NUI Galway – UL Alliance First Annual ENGINEERING AND - ARAN ...

The influence of secondary settlement on foundations supported by stone
columns
Mr. Brian Sexton and Dr. Bryan McCabe
College of Engineering and Informatics, NUI Galway
b.sexton1@nuigalway.ie and bryan.mccabe@nuigalway.ie
Abstract
A finite element/calculation based numerical model will
be developed to predict the long-term settlement
improvement offered by stone column installation in
soft cohesive soils. PLAXIS, a finite element program
specific to geotechnical engineering, will be used to
model the secondary settlement behaviour associated
with stone column-supported foundations. Small-scale
model experiments will be conducted in order to
support and authenticate the finite element results and
to gain a better understanding of the mechanisms of
behaviour involved.
1. Introduction
The use of vibro stone columns (VSCs) as a ground
improvement technique has gained popularity in recent
years. It is now widely accepted that stone columns
reduce the settlement and improve the bearing capacity
of soft cohesive soils.
However, most of the research that has been carried
out to date has been concerned with their effect on
primary settlement and very little consideration has been
given to how stone columns arrest long-term creep
settlements [1]. Secondary settlement (creep) can be
significant, if not dominant in normally consolidated
cohesive soils. Since stone columns are now being
extensively used in such deposits, secondary settlement
should be given careful consideration.
2. Numerical Modelling
Preliminary analyses using PLAXIS have focused on
a single stone column 0.6m in diameter supporting a
circular concrete footing 0.6m thick. The diameter of
the footing is the same as that of the column. The soil
profile considered for the finite element model is that of
the Bothkennar soft clay test site in Scotland. The
Bothkennar site has been a popular geotechnical test
bed for studying soft soil behaviour for a number of
years. The soil profile consists of a 1.5m crust overlying
a 13m thick layer of lightly overconsolidated clay. The
clay is in turn underlain by a gravel/bedrock layer.
The Hardening Soil Model in PLAXIS is used to
simulate the behavior of the crust and the stone column
material. The Hardening Soil Model is a hyperbolic
elastoplastic model capable of simulating advanced soil
behaviour. However, the Hardening Soil Model does
not account for viscous effects such as creep and stress
relaxation. The Soft Soil Creep Model is used for the
clay layers. The Soft Soil Creep Model is a second
137
order model that can be used to model long-term
secondary settlement behaviour. It will be used to
examine the influence that stone columns have on long
term settlement (creep). The column and the concrete
footing have been wished in place. The properties of the
surrounding soil are thus unaffected by the installation
of the stone column. Future work may involve
modelling column installation using a cavity expansion
technique.
3. Preliminary Analyses
The Soft Soil Creep Model has been used to simulate
the settlement reduction offered by column installation
over a 10 year period. Preliminary analyses have
focused on studying the effect of column installation on
the coefficient of secondary compression, Cα. The initial
studies indicate a reduction in Cα, highlighting the
effectiveness of the stone columns.
Preliminary results have indicated that Cα decreases
as column length increases. This is in comparison with
[2] who have carried out a parametric study using
PLAXIS 3D Foundation to examine the influence of a
number of different factors on the behaviour of small
groups of columns supporting rigid footings. The
authors have found that increased column lengths
resulted in improved settlement performance.
4. Conclusions
Stone Columns effectively reduce primary
settlement. However, the long-term secondary
settlement reduction offered by column installation
needs to be considered. It is hoped that the finite
element analyses and small-scale laboratory
experiments will lead to the development of a numerical
model for predicting the long-term settlement reduction.
5. Acknowledgements
The authors wish to acknowledge the support of the
Irish Research Council for Science, Engineering and
Technology (IRCSET).
6. References
[1] B.A. McCabe, G.J. Nimmons, and D. Egan, “A review of
field performance of stone columns in soft soils”, Proceedings
of the ICE - Geotechnical Engineering, 162, 6, pp. 323-334.
[2] M.M. Killeen and B.A. McCabe, “A Numerical Study of
Factors Affecting the Performance of Stone Columns
Supporting Rigid Footings on Soft Clay”, Proceedings of the
7th European Conference on Numerical Methods in
Geotechnical Engineering, Trondheim (Norway), 833-838.