March/April 2021 - 18

Alternate Excavation Support
Due to structures and utilities nearby that could be damaged by
potential vibrations and settlements associated with the
excavation, the MTA-NYCT contract suggested rigid wall
systems consisting of either secant pile walls or diaphragm
walls for construction of the new entrances for the First
Avenue Station and substation at Avenue B. At the station's
north entrance, due to limited space between the tunnel and
existing Con Edison oil-o-static lines running adjacent the
subway box, a combined king pile and sheet wall system had
been suggested. The initial MTA-NYCT contract drawings
showed extensive use of secant pile walls and jet grout vertical
curtains extending down to bedrock at depths of approximately
70 ft (21.3 m) below ground surface to provide stiffness and
groundwater cutoff while minimizing construction impacts.
MRCE, in coordination with Judlau Contracting, developed
an alternative concept for excavation support and groundwater
cutoff for use at both new station entrances and the Avenue B
substation. The alternative utilized pressed-in steel sheet pile
walls, a subgrade jet grout blanket of limited volume and
passive tie-downs to resist buoyancy forces. Internal lateral
supports for the walls consisted of conventional wales and
struts. This alternative concept provided significant savings in
cost and schedule when compared to the secant wall and jet
grout curtain scheme originally suggested in the MTA-NYCT
drawings, while maintaining the same level of protection to the
existing adjacent structures.

First Avenue Station Upgrades
After the required utility relocations, the excavation support
walls for the new entrances were installed along the new
construction perimeters on both sides of the existing tunnel.
The alternative walls consisted of AZ40-700N steel sheet
piling installed by the press-in method. This method uses
hydraulic jacking equipment to advance the piles into the
ground without vibration, minimizing impacts to adjacent
structures and utilities. Installation of the steel sheet piling
was performed using the Silent Piler machine of Giken of
Japan, the first press-in method use in Manhattan.
The sheet piles were installed with approximately 10 ft
(3 m) of embedment below final excavation level. With this
alternate scheme, it was not necessary to provide full
embedment of the excavation support walls to top of bedrock.
At the south entrance, an existing secant pile wall previously
installed for excavation of an adjacent basement was
connected to the new sheeting and reused for a portion of the
new excavation support alignment. At locations of existing
active utility lines that crossed the excavation support wall,
vertical jet grout curtain walls were installed. The jet grout at
these locations were designed to arch between adjacent sheet
pile pairs. Jet grout closures were also required between the
sheet piles and the existing subway tunnel.
18 * DEEP FOUNDATIONS * MAR/APR 2021

Then, 8 ft (2.4 m) thick jet grout blankets or " plugs " were
installed that covered the entire excavation footprint just
below subgrade. This " closed off " the space against the existing
tunnel and provided groundwater cut off. The jet grout
blankets were installed by drilling from grade level prior to any
excavation. The blankets also provided lateral support to the
sheet piling at subgrade level and provided groundwater cut off.
To provide resistance against uplift forces on the jet grout
blanket due to buoyancy, the next step was installing tiedowns with a design load of 65 tons (60 m tons). They were
installed at 10 ft (3 m) on-center spacing, and consisted of
drilled and pressure-grouted steel threadbars anchored in the
glacial soils below. Connection of the tie-downs into the jet
grout blanket was achieved by bond between the jet grout and
the anchor grout. The tie-downs significantly reduced the
thickness of the plug that would have otherwise been needed
to resist uplift forces if relying on gravity alone. Load tests were
performed on the tie-downs to verify both the geotechnical
capacity of the pressure-grouted bond zone, and the capacity
of the bond between the anchor and the jet grout plug.
A temporary deck was installed at street level prior to
excavation. The deck structure had steel beams vertically supported on the sheet pile walls, with removable timber decking
spanning between the beams. The decking served to provide a
platform for construction equipment movement while in use,
and to provide lateral support at the top of the sheet pile wall.
Excavating south entrance
to First Ave. Station

March/April 2021

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