January/February 2024 - 81

The contractor evaluated the available
laydown space, ease of vehicular
access, proximity to residential
properties, direction of the microtunnel
slope (up or down), and proximity of
sensitive facilities to determine
whether a shaft would be a launch shaft
or a receiving shaft. Several launch
shafts were used to drive the MTBM in
both directions.
Evaluation and Selection of
SOE Methods
During the tender phase, the contractor
raised the microtunnel elevations
within the allowable limits to decrease
the depth of shafts, reduce materials
and stay above the discontinuous clay
layer along the Cedar Creek alignment.
Several considerations were made in
evaluating feasible shaft construction
options: the maximum excavated depth;
minimum shaft diameter; the primarily
sandy soil conditions with high groundwater;
proximity to utilities, structures
and residences; cost and schedule for
construction; availability of equipment;
and the project performance criteria
mentioned previously.
The contractor's vertical microtunnel
profiles ranged between 24-55 ft
(7.4-16.8 m) below ground surface. To accommodate
microtunneling operations,
the team sized receiving shafts at a 21 ft
(6.4 m) minimum internal diameter and
launch shafts at a 26 ft (7.9 m) minimum
internal diameter at the bottom of the
shaft after accounting for construction
tolerances. Three shafts were sized
slightly larger to accommodate permanent
shaft features or pipe bends.
Based on predominantly sandy
ground conditions, high hydraulic
conductivity within the sand, and
depths, the project team determined
that both secant pile and cutter soil
mixing (CSM) would be effective shaft
construction methodologies for both
the Bay Park and Cedar Creek sites.
Both methods offered the advantage of
being unreinforced in a circular
configuration and provided effective
Unlike conventional soil-mixing 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.
Freeze/thaw raveling degradation of the CSM shaft walls
watertightness with the inclusion of a
bottom seal.
The primary pile drilling method for
secant construction was the use of a
cased continuous flight auger (CCFA),
which allowed for quick construction
over traditional auger methods. With
the CCFA setup, the maximum secant
depth was 63 ft (19.2 m) with a diameter
of 29.5 in (750 mm).
DEEP FOUNDATIONS * JAN/FEB 2024 * 81

January/February 2024

Table of Contents for the Digital Edition of January/February 2024

TOC
January/February 2024 - Intro
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January/February 2024 - TOC
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