Two-dimensional numerical modelling of wave propagation in soil media

Wave propagation in soil media is encountered in many engineering applications. Given that the soil is unbounded, any numerical model of finite size must include absorbing boundary conditions implemented at the artificial boundaries of the domain to allow waves to radiate away to infinity. In this work, a finite element model is developed under plane strain conditions to simulate the effects of harmonic loading induced waves. The soil can be homogeneous or multi-layered where the soil properties are linear elastic. It may overlay rigid bedrock or half-space. It may also incorporate various discontinuities such as foundations, wave barriers, embankments, tunnels or any other structure. For the case of soil media over rigid bedrock, lateral wave radiation is ensured through the implementation of the consistent transmitting boundaries, using the Thin Layer Method (TLM), which allow replacing the two semi-infinite media, on the left and right of a central domain of interest, by equivalent nodal forces simulating their effect. Those are deduced from an eigenvalue problem formulated in the two semi-infinite lateral media. In the case of soil media over half-space, the Thin Layer Method is combined to the Paraxial Boundary Conditions to allow the incoming waves to radiate away to infinity laterally and in-depth. The performance of this coupled model is enhanced by incorporating a buffer layer between the soil medium and the underlain half-space. For extensive analyses, the eigenvalue problem related to the TLM may become computationally demanding, especially for soil media with multi-wavelength depths. As the TLM involves thin sub-layers, in comparison to the wavelength, the size of the eigenvalue problem increases with increasing depth. A modified version of the TLM is proposed in this work to reduce the computational effort of the related eigenvalue problem. This dissertation work led to the development of a Fortran computer code capable of simulating wave propagation in two-dimensional soil media models with either structured or unstructured triangular mesh grids. This latter option allows considering soil-structure problems with geometrical complexities, different soil layering configurations and various loading conditions. The pre- and post-processing as well as the analysis stages are all user friendly and easy

CONTRIBUTION TO THE MODELIZATION, ANALYTICAL AND NUMERICAL, OF GENERATION AND PROPAGATION OF VIBRATIONS ORIGINATED BY RAILWAY TRAFFIC. ANALYSIS OF MITIGATION PROPOSALS

Tesis por compendioReal Herráiz, JI. (2015). CONTRIBUTION TO THE MODELIZATION, ANALYTICAL AND NUMERICAL, OF GENERATION AND PROPAGATION OF VIBRATIONS ORIGINATED BY RAILWAY TRAFFIC. ANALYSIS OF MITIGATION PROPOSALS [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/52247TESISCompendi

Field Experiments on Wave Propagation and Vibration Isolation by Using Wave Barriers

In this paper, the obtained results from the experimental Studies describe the basic characteristics of wave propagation and ability of possible measures to reduce the impact of soil vibrations on structural response for both active and passive isolation cases. A series of field tests on the foundation vibrations generated by electrodynamic shaker are performed to examine the screening efficiency of open and in-filled trench barriers which are constructed for full-scale measurement. From field measurements of amplitude with and without the barrier, the amplitude reduction ratio is estimated at different points of interest. Wave propagating characteristics and frequency-dependent screening effects of the wave barriers are investigated according to various isolation material stiffnesses. The remarkable outcomes from these experimental Studies can be briefly generalized as follows: backfilled trench with softer material than soil is more effective for the passive isolation than the active one. The reduction effects of wave barriers depend on the frequency of vibration source for both passive and active isolation cases. In-situ measurements confirm that vibration screening systems using open or in-filled trench barriers can be applied as a reduction measure for soil vibrations due to a moving load which is considered as stationary wave Source in this problem. (C) 2008 Elsevier Ltd. All rights reserved

Vibration Isolation Using In-filled Geofoam Trench Barriers

A significant amount of numerical and experimental research has been conducted to study the vibration isolation by wave barriers considering open trenches, in-filled concrete or bentonite trenches, sheet-pile walls, and rows of piles. A few studies have investigated the use of expanded polystyrene (EPS) geofoam material as wave barriers, which indicated that in-filled geofoam trenches can be used as effective wave barriers. However, no engineering design method is available to date for the design of such type of wave barriers. This dissertation presents comprehensive experimental and numerical investigations on the use of in-filled geofoam trench barriers to scatter machine foundations vibration, in order to provide some recommendations and design guidelines for their implementation in design. Two- and three-dimensional time-domain finite element models have been developed utilizing the finite element package ABAQUS. The numerical models have been verified and then used to study the effectiveness of different configurations of in-filled geofoam wave barriers. All the proposed configurations performed well in scattering surface waves. However, the single-continuous wall system was considered to be more economic and practical alternative for wave scattering. Based on the findings of the preliminary numerical investigations, a full scale field experimental study has been conducted to investigate the performance of in-filled geofoam trenches. An innovative approach to construct geofoam trenches involving hydro-dig technology was utilized. A series of experimental tests have been conducted to evaluate the performance of both open and in-filled geofoam trench barriers considering their geometry and distance from the source of disturbance. The results of the field experimental investigations were analyzed and interpreted to provide recommendations for implementation in design. Experimental results confirmed that in-filled geofoam trench barriers can effectively reduce the transmitted vibrations and its protective effectiveness is comparable to the open trench barrier. An extensive numerical parametric study was conducted to investigate the behaviour of in-filled geofoam wave barrier under different soil conditions and to point out the key parameters that dominate the performance of in-filled geofoam trench barriers. The influence of various key parameters on the screening performance were carefully analyzed and discussed. A model using Multiple Linear Regression (MLR) analysis was developed for design purpose. Finally, an artificial neural network (ANN) model has been developed, which aims at extrapolating the parametric study results to predict the in-filled geofoam wave barrier protective effectiveness in different soil profiles with different geometric dimensions

VERTICAL VIBRATIONS OF BASE ISOLATED MACHINEFOUNDATIONS

Vibration of base isolated machine foundations has been studied using the Scaled Boundary Finite Element Method (SBFEM) and the cone model method. The dynamic stiffness of soil supporting rigid massless foundation was determined. This stiffness is of complex value. The real part represents the reflected energy of the restoring and inertial forces while the imaginary part represents the energy dissipated within the endless extent of the soil as a geometric damping. The effect of geometric and material properties of soil upon the real and imaginary parts of the dynamic stiffness was determined and represented in terms of dimensionless charts for the frequency range of interest. Results have shown that increasing the embedment ratio has a significant effect on the dynamic stiffness, it increases the dynamic stiffness considerably. The effect of stiffness ratio(stiffness of isolator/ stiffness of soil) was demonstrated for isolated machine foundations. The use of soft isolators reduces the dynamic response of foundation and the soil reactio

Vibration from underground railways: considering piled foundations and twin tunnels

Accurate predictions of ground-borne vibration levels in the vicinity of an underground railway are greatly sought after in modern urban centers. Yet the complexity involved in simulating the underground environment means that it is necessary to make simplifying assumptions about this system. One such commonly made assumption is to ignore the effects of nearby embedded structures such as piled foundations and neighbouring tunnels. Through the formulation of computationally efficient mathematical models, this dissertation examines the dynamic behaviour of these two particular types of structures. The effect of the dynamic behaviour of these structures on the ground-borne vibration generated by an underground railway is considered. The modelling of piled foundations begins with consideration of a single pile embedded in a linear, viscoelastic halfspace. Two approaches are pursued: the modification of an existing plane-strain pile model; and the development of a fully three-dimensional model formulated in the wavenumber domain. Methods for adapting models of infinite structures to simulate finite systems using mirror-imaging techniques are described. The interaction between two neighbouring piles is considered using the method of joining subsystems, and these results are extended to formulate models for pile groups. The mathematical model is validated against existing numerical solutions and is found to be both accurate and efficient. A building model and a model for the pile cap are developed, and are attached to the piled foundation. A case study is used to illustrate a procedure for assessing the vibration performance of pile groups subject to vibration generated by an underground railway. The two-tunnel model uses the superposition of displacement fields to produce a fully coupled model of two infinitely long tunnels embedded in a homogeneous, viscoelastic fullspace. The significance of the interactions occurring between the two tunnels is quantified by calculating the insertion gains that result from the existence of a second tunnel. The results show that a high degree of inaccuracy exists in any underground-railway vibration prediction model that includes only one of the two tunnels present

Active Vibration Isolation by Open and In-filled Trenches: A Comparative Study

Active isolation of steady-state surface vibrations by open and softer in-filled trenches in an elastic and homogeneous half-space is investigated in this study using PLAXIS 2D. Percentage isolation by barrier is estimated from amplitude reduction factors. Effects of in-fill material parameters are investigated in terms of impedance ratios against the case of a trench of definite geometry. Several cases of softer backfilled trenches with impedance ratios ranging from 0.08-0.42 are studied and their effectiveness is compared with that of an open trench of identical dimension. It is found that softer in-filled trench barriers of impedance ratios lying between 0.08-0.17 exhibit isolation effectiveness comparable to open trenches. Isolation efficiency of in-filled trenches increases with decrease in impedance ratio. Both open and in-filled trenches are found more effective in reducing the vertical vibration component than the horizontal.Keywords:vibration isolation, in-filled trench, impedance ratio, Rayleigh wavelengt

Reduction in ground vibrations by the use of wave obstacles

The increasing size of the population results in that unbuilt spaces needing to be used for the construction of new facilities. Large construction sites can generate disturbing vibrations to nearby buildings, both while construction is underway and afterwards through the operation of subways, for example. The establishment of new areas close to, for example, motorways and railways increases the risk of disturbing vibrations being propagated to the new buildings. It is important that efficient methods for reducing ground vibrations be available when densely built areas are being planned. Reduction in ground vibrations by use of wave obstacles is investigated here by use of numerical simulations, trenches and shaped landscapes being considered as wave obstacles. The effects of geometric parameters on open trenches, material parameters of filled trenches, and of infiltrated water in open trenches, were examined in appended Paper A. The finite element method involving use of both finite and infinite elements in the frequency domain was employed. In investigating the effects of the infiltrated water, account was taken of fluid-structure interaction. The finite element model, in which plane strain conditions were assumed, was applied to a road, the bedrock, two layers of soil and a trench. The depth of the trench and the elastic modulus of the solid material that was inserted into it were found to be the most important parameters to consider. The results concerning the infiltration of water into an open trench indicated the presence of water there to increase the vibration levels. Reduction in traffic-induced ground vibrations by use of shaped landscapes is investigated in appended Paper B, the effects of shaping the landscape surrounding a high-tech facility and using the landscape as a wave obstacle being studied. The effects of the geometric parameters of a shaped landscape were examined in parametric studies. An architectural landscape design was also investigated in terms of its effectiveness in reducing trac-induced ground vibrations. The finite element method involving use of both finite and infinite elements in the frequency domain was employed, the finite element models employed concerning a layer of soil and the underlying bedrock. It was found that anywhere from an appreciable reduction to an appreciable amplification of the vibrations can occur, depending upon the geometric parameters of the shaped landscape. Both types of ground modifications that were investigated were shown to be able to achieve an appreciable reduction in the level of vibration. Both the use of a trench filled with a solid material and use of a shaped landscape were found to result in a reduction in the level of vibrations of approximately 35 %. Both these types of methods can thus be regarded as being suitable for making it possible in this respect for buildings to be constructed close to vibration sources

Reduction of train-induced vibrations by using a trench in a layered half-space

A coupled model of a track-layered ground-in-filled trench system is developed to investigate the isolation effects of an in-filled trench on reducing vibrations generated by moving train loads. By using the substructure method, the whole model is decomposed into two parts: the layered ground containing the in-filled trench and the track. Firstly, the flexibility coefficient for moving uniformly distributed loads applied on the layered ground containing the trench is obtained by using the 2.5D indirect boundary element method. Then, the dynamic equilibrium equation of the track under the moving train loads and uniformly distributed loads are established. Finally, the layered ground and the track are coupled according to the equivalence between the deformation of the track and the vertical displacement of the layered ground. The validity of the method is confirmed by comparing its results with the published ones. Numerical calculations are performed by embedding an in-filled trench in a homogenous ground, in a single layered ground and also in the real site at Ledsgard as examples. The results show that the isolation effects are different for different ground conditions and for different geometric parameters such as the depth, width and location of the in-filled trench