2418
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2 GI WITHOUT ADMIXTURES IN COHESIVE SOILS
In the present Technical Session, six papers can be put in the
category B: GI without admixtures in cohesive soils. They are
mainly related to the subject of consolidation acceleration by
vertical drains combined with surcharge or Vacuum. The
interest seems to be oriented to the approach of “smear”.
Parsa-
Pajouh et al. (2013)
address this delicate topic so difficult to
model due to the lack of field parameters. According to the
authors, the smear zone varies between 1.6 and 7 times the drain
radius or 1 to 6 times the mandrel equivalent diameter.
Numerical models are used within the framework of case
studies. Parameters studies confirm their validity. As a result of
their researches, it is recommended to assess the smear zone on
the basis of trial construction with the help of back calculation
process.
Chai and Carter (2013)
present a theoretical approach of
Prefabricated Vertical Drains (PVD) and consolidation
combining vacuum pressure and surcharge loading. Using
Hansbo’s (1981) solution, consolidation parameters of the
smear zone and the undisturbed zone were derived using a
simple equation. Adopting an average well resistance and with
some approximation, the dimensionless parameter µ quantifying
the effects of PVD spacing, smear zone and well resistance can
be expressed. The study was performed in uniaxial
consolidation condition, which is not in agreement with the real
isotropic character of deformation under Vacuum. Moreover,
the classical assumption of uniform smear zone cannot be
measured. However the pore pressure measurements of the
tested samples are in extreme close concordance with the
prediction confirming the validity of the approach and the
selected parameters.
Indraratna et al. (2013)
treat similar subject in conjunction
with a real construction site in the Port of Brisbane where the
consolidation of thick Holocene clays was performed with
PVD’s under surcharge and/or Vacuum loading. Variable drain
spacing was selected and analytical solutions were proposed.
For the excess pore pressure dissipation, the same equation as in
Chai and Carter (2013) was adopted. The results demonstrate
that Vacuum combined with preloading would speed up
consolidation compared to preloading alone. Moreover,
Vacuum results in isotropic consolidation increasing the
stability of the surcharge fill (decreasing lateral displacements).
In a similar way,
Lee et al. (2013)
have also studied the
effect of the smear zone for a consolidation case history in
Busan (South Korea). Modification of Hansbo's analysis is
proposed to study the degree of consolidation considering the
properties of the soil within the smear zone.
As another case history,
Islam and Yasin (2013)
present an
application of PVD’s coupled with preloading used for the
construction of a large container yard in Bangladesh. The soil
profile consists of 4 to 6 m thick silty clay, 8 to 10 m of sand
and silt and 16 m of clayey silt. On the basis of design
requirements, GI of the upper soft clay layer was considered
essential. Five alternatives were assessed and compared. A
solution combining PVD and preloading was adopted for this
site. The settlement under preloading was monitored during the
consolidation phase. Pre and post consolidation SPT tests are
presented to illustrate the efficiency of the technique. It is
believed that dynamic compaction although economical would
not have been technically feasible due to the clayey nature of
the upper fill. However, dynamic replacement in the upper 4 m
with densification of the lower silty sand might have been
technically and financially optimal.
For their part,
Jebali et al. (2013)
have assessed the theory
of Carillo using three different oedometer tests carried on Tunis
soft soil. Oedometer tests were conducted, conventionally (NF
P94-90-1) for the first test, with a vertical drain allowing only
radial drainage for the second one and finally with a drain
allowing vertical and radial drainage for the last one. Defining
C
r
and C
v
as the radial and vertical coefficients of consolidation
and K
r
and K
v
as the coefficients of radial and vertical
permeability, they observed that the often-made assumption of
the equality between the ratio’s C
r
/C
v
and K
r
/K
v
is only valid at
high levels of stress conditions. Moreover, on the basis of
experimental results, the authors demonstrated that the global
degree of consolidation computed with respect of the Carillo’s
theory can lead to underestimated consolidation times.
The paper of
Weihrauch et al. (2013)
describes a
combination of GI methods for the improvement of roads in the
HafenCity area in Hamburg. Indeed, in the Hamburg Harbour
area, many roads are lifted with almost 3 m to ensure safety in
case of flooding. Special measures are necessary when the
subsoil contains compressible layers. At the Hongkongstrasse,
three different construction methods have been applied, namely:
- installation of PVD and preloading with sand (settlements of
more than 30 cm have been measured);
- filling with lightweight aggregate: expanded clay (almost no
settlement was observed);
- pile supported embankment including geogrid-reinforced
sand layer (measurements are discussed in another paper).
The different aspects of each method are described. The
conclusion is that when comparing different methods, not only
the absolute costs must be ascertained, but also the project
specific reconstruction, protection and follow-on measures, as
well as the time and flexibility for individual measures, and
their technical feasibility under local conditions.
3 GI WITH ADMIXTURES OR INCLUSIONS
3.1
Rigid inclusions
Moving towards category C, GI with admixtures or inclusions,
the paper presented by
Kirstein and Wittorf (2013)
is an
interesting transition between categories B and C. Indeed, the
authors describe the improvement of soft fat clay using rigid
inclusions combined with vertical drains, preloading and the use
of geotextile. The aim of the project was the construction of a
bridge for a new road in Germany including 1.5 to 7 m high
embankments. Vertical drains were first used to accelerate the
consolidation under the embankments (preloading condition).
Even using 600 kN/m woven geotextiles, vertical settlement of
around 1.5 m and horizontal displacement up to 27 cm were
measured throughout one year of monitoring. Because the
bridge could not tolerate residual settlements, Controlled
Modulus Columns (CMC) were designed and executed. The
design of the transition interface between the bridge and the
embankment, referred as the Load Transfer Platform (LTP), was
confirmed by the monitoring.
Cirión et al. (2013)
set the constructive procedures and
bases of design of rigid inclusions including the LTP. The
ASIRI guidelines (IREX, 2012) were not yet published at the
time of preparation of this paper. The paper highlights the
difference with pile foundation. In rigid inclusion solutions,
there is no mechanical link between the pile and the structure. A
LTP is usually placed between the inclusions and the structure.
This distribution layer spreads the acting loads from the
structure towards the underlying soil-inclusions setup. As
indicated by the authors, isolated or continuous footings can
possibly be used to directly transmit the loads to the soil-
inclusions setup. This GI technique can also be applied for
embankments and landfills.
The following paper constitutes a good transition with the
next topic concerning stone columns. According to
Carvajal et
al. (2013)
, dealing with the design of Column Supported
Embankments (CSE), a clear distinction has to be made
between rigid inclusions (e.g. concrete type columns)
characterized by a brittle behavior in its Ultimate Limit State
