Abstract

Quail Creek Dam and Reservoir is owned and operated by Washington County Water Conservancy District (District) of St. George, Utah. The reservoir is located approximately 15 miles (24 km) northeast of St. George. The 40,000 acre-foot (49,345,000 m³) reservoir was originally formed by two dams, known as the main dam and the dike. Construction of the dike was completed in April 1984, the main dam was completed in January 1985, and reservoir filling began in April 1985. The dike had severe leakage problems and grouting programs were undertaken three different times from 1986 through 1988. However, on January 1, 1989 the dike failed.

Morrison Knudsen Engineers of San Francisco and James M. Montgomery Consulting Engineers of Salt Lake City were retained by the District to design a dam to replace the failed dike. Geotechnical investigations began in June 1989 for the purpose of evaluating foundation conditions and borrow materials. The foundation bedrock consists of thinly bedded sediments of silty dolomicrite, gypsiferous siltstone, gypsum and sandstone. The sequence is a portion of the upper Moenkopi Formation. Seventeen rock units were identified.

Rock quality, the amount of gypsum, hydraulic conductivity data, rock core recovery and previous grouting records were evaluated to aid in the selection of type and depth of cutoff. A positive concrete cutoff was selected which would have a maximum depth of approximately 75 feet (23 m) in the breach area, tapering to 50 feet (15.2 m) at the left abutment and 25 feet (7.6 m) toward the right abutment. The rebuilt dam has been named the Quail Creek South Dam and is a concrete gravity dam constructed by the roller compacted concrete (RCC) method. The dam has a maximum height of 80 feet (24.4 m), a crest length of approximately 2,000 feet (610 m) and was completed in June 1990.