Abstract
During the recent earthquakes in Kobe, Taiwan,
Turkey and India, foundation failures have contributed heavily to the resulting
damages. In many cases, liquefaction of the underlying soil has resulted in
tilting and settlement of supposedly ‘safe’ rigid structures. These types of
failures of rigid foundation in different types of soil stratifications are
investigated in this thesis with special emphasis on the seismic soil structure
interaction during such failures.
In this work, a series
of dynamic centrifuge tests were performed on a rigid foundation supporting a
containment structure. This foundation was supported on different soil
stratifications. The seismic response of the soil bed was compared for these
soil stratifications. It was observed that seismic response of layered,
inhomogeneous and remediated soil affects the overall behaviour of the rigid
foundation. It was concluded that inherent layering could change the
acceleration experienced at the base of the structure. This effect varied from
negligible to profound. Tilt and ultimate settlement of the foundation was
strongly dependent on the residual resistance provided by the sheared soil,
which was a function of the degree of softening, intensity of shaking, local
permeability variations and transient flow conditions from the surrounding
soil. The likely zone of influence affected by the interaction between the soil
and the structure was identified.
It was shown that the
‘vibration isolator’ capabilities of the liquefiable layer can be exploited
while designing remediation schemes for liquefiable sites. This result can have
important implications to the practising engineer. It
was concluded that the degree of heterogeneity of the soil profile has a
greater role to play in site-specific response studies than presently accounted
for. As part of the experimental work a ‘miniature air hammer’ was designed
which was used ‘in flight’ to characterize the soil softening during
consecutive earthquakes providing valuable information.
The role of
Soil-Structure interaction was also investigated and it was clearly seen that
the foundation input motion was different from the free field motion and it was
often higher in magnitude. No visible input loss was seen for low intensity
earthquakes. In high intensity earthquakes such assumptions of neglecting kinematic
interaction may be justified. Site response analysis for the layered soil also
showed that there was visible advantage of using a 2D non -linear program over
1D program, as this would enable an economic design.
Numerical simulation was successfully carried out
for the reported centrifuge test results by using the finite element code
SWANDYNE. The success was due to incorporating the criteria developed in the
numerical work presented in this thesis for proper mesh selection and time
discretization. It was shown that most of the dynamic simulations reported in
the literature use a far coarser time step than required for non-linear
liquefaction- type problems. Numerical analysis has also shown the effects of
frequency content on the rate of pore pressure generations in liquefiable soil.
Careful modelling and selection of sensible parameters have shown that the
present ‘u-p’ formulations can be used for predicting the deformations,
accelerations and excess pore pressure for different strength earthquakes and
bearing pressures in layered soils.
The seismic soil structure interaction
of a rigid foundation embedded in non-homogeneous, layered and liquefiable soil
is a complex problem. This boundary value problem has been investigated using
dynamic centrifuge modelling and
numerical modelling
based on finite element.
In this thesis, it is shown that these methods can complement each other in
providing an insight into the SSI interaction during earthquake shaking. The
model test data obtained from the present centrifuge tests can be used to
improve the design of the foundation of a high risk structure like a nuclear
containment in an earthquake zone founded in inhomogeneous soil. These results
are equally relevant for other heavy foundation structures like power plants,
industrial structures and buildings resting on raft foundations.