Design of Supplementary Intake from Lake Sakakawea for the Southwest Pipeline Project

The Southwest Pipeline Project (SWPP) in southwestern North Dakota is a large state-owned regional water system providing water to 32 communities and more than 5,000 rural water users through a network of more than 4,200 miles of main transmission and distribution pipelines and pump stations. The project supplies raw water to the 3.5 MGD Oliver-Mercer-North Dunn (OMND) Water Treatment Plant (WTP) located 11 miles from the source and the 12 MGD Dickinson WTP located 87 miles from the source. System improvements to increase the project capacity were identified in a 2008 report. A key component of those improvements is the supplementary raw water intake. The ND State Water Commission (SWC) authorized Bartlett & West/AECOM to perform the tasks necessary to complete final design and preparation of bid-ready documents for a secondary raw water intake for the Southwest Pipeline Project. The secondary intake will provide additional capacity above the capacity of the existing raw water intake facility. The supplementary intake facilities will consist of a 154.5 ft. deep vertical caisson underneath a pump station. An intake pipeline will be constructed to connect the caisson with the screened intake structure located approximately 2,700 ft away on the bottom of Lake Sakakawea's Renner Bay. The pump station and caisson will be located approximately 500 ft west of the existing SWPP Intake Booster Pump Station and the Basin Electric Power Cooperative (BEPC) Intake Pump Station. The new supplementary intake will be connected to the existing 30-in. diameter steel SWPP raw water transmission pipeline on property owned by the U. S. Army Corps of Engineers (COE). Construction of the intake pipe was proposed to be accomplished using the horizontal directional drilling (HDD) method. The bore pit would have been located on private property to the southwest of the pump station. The HDD would intersect the caisson and the intake pipe, made of either fusion-bonded epoxy-lined and coated steel pipe, or high-density polyethylene (HDPE) pipe, would be pulled back to the caisson from the lake insertion point. The project was advertized for bids in July of 2013 and three bids were received from the prequalified list of five contractors. In a surprise to the design engineers, the selected contractor has proposed to construct a much larger-diameter caisson and use microtunneling to install a much larger-diameter intake pipe. This paper describes the project to date.