9-foot-high, 4-foot-wide tire. One-of-a-kind 125-ton vehicle that does nothing but push another machine along a rocky canyon floor about 45 miles northwest of Los Angeles. They are part of a fleet of giant earth-moving equipment, mobilized by the Western Contracting Corp. of Sioux Falls, S.D., to work on the monumental Castaic Dam and Reservoir, the southern terminus of the 44-mile California Aqueduct being built in a $2.8 billion State Water Project.