure (Pf) for footing deformation limits was subsequently estimated. Proposed hazard curves for two anisotropy and three variability categories were developed employing the estimated Pfs. The method proposed has been validated using an independent database of in-situ results, and a worked example was provided to illustrate the implementation of the process. The key contribution of the research is to form hazard curves for shallow foundations considering elasto-plastic soil behavior with the impact of all influencing parameters, respecting the limit values for foundation deformation in the design codes. The proposed technique offers a probabilistic evaluation of strip footings with spatial variation of clayey soils and a valid method for the reliability-based design of foundations in the serviceability limit state.

An analytical method for predicting the soil-structure interaction of axially loaded piles in sands incorporating the two-surface plasticity model

The paper proposed a novel and practical analytical method for predicting the soil-structure interaction in sands. To well capture the stress softening and soil dilatancy behaviors during loading, a developed two-surface plasticity model for interface shearing is adopted to determine the 2-D stress–strain state of the soil-structure interface and the shear band, while the 3-D elastoplastic behavior of soil around the shear band is determined by another developed two-surface plasticity model for triaxial compression. Following a standardized solving procedure, the rigorous derivation for the stress–strain relationship in plastic zone is formulated as a system of first-order differential equations, which are solved as an initial value problem. Combined with the finite difference method (FDM), the load–displacement response of the axially loaded pile and the stress–strain state around the pile are presented. To verify the proposed analytical approach, a FEM simulation is performed using a user-defined subroutine. The soil with different initial void ratios ranging from very dense to loose is also modeled to demonstrate the effect of the initial void ratio on soil-structure response. The proposed analytical approach is also validated against centrifuge tests and field tests, manifesting the validity and capability of the proposed method.

Effective foundation damping prediction of monopile-supported offshore wind turbines based on integrated fitting equation and PSO–SVM algorithm

Foundation damping affects the fatigue life of offshore wind turbines. However, the effective damping ratio determined using a finite element analysis is not always consistent with the input. A series of simplified finite element models were established using realistic structural properties for a monopile-supported wind turbine to analyze the influence of wall thickness, pile embedment length, structure density, and input damping ratio on the calculated effective damping ratio. An empirical fitting equation was proposed using a power function based on the comprehensive laws between various factors and the effective value. An integrated prediction approach combining this equation with a hybrid support vector machine and particle swarm optimization algorithm was subsequently described and its results compared with those of other intelligent algorithms. The results showed that the effective damping ratio was much smaller than the input value, with the former reaching a maximum of only 15% of the latter. The integrated prediction approach was shown to strike an equilibrium between computing efficiency and forecast accuracy. Considering the soil–structure interaction and elastoplastic behavior of soil only increased the effective damping ratio to 35% of the input value, and also obfuscated the roles of the various factors in the effective damping.

Undrained semi-analytical solution for cylindrical cavity expansion in anisotropic soils under biaxial stress conditions

This paper presents an undrained semi-analytical elastoplastic solution for cylindrical cavity expansion in anisotropic soil under the biaxial stress conditions. The advanced simplified SANICLAY model is used to simulate the elastoplastic behavior of soil. The cavity expansion is treated as an initial value problem and solved as a system of eight first-order ordinary differential equations including four stress components and four anisotropic parameters. The results are validated by comparing the new solutions with existing ones. The distributions of stress components and anisotropic parameters around the cavity wall, the expansion process, the stress yield trajectory of a soil element and the shape and size of elastoplastic boundary are further investigated to explore the cavity expansion response of soils under biaxial in situ stresses. The results of extensive parameters analysis demonstrate that the circumferential position of the soil element and the anisotropy of the soils have noticeable impacts on the expansion response under biaxial in situ stresses. Since the present solution not only considers the anisotropy and anisotropy evolution of natural soil, but also eliminates the conventional assumption of uniform radial pressure, the solution is better than other theoretical solutions to explain the pressure test and pile installation effect of shallow saturated soil.

A comparative study to determine seismic response of the box culvert wing wall under influence of soil-structure interaction considering different ground motions

The present study deals with evaluation of the effects of different ground motions, backfill-structure interaction (BSI) and soil-structure interaction (SSI) on dynamic response of a box culvert wing wall. In obtaining numerical results, five different earthquake records with different frequency contents, five different backfill types and four different subsoil types are taken into account. Firstly, a finite element model (FEM) and three analytical models are proposed for analysis of the wing wall-backfill system under fixed-base case, and these fixed-base models are compared with each other through both modal analysis and displacement time-history analysis under the assumption of linear-elasticity. After validating the ability of the FEM approach, by extending the fixed-base FEM to take account of elastoplastic behavior of soil and SSI, a three dimensional (3-D) model has been described, and 3-D nonlinear analyses have been carried out in time domain. The numerical results show that the seismic response of the wing walls are affected remarkably by the SSI, BSI and frequency content of the earthquake.

Prediction of precise subsoiling based on analytical method, discrete element simulation and experimental data from soil bin

Prediction of a precise subsoiling using an analytical model (AM) and Discrete Element Method (DEM) was conducted to explain cutting forces and the soil profile induced changes by a subsoiler. Although sensors, AMs and DEM exist, there are still cases of soil structure deformation during deep tillage. Therefore, this study aimed to provide a clear understanding of the deep tillage using prediction models. Experimental data obtained in the soil bin trolley with force sensors were used for verification of the models. Experiments were designed using Taguchi method. In the AM, the modified-McKyes and Willat and Willis equations were used to determine cutting forces and soil furrow profile respectively. Calculations were done using MATLAB software. The elastoplastic behavior of soil was incorporated into the DEM. The DEM predicted results with the best regression of 0.984 R^{2} at a NRMSE of 1.936 while the AM had the lowest R^{2} of 0.957, at a NRMSE of 6.008. All regression results were obtained at p < 0.05. The ANOVA test showed that the p-values for the horizontal and vertical forces were 0.9396 and 0.9696, respectively. The DEM predicted better than the AM. DEM is easy to use and is effective in developing models for precision subsoiling.

Behaviour of Square Footing Due to the Effect of Adjacent Excavation in Sand

In today scenario, urbanization and related infrastructure development in the form of an excavation adjacent to the existing structure are inevitable. In this case, the stability of an existing structure closer to the excavation is a serious concern. The main issue here is that, what is the safe distance at which new construction can take place from the existing structure? There are many research articles that states on the failure of an existing structure due to new construction adjacent to it in the form of excavations. Given the above, the paper presents the numerical results based on the finite element analyses that were performed for various parameters to examine the behaviour of existing square footing due to the effect of adjacent excavations in loose sand using PLAXIS3D software. In the finite element analysis, the behaviour of the footing and sheet pile walls were modelled using a linear elastic material and the elastoplastic behaviour of soil was modelled using a Mohr-Coulomb model under drained conditions. The literature available on this topic reveals that the excavation dimensions, shape of the footing, depth of the footing, location of the footing from the excavation, relative stiffness factor and relative density of sand are influencing the response of existing footing. Two different shapes of excavations such as a long length with narrow excavation under plane strain condition and short length of excavation under three-dimensional conditions were considered in this study. Numerical results indicate that the response of the square footing due to the effect of adjacent excavations are very much significant and mainly depends on the size of the excavation. The vertical and horizontal displacement, bending moment of the footing is increasing with an increase in depth of excavation. The footing located very closer to the excavation is significantly influenced as compared to the footing located far away from the excavation. The safe location of the square footing based on the settlement criteria is found to be 1.3 m from the excavation for shorter length excavation under three-dimensional conditions and 10 m for longer length of excavation under three-dimensional conditions.

A Macro-element for Modeling the Non-linear Interaction of Soil-shallow Foundation under Seismic Loading

This paper presents a macro-element for simulating the seismic behavior of the soil- shallow foundation interaction. The overall behavior in the soil and at the interface is replaced by a macro-element located at the base of the superstructure. The element reproduces the irreversible elastoplastic soil behavior (material non-linearity) and the foundation uplift (geometric non-linearity) at the soil- foundation interface. This new macro-element model with three degrees-of-freedom describes the force-displacement behavior of the footing center. The single element is restrained by the system of equivalent springs and dashpots. The footing is considered as a rigid body. It is solved by a suitable Newmark time integration scheme and implemented in Matlab to simulate the nonlinear behavior of soil-shallow foundation interaction under seismic loading. A reduce scaled soil-foundation system has been tested on a shaking table at the University of Transport and Communications, Hanoi, Vietnam. Five series of earthquake motions were used with maximum acceleration increased from 0.5 to 2.5 . The comparison of numerical results obtained from the simulation and experimentations shows the satisfactory agreement of the model. The proposed macro-element can be used to predict the seismic behavior of a wider variety of configurations.

Investigation of the torsional behaviour of circular piles in double-layered nonhomogeneous soil

Response of piles subjected to torsional loading is significantly affected by soil profile and the interaction between soil and pile, which has not yet been fully understood. This paper describes a new analytical method for predicting the nonlinear response of torsional loaded piles in a double-layer soil profile, which considers the effect of elastoplastic behaviour of soil and the nonlinear variation of soil profile. The results from the new method have been compared with experimental and other published data, with good agreement obtained. Finally, an illustrative example has been carried out to explain how the new method can be applied to a practical situation.

A new simplified meta-model to evaluate the transmission of ground movements to structures integrating the elastoplastic soil behavior

Many buildings may suffer damage due to ground movements associated to underground excavations like tunneling, mines or urban excavations. The level of damage depends upon the rate of ground movements transmitted to structures that is affected by the soil-structure interaction (SSI) conditions and parameters such as the soil elastoplastic behavior. The purpose of this paper is to integrate the soil elastoplasticity while investigating the response of a structure sitting on a soil subjected to a ground movement. Analytical and numerical approaches are applied to examine the effect of the non-linear soil behavior. The analytical approach is based on modified Winkler and Pasternak elastic models under the assumption of limiting the soil reaction by its bearing capacity; the numerical approach is based on finite element models, using Plaxis 2D that consider an elastoplastic soil behavior. Results display the discrepancy in the structural response between considering an elastic or an elastoplastic soil behavior. Based on a significant number of iterations, the artificial neural network technique is used to propose a new simplified meta-model that integrates the soil elastoplasticity to evaluate the transmission of ground movements to structures. In addition, a procedure, which can be used by engineers and designers, is developed to evaluate the transmission ratio for any structure sitting on any type of soil.

Application of the particle finite element method for large deformation consolidation analysis

Purpose Large deformation problems are frequently encountered in various fields of geotechnical engineering. The particle finite element method (PFEM) has been proven to be a promising method to solve large deformation problems. This study aims to develop a computational framework for modelling the hydro-mechanical coupled porous media at large deformation based on the PFEM. Design/methodology/approach The PFEM is extended by adopting the linear and quadratic triangular elements for pore water pressure and displacements. A six-node triangular element is used for modelling two-dimensional problems instead of the low-order three-node triangular element. Thus, the numerical instability induced by volumetric locking is avoided. The Modified Cam Clay (MCC) model is used to describe the elasto-plastic soil behaviour. Findings The proposed approach is used for analysing several consolidation problems. The numerical results have demonstrated that large deformation consolidation problems with the proposed approach can be accomplished without numerical difficulties and loss of accuracy. The coupled PFEM provides a stable and robust numerical tool in solving large deformation consolidation problems. It is demonstrated that the proposed approach is intrinsically stable. Originality/value The PFEM is extended to consider large deformation-coupled hydro-mechanical problem. PFEM is enhanced by using a six-node quadratic triangular element for displacement and this is coupled with a four-node quadrilateral element for modelling excess pore pressure.

Investigation of the structural performance of two flexible pipes set in one trench with a new placement method for separate sewer systems

Substantial research has been conducted on single flexible pipes buried in a trench. In contrast, the objective of this study is to determine the structural performance of two buried flexible sewer pipes positioned one over the other in a single trench. An innovative configuration is designed, based around the use of an innovative manhole structure which can accommodate both foul and surface water, to solve the challenges associated with constructing separate sewer systems in narrow streets while providing additional space for other infrastructure services. The behaviours of the two flexible pipes were tested using a 3D finite element (FE) model validated with experimental data from a laboratory investigation. A modified Drucker–Prager cap soil constitutive model was used to simulate the elasto-plastic soil behaviour. The results show that this approach comprising the use of a large-diameter flexible pipe set above a small-diameter flexible pipe mitigates the strain on the smaller pipe and decreases the total deflections of both pipes and the soil.

Nonlinear elastoplastic formulation for tunneling effects on superstructures

The paper presents a formulation for evaluating the effect of tunneling on existing buildings. The formulation involves the matrix condensation method to represent the response of a linear elastic building and macroelements to represent the nonlinear elastoplastic soil behavior. The formulation includes new features that allow interaction between macroelements, both through the soil continuum and the structure, to result in the final displacements of the foundations due to tunneling. One of the advantages of the formulation is its ability to incorporate a general input of a greenfield field displacement for the interaction analysis, allowing consideration of various tunneling scenarios. The formulation is evaluated by a comparison with a continuum-based solution obtained using the finite difference method. The formulation is then used to conduct a parametric analysis of tunneling–soil–superstructure interaction, considering three different approaches: (i) the suggested elastoplastic formulation, (ii) purely elastic analysis, and (iii) simplified analysis in which the foundations are forced to displace as a greenfield. It is shown that the vertical settlements of the foundations, due to tunneling, are the greatest when the first approach is considered. This is an outcome of the combined vertical and horizontal yielding, depicted in the formulation by the coupled yield function and plastic flow potential. Yet damage, which relates to differential settlement, appears to be smaller in the elastoplastic formulation.

Determination of in-situ engineering properties of soil using an inverse solution technique and limited field tests

The goal of this research is to develop a response surface based inverse solution technique to determine in-situ engineering properties of soil for use in mobility and traction prediction models from the force displacement data generated by a cone penetrometer device. The nonlinear elasto-plastic behavior of soil was characterized by a six-parameter constitutive model – two elastic parameters (i.e., bulk modulus, K and the Poisson’s ratio, υ), three plastic parameters (i.e., angle of internal friction φ, cohesion c, and soil hardening parameter λ), and one soil physical condition parameter (i.e. initial void ratio, e∗). Soil failure was represented by the Drucker-Prager yield criterion and associated flow rule. LS-DYNA FEM software package was used to model the soil-cone interaction problem. FEM simulations were conducted for a set of soil properties properly selected within the defined parameter space. The analysis of FEM simulations indicated that the cone penetration force-displacement curves could be represented by two piecewise smooth functions – a parabola followed by a straight line. The coefficients of these two curves were used to create fourth order response surfaces using a stepwise multiple linear regression technique. The results showed both cohesion and soil hardening parameter could be predicted using this methodology.

A stable Maximum-Entropy Meshless method for analysis of porous media

Consolidation analysis of saturated porous media demands the coupling of solid displacements with the pore fluid pressure via the equilibrium and the continuity of mass. In this paper, a stable numerical procedure is presented for coupled analysis of consolidation problems in geotechnical engineering. The numerical framework is based on the Element-Free Galerkin method and the principle of Maximum Entropy. Identical shape functions are employed for approximating the displacement field as well as the pore fluid pressure field. The proposed method is used for analysing several consolidation problems assuming elastic and elastoplastic soil behaviour. The numerical results indicate that the proposed Maximum-Entropy Meshless method based on the maximum entropy shape functions is able to provide stable and robust solutions for consolidation problems in porous media.

Three-dimensional smoothed-particle hydrodynamics simulation of deformation characteristics in slope failure

This paper presents the development, validation and application of a smoothed-particle hydrodynamics model for three-dimensional (3D) simulation of the evolution of large deformation failure in geo-materials. The Drucker–Prager model with non-associated plastic flow rules is implemented into the smoothed-particle hydrodynamics formulations to describe elasto-plastic soil behaviour. Two typical numerical examples – a two-dimensional (2D) analysis of cohesive slope instability and a 3D simulation for instant collapse of a granular slope – are shown to demonstrate the effectiveness of the method for modelling large deformation of slope failure. Good agreement with experimental observations and previous simulated results is obtained in terms of the profile and internal deformation, respectively. The method is then applied to two special 3D slopes with different geometric configurations, including a curving slope surface and a slope that turns corners. The results suggest that 3D effects should be considered for natural landslides. By influencing the stress status, slope geometries have a significant effect on the final profile, slip surface and distance. The results provide a more accurate and detailed reference for landslide evaluation and foundation ditch design.

Application of Composite Wraps for Strengthening of Buried Steel Pipelines Crossing Active Faults

In the paper, the efficiency of strengthening of a buried steel pipeline with a composite wrap subjected to an active faults action is analyzed. A three-dimensional numerical model of the pipeline is developed. The pipeline is considered as an elastoplastic steel shell, while the composite wrap is represented as an orthotropic elastic shell. The model takes into account the elastoplastic behavior of soil, contact interaction between the soil and the pipe, large inelastic strains, distortion of the pipeline cross section, and local buckling formation. A normal-slip fault kinematics with large fault offsets is considered in numerical modeling. The effect of the wrap thickness, length, and position relative to the fault plane is analyzed.

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.

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.

Numerical simulation of liquefaction in porous media using nonlinear fluid flow law

SummaryIt is well known that for a sufficiently high seepage velocity, the governing flow law of porous media is nonlinear (J. Computers &amp; Fluids 2010; 39: 2069–2077). However, this fact has not been considered in the studies of soil‐pore fluid interaction and in conventional soil mechanics. In the present paper, a fully explicit dynamic finite element method is developed for nonlinear Darcy law. The governing equations are expressed for saturated porous media based on the extension of the Biot (J. Appl. Phys. 1941; 12: 155–164) formulation. The elastoplastic behavior of soil under earthquake loading is simulated using a generalized plasticity theory that is composed of a yield surface along with non‐associated flow rule. Numerical simulations of porous media subjected to horizontal and vertical components of ground motion excitations with different permeability coefficients are carried out; while computed maximum pore water pressure is specially taken into consideration to make the difference between Darcy and non‐Darcy flow regimes tangible. Finally, the effect of non‐Darcy flow on the evaluated liquefaction potential of sand in comparison to conventional Darcy law is examined. Copyright © 2014 John Wiley &amp; Sons, Ltd.