January/February 2024 - 90

8. The concrete footing was poured,
being centered above nine piers. A
total of five settlement points were
established on top of the concrete
footing. One point was set in each
corner and one in the center of the
footing. The settlement from the
weight of the concrete footing was
measured to be 7.5 mm (0.3 in). The
concrete blocks were stacked on top
of the footing to apply the prescribed
design load to the footing.
The footing was loaded in 25% load
increments using 5-ton concrete blocks
until the target load of 150 kPa (22 Psi)
was obtained, which pertained to 100%
of the design load. Settlement was
recorded at every stage before loading
was resumed. The 100% design load was
achieved 16 hours after commencement
of loading.
The movement of the five settlement
points was monitored using two
precision survey levels. The levels were
calibrated with an established reference
point outside the test zone prior to
each reading. Settlement readings were
taken for a total period of 24 hours after
the 100% load was achieved, in
Concrete loading blocks placed on top of test footing
accordance with the approved method
statement for the test. Additional
readings were taken up to five days
after commencement of loading.
Zone Load Test Results
A total average settlement on the order
of 30 mm (1 in) was recorded from the
design load of 150 kPa (22 Psi), and the
largest differential settlement recorded
between two settlement points was
1 cm (12 in) over a 10.4 m (34 ft) distance.
The pier design successfully
controlled settlement of the zone load
test to meet project specifications.
Consolidation occurred rapidly
during loading of the footing. Using
Boussinesq plots of stress with depth as
reference, most of the footing loads
Plot of overlain pre-installation and post-installation CPT results at the test site
were likely concentrated in the
densified sands to 5 m (16 ft) depth
below the footing. The settlements on
the silt layer were, therefore, much
lower than on the densified upper
sands. The authors believe that the high
permeabi l i ty of the pi ers a l so
contributed to rapid consolidation of
the silts. The high permeability of the
gravel likely facilitated radial drainage
into the piers and provided a rapid
drainage pathway to alleviate excess
pore pressures.
The 100% design load was
achieved 16 hours after
commencement of loading.
Elastic Modulus BackCalculation
Given
the relative scarcity of largescale
zone load tests on ground
improvement elements, the team set
out to back calculate the elastic
modulus values used for design.
Settlements were estimated using a
composite elastic modulus approach.
This approach consists of summing the
estimated settlement in the reinforced
zone (the " upper zone " ) and the
estimated settlement in the zone of soil
below the bottoms of the elements (the
" lower zone " ).
The compression of the upper zone
is estimated using conventional settlement
calculations as shown in the
following equation:
Where q is the average bearing
pressure, Iσ is the influence factor at
depths within the compressible layer, H
is the thickness of the compressible
layer, and Ecomp is the composite
elastic modulus value within the
aggregate pier-reinforced zone.
In cohesive soils, the equivalent
constrained modulus (or Eequivalent)
can be back calculated from either compression
ratios (Cεc) or recompression
90 * DEEP FOUNDATIONS * JAN/FEB 2024
January/February 2024

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