January/February 2024 - 82

The CSM method mixes in situ soil
with cement and water to form
rectangular soil-cement (soilcrete)
panels. The individual panels can be
interlocked to provide structural rings
for circular shafts. The CSM system is a
modified trench cutter hydromill type
machine. Unlike conventional soilmixing
tools, such as augers that are
mounted in a vertical axis and turn
along a horizontal axis, the cutters for
the CSM method are mounted on a
horizontal axis and turn on a vertical
axis. In this case, the panels were
3.2 ft (1 m) thick by 9 ft (2.7 m) long and
could reach a maximum depth of over
140 ft (42.7 m).
CSM was not a method listed within
the tender documents and therefore
was proposed by the contractor as an
alternative technical concept. CSM
would increase watertightness because
there are fewer joints. In addition,
spoils disposal would be reduced
because cement would be mixed with
in situ soils, thereby reducing carbon
emissions and creating less impact to
the community.
Other allowable shaft construction
methods such as slurry wall, sunken
caisson and vertical shaft machine
were eliminated for cost reasons.
For tender evaluation, cost and
schedule were assessed in the design
for both shaft types. Ultimately, the
contractor proposed circular CSM for 12
of the microtunnel shafts, with most
being a single ring of CSM panels. Three
of the 12 shafts are deeper, larger shafts
and were designed as a double ring of
CSM panels. CSM design strength was
generally 400-600 Psi (2.8-4.1 MPa)
with the range observed during
construction between 400-2,800 psi
(2.8-19.3 MPa), and averages generally
around 860-1,170 psi (5.5-8.1 MPa)
excluding the high and low strength
outliers. The shafts have performed
successfully, although a few required
protection from freeze/thaw to prevent
degradation of the soilcrete.
82 * DEEP FOUNDATIONS * JAN/FEB 2024
Secant pile construction was
selected for three shafts. One location
was incorporated into the permanent
building foundation as proposed
through a separate alternative technical
concept. In this case, a wide flange
beam or a single large-diameter allthread
bar placed within the piles was
used for foundation support.
During construction, one shaft was
redesigned from CSM to secant pile
because CCFA installation proved to be
both effective and efficient. Further, one
shaft was modified to a braced sheet
pile excavation.
Shaft Bottom Seals
During tender, the designer evaluated
options that would limit groundwater
inflows to the allowable limits. These
options included (1) extending the
panels to a significantly greater depth to
reduce the inflow rate, (2) using a jet
grout bottom seal, (3) using a tremie concrete
bottom seal and (4) toeing into the
clay layer along the Cedar Creek profile.
The option selected for most of the
shafts was a top-hat style jet grout plug,
installed after the secant piles or CSM
panels. The plug would bear on the
bottom of the piles or panels and extend
upward into the shaft to lengthen the
potential path for water infiltration. Design
jet grout strengths were generally
750 psi (5.2 MPa). The range of jet grout
strengths observed during construction
was 130-4,640 Psi (0.9-32 MPa), with an
average of 1,275 Psi (8.8 MPa). All jet
grout plugs were successful.
The geotechnical baseline report for
the project noted the clay layer along the
Cedar Creek alignment to be potentially
discontinuous. Given the artesian
conditions and after a review of the
boring logs and further investigation, it
was judged that three of the shafts could
potentially use this clay to seal off water
inflows. However, investigations during
the design phase revealed that two of
these locations either did not have
enough thickness to resist artesian
pressures or had silt lenses. These two
sites were deemed too risky, and
consequently hybrid jet grout and clay
plugs were selected for bottom seals.
At the Cedar Creek Shaft 1 location, a
20 ft (6 m) consistent clay layer was
confirmed with additional geotechnical
investigation. The strength of this clay
was sufficient to safely use this layer as
The option selected for most of the shafts was a tophat
style jet grout plug, installed after the secant piles
or CSM panels.
Top hat style bottom seal
January/February 2024

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