March/April 2022 - 76

the southwest corner. These structures
were demolished many years prior to
current construction and the site has
since been used as paved surface parking.
Subsurface Conditions
To explore the subsurface conditions at
the site, the geotechnical engineer of record
for the project, Haley & Aldrich, conducted
several rounds of test borings,
geoprobes and test pits. Data from the
exploration programs indicated that the
subsurface soil conditions generally
consisted of five layers. Those subsurface
layers consisted of fill, or debrisladen
fill, extending between 8 and 17 ft
(2.4 to 5.2 m) below the ground surface,
followed by organic/estuarine deposits
from 1.5 to 8.5 ft (0.5 to 2.6 m) thick, to
marine (clay) deposits from 34.5 to 47.5 ft
(10.5 to 14.5 m) thick, to glacial deposits
of 15.5 to 36 ft (4.7 to11.0 m) thick. Below
the glacial deposits, bedrock was
encountered starting at elevations of -56
to -80.5 ft (-17.1 to -24.5 m).
The fill generally consisted of medium
dense silty sand or sandy lean clay
with various amounts of silt, gravel,
brick, concrete, cinders and ash. Explorations
conducted in the southwest corner
of the site encountered significant
debris consisting of concrete, reinforced
concrete, steel, granite blocks and other
building rubble from depths of 4 to 19 ft
(1.2 to 5.8 m) below the existing site grades.
The organic/estuarine material was
described as soft organic soil, fibrous peat
or poorly graded sand with trace organic
matter. The marine deposits consisted of
lean clay that was slightly overconsolidated
at the top of the layer and
then became softer with depth. The glacial
deposits transitioned twice: from a hard to
very stiff sandy, lean clay (glaciomarine
deposits) to a medium dense to very dense
sand with silt and gravel (glaciofluvial
deposits), and then to a dense to very
dense sandy silt with gravel (glacial till
deposits). Below the glacial deposits was
bedrock consisting of an argillite or
diabase material. Groundwater was
generally observed to be approximately
8 ft (2.4 m) below the existing site grades.
76 * DEEP FOUNDATIONS * MAR/APR 2022
Addressing Foundation
Challenges
The fill and organic/estuarine deposits
were not considered suitable for
foundation bearing in their current
condition. Haley & Aldrich evaluated the
feasibility of improving the near-surface
fill and organic/estuarine deposits using
shallow ground improvement to
transfer the building loads to the top of
the marine clay deposit. However,
foundations bearing above or in the
marine clay would have been subject to
excessive long-term settlements due to
consolidation of the clay; this would
have resulted from loadings imposed
from the building and the weight of fill
used to raise site grades.
To reduce the predicted foundation
settlements, the geotechnical engineer
also evaluated several foundation options.
They included deep driven piles
or micropiles bearing in the glacial
deposits/rock, improvement of site soils
using deep ground improvement elements
or site surcharging. In addition to
evaluating foundation settlements,
long-term settlements of the lowest
level floor slab were evaluated. Ultimately,
Haley & Aldrich recommended
the building be supported on conventional
spread footing foundations sized
for 8,000 psf (384 kPa) allowable bearing
pressure; these foundations would follow
the installation of ground improvement
elements consisting of Geopier
GeoConcrete Columns (GCCs) that
would stiffen the near-surface soils and
transfer loads to natural, inorganic
glacial deposits. The geotechnical
engineer also recommended that the
lowest level slab be constructed as a
conventional slab-on-grade, following
improvement of the existing fill and
organic/estuarine deposits using shallow
ground improvement (rammed
aggregate piers). To further mitigate
long-term slab settlement, there would
also be a site-wide preloading program
with placement of a soil surcharge to 6 in
(152 mm) above the elevation of the
lowest-level floor slab. Surcharge was to
be placed for a period of at least 6
Soil conditions across the site
March/April 2022

Table of Contents for the Digital Edition of March/April 2022
