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Technical Committee 214 /
Comité technique 214
illustrates the major steps in the heavy tamping ground
improvement method adopted in the project.
Figure 2. Major steps in heavy tamping ground improvement
4.2 Vacuum consolidation method
In the application of vacuum consolidation method, about a 1.0
m to 1.5 m thick fill was constructed on the original ground
surface to form a working platform for the band drain
installation machine. Band drains were installed by a machine
up to a designed depth from the original ground surface in a
square pattern with a spacing of 1 m. Thereafter, flexible
horizontal drains (300 mm wide and 4 mm thick) were laid on
top of the fill with a horizontal spacing of 1 m and then
connected to the vertical band drains in order to ensure adequate
horizontal drainage capacity. Subsequently, the tank system was
installed and connected to the designed pipe systems. Small
ditches were excavated perpendicular to the horizontal drains at
20 m intervals and filled with aggregates after placing
perforated pipes. Instrumentation such as settlement plates,
displacement stakes, electrical piezometers and differential
settlement gauges were also installed at the designed depths.
After installation of vertical drains, horizontal drains, perforated
pipes and separator tanks, the surface of the treatment area was
covered by a protection sheet. Thereafter, an air tight sheet was
laid on top and the periphery trench system was constructed to
provide air tightness and the necessary anchorage at the
boundary of the treatment area. Vacuum pressure was then
applied using a vacuum pumping system patented by Maruyama
Industry Co. Ltd, Japan by connecting the suction and water
hoses to the vacuum pump. After confirming that there were no
leaks through the air tight sheet, filling was commenced.
It was expected to apply the surcharge by means of a
vacuum pressure of 70kPa to compensate the primary
consolidation settlements and to minimize the secondary
settlements that can take place in the proposed highway
embankment. However, in many areas the applied vacuum
pressure was less than the designed value and therefore the
above designed surcharge was applied by means of both
vacuum pressure and embankment fills. The designed load was
kept until the expected settlement completed.
5 ASSESSMENT OF THE SOFT GROUND
IMPROVEMENT
The continuous assessment of the improvement of soft ground
was carried out by conducting the field monitoring program. In
addition, the soft ground improvement was assessed by
conducting appropriate field and laboratory testing.
5.1 Field monitoring program
The improvement of the soft ground was monitored through the
measurement of settlement and the excess pore water pressure
during the construction period. Settlement plates were installed
at the top of the soft layer or on top of the pioneer layer and
piezometers were installed at the middle of the soft layer. The
settlement stakes were installed near the toe of the
embankments to check the stability during the construction. In
addition to the above, in the areas improved by vacuum
consolidation, a vacuum pressure monitoring unit was used to
measure the vacuum pressure at the pump and under the air
tight sheet. Also, a water discharge meter was used to measure
the rate and the total discharged water flow due to the vacuum
operation. An automatic data acquisition unit was connected
with the piezometer, vacuum pressure monitoring unit and
water discharge meter to keep continuous records.
The decision to remove the surcharge was made on the basis
of the monitoring data obtained during the surcharge period.
The aim was to eliminate 100% of primary consolidation
settlement and enough secondary settlement such that the
residual settlement was within acceptable performance limits.
The primary consolidation settlement was assessed by
estimating the degree of consolidation and in this project it was
estimated by the method outlined by Asaoka (1978). The degree
of consolidation was also calculated based on the pore water
pressure (PWP) measurements, and laboratory consolidation
testing of peaty samples after the treatment program. The
comparison of the degree of consolidation for each method for
some areas improved by the vacuum consolidation method is
shown in Table 1.
Table 1. Estimation of the degree of consolidation
Degree of Consolidation
Location
Asaoka
Method
Laboratory
Data
PWP
83.10%
Ch. 45.380 –
Ch. 45.430
97.83%
73.87%
79.46%
100.00%
Ch. 47.850 –
Ch. 47.920
97.10%
100.00%
100.00%
80.21%
Ch. 52.950 –
Ch. 53.000
97.57%
90.91%
100.00%
96.70%
Ch. 53.660 –
Ch. 53.730
96.65%
83.62%
68.71%
If the degree of consolidation from the PWP measurement is
assumed to be accurate, Asaoka Method accurately estimates
the degree of consolidation in treatment areas Ch.47.850 to
Ch.47.920 and Ch.52.950 to Ch.53.000 whereas Asaoka method
over predicts the degree of consolidation in treatment areas Ch.
45.380 to Ch. 45.430 and Ch.53.660 to Ch. 53.730. However, in
treatment area Ch.53.660 to Ch.53.730 the degree of
consolidation from the laboratory test results agreed very well
with the same estimated from the Asaoka method. Therefore,
based on this investigation it can be concluded that the degree
of consolidation estimated from the Asaoka method is
reasonably accurate.
In order to assess the secondary settlements, for each
monitoring point, the long-term settlement was predicted by
extrapolating the secondary settlement rate over a period of 3
years. Predictions were made by preparing a plot of
displacement against log (time) for each settlement plate, with
the best-fit line through the data extended to define the likely
settlement after 3 years. The surcharge was removed only after
confirming the residual settlement by considering both the
primary and secondary consolidation settlements as described
above.
5.2 Investigation to confirm the ground improvement
Site investigation was carried out to assess the actual ground
improvement in the areas improved by the vacuum
consolidation method just before the removal of surcharge.
Investigation was carried out in the improved as well the
adjacent unimproved area in order to assess the ground
improvement. Investigation revealed that initial thickness of the
peat layer has been reduced by 50%-60% after ground
improvement. The above reduction agreed reasonably with the
