Abstract

The Quail Creek Dike near St. George, Utah, was constructed in 1984 as a 24 m (78 ft) high, 604 m (1980 ft) long embankment. On January 1, 1989, the dike failed catastrophically after undergoing repeated episodes of increased seepage, discolored seepage, and developing small sinkholes over the previous three years. Downstream evacuation prevented fatalities, but flooding caused millions of dollars of damage and disrupted major highways and bridges.

The dike was constructed approximately perpendicular to the axis of a doubly-plunging anticline in thin-bedded sediments containing gypsum-rich beds. Gypsum content exceeded 50% in some beds. The combination of fracturing and bedrock orientation caused by arching of the anticline and presence of significant amounts of soluble salts presented a complex and challenging foundation for the dike. The failure generally resulted from seepage water removing embankment or in-situ soils at the dike/foundation contact. As seepage and erosion accelerated, a process of backward erosion caused caving and the eventual breach. The combination of factors contributing to the failure include:

1. Erodible materials existed or were placed at the dike foundation contact.

2. Post-failure investigation revealed an extensively jointed and weathered rock foundation with discrete seeage paths, particularly near the surface. Open beds caused by expansion, disolving and erosion of gypsum also existed within the weathered zone.

3. Participation of an engineering geologist was limited during all phases of the project, resulting in flaws in each stage of the dikes development and operation. Errors contributing to the dike failure included an underestimation of gypsum content in the foundation; undiscovered geological factors relating to foundation rock solubility and erodibility; a mistaken, generalized assumption of low permeability in the foundation; and inadequate preparation and treatment of the foundation.