Underground Construction - August 2021 - 21

installed large-diameter Hobas pipe in
the Washington, D.C. Clean Rivers First
Street Tunnel project, and were familiar
with its characteristics and installation.
Work began on the Historic Broadway
Station in 2016. Soldier piles were drilled
into the Fernando Formation bedrock
along 2nd Street and deck beams were set
to support the decking for the restoration
of the 2nd Street roadway for continued
public use during construction below.
The traffic decking, constructed with an
entrance structure, enabled crews to access
the subterranean construction zone.
With preparations completed, workers
began excavation to expose utilities down
to the invert of the existing storm drain
at 30 feet below grade. The contractor
had taken delivery of 320 linear feet
of 126-inch-diameter, 46-psi stiffness
rated pipe in an off-site staging yard.
Then, during non-peak traffic time, RCC
crews were able to take a lane closure
to bring in and lower the sections of
Hobas pipe through the access window.
Making it happen
A temporary support system for the pipe
installation was devised using vertical
supports and a series of threaded rods
to attach nylon web slings to suspend
the pipe from the temporary ceiling/
supporting deck beams for the 2nd
Street decking. As each 20-foot section
of pipe was lowered through the deck
opening, it was moved into place with
telescoping material handlers, suspended
below the traffic decking with web slings
along the north side of the existing
RCP storm drain, and jacked into
connection with the previous section.
The pipe was secured at grade with
turnbuckles and ultimately with lateral
and longitudinal braces. There it would
remain suspended for the remainder
of the excavation, station build-out
and backfill to the pipe's grade.
As the new 300-foot line neared
completion, the existing RCP storm drain
was removed up to a couple of feet above
its invert. This work was done during Los
Angeles' dry season, from April to October,
to take advantage of the low flow at that
time of year. A sandbag dam and pump
were able to handle the minimal base flow
during the construction of cast-in-place
transition structures to connect the new
line to the existing storm drain at either
end of the cavern. Upon completion of
the connection, the remaining lower
portion of the RCP was removed.
Below these suspended utilities, more
than 70 workers used compact and midsized
excavating equipment to dig down
another 54 feet to the invert of the already
bored rail tunnels, creating a cavern
approximately 440 feet long by 55 feet
wide. The 31,500 cubic yards of material
was removed through the access opening
in the decking and trucked offsite. More
extensive than the excavations associated
with the other two underground stations,
this " cavern " under 2nd Street will
accommodate cross-over switches between
the tracks. All the while, the suspended
storm drain system remained in service.
With build-out of the station's concourse
level and installation of the station roof
concrete slab, the cover phase of the " cut
and cover " operation commenced with
replacement of the fill material to within
inches of the pipe, just to the point where a
skid steer could safely perform compaction.
At this point, the fill material was changed
to 100 psi slurry surrounding the new pipe
by approximately 18 inches, which after
curing, provided an equivalent E' (Msb)
of at least 25,000 psi per AWWA M45.
Split installs were not recommended due
to the potential of stress concentrations
at the interphase of rigid to flexible
surround. Therefore, incorporating a
full-surround slurry for pipe support
was a sound engineering decision.
Above the slurry, the backfill material
reverted back to compacted soil, topped
with the pavement base course for
the reconstruction of 2nd Street.
The new section of 126-inch Hobas
CC storm drain, totally and permanently
encased, will continue serving the LA
Flood Control Department and the
citizens of Los Angeles indefinitely.
FOR MORE INFORMATION:
Hobas Pipe USA, (800) 856-7473,
(281) 821-2200, hobaspipe.com
UCONonline.com | AUGUST 2021
21
http://www.hobaspipe.com http://www.UCONonline.com
Underground Construction - August 2021

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