CS27C - Concurrent Session 27C: Replacing a Siphon with a Low-Level Delivery Conduit on a Soft Compressible Foundation

The Weber Basin Water Conservancy District (WBWCD) operates a siphon near the southwest corner of the A.V. Watkins Dam, which is a Bureau of Reclamation facility. The A.V. Watkins Dam is a 14.5-mile long earth embankment, with a maximum height of 40 feet, that impounds the 250,000 ac-ft Willard Bay Reservoir. The dam was constructed near the eastern shore of the Great Salt Lake on soft to firm lacustrine clay soils interbedded with alluvial and beach sand deposits. The existing dam has settled up to 18 feet since it was constructed. The sandy alluvial and beach sand deposits are susceptible to liquefaction due to a seismic peak ground acceleration of 0.12g or greater, which has an estimated recurrence interval of 150 years.

The existing siphon crest is about 2 feet below the normal high-water reservoir elevation, and does not have adequate hydraulic capacity to meet water delivery requirements when the reservoir is more than about 12 feet below the normal high-water elevation. WBWCD incurs significant pumping costs to satisfy water delivery requirements during low reservoir periods.

Options evaluated during project design to improve the hydraulic capacity of the water delivery system included a new but more hydraulically efficient siphon similar to the existing, a new siphon with a lower crest elevation, and a low-level conduit. A low-level conduit was selected as the preferred project alternative because it offered greater hydraulic reliability and structural resilience under static and seismic loads.

The new low-level conduit was designed to be constructed while retaining approximately 95,000 ac-ft of water in the reservoir, utilizing a temporary 840-ft long cofferdam with a maximum height of 25 feet to retain the drawn-down reservoir during construction. The design evaluations included 3D analyses to estimate the settlement which will occur under the cofferdam loads, rebound which will occur during excavation of the conduit alignment, and settlement under new embankment loads.

The project was designed in partnership with Reclamation using a risk informed approach. Chimney filters and drains, permanent vinyl sheet piling, butt-welded 1” thick steel conduit encased in reinforced concrete, crack stopper concrete keyways, an LLDPE liner membrane, and instrumentation were included in the project design to reduce risks under static and seismic loading conditions. A risk analysis performed by the Bureau of Reclamation determined that the project design provides a net decrease in dam safety risks compared to the existing siphon.