Emergency Construction of a New Dam on Pinto Creek, Gila County, Arizona

On October 21, 1997, the Middle Goose Tailings Dam at the Pinto Valley Mine failed spilling approximately 300,000 cubic yards of copper tailings into Pinto Creek on lands of the Tonto National Forest. Pinto Valley Mine, owned by BHP Copper Inc. (BHP), is located in southern Arizona about 68 miles east of Phoenix. Pinto Creek is an intermittent stream where the tailings failure occurred and was dry at the time of the failure. However, the creek is perennial downstream of the tailings spill and has significant environmental values. Pinto Creek is also a tributary to Roosevelt Lake which is a major source of water to the Phoenix metropolitan area. To prevent migration of the tailings downstream during the impending winter storm season and to keep the creek dry during the cleanup process, BHP needed to construct a temporary flood control dam of jurisdictional size on Pinto Creek upstream of the tailings spill to retain storm water; the dam will be removed following cleanup of the spilled tailings. To be effective in controlling runoff on Pinto Creek construction of the dam had to start immediately without time for the usual design review and permitting process. This paper describes the design-build-review process of the dam's construction and the unprecedented role of the Arizona Department of Water Resources (ADWR) dam safety staff in the process.

BHP, their consulting engineer and the various federal and state agencies involved worked together to develop a design concept for the dam and work was started on November 2, 1997. Foundation exploration and materials investigation were done as part of the construction process. The original design concept was modified several times during the construction of the dam. Because there was no time for the usual dam design process followed by ADWR review, ADWR took a continuously proactive role in the design and construction of the dam. ADWR staff took part in all design decisions in lieu of the usual design review, and made frequent inspections of the work during construction. Although winter storms frequently occurred none were intense enough to cause overtopping of the dam during its construction, and the embankment was completed to its design height at the end of December 1997. The completed dam is a zoned earthfill 50 feet in height with a crest length of about 700 feet. Three 36-inch diameter outlet pipes were installed through the dam. The site did not lend itself easily to an emergency spillway apart from the dam, so ADWR allowed a spillway on the embankment to be constructed because of the temporary nature of the structure. The spillway presented some of the greatest challenges technically, and the design process was difficult with lengthy discussions between the various parties involved. Eventually, a cast-in-place concrete channel 100-feet wide on the embankment was completed in March 1998.

ADWR's unprecedented role in the design and construction of a new dam under emergency conditions resulted in several lessons learned: (1) Arizona's dam safety law should be changed to include procedures for the design and construction of a new dam under emergency conditions; (2) emergency design and construction of a dam requires proactive involvement between the owner, owner's engineer, regulatory agencies and the contractor to achieve the most effective combination of constructability, conservative design, and flexibility to accommodate potential changes in design during the construction process. 8 pp., 6 references.