Twenty-one foot diameter buried steel pipe was installed on the Central Arizona Project by the US Bureau of Reclamation and the Central Arizona Water Conservation District. The project conveys water from the Colorado River through Phoenix to Tucson, Arizona. The 336 mile long project includes open canals, tunnels, pumping plants, and seven 21-foot inside diameter inverted siphons. Some of the siphons were constructed of precast prestressed concrete pipe. These siphons were built in 1975 through 1978. Investigations begun in 1990 revealed distress to the mortar coating and prestressing wire. Two of the siphons were replaced by new parallel steel pipelines. The Salt River Siphon replacement is about 8025 feet of new pipe and the Aqua Fria River Siphon replacement is about 9875 feet of new pipe. The maximum soil cover over the pipe is 45 feet and the maximum internal pressure head is 275 feet. Unique design issues for pipe of this size included selection of pipe type options, coatings and linings, soil stiffness selection, dewatering plans, flotation, and depth of cover. Two of the pipe options were steel pipe and cast-in-place monolithic reinforced concrete pipe. Steel pipe was selected for both replacement siphons and they were constructed in 1993 through 1997. Details of the construction are discussed in a companion paper.
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