2950
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 1. Distribution of soft ground areas
In the flood plains of Panape, Kalu Ganga, Welipenna and
Bentota river areas sub soil consisting of mainly peat, organic
clay, very soft inorganic clay and silt layers was found. The
total average thickness of the compressible layer was in the
range of 4 m to 11 m. In some areas loose silty sand layers were
present under the above compressible layers. In the valley areas
between hillocks, instead of cohesive inorganic clays, very
loose to loose silt and sand were found ranging from 0.5 m to
4m thickness. The details of the Geotechnical properties of the
subsoil have been given in Karunawardena and Nithiwana
(2009) and Karunawardena and Toki (2011).
3 SOFT GROUND IMPROVEMENT DESIGN
Soft ground improvement design had to be carried out in order
to control the settlements and to ensure the stability of the
highway embankment as required in the technical specification.
According to the technical specification, the embankment had to
be designed and constructed by improving the soft ground in
order to control the continued settlement to 15cm at the road
center after a period of 3 years following the acceptance of the
paving. In addition, the maximum residual differential
settlement had to be not more than 0.3% change in grade over
longitudinally within 3 years after construction. In order to
achieve the above criteria, most or all of the primary settlement
and some of the secondary settlement that would have occurred
under the final embankment height alone were forced to take
place by improving the soft ground.
The soft ground was improved mainly by using the
following methods based on the subsoil conditions. Soft clay
of shallow thickness was improved by placing a surcharge load.
Shallow peat and organic clay deposits were removed and
replaced with rock in order to support the embankments. The
subsoil with relatively thick soft clay layers were improved by
installing vertical drains and placing a surcharge load. The
embankments on the relatively thick peat and organic deposits
were constructed by improving the ground by heavy tamping
method and the vacuum consolidation method from 0.0 km to
34.5 km and from 34.5 km to 66.5 km respectively.
In rock replacement method, all compressible layers of the
sub soil were removed and replaced with rock, completely
eliminating the settlements. In the ground improvement method
of application of surcharge load with or without vertical drains,
future settlement of the highway embankment was controlled as
required in the contract by designing an appropriate surcharge
load. Most or all of the primary settlement and some of the
secondary settlement that would have occurred under the final
embankment height alone were forced to take place under the
surcharge load. In addition, it was expected that the soil beneath
the embankment would become over consolidated or stiffer due
to the surcharging of ground. The aim of applying the surcharge
was to eliminate 100% of primary consolidation settlement and
enough secondary settlement such that the residual settlement is
within acceptable performance limits. The residual settlement
for a given length of time after construction was estimated as
the remaining secondary settlement that occurs during the
required time after the eliminated equivalent time of secondary
compression has elapsed. In the design of surcharge, it was
expected to have 1.1 over consolidation ration (OCR) for
inorganic clays and 1.2 to 1.3 OCR for peat and organic clays in
order to reduce the secondary settlements during the operation
period.
4 EMBANKMENT CONSTRUCTION ON PEATY SOILS
Embankments over peaty deposits in the Southern Expressway
between Ch. 0.000 km to Ch 34.500 km were constructed by
improving the peaty soil using the heavy tamping method
whereas vacuum consolidation technique was applied to
improve the peaty soil in the Section between Ch.34.500 km to
66.500 km. This Chapter presents the details of the heavy
tamping method and the vacuum consolidation techniques
applied in the project.
4.1 Heavy Tamping Method
Heavy tamping method was designed to enforce the settlements
that would be caused by the construction of earth embankment
on soft ground by applying impact energy. Different energy
levels had to be imparted by considering the anticipated
settlement of the compressible layer under the respective
designed embankment heights. In the estimation of settlements,
all primary consolidation settlements and secondary settlements
at the end of 3 years after construction were considered. First,
the soft soil which was to be consolidated, was overlain by a
working platform of lateritic soil to facilitate the movement of
machinery. Then, a strong type fibre drain (band drain) was
installed by a machine in the soft subsoil in a square pattern
with a spacing of 1 m in order to prevent high excess pore water
pressure development in the underneath soil due to the applied
energy. The required energy was applied to the soil by dropping
a large weight on the ground surface repeatedly in phases on a
grid pattern over the entire full base width of the embankment
using multiple passes.
During tamping, once the depth of the crater formed by
pounder exceeded the height of the pounder, the crater was back
filled and leveled with soil. The dimension of the crater was
recorded in order to calculate the volume of soil introduced. The
above process was continued in all phases of the tamping
operation. Using the crater fill volumes, the enforced settlement
was calculated and if the enforced settlement was less than what
was required then another phase of tamping was introduced
until the required settlement was achieved.
After application of heavy tamping, borehole investigation
was carried out in order to assess the ground improvement.
Investigations revealed that the layer thickness of peat has been
reduced to 20% to 50% of its original thickness after the heavy
tamping. The SPT values of peat layer at a 3m to 4m depth
increased from 0 to a range of 4 to 8. Consolidation test results
showed that the value of the compression index (c
c
) and
coefficient of secondary consolidation (c
α
) has decreased
significantly. It was also noted that the pre-consolidation
pressure,
P
c
, of peaty soil has increased from 32, to as high as
85. This increase of pre-consolidation pressure means the peaty
soil is in an over-consolidated state during the service life of the
highway. All these observations confirmed that the expected
primary and secondary consolidation settlements due to the
embankment load would be very small in the areas improved by
heavy tamping.
However, it was observed that the peat layers at the deeper
depths had not achieved the above improvement. This was
investigated and it was found that the practically possible
improvement depth that could be achieved in the present
operation was about 3.5 m to 4 m. These underneath deeper soft
layers were improved after the heavy tamping operation by
keeping a surcharge load for a sufficient period of time as
reported by Karunawardena and Toki (2011). Figure 2
