 Excavation for the powerhouse was carried out simultaneously with the excavation for the dam foundation and abutments . A U @-@ shaped structure located at the downstream toe of the dam , its excavation was completed in late 1933 with the first concrete placed in November 1933 . <unk> of Lake Mead began February 1 , 1935 , even before the last of the concrete was poured that May . The powerhouse was one of the projects uncompleted at the time of the formal dedication on September 30 , 1935 — a crew of 500 men remained to finish it and other structures . To make the powerhouse roof <unk> , it was constructed of layers of concrete , rock , and steel with a total thickness of about 3 @.@ 5 feet ( 1 @.@ 1 m ) , topped with layers of sand and tar . 
