NORTHERN BLVD
ESA’s SEM
challenge
In a project full of demanding underground
contracts, the short 125ft (38m) section of SEM
tunnel under the Northern Boulevard, has the
accolade of being one of the most technically
challenging elements of New York’s East Side
Access Project. Kyle Ott, Senior Engineering
Manager for Parsons Brinckerhoff, Paul Madsen,
Project Manager for Kiewit Infrastructure, and
Paul Schmall, Vice President and Chief Engineer
for specialty contractor Moretrench, explain
THE EAST SIDE ACCESS (ESA) project, the
largest capital construction project ever
undertaken by New York’s Metropolitan
Transportation Authority (MTA), will connect
the Long Island Rail Road’s (LIRR) Main and
Port Washington lines, in Queens, to a new
terminal beneath Grand Central Terminal, in
Manhattan. The project will increase the
LIRR's capacity into Manhattan, and
significantly reduce commuter travel time
from Long Island and eastern Queens into
east Manhattan.
The overall project includes four tunneling
contracts between the Sunnyside Rail Yard
complex in Queens and Grand Central
Terminal. In Queens, four soft ground tunnels
have been completed linking the LIRR
mainline tracks beneath Sunnyside and LIRR's
Existing Rail Yards to the Plaza Structure. In
Manhattan, new tunnels and caverns have
been mined in Manhattan schist from the
existing Bellmouth structure at Second
Avenue and 63rd Street, west and then south,
under Park Avenue and Metro-North
Railroad's four-track right of way (see NATJ,
April/May, p20).
The Northern Boulevard Crossing in Queens
is the keystone that connects the Plaza
Structure to the existing 63rd Street Bellmouth
shaft on the other side of Northern Boulevard.
It is by far the shortest of the tunnels at just
38m (125ft). Nevertheless, it presented project
designer General Engineering Consultant (a tri
venture of Parsons Brinckerhoff, STV and
Above: MIDAS finite element model developed for analysis of the SEM
excavation sequence. Right: The crossing was successfully mined by SEM
methods under a protective arch of frozen soil
24 NORTH AMERICAN TUNNELING JOURNAL
Horizontal drilling for the bottom hole of
the frozen arch was accomplished from a
pit excavated in the rock
Parsons Transportation Group), the
construction management team (URS/HMM),
tunnel contractor Schiavone/Kiewit JV, and
specialty geotechnical contractor Moretrench
with design and construction challenges.
Methodologies considered
The Northern Boulevard Crossing was
constructed using the sequential excavation
method (SEM), and is the first SEM tunnel
constructed to date within the five Boroughs
of New York. The tunnel passes beneath an
active five-track subway box structure, with
only 12ft to 16ft (3.6m to 4.8m) of ground
above the SEM excavation. Above the fivetrack subway is the Northern Boulevard itself
(six lanes), and a pile-supported elevated rail
structure for New York City Transit’s BMT Line.
The tunnel was mined through an upper
stratum of mixed glacial silty sands, a thick
deposit of bulls liver silt and silty sand, glacial
till/reworked till/outwash, decomposed rock,
and bedrock. Depth to bedrock ranges from
approximately 70ft to 90ft (21m to 27m)
below ground level, with groundwater located
about 16ft (5m) below the surface.
Groundwater control and stabilization of the
NORTHERN BLVD
Above: As-built
sequence of drift
excavation
Above left: Thermal
modeling of the
frozen arch with heat
pipes after 41 days of
operation
Left: Thermal model
of as-built pipe array
showing
temperature
distribution after 59
days of freezing
soils beneath the subway were required. In
accordance with New York State Department
of Environmental Conservation (NYSDEC)
Long Island Well Permit requirements,
dewatering was not allowed outside of the
groundwater cut-off system so as not to cause
movement of contaminant plumes that exist
within the adjacent Sunnyside Rail Yard. Initial
concepts for the crossing therefore included
various means to cut off water locally without
the need for groundwater lowering.
At the adjacent New York City Transit 63rd
Street to Queens Boulevard Connection Project
constructed in the 1990’s, such methods had
included micropile underpinning and jet grout
cut-off walls, which involved drilling through
and from within the subway box. However, for
the CQ039 Northern Boulevard Crossing,
construction from within the subway box or
from ground surface at Northern Boulevard
was not permitted so the previously adopted
methods and other various cut-off methods
considered were discarded in favor of a
horizontally constructed frozen arch around
the tunnel perimeter extending to bedrock.
This approach was considered to meet the
project criteria of groundwater cut-off and
structural settlement control while meeting the
access limitations.
Tunnel excavation sequence modeling
The design called for installing freeze pipes
along the tunnel perimeter to develop a
minimum 6ft (1.8m) thick frozen soil arch
followed by sequential excavation of six drifts
for the tunnel excavation. For the SEM design,
the frozen arch would function as a
groundwater cut-off; ground support to
provide standup time to place the initial
shotcrete liner; ground support as part of the
initial support system to limit deformations
within the shotcrete liner system during
excavation; and ground support as part of the
initial support system to limit ground
deformations (settlement) at the subway box
and at the ground surface. The frozen arch
was required to be maintained through
completion of the final liner placement and
curing to 28-day strength.
To verify the SEM design, the GEC
performed 3-D numerical modeling using the
MIDAS software. Based on the MIDAS
modeling results, an initial liner was
developed of 3in (76mm) of plain shotcrete
placed against the frozen soil, and 12in
(300mm) of welded wire fabric reinforced
shotcrete with lattice girders at 4ft (1.2m)
centers to support the soil load and subway
box along with the frozen arch.
In situ soil samples were obtained by
ground freezing contractor Moretrench and
laboratory testing was performed to evaluate
the strength and deformation behavior of the
frozen and thawed soils, thermal conductivity,
and frost heave potential to enable
finalization of the required ground freeze
temperatures.
Ground freezing design challenges
The same site constraints that precluded
vertical drilling for pre-drainage dewatering
and other groundwater control methods also
applied to the installation of freeze pipes.
Drilling of horizontally oriented pipes below
the water table, through the glacial silty
sands and “bulls liver-like” silts, was likely to
result in some loss of ground, potentially
causing settlements and movement of the
installed pipes.
The bull’s liver material would also be
susceptible to the formation of ice lenses and
heave. Ice lenses typically exert forces in the
direction of the temperature gradient, which
is normal to the orientation of the freeze
pipes and the freezing plane. For vertical
freezes, the forces are lateral, counteracted by
greater lateral earth pressures at depth, and
are rarely an issue, but for horizontal freezes
they are vertical, which could exacerbate
heave of the structure, particularly at
shallower depths.
With minimal deviation of the drilled freeze
pipes, there would still be only several feet
between the top of the frozen arch and the
bottom of the existing subway structure, and
it was from this horizon that the growth and
heave of the freeze had to be controlled.
Heat pipes, i.e. pipes with heating
elements, were installed above the uppermost
pipes of the freeze arch to control the upward
growth of the freeze towards the five-track
subway structure. Thermal modeling was
performed to evaluate the effectiveness of the
heat pipes. Thermal modeling was also
performed with the as-built freeze pipe
surveys to evaluate the time to closure and
the structural thickness of the freeze.
NORTH AMERICAN TUNNELING JOURNAL 25
NORTHERN BLVD
As with any freeze, placement of the freeze
pipes within the design tolerance is
important. Several significant conditions
would make alignment of the pipes very
challenging. Four clusters of 16 concrete filled
pipe piles had to be penetrated (without
damaging the piles), as well as cobbles and
boulders, and an undulating rock surface.
range of ground and obstruction conditions.
In areas where obstructions were not
anticipated, casing was advanced by duplex,
cased-hole, positive flush methods. All pipes
were surveyed with a gyroscope immediately
upon completion.
SEM tunneling
Upon closure of the frozen arch, tunneling
proceeded. The tunneling process was
accomplished using a step-by-step approach,
with the successive excavation and
installation of support. The three upper drifts
were excavated in 4ft (1.2m) rounds, and the
lower drifts were excavated in 8ft (2.4m)
rounds. A 9in (230mm) thick shotcrete initial
lining was installed as a temporary invert in
Settlement and heave control
In order to address any settlement that may
occur during drilling of the horizontal holes,
and any heave that may occur during the
freeze, specific mechanisms were developed.
Compensation grout holes were installed
enabling grouting to be performed to
mitigate settlement of the overlying
structures during installation of
the freeze pipes.
Provision was also made for
soil extraction from the zone
between the frozen arch and the
base of the subway box to
counteract heave of the structure
during freeze formation and
freeze maintenance, and heat
pipes were installed around the
top of the arch to control the
outward growth of the freeze.
During thawing of the frozen
ground following completion of
the SEM tunneling,
compensation grouting could be
performed through the preinstalled grout pipes if necessary
to mitigate against settlement.
Since pre-conditioning/void
filling and compensation
Frozen ground visible during drift excavation
grouting with conventional
cement based grouts could
the upper sidewall drifts. For initial support of
render the soils unworkable for subsequent
sequential excavation, the outer shotcrete
soil extraction, a specially formulated nonliner segments consisted of a 3-bar lattice
cementitious grout was developed that
girder embedded in a 1ft (300mm) thick
mimicked the strength and consistency of the
shotcrete layer. Shotcrete strength
in situ soils.
requirements were 1,800 psi (12MPa) at 24
hours, 2,500 psi (17MPa) at 3 days and 5,000
Installation of freeze pipes
psi (34.5MPa) at 28 days. Additionally, two
Maintaining drill alignment while penetrating
layers of welded wire fabric were placed with
through concrete-filled pipe pile clusters,
each 4ft (1.2m) long round. For temporary
cobbles and boulders, and an undulating rock
sidewalls, steel lattice girders were installed
surface was challenging. Accomplishing that
along with two layers of welded wire mesh.
from below the water table, through very
Only one layer of welded wire mesh was
sensitive silty soils, compounded the
installed at temporary inverts. Drift sequence
complexity of the work as well as restricting
and stagger between headings were altered
drilling methodologies. Measures were
during construction to accommodate the
therefore put into place by the ground
conditions encountered.
freezing contractor to install the pipes within
The design included a 3in (76mm)
permissible tolerances while preventing
insulating layer of flashcrete prior to lattice
ground loss.
girder installation. However, the heat of
These included groundwater control
hydration from the flashcrete would thaw a
devices (blow-out preventers) with design
thin layer of frozen soil, causing the unfrozen
features that eliminated ground loss during
soil and the flashcrete to delaminate from the
drilling as well as being able to advance
frozen ground. To resolve this issue, it was
multiple casings to overcome obstructions.
decided to apply the insulating layer together
Platform-mounted core drills could advance a
with the initial structural shotcrete layer. The
wide range of casing and tool diameters at
full thickness of shotcrete was applied in two
high or low rotation speeds to address a wide
26 NORTH AMERICAN TUNNELING JOURNAL
passes so that the inside wire mesh could be
applied by the crews under the protection of
the first pass.
The main piece of excavation equipment
was a tunnel excavator outfitted with a quick
connect that allowed for easy exchange of
four different tools: grinding head, hydraulic
hammer, aggressive bucket with tiger teeth
and a bulk excavation bucket. Shotcrete was
applied using a small track mounted robot.
The shotcrete was pumped from the surface
to the headings through a 4in (100mm) slick
line using shotcrete pumps with integrated
accelerator dosing system. All shotcrete was
delivered by a ready mix supplier, retarded
and accelerated on site. Following completion
of the SEM excavation and initial liner, a PVC
waterproofing membrane was
installed followed by a 30in
(760mm) reinforced concrete
final liner. The interior dimensions
of the final liner are
approximately 53ft (16m) wide
by 33ft (10m) high.
The SEM modeling indicated
the settlement at the crown of
tunnel to be 1.2in (30mm) and
the base of the subway box to
settle approximately 0.6in
(15mm). Slight 0.1in to 0.2in
(2.5mm to 5mm) movement was
observed within the overlying
five-track subway settlement
monitoring during tunnel
excavation and removal of the
temporary sidewalls. Following
excavation of the center drift,
only 0.3in to 0.6in (7.5mm to
15mm) of movement was
observed in the crown. No
further movement was observed during or
after the removal of the temporary sidewalls.
Given the very limited amount of
movement, ultimately, soil extraction during
freezing was not required. Currently, after five
months of thawing of the freeze, the
maximum movement at the surface is
approaching 2in (50mm). Compensation
grouting will be forthcoming to address this.
Close cooperation and teamwork between
the design engineers, owner and contractors
was key to the success of this challenging
project. Modeling of the SEM tunneling and
the frozen arch provided a sound approach to
mitigate many geotechnical uncertainties.
The ground freezing provided excellent
groundwater cut off and ground support.
SEM excavation was performed in a safe and
satisfactory manner, with actual ground and
structural deformations less than the
numerical modeling indicated. The ground
freezing installation was performed with
negligible ground loss, and no compensation
grouting was required during system
installation. Ground heave due to ice lensing
was also within acceptable limits and no soil
extraction was necessary.