July/August 2021 - 18

capacity to demand; this allowed for just three overall types of
anchors with a consistent design load for each type over the
approximate 3,000 total anchors required for the project. The
soil nails were Grade 75 tendons installed in 6 or 8 in (150 or
200 mm) diameter cement-grouted holes and were not
tensioned. The rock anchors were 1.38 in (35 mm) diameter,
150 ksi (1,000 MPa) tendons installed in 4 in (102 mm)
diameter cement-grouted auger drill holes. Rock anchors
adjacent to structures were prestressed to limit the potential
for movement, while passive anchors were utilized otherwise.
Ultimate bond stresses varied from 10 psi (70 kPa) in the soils,
to up to 130 psi (900 kPa) in the limestone.
Performance and proof test loads varied from 20 - 200 kips
(90 - 900 kN) and were based on the material into which the
anchors were bonded, and the actual grouted bond length: a
5 ft (1.5 m) minimum to 15 ft (4.6m) maximum. Lateral
deflections for the retention system were restricted to 0.5 in
(12 mm) adjacent to structures and 1.0 in (25 mm) elsewhere.
Surcharge loads varied from 250 psf (12 kPa) for street traffic
to 1,200 psf (57 kPa) for heavy cranes.
Foundation loads from adjacent structures
with influence on the retention system
were considered on an individual basis.
In total, over 50 unique cross sections
were developed over the excavation
perimeter. Design computations were
performed with SLOPE/W software for
local and global stability analyses, and an
internally developed spreadsheet to
evaluate potential planar sliding within
the rock mass, based on the estimated fault
location and orientation.
Excavation began in July 2018 for Phase
1A, in which the length of the soil nails in
the surficial soils was limited to 10 ft (3 m) to
stay within the property limits (i.e., outside
the public right-of-way), as construction
was anticipated to start prior to final license
agreements being obtained. This restriction
required a tight 4 ft by 4 ft (1.2 m by 1.2 m)
pattern of the 8 in (203 mm) diameter drilled
hole soil nails to maintain local stability in
the surficial soils with the very low available
pullout strength. While a soldier piling
alternative was evaluated, the soil nail
option proved more economical to construct
and provided a significant schedule advantage. In Phases 1B
and in 1C (excavated from February 2019-September 2019, and
August 2019-August 2020, respectively), longer soil nails could
be used, permitting a larger pattern spacing.
In locations with utilities immediately behind the
excavation face that precluded soil nails, double channel
soldier piling was utilized in certain portions of each sub18
* DEEP FOUNDATIONS * JULY/AUG 2021
phase of excavation to permit the upper level of global
stability anchors to be installed beneath the utilities, while
still providing facing support. A 5 ft (1.5 m) minimum
clearance between the anchors and existing or proposed
utilities was required. As soldier piling was not required
within the rock stratum, the length could be kept short -
pilings were only installed in the upper 20 ft (6.1 m) of the
excavation depth. Below the piling, a general section of rock
anchors and shotcrete was utilized.
In several locations, the retention system anchors were
detailed to accommodate future improvements. Where these
improvements could not be installed until after the retention
system was abandoned, fiberglass tendons were utilized to
minimize the effort required to cut through the abandoned
anchors. A reinforced shotcrete facing was required to
stabilize the surficial clays and weathered rock between the
soil nails. Although shotcrete was not typically required to
serve a structural function in the rock, it was provided as a
substrate for the basement's blindside waterproofing system.
General excavation cross section
Relocated Utility Support Structure
A critical part of the excavation was temporary support of
existing utilities over the planned Phase 1C excavation. Fiber
optic and chilled water supply/return lines (for climate control
of adjacent buildings), as well as storm and wastewater lines,
all had to remain in service during construction. The structure
developed for this purpose, called the Temporary Relocated
July/August 2021

Table of Contents for the Digital Edition of July/August 2021
