Choosing the High Road: Large-Diameter Waterline Shallow Crossing versus Tunneling under the Storm Box

The North Harris County Regional Water Authority (NHCRWA) retained Arcadis in 2019 for the fast-paced design and construction oversight of a large diameter pipeline project. The project, titled 28G, is part of a larger pipeline project to supply portions of Harris County with surface water and take users off groundwater wells. 28G consists of 3,800 LF of 84-in. steel water pipe connecting a proposed regional booster pump station in the south to a proposed 84-in. pipeline in the north. A portion of the 28G pipeline is in close proximity to an existing detention basin to the west of the pump station. Several alternatives were considered and analyzed, with the route chosen to be the shortest distance but with many existing utilities, including three (3) outfalls to the detention basin. Preliminary design proposed tunneling the waterline under the existing culverts. Evident during design were several construction risks: tunnel depth, length, and cost due to potential scarcity of tunneling contractors, which drove analysis into alternative alignments. Survey data later determined the storm boxes to be several feet deeper than initially estimated. This allowed for a shallow pipe alternative, placing the pipes over the storm boxes in lieu of under. Alternatives analysis encompassed construction cost, constructability, O&M cost/feasibility, stakeholder impact, and discussions with the owner. The shallow pipe alternative provided its own set of hurdles, including pipe protection in shallow cover, impact to basin and existing storm culverts, and required appurtenances. This project provides a case study in the alternative of shallow pipe burial versus tunneling, benefits to client’s construction cost (savings of $1.3 million), and long-term maintenance while providing a safe, reliable water delivery system.