January/February 2022 - 80

As part of the first phase of construction,
the ground level of the
existing building was lowered 8 ft
(2.4 m) to accommodate a large,
commercial grocery store, which will
anchor the new development. The
project scope included shoring for the
existing building, deep foundations to
support new structures and the support
of excavation for the site. Berkel, which
has successfully completed numerous
similar projects locally, was selected by
the design team for this unique project,
in part, for its background in accomplishing
the lowering of below-grade
structures.
Site Geology
The soils at the project site were
generalized into five strata: (I) fill, (II)
natural alluvial soils, (III) natural
residual soils, (IV) weathered rock, and
(V) less weathered rock. Strata IV and V
provided the bulk of the deep foundation
capacity for the project.
* Stratum I - fill/possible fill - Fill soils
at the site ranged from 4.5 to 22 ft
(1.37 to 6.7 m) below existing grades
with varying standard penetration
test (SPT) N values from 2 to 58 blows
per 1 ft (300 mm). The higher SPT N
values were attributed to the presence
of gravel and other debris.
* Stratum II - natural alluvial soils -
Natural alluvial soils were encountered
from 22 to 47 ft (6.7 to 14.3
m) below existing grade. These soils
consisted of sands, gravel and clays.
SPT N values ranged from 2 to 48
blows per foot.
* Stratum III - natural residual soils -
Natural residual soils were encountered
below Stratum II or
directly beneath Stratum I in some
cases. These soils mostly consisted
of sand or silt with some clay. SPT N
values ranged from 3 blows per foot
to 50 blows over 3 in (75 mm). Rock
fragments found in core samples
were the likely cause of the high
SPT N values in this layer.
80 * DEEP FOUNDATIONS * JAN/FEB 2022
* Stratum IV - weathered rock -
Weathered rock was observed
below Stratum III to depths of 50 to
72 ft (15.2 to 21.9 m) below the
existing grade. SPT N values typically
exceeded 50 blows per foot.
* Stratum V - less weathered rock -
Boring refusal occurred at multiple
boring locations on less weathered
rock that ranged in depth from 52.5
to 72 ft (16 to 21.9 m) below existing
grade.
difficult access and low-headroom
installation requirements, steel-cased
micropiles were selected as the
primary deep foundation system for the
existing structure. The use of micropiles
would not only meet the service
requirements of the project, but would
also be appropriate for the logistical
constraints present at this site.
Micropiles with an outside diameter of
7 in (17.8 cm) were used to support the
gravity-only column locations, and
City Ridge general subsurface profile
Foundation Choice and
Construction
As part of the initial geotechnical investigation
in summer 2016, the project
geotechnical engineer of record, ECS
Mid-Atlantic (ECS), provided recommendations
for shallow foundations,
micropiles and drilled shafts. Shallow
foundations would primarily be used for
the below-grade parking structure,
while deep foundations would be
needed for the existing structure and
future at-grade construction.
In addition to supporting the loads of
the final structure, the selected deep
foundation system would need to
support the structure during excavation.
Because of the load demands,
stringent settlement requirements,
micropiles with an outside diameter of
9 5/8 in (24.4 cm) were used to support
columns at braced frame locations. The
micropiles - with an allowable
compression load of 100 tons (890 kN) -
were installed adjacent to the existing
footings from the existing basement
slab using modified tieback rigs. Each of
the 94 existing columns were supported
by at least 4 micropiles. In total,
385 micropiles were installed in and
around the existing structure.
Deep foundat ions were al so
required for several of the new structures
being built at the site. Berkel
provided a value-engineered design
that involved installation of 16 in
(40.6 cm) diameter, 100 ton (890 kN)
auger pressure grouted (APG) piles for
the locations where limited access
January/February 2022

Table of Contents for the Digital Edition of January/February 2022
