August 2021 Vol.76 No. 8
Features
Taming a Mammoth: Innovative Interconnect Improves Houston Water Operations
By Christine Kirby and Ram Chakradhar
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).
The NETL will transmit 365 million gallons a day from the newly expanded Northeast Water Purification Plant (NEWPP) through 16.5 miles of primarily 108-inch steel waterlines to serve approximately 900,000 homes in Harris County.
The eastern 108-inch waterline projects were constructed using primarily open-cut methods within an existing 90-foot-wide waterline easement, which already contained a parallel 84-inch-diameter waterline. This steel, tape-coated waterline was built in 2004, and a stub-out with butterfly valve was installed near the busy intersection of Beltway 8 and Wilson Road.
The 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 traffic 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 butterfly 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 butterfly 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 fittings with ASME steel wrappers for reinforcement, eliminating the need for crotch plates.
With the shallow depth of the existing stub-out requiring the proposed 84-inch gate valve to be mounted horizontally, LAN designed a vault and foundation to provide support and evenly distribute the weight of the valve as it operated from open to close. The shallow depth also meant that the air release valve would extend above grade. The project team adapted by designing a steel cabinet enclosure around the air release valve with doors on both sides for access.
Complex challenges
Construction for the interconnect commenced in April 2020. The general contractor, Main Lane Industries, began excavation for the proposed foundation and vault.
“One of the biggest challenges was the limited workspace. We had to come up with a plan to make the best use of space that we had,” said Jeff Heflin, project manager at Main Lane Industries. “We went with a 200-ton Hitachi crane. Once we had a plan for the crane in position, everything else fell into place. Planning was key.”
Soil conditions varied from lean clay soil with gravel to very stiff lean clay or fat clay with sand. Groundwater was found initially at 15 feet below grade, but this level fluctuated. A well-point dewatering system was used for excavation to install the structural slab that was placed approximately 30 feet below grade. The slab was placed first, then the walls.
“It got tricky because the back side of the vault was also the edge of the adjacent Wilson tunnel shoring, which was a circular shaft next to the rectangular vault excavation,” said Heflin. “We planned the rectangular shoring dimensions to avoid conflict with the adjacent tunnel shaft shoring.”
To add another layer of complexity, the existing 84-inch main waterline extending parallel within a few feet from the southern edge of the open shaft, was in full service with no possibility for interruption.
Upon unearthing the existing 84-inch stub-out, suspicions were confirmed that the dished head plug was external, not internal. The contractor implemented a temporary support system using nylon straps to support the stub-out piping while the surrounding soil support was excavated.
Connecting to the external dished head plug would be easier said than done, with the outer diameter of the existing 84-inch being different than the outer diameter of the new 84-inch connection piping. After grinding off the existing tape coating, Main Lane Industries used a butt strap to connect a new 84-inch spool piece to the existing 84-inch stub-out. Field modifications to the butt strap were needed to make it work.
Special delivery
Good things come to those who wait. After a nine-month lead time, the huge gate valve made its final journey from Alabama to Texas in July 2020. The valve, approximately 22 feet long and 4 feet wide, weighed a whopping 34.6 tons, the equivalent of a brontosaurus, or three great woolly mammoths.
On a hot and sunny summer day, a small crowd of engineers, managers and representatives patiently waited as the valve was lifted from the 18-wheeler flatbed with the crane and lowered into place. The bolted fasteners, 130 in total, took a day and a half for crews to complete.
Once the piping and valve were in place, the contractor completed the concrete foundation and vault. The structure itself was more than 400 cubic yards of concrete. For vault cover and access, LAN designed a four-panel access door, with access points that would permit the 84-inch gate valve operator and bypass to operate without removing the panels.
Once complete, these vault panels were sealed with caulk and covered in a shallow layer of soil and grass sod, so all that could be seen from the ground surface was the air release valve cabinet.
Conclusion
“Design and construction of such a massive interconnection was a big challenge to the project team. It required innovation, teamwork and communication from every member of the team,” said Kevin Tran, P.E., supervising engineer at Houston Public Works.
The interconnection was successfully completed with site restoration in January 2021. Houston’s legacy continues with its groundbreaking water infrastructure expansion programs. The city of Houston’s drinking water system maintains a “superior” rating, the highest rating for water quality issued by the Texas Commission on Environmental Quality.
CHRISTINE KIRBY, P.E. is a senior associate and senior project manager at Lockwood, Andrews & Newnam, Inc. (LAN), a national planning, engineering and program management firm. She can be reached at chkirby@lan-inc.com. RAM CHAKRADHAR, P.E. is a project manager at City of Houston Public Works, Capital Projects, Facilities Delivery Line, Surface Water Transmission Program. He can be reached at ram.chakradhar@houstontx.gov.
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