Three-dimensional numerical and physical modelling of soft soil improvement using concrete injected columns
- Publication Type:
- Thesis
- Issue Date:
- 2019
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Concrete injected columns (CICs) are a popular method for improving soft
soil properties to support road and bridge approach embankments due to quick
construction, absence of spoil, and limited post-construction settlement. While the
limited settlement of CICs makes them attractive for cases where there are stringent
settlement criteria, low-cost methods of improving soils are used where there are no
such limitations. The lack of comprehensive experimental studies on CICs in the
available literature showed the necessity of further laboratory modelling. Moreover,
the equivalent comparison between frictional and socketed CICs has not been
thoroughly studied.
In this study, a well-instrumented physical modelling of soft clay improved
with CICs was performed. A granular layer was used to model the load transfer
platform (LTP), and a geotextile layer was utilised to model the geosynthetic
reinforcement (GR) layer. The load was applied and controlled in stages using a large
loading frame on top of the granular layer. Pore pressure dissipation, stresses
transferred to the soft soil and CICs, and the strains in the geotextile were monitored
with time. A three-dimensional numerical model was also developed using finite
difference software FLAC³ᴰ, and the results were validated against the experimental
data. The numerical model considered coupled flow-deformation allowing prediction
of the excess pore water pressure (EPWP) dissipation with time, while the permeability
of the soft soil varied with time. Modified Cam-Clay (MCC) soft soil model was used
as the constitutive model for the soft clay deposit, while elastic-perfectly plastic Mohr-
Coulomb failure criterion was used to simulate the LTP layer. Hoek-Brown
constitutive model was used to model the unreinforced concrete used for CIC
construction. A good agreement was perceived between the numerical results and the
measurements from the experiment. Referring to both measurements and predictions,
despite the low permeability of the soft clay, a rather quick dissipation in the EPWP
occurred due to the load transfer mechanism between the soft soil and CICs. The stress
concentration ratio decreased at the beginning of the loading stages and then later
increased with time, and was higher for higher applied loads.
This thesis also sets out to investigate the options available for the transition
zone from CICs to other ground improvement methods away from the abutment. Two
possible alternatives were numerically simulated using FLAC3D software considering
the dissipation of pore water pressure and variation of soil permeability with time. A
geosynthetic layer was introduced into the load transfer platform (LTP) located above
the CICs, and interface elements were incorporated to simulate CIC-soil interaction.
The first option for the transition zone was widely spaced CICs socketed into stiff
material and the second was using shorter, closely spaced, frictional CICs. A
comparison was then made between the predicted ground settlement, the force
mobilised in the geosynthetic, the excess pore water pressure, and stresses in the CICs
for the two scenarios. The total length of the CICs and thus the total volume of the
concrete used for their construction were kept the same for both alternatives. Indeed,
the embankment on frictional CICs experienced less settlement, the forces mobilised
in the geosynthetic were reduced, and the bending moments and shear forces generated
in the columns were less than the corresponding values for the case of socketed CICs.
This study showed that for a given volume of concrete, shorter, frictional CICs perform
better than longer CICs socketed into stiff strata.
Furthermore, a comparison was made between drained and coupled flowdeformation
numerical analyses. This study revealed that while performing drained
analysis by simply assigning drained parameters to the material was less
computationally demanding, it lead to inaccuracies in the predictions. The perceived
discrepancies were attributed to the difference in the stress-path of drained and coupled
analyses.
The results from this study can be beneficial for the practicing engineers for
designing structures on CIC-improved grounds, particularly for predicting the timedependent
performance of the system.
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