Piute Dam, located approximately 10 miles north of Junction, Utah was constructed during the early 1900’s by pioneer farmers and ranchers to provide a more stable irrigation water supply for their agri-businesses. Construction of the 90 foot high dam with a 1400 foot long crest, impounding 72,000 acre-feet of water, was completed using horse-drawn fresno’s and wagons and a steam– driven pump. Soils from nearby borrow areas were sluiced into central portions of the dam to provide an “impervious” core and tied into meta-conglomerate rock abutments. Downstream toe drains consisted of coarse rock drains and wood stave pipes.

Geotechnical studies conducted upstream and downstream of the dam crest, in accordance with State of Utah Dam Safety requirements, discovered the existence of an active fault just downstream of the dam toe. Seismicity studies concluded that the Maximum Credible Earthquake event would be of moment magnitude 7.0 with peak ground accelerations of 0.45 g, and that the fault located at the downstream toe would be activated by such an event. The presence of liquefiable layers in both the upstream shell and foundation areas led to Phase I construction activities that included constructing a stability berm, flattening the upstream dam slope and replacing riprap on the upstream face. Work took place during the late summer, fall and early winter months of 2002 and required evacuating the reservoir. Evacuating agribusiness reservoirs is dimly viewed at any time, and in 2002, Utah was already 2 years into what would turn our to be a 5 year drought.

Pockets of liquefiable embankment and fine to coarse grained alluvial deposits, downstream of the dam crest, were also identified during field studies, although it was judged that their apparent random presence did not warrant specific remedial activities associated with reducing this risk. What was of concern, downstream of the crest, was the existence of unfiltered coarse rock drains with wood stave outfall pipes. Given the nearly 100 year history of the dam, static piping of materials into the old toe drain system did not appear to represent a significant risk. However, given: (a) the close proximity to an active fault, (b) steep rock abutments, (c) the lack of significant compaction in lower, saturated embankment zones, (d) potentially liquefiable pockets in both lower embankment and underlying alluvial zones, and (e) earthquake induced alluvial material settlement; the decision was made to search for a solution that would reduce potential seismically induced piping risks to acceptable levels.