 The cleared , underlying rock foundation of the dam site was reinforced with grout , called a grout curtain . <unk> were driven into the walls and base of the canyon , as deep as 150 feet ( 46 m ) into the rock , and any cavities encountered were to be filled with grout . This was done to stabilize the rock , to prevent water from seeping past the dam through the canyon rock , and to limit " <unk> " — upward pressure from water seeping under the dam . The workers were under severe time constraints due to the beginning of the concrete pour , and when they encountered hot springs or cavities too large to readily fill , they moved on without resolving the problem . A total of 58 of the 393 holes were incompletely filled . After the dam was completed and the lake began to fill , large numbers of significant leaks into the dam caused the Bureau of Reclamation to look into the situation . It found that the work had been incompletely done , and was based on less than a full understanding of the canyon 's geology . New holes were drilled from inspection galleries inside the dam into the surrounding bedrock . It took nine years ( 1938 – 47 ) under relative secrecy to complete the supplemental grout curtain . 
