Stijn François

Short Note: EDT: An ElastoDynamics Toolbox for MATLAB

The ElastoDynamics Toolbox (EDT) version 2.1 offers an extensive set of MATLAB functions to model elastodynamic wave propagation in horizontally layered media. The toolbox is based on the direct stiffness method and the thin layer method. These methods provide stiffness matrices for a homogeneous layer and a homogeneous halfspace, which are formulated in the frequency-wavenumber domain. EDT 2.1 can be used to solve a variety of problems governed by wave propagation in the soil, such as (1) site amplification, (2) the computation of dispersive wave modes in layered soils, and (3) the calculation of the forced response of the soil due to harmonic and transient loading. The toolbox serves as an electronic learning environment for the simulation and processing of seismic wave propagation in layered media. It has also been used by various authors to model wave propagation in layered soils

Using the PiP Model for Fast Calculation of Vibration from a Railway Tunnel in a Multi-layered Half-Space

This paper presents a new method for calculating vibration from underground railways buried in a multi-layered half-space. The method assumes that the tunnel’s near-field displacements are controlled by the dynamics of the tunnel and the layer that contains the tunnel, and not by layers further away. Therefore the displacements at the tunnel-soil interface can be calculated using a model of a tunnel embedded in a full space. The Pipe-in-Pipe (PiP) model is used for this purpose, where the tunnel wall and its surrounding ground are modelled as two concentric pipes using elastic continuum theory. The PiP model is computationally efficient on account of uniformity along and around the tunnel. The far-field displacement is calculated by using another computationally efficient model that calculates Green’s functions for a multi-layered half-space using the direct stiffness method. The model is based on the exact solution of Navier’s equations for a horizontally layered half-space in the frequency-wavenumber domain.

Ground-Borne Vibration due to Railway Traffic: A Review of Excitation Mechanisms, Prediction Methods and Mitigation Measures

The aim of this paper is to provide a comprehensive overview of the state of the art on railway-induced ground vibration. The governing physical mechanisms, prediction methods, and mitigation measures of ground-borne vibration are discussed, with focus on low frequency feelable vibration and the case of railway traffic at grade. In order to clarify the importance of quasi-static and dynamic excitation, the basic problems of a moving load with constant magnitude and harmonic magnitude are discussed first. Dynamic excitation due to wheel and track unevenness and parametric excitation is shown to be the dominant source of environmental vibration in most cases. Next, an overview of prediction methods for ground-borne vibration is given. The advantages and limitations of numerical methods, based on physical or mechanical models, and empirical models, derived from measured data, are discussed. Finally, the mitigation of railway-induced ground vibration is considered, where the focus goes to mitigation measures at source (wheel and rail unevenness, rolling stock, track) and measures on the transmission path (trenches and barriers, wave impeding blocks, subgrade stiffening, and heavy masses next to the track). In conclusion, a number of open points requiring further research is given.

Geotechnical characterization of a river dyke by surface waves surface waves

The need for effective and reliable methods to survey and monitor the structure of earth-fill dams recently became pressing in light of the increasing number of flood events in central Europe. Among geophysical techniques, dam imaging using electrical resistivity methods is applied in most cases. Occasionally, ground-penetrating radar is applied in the framework of the search for subsurface facilities. Seismic methods are rarely used. This paper focuses on the multichannel analysis of the surface waves (MASW) method to determine dynamic soil properties and aims to extend its application field to dyke and dam structures. The standard processing procedure of the MASW assumes a flat free surface of infinite extension. The flat surfaces of a dyke, in contrast, are in the order of 1–10 times smaller than the wavelengths in the soil; disturbing side reflections will occur. Even though MASW has already been applied on a few dyke sites, the effect of such an obvious breach of preconditions needs to be studied before the method can be recommended. In this paper the influences of the dyke’s topography on the test results are studied by means of a numerical analysis. Typical cross-sections are modelled using 2.5D finite and boundary elements. The results of models taking the topography into account are compared with models neglecting the topography. The differences are evaluated on the level of the dispersion curves and for one cross-section on the level of the S-wave velocity. They were found to be insignificant for dykes with a width-to-height ratio larger than four. A testing campaign was conducted providing the chance to collect experience in the practical use of the MASW method on dykes. Test results obtained at two test sites are selected and compared to the results of borehole logs and cone penetration tests. A remarkable relation between the S-wave velocity and the consistency of the clay sealing was found at one site; a distinct positive correlation to the measured cone tip resistances was achieved on the other test site. Valuable information on the composition of the dyke body and base could be obtained but the resolution of the method to identify small areas of inhomogeneity should not be overestimated.

Verification of an Empirical Prediction Method for Railway Induced Vibration

The Detailed Vibration Assessment is an empirical procedure developed by the U.S. Federal Railroad Administration (FRA) for the prediction of railway induced vibration and re-radiated noise. The vibration velocity level in the free-field is predicted with a force density, characterizing the source, and a line transfer mobility, characterizing the transfer of vibration due to a line load. The line transfer mobility is determined with in situ measurements of transfer functions. The force density is obtained by subtracting the line transfer mobility from the vibration velocity level due to a train passage. It is assumed that the resulting force density can be used to predict the vibration velocity level at other sites with similar train and track characteristics. In this paper, the influence of the soil characteristics on the force density and the resulting vibration velocity level predicted with the FRA procedure is investigated. Numerical simulations are used to compute the vibration velocity level and the line transfer mobility at three sites with different soil characteristics. From these results, the force density due to a train passage is determined for each site. Finally, the three force densities are used to investigate the influence of the soil characteristics on the predicted vibration velocity level due to a train passage.

ANALYSIS OF STOCHASTIC DYNAMIC SOIL-STRUCTURE INTERACTION PROBLEMS BY MEANS OF COUPLED FINITE ELEMENTS-PERFECTLY MATCHED LAYERS

This paper presents a probabilistic computational model for the dynamic soilstructure interaction problem. Specifically, the structure and a limited bounded volume of subsoil in its vicinity are modeled with functionally graded finite elements, allowing to consider heterogeneous local subsoil conditions. This part of subsoil is modeled as a three-dimensional constrained stochastic field. The unboundedness of the surrounding soil is accounted for by coupling the finite element model with perfectly matched layers. An incident wave field is incorporated in the finite element-perfectly matched layers model without explicitly including the source in the computational domain by decomposing the displacement field of the soil in accordance with the subdomain formulation developed for the soil-structure interaction problem. The proposed methodology is illustrated with a case study in which the overall uncertainty is propagated by means of Monte Carlo simulation. The analysis shows that the uncertainty of the structural response increases at specific frequency bands and generally for higher frequencies. These results illustrate how the stochastic variability of the subsoil properties affect both the incident wave field and the structural response in a wide frequency range, and sets the base for future investigations.

THE INFLUENCE OF IMPERFECTLY KNOWN LOCAL SUBSOIL CONDITIONS ON THE PREDICTION OF GROUND-BORNE VIBRATIONS IN BUILDINGS

THE INFLUENCE OF IMPERFECTLY KNOWN LOCAL SUBSOIL CONDITIONS ON THE PREDICTION OF GROUND-BORNE VIBRATION IN BUILDINGS Manthos Papadopoulos, Stijn François, Geert Degrande and Geert Lombaert KU Leuven, Department of Civil Engineering, Structural Mechanics Section Kasteelpark Arenberg 40, 3001 Leuven, Belgium e-mail: {manthos.papadopoulos, stijn.francois, geert.degrande, geert.lombaert}@kuleuven.be

Dynamic behavior of silica sand under repeated cyclic loading

The nature-inspired concept of self-healing materials in construction is relatively new and has recently attracted significant attention as this could bring about substantial savings in maintenance costs as well as enhance the durability and serviceability and improve the safety of our structures and infrastructure. Much of the research and applications to date has focused on concrete, for structural applications, and on asphalt, with significant advances being made. However, to date no attention has been given to the incorporation of self-healing concepts in geotechnical and geo-environmental applications. This includes the use of concrete and other stabilising agents in foundations and other geotechnical structures, grouts, grouted soil systems, soil-cement systems and slurry walls for ground improvement and land remediation applications. The recently established Materials for Life (M4L) project funded by EPSRC has initiated research activities in the UK focussing on those applications. The project involves the development and integration of the use of microcapsules, biological agents, shape memory polymers and vascular networks as healing systems. The authors are exploring development of self-healing systems using mineral admixtures, microencapsulation and bio-cementation applications. The paper presents an overview of those initiatives to date and potential applications and presents some relevant preliminary results.By contrast to studies in petroleum geology and, despite their world-wide occurrence, geotechnical studies of ancient fluvial sediments are rare. This paper introduces the main characteristics of these sediments by reference to a classic UK example. Attention is then drawn to a number of major overseas examples where, although the principal features can be recognised, large differences arise as a result of factors such as the tectonic setting, the volume and mineralogy of the source material and the climate at the time the sediments were deposited. The first, over-riding problem for their engineering evaluation comes during the site investigation phase with the difficulty of deducing the geological structure and distribution of the widely varying lithologies.Strain accumulation in granular soils due to dynamic loading is investigated through long term cyclic triaxial tests and cyclic triaxial tests according to ASTM D 3999-91. Soil parameters, test equipment and loading conditions have a significant influence on strain accumulation, therefore a parameterization of the silica sand and a description of the cyclic triaxial test device are explained. Cyclic triaxial tests are performed and test results are presented illustrating the evolution of Young’s modulus during long term cyclic loading. The influence of the width of the stress-strain loop and the initial void ratio on strain accumulation is investigated and validated with existing accumulation models. The usefulness of Miner’s rule on sand subjected to cyclic loading is demonstrated by two tests with different packages of loading cycles.

A NUMERICAL STUDY OF SUBGRADE STIFFENING AS A MITIGATION MEASURE FOR RAILWAY INDUCED VIBRATIONS THROUGH 2.5D AND 3D FE-BE MODELS

This paper studies the efficiency of subgrade stiffening next to the track as a mitigation measure for railway induced vibrations by means of a two-and-a-halfdimensional coupled finite element – boundary element methodology. The pyhsical mechanism is interpreted in the frequency–wavenumber domain, illustrating that the stiffened block of soil next to the track can act as a wave impeding barrier. The existence of a critical frequency from which this mitigation measure starts to be effective, as well as a critical angle delimiting the area where the vibration levels are reduced, is demonstrated in this paper. Two realistic ca se studies are discussed to indicate that the effectiveness of the proposed mitigation measure depends on the stiffness contrast between the soil and the stiffened block of soil. Keywords— Dynamic soil-structure interaction, railway induced vibrations, mitigation measures on the transmissi on path, frequency–wavenumber analysis

Coupling of finite elements and hierarchical boundary elements for dynamic soil-structure interaction problems

Abstract. This paper discusses the coupling of finite element and fast boundary element methods based on hierarchical matrices to solve problems of visco–elastodynamic wave propagation involving dynamic soil–structure interaction in the frequency domain. Three coupling methodologies are presented and their computational performance is assessed through numerical examples. It is demonstrated that a direct coupling approach, in which the boundary element domain is condensed into an equivalent dynamic stiffness matrix, is the least efficient. Iterative procedures provide a valuable alternative; the efficiency of these algorithms strongly depends on the kind of boundary conditions applied to each subdomain, however. The fastest convergence is observed if Neumann boundary conditions are imposed on the most stiff subdomain. Aitken’s ∆–method is employed in these schemes for the calculation of an optimized interface relaxation parameter in order to ensure and speed up the convergence. A monolithic coupling approach is presented as well, providing a simultaneous solution of the governing equations while avoiding the assembly of a dynamic soil stiffness matrix.