Abstract :
[en] In urban areas where the infrastructure is dense and construction of new structures is near
existing and sensitive buildings, frequently vibrations, caused by human activities, occur.
Generated waves in the soil may adversely affect surrounding buildings. These vibrations
have to be predicted a priori by using currently available knowledge of the soil dynamics.
Current research, conducted by Deltares research institute, showed that the reliability of
methods for prediction of man-made vibrations is disappointingly low. Therefore the models
for vibrations in the soil should be improved in order to get more accurate predictions.
The main aim of this thesis is to increase the knowledge on dynamic soil behaviour with
respect to the fundamental geotechnical aspects of the soil, like non-viscous damping,
inhomogeneity, anisotropy, variable degree of saturation, etc. and to give an improved
prediction method.
The scientific investigations of this thesis started with the following setup: an oscillating plate
on an elastic, homogeneous and isotropic half-space, where the plate oscillates harmonically
in vertical direction and the soil is unsaturated. In this way, the geotechnical aspects have
been left aside in order to check first whether it is possible to predict the vibration amplitudes
of the oscillating plate and of the soil surface, without additional complexities.
This setting allowed to compare the present analytical methods with the results, obtained
from the finite element method (FEM) calculations, and showed that the analytical methods
have their limitations. Therefore the wave-field near an oscillating plate had to be
investigated more carefully. Unfortunately the state of the art in soil dynamics is such that
only the particle vibration velocities are measured without knowing which part of the
velocities/vibrations belongs to which type of basic wave (compressional, shear or Rayleigh
wave). Therefore first of all, a technique to decompose the measured signal into its basic
waves was developed. This new technique showed remarkably that all three basic waves have
phase shifts and these phase shifts are all different from each other. The decomposition
technique is an important tool for researching soil dynamics. Also a qualitative evaluation of
the energy transmission between the basic waves near the vibration source was given, which
showed that the R-wave energy starts at zero just at the source and grows in the near-field
zone due to an energy transmission (body waves are transferring energy to the R-wave). This
means that even without uncertainties in the soil body, there is a lack of understanding of the
behaviour of the different waves.
A real field test is performed with a shaker on a soft peaty site in the Netherlands, as an
attempted to replicate the FE model experiments. It showed the limitations of the analytical
methods and highlighted the indispensability of the FEM. Still, for engineering purposes, an
improved analytical method is suggested, which is able to predict the geotechnical vibrations
with good accuracy. Herein, one of the fundamental aspects, the material damping, was used
and a hypothesis was made, that with a more correct physical model of the soil material
damping, the vibration predictions with FEM can be improved.
The 1D frictional damping model, first suggested by Van Baars (2011), was extended for the
3D and incorporated into the FEM software Plaxis as a User Defined Soil Material model.
The results are very interesting scientifically, but do not give much better results as the
already existing Rayleigh damping model.
