September/October 2024 - 92

Site history
maps, the site was developed and
redeveloped multiple times since the
mid-1800s including use as a lumber
yard, various storage facilities and a
construction yard. Multiple layers of
abandoned foundations and dilapidated
waterfront structures of various types
including timber, concrete, brick and
stone existed at the site. The immediate
vicinity of the site is a designated
Superfund Cleanup Site by the EPA
primarily because of contamination
from manufactured gas waste.
Subsurface Conditions
The site falls within the extents of a
buried marsh and near the southern
extents of the Wisconsin glaciation.
Overburden at the site consists of a
miscellaneous granular fill stratum,
soft organic clay, loose to medium
compact sand with trace to some fines,
medium compact to compact sand with
trace to some fines, and bedrock at a
depth of greater than 200 ft (61 m).
General Design
To connect the buildings below grade
wi th e l eva tor access r equi r ed
excavation through the organic clay
stratum up to about 15 ft (4.5 m) below
groundwater level and within the
stratum of loose to medium compact
sand in two 40 sq ft (4 sq m) areas.
Regional data available from the U.S.
Geological Survey has documented the
highly permeable nature of the
granular soil strata identified at the site.
Based on this data, the anticipated
dewatering volume was expected to be
on the order of 1,000 gal (3,785 L) per
minute for each of the two excavations
with commensurate influence radii
extending beyond the site and into
existing contaminant plumes.
Based on the anticipated costs of
pumping, treatment and discharge as
well as the potential for contaminant
transport, Mueser Rutledge Consulting
Engineers proposed an SOE solution
that created a watertight excavation by
cutting off water inflow from the
excavation bottom and sides. The
project team considered three
watertight design solutions:
driven steel sheet piles with a jet
grout cutoff, concrete secant piles
with a jet grout cutoff or soil mix
secant piles with a soil mix
cutoff. The first two solutions
came with a high price tag and
the third solution was relatively
unproven in the area.
Subsurface conditions
Initial Design
The selected preliminary SOE design
utilized a soil mix blanket across the
bottom of the proposed excavation and
surrounded by a soil mix secant pile
wall reinforced with steel W sections
with one level of internal steel bracing.
Initial designs contemplated soil
mix that required an unconfined
compressive strength (UCS) of about
300 Psi (2 MPa). For the required
strength and a span of about 40 ft (12 m),
this required a soil mix blanket of up to
about 10 ft (3 m) thick to resist the
hydrostatic uplift forces on the
watertight soil mix block.
Design-Build
The SOE contractor proposed modifying
the design by replacing the soil mix
secant wall by extending the soil mix
walls to a minimum of 4 ft (1.2 m)
beyond the excavation extents utilizing
10 ft (3 m) diameter columns spaced at
8.5 ft (2.5 m) on center in a triangular
pattern and reducing the required UCS
by 50% to 150 Psi (1 MPa). This was
determined to be acceptable due to the
expansion of the soil mix extents such
that the wall thickness of soil mix was
doubled. The bottom blanket was
checked for thickness and reduced to
5 ft (1.5 m) thick.
In a further effort to reduce costs
and accelerate construction, the SOE
contractor suggested eliminating a test
section and proposed verifying the
92 * DEEP FOUNDATIONS * SEPT/OCT 2024
September/October 2024

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