March/April 2021 - 17

Mueser Rutledge Consulting Engineers (MRCE) served as
the geotechnical engineering subcontractor to the JV that
developed alternate excavation and groundwater cutoff
systems. This approach replaced the suggested system shown
on the contract documents, and involved MRCE working with
Judlau Contracting. During project bidding, Judlau considered
the cost and schedule savings of the innovative designs in the
subcontractor's bid, which together with other factors, resulted
in the lowest qualified project bid. To minimize the risk
associated with the alternate systems, a budget for additional
subsurface geotechnical investigations was included in
Judlau's bid price to develop and construct the new belowgrade structures.

Station Improvements and
Electrical Substation

The subsurface investigations indicated bedrock at
approximately 70 ft (21.3 m) and deeper below existing grade
at the First Avenue Station and Avenue B construction areas.
The typical subsurface profile consists of approximately 10 to
25 ft (3 to 7.6 m) of miscellaneous fill followed by
discontinuous, thin layers of organic deposits and alluvial
river sands. Underlying these strata and generally near the
base of the required excavations, the site is covered with a
thick layer of glacial lake sand and silt sediments. A layer of
glacial till, underlain by decomposed rock, lies below the
glacial lake deposits and above the bedrock.
Test pits were also performed to investigate the
foundation conditions of abutting structures on the station's
south side and to locate critical utilities requiring special
protection within the support-of-excavation limits for the
station's north entrance. Of high importance to the MTANYCT was minimizing impacts to adjacent Con Edison oil-ostatic lines, as well as minimizing groundwater drawdown
outside the excavation that could cause detrimental
settlement of adjacent structures and utilities.

The subway structure under 14th Street was originally
constructed in the 1920s. The rectangular, steel-framed,
concrete tunnel box carries two train tracks and widens from
1st Avenue to Avenue A to accommodate station platforms and
stairways. The base of the structure
rests on permeable glacial soils at a
depth of approximately 30 ft (9.1 m)
below street level.
The First Avenue Station improvements included construction of added
stairway entrances and ADA elevators,
requiring excavations of approximately 20 ft wide by 140 ft long
(6.1 m by 42.7 m) on the north and south
sides of the station (extending to the
base of the existing structure). At the
Avenue B location, constructing the
Manhattan tunnel upgrades for accessing First Avenue Station below street, and to add nearby substation
new electrical substation required an
approximately 40 ft wide by 190 ft long
(12.2 m by 57.8 m) excavation adjacent to the north side of the
running tunnel, also extending to the existing subway
structure's base. In both cases, the excavation subgrade
was at a depth of approximately 20 ft (6.1 m) below the
groundwater level. The work completed in November 2019
needed to be performed while maintaining 14th Street open
to traffic throughout construction. Sidewalk access also was
needed to many businesses and residential buildings lining
the street on both sides.

Subsurface Conditions
For this challenging portion of the project, the first step was to
perform additional subsurface geotechnical investigations
for optimal design of the excavation support and groundwater
cutoff systems that supplemented prebid phase information.
Additional borings were performed, groundwater levels were
monitored using piezometers, and soil samples and rock cores
were taken and classified.

Sheet piling installation for excavation support of new station entrances
DEEP FOUNDATIONS * MAR/APR 2021 * 17

March/April 2021

Table of Contents for the Digital Edition of March/April 2021