DFI Magazine January/February 2020 - 98

For shallow columns where exhumation is possible, visual examination is the
best means of confirmation. Due to the
consistent ground conditions with depth, it
was determined that the shallow exhumation and observation were adequate for this
test program. Coring could have been
performed but would have required a wait
time of at least a week, if not longer, to
obtain intact samples.

columns that are still fluid or from spoils at
the ground surface, as mandated by the
project requirements. To retrieve wet grab
samples at specific depths, a specific
sampling device is lowered into the jet
grout column, which can prove difficult if
the jet grout column does not extend all the
way up to the ground surface. For example,
jet grouting may only be required for a 10 ft
(3 m) thick bottom plug that is 50 ft

Jet grouting can be used as a
form of structural
underpinning as well as an
earth retention system for
building excavations. It is
effective at controlling soil and
building movement during
excavation and building
movements during installation.

Exhumation of three test columns in a linear array

Wet grab samples are commonly
obtained during installation and are used
as a means of testing the soil-cement
material for strength and hydraulic
conductivity. Wet grab samples can be
taken at depth in recently installed
98 * DEEP FOUNDATIONS * JAN/FEB 2020

(15.2 m) below the ground surface.
The drill hole is typically about 6 in
(152 mm) in diameter, which makes it very
difficult to insert a sampling device from
the ground surface. Furthermore, if a
working slab (concrete) is present, it may

be impractical to insert a larger sampling
device into the drill hole made through the
working slab. In these instances, wet grab
samples retrieved from the ground surface
have proven to be sufficient for predicting
strength and hydraulic conductivity.
Cylinders to be tested for unconfined
compression strength (UCS) were made
from the spoils collected at the ground
surface while the jetting was being
performed at prescribed depths. Since the
in-situ soil was being turbulently mixed
with the cement grout being injected, it was
assumed that the return spoils were of
similar consistency to the jet grouted
material left in the ground. In general,
comparisons have shown that wet grab
samples achieve lower values of UCS than
cores obtained after installation. This is
most likely due to the enhanced in-situ
curing environment and the fluid head
pressures that may facilitate pressure
filtration and subsequent lowering of the insitu water/cement ratio of the soil-cement.
The design strength of the jet grout
column is based on the 28-day UCS of the
soil-cement, similar to the design approach
for concrete structures. Due to the heterogeneity of the ground, it is typical to get a
wide range of measured UCS. The 28-day
UCS obtained from compression testing
ranged from 1,000 to 2,800 psi (6.90 to
19.31 MPa), with the majority of the UCS
falling between 1,200 and 2,300 psi (8.27
to 15.86 MPa). This wide range of UCS
should be expected, especially as the target
design strength is lower.
DFI Magazine January/February 2020

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