Beena Sukumaran

Efficient finite element techniques for limit analysis of suction caissons under lateral loads

Abstract This paper documents the use of finite element analyses techniques to determine the capacity of suction caisson foundations founded in soft clays under undrained conditions. The stress–strain response of the soft clay is simulated using an elasto-plastic model. The constitutive model employed is the classical von Mises strength criterion with linear elasticity assumed within the yield/strength surface. Both two- and three-dimensional foundation configurations are analyzed. The three-dimensionality of the failure surface of the actual caisson requires that computationally intensive three-dimensional models be used. Suggestions are given on how to improve computational efficiency by using quasi three-dimensional Fourier analyses with excellent results instead of true three-dimensional analyses. The finite element techniques employed are verified against available classical limit solutions. Results indicate that both hybrid and displacement-based finite element formulations are adequate, with the restriction that reduced-integration techniques are often required for displacement-based formulations.

Three Dimensional Finite Element Modeling of Flexible Pavements

Pavements currently in existence in most airports have not been designed to sustain the loading imposed by heavier and bigger aircraft, like the Boeing 747 -400, Airbus A-380 and Boeing 777. With the help of the National Pavement Test Facility of the Federal Aviation Administration based in Atlantic City , a finite element model will be created capable of predicting shear failures in the subgrade . The pavement test facility at Atlantic City will serve as a valuable sour ce of data for the study and at the same time, the finite element modeling study will enhance the understanding of pavement behavior. Finite element analyses techniques will be used to determine the failure mechanism in a pavement system under moving aircraft loads. The threedimensionality of the failure surface under actual wheel loads with wander requires that computationally intensive three -dimensional models be used. The finite element techniques employed are verified against available failure data fro m the National Pavement Test Facility.

Experimental analyses of the behaviour of saturated granular materials during axisymmetric proportional strain paths

Instability of granular materials is usually studied using undrained triaxial compression tests or constant shear drained tests. These tests are usually performed on fully undrained or fully drained samples respectively. In this paper, experimental investigations of the behaviour of loose saturated sand under proportional strain paths performed by imposing a partial drainage condition are presented. Partially drained conditions observed in situ are due to potential pore pressure variations. Different drainage condition changes can lead to different stress–strain responses, and it is shown that the strength mobilisation for expansive drainage conditions is lower than what is usually obtained for undrained conditions. The stability analyses based on experimental results have shown that the second order work criterion, rewritten using the relevant control parameters, is a good tool to investigate the onset of instability. L’étude des instabilités dans les milieux granulaires est souvent menée en utilisant des résultats d’essais triaxiaux de compression non-drainés ou des essais à contrainte déviatoire constante. Ces essais sont réalisés respectivement en condition totalement non-drainée ou totalement drainée. Nous présentons dans cet article une étude expérimentale réalisée sur sable d’Hostun lâche saturé sous chargement proportionnel en déformation en imposant un drainage partiel. Ce drainage partiel est dû à une augmentation de pression interstitielle et reproduit des conditions in situ. Ces différentes conditions de drainage conduisent à des réponses contraintes-déformations différentes et l’on montre que la résistance mobilisée dans le cas d’un essai à dilatance imposée est inférieure à celle classiquement obtenue durant les essais non-drainés. L’analyse de stabilité réalisée en utilisant les résultats expérimentaux montre clairement que le critère du travail de second ordre, réécrit en utilisant les variables de contrôles adéquates, est un indicateur pertinent pour l’étude du déclenchement de l’instabilité.

Measurement of Porosity in Granular Particle Distributions Using Adaptive Thresholding

It has been shown that the flow and shear characteristics of granular particles such as soils are significantly dependent on the shape of the particles. This is important from a practical viewpoint because a fundamental understanding of granular behavior will lead to an improved understanding of soil stability and influence the design of structural foundations. Furthermore, the calculation of soil stability and, consequently, structural stability is particularly useful during earthquake events. In previous work, we have demonstrated the applicability of X-ray and optical tomography measurements for characterizing 3-D shapes of natural sands and manufactured granular particles. In this paper, we extend the work to measure the arrangement and the orientation of an assemblage of such particles. A combination of X-ray computed tomography (CT) for measuring the coordinates of the individual particles and an iterative adaptive thresholding technique for computing the local variations in porosity is employed to generate porosity maps. Such maps can be used to gain a more fundamental understanding of the shear characteristics of granular particles. In this paper, we demonstrate the success of our technique by exercising the method on several sets of granular particles-glass beads (used as a control), natural Michigan Dune and Daytona Beach sand, and processed Dry #1 sand.

Measurement of Granular Material Packing

Granular material packing is of considerable importance when considering the stability and shear strength of granular media. It has been shown in various studies that the shear strength of granular media is influenced by inherent particle characteristics such as shape, angularity, surface texture and mineralogy. In this study, granular material packing at the grain scale is studied to see how packing is influenced by these inherent particle characteristics and if there is any evidence of an intermediate scale larger than the grain scale. Studies will also show clear evidence of the influence of shape, angularity and surface texture on the granular packing obtained.

Investigation of the Performance of Flexible Airport Pavements under Moving Aircraft Wheel Loads with Wander using Finite Element Analysis

The introduction of larger and heavier aircraft with more complex wheel configurations is making the current design methods inadequate for airfield pavements. In addition, airport pavements experience significant wander. However, the effect of wander on airport pavement performance has not been evaluated. In previous studies, the stress interactions between each tire of a triple-dual-tandem (TDT) axle used on B-777 and A380 aircraft cannot be captured using a two-dimensional model. In addition, many of these studies have assumed a linear-elastic material behavior of the pavement layers. The purpose of this study is to conduct a three-dimensional finite element analysis to quantify and evaluate the effects of wander and aircraft wheel configurations on the mechanical response of the pavement layers. The flexible pavement system that is modeled in this study is comprised of a medium and low strength subgrade. The stress-strain response of the base, subbase, and subgrade layers are simulated using an elasto-plastic model and the asphalt layer is modeled separately as a viscoelastic and elasto-plastic material. The model parameters are validated using results from laboratory and field tests. The results of the study show how flexible airport pavements are affected when wander and complex gear configurations are considered. Correlations between deformations from a single wheel and 4- and 6- wheel configurations are also studied to understand the effect of gear configuration on flexible airport pavements. Where possible, the results from the analysis are also compared against full scale results available from the National Airport Pavement Test Facility (NAPTF).

Three-Dimensional Finite Element Analysis of Flexible Airport Pavements for the Next Generation of Aircrafts

With the advent of later and heavier aircraft with more complex wheel configurations, current design methods of airfield pavements are becoming inadequate. Current design procedures are typically conducted using a layered elastic approach, which assumes a two-dimensional environment with linear elastic material responses with theory suspension to achieve the effect of multiple wheel loads. With the release of heavier aircraft with more complicated landing gears, the analysis situation has become more complex. The stress interactions between separate tires is not as simple to model; a two-dimensional model is incapable of modeling the three-dimensional problem of a triple-dual-tandem (TDT) axle used on a B-777 and A380 aircraft. This paper documents the use of finite element analyses techniques to determine the failure mechanism in a pavement system under moving aircraft loads. The three-dimensionality of the failure surface under actual wheel loads with wander requires that computationally intensive three-dimensional models be used. The flexible pavement system that is modeled is comprised of a medium and low strength subgrade. The stress-strain response of the material is simulated using an elasto-plastic model. The finite element techniques employed are verified against available failure data from the National Airport Pavement Test Facility (NAPTF) of the Federal Aviation Administration based in Atlantic City. Several conclusions can be drawn from the analysis. The response of pavement structure using elastic properties does not accurately predict the pavement response. On the other hand, the elasto-plastic material model is able to emulate the response pf the pavement material. It is also seen that wander causes greater stresses and strains with subgrade layer as well as greater upheaval when compared to models that neglect wander. The field of influence is also increased. The analysis which modeled the aircraft landing gear trafficking across the pavement section also indicates that pavements with less than 20 inches of pavement structure over the subgrade result in high levels of stress on the subgrade and adding additional subbase has diminishing returns in improving the pavement performance.

Imaging Systems and Algorithms for the Numerical Characterization of 3D Shapes of Particle Aggregates

The properties of geomaterial aggregates, mainly flow and shear characteristics, are significantly dependent on the shapes of the particles in the mixture. 3D particle models that have been previously developed to quantify this dependence have proved complex and expensive. This paper describes an optical tomography technique for the acquisition of real particle data that can be characterized using 3D shape descriptors. The algebraic reconstruction technique (ART) is used to synthesize 3D particle shapes from 2D tomography projections. Results demonstrating the efficacy of the method on a set of sand mixtures are presented.

Visual beams: tools for statics and solid mechanics

A team of faculty and students in the College of Engineering at Rowan University are developing hands-on and visualization tools for use in mechanics courses. The developed tools consist of physical simply-supported and cantilever beams that are instrumented with load cells to which students can apply various loading conditions. Measurements from the load cells and displacement transducers are used in a Labview graphical user interface allows the user to find reaction loads and plot deflections, stresses, and shear and bending diagrams. The tools are designed to help students overcome difficulties in working with forces, moments, displacements and stresses in courses the mechanical and civil engineering courses as well as benefit students with various learning styles.

Imaging Systems and Algorithms for the Numerical Characterization of Three-Dimensional Shapes of Granular Particles

The shear properties of natural granular particles such as sand are significantly dependent on the shapes of the particles in the mixture. This is important from a practical viewpoint, because a measurement and characterization technique for the 3-D shapes of such particles can lead to an improved understanding of soil stability and influence the design of structural foundations. Previous techniques that have been developed for this purpose have proven to be complex, and the associated instrumentation has proven to be expensive. Furthermore, conventional 2-D shape measurement and description methods do not readily lend themselves to parsimonious 3-D representations. The situation is further complicated by the fact that, to parameterize the relationship between shape and shear characteristics, a single numerical descriptor is required to model the 3-D shapes of multiple particles in a natural sand particle mixture. This paper describes an optical tomography technique for the measurement of particle data that is then characterized using statistical 3-D shape descriptors. The algebraic reconstruction technique (ART) is used to synthesize 3-D particle shapes from 2-D tomography projections. It is shown that the measurement and characterization techniques used can provide distinct features for differently shaped particle mixtures and can be used to synthesize 3-D composite particles representative of the entire mix. The novelty of the technique described in this paper is that numerical shape descriptors can be obtained for not only a single 3-D object but also an entire collection of 3-D objects. Furthermore, the statistical nature of the 3-D shape descriptor of a particle mixture can be used to synthesize a mixture containing an arbitrary number of particles that have similar but not identical shapes. Results demonstrating the efficacy of the method on a set of natural sand particle mixtures are presented.