Underground Construction - August 2021 - 15

LEAD PHOTO: Aerial photograph of 84-inch diameter double disc
gate valve being lowered into position. (Photo Credit: LAN)
ABOVE: 108-inch waterline with 84-inch tee branch positioned
upwards for interconnect piping. (Photo Credit: LAN)
RIGHT: 84-inch by 84-inch tee with steel wrapper reinforcement
to save space. Existing 84-inch stub-out with tape coating is exposed.
With an astounding 21.6-percent population growth over the
last decade, the greater Houston region is expected to double in
population by 2050. To meet future demands, and in response
to the Harris-Galveston Subsidence District mandate to reduce
groundwater pumping along the Texas coastal region, Houston
Public Works partnered with regional water authorities to build
the Northeast Transmission Line (NETL).
Th e NETL will transmit 365 million gallons a day from the
newly expanded Northeast Water Purifi cation Plant (NEWPP)
through 16.5 miles of primarily 108-inch steel waterlines to
serve approximately 900,000 homes in Harris County.
Th e eastern 108-inch waterline projects were constructed
using primarily open-cut methods within an existing 90-footwide
waterline easement, which already contained a parallel
84-inch-diameter waterline. Th is steel, tape-coated waterline was
built in 2004, and a stub-out with butterfl y valve was installed
near the busy intersection of Beltway 8 and Wilson Road.
Th e interconnection between the 108-inch and existing
84-inch waterline was an integral part of the project to secure
reliable isolation for future maintenance. With not a lot of room,
heavy traffi c and big businesses in the vicinity, the ambitious
interconnection took a lot of research, planning, innovation and
coordination, during design and construction, to make it a reality
A butterfl y valve was already in place on the stub-out, but a
second valve was needed to provide double isolation between the
proposed 108-inch and existing 84-inch lines. Houston Public
Works and Lockwood, Andrews & Newnam Inc. (LAN), the
design engineer for the interconnect, considered several options.
With this interconnection being considered critical to the
system, Houston Public Works chose a combination of a new gate
valve with the existing butterfl y valve as the preferred option for
a secure and robust double isolation. As such, the city opted
to install a new 84-inch gate valve with the interconnect.
Innovative solution
When the original 84-inch waterline and stub out were
constructed, the area was mostly undeveloped, vacant
land. Fast-forward to 2020, the area has developed into a
prospering suburban master-planned community. Designing
and constructing a project of this scale within a fully
developed and thriving area was a big challenge. Situated
along a freeway frontage road, in the immediate vicinity of the
project, was a gas station, bank and a popular restaurant.
To say that the project had space constraints is an
understatement. With less than 30 feet horizontally and
14 feet vertically between the two main lines, the small
footprint made this complex, large-diameter connection
that much more of a challenge. A typical crotch plate design
used to reinforce the tees would be too large; the dimensions
simply would not allow room for the proposed 84-inch
gate valve. LAN engineers used an innovative solution to
design the steel pipe fi ttings with ASME steel wrappers for
reinforcement, eliminating the need for crotch plates.
UCONonline.com | AUGUST 2021
15
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Underground Construction - August 2021

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