SEEPAGE EVALUATION OF QUAIL CREEK SOUTH DAM IN SOUTHWEST UTAH

Quail Creek reservoir is located in southwest Utah near St. George. The Quail Creek South Dam was constructed along the eastern limb of the Virgin anticline and is located on Triassic age Moenkopi Formation deposits of predominately highly gypsiferous, siltstone and dolomicrites.

After the failure of the original earthfill dike in 1989, the dam was reconstructed as a roller compacted concrete (RCC) structure with a concrete and RCC cutoff trench which reached a depth of about 75 feet (22.9 m) through the maximum section, a maximum dam height of about 80 feet (24.4 m), and a crest length of about 2150 feet (655 m).

Since completion of the new dam in 1991, seepage has gradually increased. Seepage had been most notable along the left side of the dam, leading to the installation of a toe drain system. During the past few years, subsidence features have been noted down stream of this area. Since January 2002 seepage along the right side of the dam has increased significantly. About 800 feet (244 m) downstream of Station 5+00, water has been flowing out of several closely grouped open eroded fractures in a highly fractured, light greenish gray dolomicrite unit with about 20% visible gypsum. During the Spring, flows from this unit reached peaks of 4.9 cfs and were slightly discolored and cloudy. Analysis showed the flow to be carrying up to 1.4 tons of sediment per day, dropping to about 760 lbs/day 6 days later.

Based on a review of the previous grouting programs for the original dike and the logs from the cutoff trench and test holes for the new dam, it is our opinion that there are two primary sources of seepage. Seepage on the left side may be associated with shear zones as the bedrock approaches the axis of the anticline. Seepage on the right side is likely due to the dissolution of gypsum and erosion along one or more of the numerous joints. As the velocity of the flow increases, more rapid erosion and piping of gypsiferous bedrock is taking place.

A series of exploratory holes are currently being drilled in target areas, along with research into various geophysical methods which may assist in finding the locations of seepage beneath the cutoff trench so that an appropriate grouting plan or other remediation procedures can begin.