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Houston has experienced significant population growth over the past decade, with the population expected to double by 2050. With a need to meet current and projected water demands, the city began an ambitious 6-year pipeline project to move treated drinking water from the Northeast Water Purification Plant across 16.5 miles of the densely populated Northeast Houston urban landscape.
The Northeast Transmission Line (NETL) is an integral part of the region’s water plan and will provide water to approximately nine hundred thousand homes, including Bush Intercontinental Airport. It is split into 13 segments with a total construction cost of nearly $450 million. As part of this critical project, HVJ provided geotechnical engineering for the tunnel at IH-69, which was designed by Lockwood, Andrews and Newnam (LAN). The tunnel had significant challenges to be installed safely and without damage to the existing freeway bridge structure.
An ever-present challenge in densely urban settings is overcoming the logistics of space and constraints of existing construction. The NETL tunnel at the crossing of IH-69 crosses beneath a major freeway overpass consisting of 15 elevated main lanes and eight at grade frontage road lanes at a depth of about 20 feet to the crown. The minimum 128-inch diameter, 1,150-foot long tunnel bore passes within 30 feet of the bridge foundations which comprised drilled shafts. Soil conditions along the bore vary from full face clay to full face sand. The sandy soils extended under the eastern half of the freeway overpass and varied from relatively clean, fine sand to silty sand below groundwater. These ground conditions created significant risks to the existing overpass structure due to the potential for large settlements and potential flowing ground conditions during tunneling. The tunnel design also had to satisfy Texas Department of Transportation (TxDOT) requirements which include steel casing as a primary liner for tunnels crossing their right of way. These multiple challenges required a thorough evaluation and out-of-the-box solution to ensure safe and effective tunnel construction.
Our goal was to design a strategy to mitigate risk to the freeway bridge and enable the tunnel to be installed safely. We evaluated several methods to determine which would allow for successful and safe installation despite the unstable soil.
A Pressurized Face Tunnel Boring Machine (TBM) would be the preferred method of constructing the tunnel, however, there are few TBMs of the required size available locally. In order to use locally available equipment, the stability of the sand layer had to be improved. Dewatering the sand would improve the ground conditions but was not a viable option due to the presence of at grade frontage road lanes crossing the bore alignment. Our analysis found that chemical grouts of sodium silicate and acrylate could stabilize the soils and allow for the necessary tunnel excavation without impacting the freeway bridge. The method included in the design included stabilization of the sands along the tunnel alignment with chemical grout coupled with an open face TBM with face closure abilities. In addition, we developed a monitoring program for the bridge to detect movement and allow for corrective action prior to any damage to the structures.
Once the project was awarded to Harper Brothers Construction, they offered to procure an earth pressure balance tunnel machine in time to meet the contract schedule. Instead of grouting the sands they proposed to install a casing pipe installed by pipe jacking with two intermediate jacking stations. Liner plates with steel ring beams had been included in the design approved by TxDOT in lieu of steel casing due to the size of the bore, so this approach also allowed the tunnel to be constructed with steel liner per TxDOT requirements. Harper Brothers proposal was accepted.
The tunnel was successfully constructed in 2020, and no unexpected challenges arose from soil conditions. Bridge movement monitoring revealed no tunneling related movement of the bridge structures. The successful result was attributed to technical expertise and strong collaboration among stakeholders to ensure that the tunnel was installed with the least possible risk. The tunnel’s on-time and successful completion enabled this segment of the NETL to be completed on schedule and on budget.
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