By Mackrena L. Ramos, P.E.
In 1985, the City of Houston created the Surface Water Transmission Program in response to a mandate issued by the Harris-Galveston Subsidence District to decrease the use of groundwater and increase surface water usage. The program’s goals included increasing surface water usage, decommissioning and minimizing the use of groundwater pump stations, increasing water system pressures, and addressing other large-diameter water line issues.
Since the program’s inception, the city has built hundreds of miles of large-diameter water lines to reduce the effects of subsidence and expedite the switch from groundwater to surface water. As part of these continuing efforts to meet subsidence goals and to comply with the region’s water demands spurred by population growth, the city is currently designing and constructing the Northeast Transmission Water Line (NETL).
The 16.5-mile NETL, which will be built in 13 segments starting this summer, will transmit surface water from the Northeast Water Purification Plant (NEWPP) to the North Harris County Regional Water Authority’s meter station, west of IH-45. Upon completion, the $450 million water line can transmit as much as 96 million gallons per day (mgd) to the City of Houston and 269 mgd to four regional water authorities — North Harris County Regional Water Authority, Central Harris County Regional Water Authority, West Harris County Regional Water Authority, and the North Fort Bend Water Authority — as well as several municipal utility districts.
Along the way, the water line will also provide additional water to Bush Intercontinental Airport, the Greater Greenspoint Area, and surrounding areas currently receiving water from the existing 66-inch water line extending along the Greens Road corridor. Altogether, the water line will serve approximately 900,000 homes in Harris County.
Concurrently, the City of Houston and regional water authorities are enlarging the treatment capacity of the NEWPP from 80 mgd to 400 mgd, and the NETL will carry water produced by the planned expansion. Both the NETL and NEWPP expansion are needed to meet the region’s projected 2040 water demand.
Planning for this project began as early as 2010 when the city started discussions with regional water authorities to build a single water line that would meet its combined future water demands and groundwater mandates. Shortly afterward, the city selected Lockwood, Andrews & Newnam, Inc. (LAN), a planning, engineering, and program management firm, as its technical adviser and asked for a feasibility study.
To this end, LAN evaluated potential corridors for 22 miles of large-diameter water transmission lines from the NEWPP to the Acres Home Pump Station. The firm then performed hydraulic modeling for the preferred alignment to size and evaluate the pumping requirements from the NEWPP. The modeling scenarios examined a range of pipe diameters from 102 inches to 156 inches. Cost estimates were also developed after taking into account the easement and construction costs for each scenario.
“Selecting a route that would enable the city to build such a massive water line in an urban environment was a big challenge,” said Michael Liga, Ph.D., P.E., LAN associate and project manager. “We spent a lot of time researching and refining the route to choose the most cost-effective alignment with the least impacts to the community.”
With the majority of the pipeline alignment located in easements outside City of Houston limits, the city subsequently embarked on an intensive, multi-year effort to acquire easements before initiating design work. This required significant planning and coordination with numerous stakeholders, including homeowners, businesses, municipalities, and regional regulatory agencies. Approximately 150 required easements have been identified as part of this effort. During this time, the city also partnered with the four regional water authorities and the Texas Water Development Board to negotiate a cost-sharing agreement.
“From feasibility to construction, we have spent a significant effort planning every aspect of these projects,” said Venus Price, P.E., supervising engineer for the City of Houston’s Department of Public Works and Engineering.
The NETL is the largest water line to be built in the program’s history. Eleven of the 13 segments of the water line that will be built to the North Harris County Regional Water Authority’s meter station at IH-45 range from 108 inches to 120 inches. The other two segments are 54-inch and 66-inch interconnections.
“From a size and scope standpoint, this is the largest water line undertaken up to this point by the city,” said Price. “Agencies we were coordinating with such as the Texas Department of Transportation and the Union Pacific Railroad had to go back to their engineering staff in-house to work through the design requirements and specifications since their standards did not apply to the diameters that we are building.”
“The largest water line that the city currently has is 96 inches,” Liga said. “So, we had to develop brand new details and specifications for the 108- and 120-inch segments. In addition, the pipeline traverses from the east side to the west side of the city, so its length is also unique.”
Consequently, the city will be using Competitive Sealed Proposal (CSP) in lieu of low bid to select contractors for the NETL projects.
“Typically, the city awards projects based on low bid,” Price said. “But we are planning to use CSP for the NETL projects due to their size, scope, and the proximity to each other. Therefore, in addition to the pricing, we will also be looking at contractors’ technical qualifications, experience, and other criteria to select the contractor who will provide the best value for the project.”
The 13 segments will be built in three phases. In the first phase, construction will begin close to the middle of the alignment. The first phase — also called the Workaround Solution — was selected to enable the shut down and internal repair of a parallel 66-inch water line, which currently carries water from the NEWPP to the Greenspoint Area. The 66-inch water line is a major source of water for the city, the North Harris County Regional Water Authority, the Central Harris County Regional Water Authority, and George Bush Intercontinental Airport.
“A portion of the existing 66-inch water line was damaged by a non-city contractor several years ago,” said Price. “Although the pipeline has continued to function properly, building the parallel section of the NETL line will give us an opportunity to isolate the damaged portion of the 66-inch line so we can assess and do internal repairs.”
The Workaround Solution consists of three segments: two segments of 6,500 linear feet each of 108-inch water line and one segment of 3,300 linear feet of 54-inch water line. Hot tap connections will be made to the existing 66-inch water line on both ends of the project, forming what is essentially a long bypass.
The first phase will be constructed using a combination of open cut and tunneling approaches. Approximately 2,400 linear feet of the 108-inch segment will be tunneled. Two tunnel sections were particularly challenging during design — crossing the Union Pacific Railroad (UPRR) and the US-59/IH-69 overpass.”
The UPRR tunnel alignment crosses over an aging 72-inch-diameter concrete sanitary sewer that showed signs of failure near the crossing. Sinkholes were discovered during the field investigation phase. The failing sewer was a concern both for its potential to contaminate the water and for its potential to impact the structural integrity of the pipeline. The city fast-tracked an already planned cured-in-place pipe rehabilitation of the sewer to prepare the site for construction of the water line.
In addition to crossing over the sanitary sewer, the tunnel crosses under an existing 42-inch ductile iron water line, which is parallel to and offset from the sewer. Mechanical joint restraints will be applied to the 42-inch water line prior to construction of the tunnel to prevent failure should unanticipated settlement under the water line occur.
The settlement analysis performed for the UPRR tunnel indicated that it could be constructed and meet the strict UPRR settlement criteria (maximum 1/4-inch movement of rails in any direction) with careful tunneling practices and strict control against loss of ground. UPRR typically only allows casing pipe be used as the primary tunnel liner in its right-of-way. However, steel liner plate and steel ring beam with steel lagging were requested and ultimately approved by UPRR in order to provide the contractor with more options and an expandable liner system to help immediately fill voids, should any occur.
The second particularly challenging tunnel was the crossing of the US-59/IH-69 overpass, which is under Texas Department of Transportation (TxDOT) jurisdiction. This tunnel is approximately 1,200 feet long to span the highway plus businesses on either side. The overpass contains 15 lanes of traffic, and the frontage roads contain an additional eight lanes.
The TxDOT Houston District typically requires steel casing pipe as the primary tunnel liner under its right-of-way. Due to constructability issues associated with installing such a long casing, steel liner plate and steel ring beam with steel lagging were requested and ultimately approved by TxDOT. These provide the contractor with more constructible options and an expandable liner system to help immediately fill voids, if needed.
The horizontal alignment was set to maintain the maximum distance possible between the tunnel and the bridge and abutment columns. Soil conditions were problematic for approximately 200 linear feet underneath the bridge and 150 linear feet under the frontage road. The soil profile contains saturated silty-sand along the tunnel alignment, creating the potential for excessive settlement and groundwater control issues. Subsurface chemical grouting of the soil formation was specified as the best solution to stabilize the sandy layer and eliminate the need to dewater the formation. In order to minimize pavement impacts in the TxDOT right-of-way, the formation under the frontage road was required to be grouted using diagonal grout injection bores outside the travel lanes. Underneath the bridge, TxDOT allowed pavement corings to the U-turn lane, and grouting activity was limited to this area.
Despite these challenges, the City of Houston is on track to begin construction on the first phase of the NETL this summer. Phase two of the project will include three 108-inch lines and one 120-inch line, and will be awarded between fall 2017 and spring 2018. Phase three, which includes five 108-inch lines and one 66-inch interconnection, will be awarded between fall 2018 and spring 2019. The entire water line is scheduled to be completed in 2021, simultaneously with the expansion of the NEWPP.
“The city has been working closely with our partners for several years and it has taken a lot of discipline, patience, and solving many challenges to get to this point,” Price said. “We are excited that construction will soon begin on this project.”
Mackrena L. Ramos, P.E., is a senior associate and team leader at Lockwood, Andrews & Newnam, Inc. (LAN; www.lan-inc.com), a planning, engineering, and program management firm. She can be contacted at email@example.com.