The California Department of Water Resources says most of the prep work for new construction on the Oroville main spillway has been completed.
DWR says crews working on the bottom 2,270 feet of the main spillway have blasted and cleaned about 95 percent of the surface to be used for the new chute.
DWR's Erin Mellon says they have begun to pour nearly a million cubic yards of concrete and will install 55,000 feet of drainage pipe.
Improper drainage and concrete problems are the most frequently mentioned possible causes of failure by the Independent Forensic Investigation Team.
"They have access to all the records and all the information drilling and coring to see the concrete and the rock beneath that spillway and they're interviewing DWR employees as well to gather more information," says DWR's Erin Mellon.
Mellon says the team's findings will be released this fall.
The main spillway failed the first week of February. When DWR made the decision to stop releasing water down the main spillway, Lake Oroville exceeded its listed capacity and overtopped the emergency spillway.
DWR estimates it cost $200 million to respond to the incident and will cost another $300 million this year for construction.
The work on the bottom of the spillway is scheduled to be complete by Nov. 1. DWR plans to replace the top 830 feet of the spillway next year.