The first construction milestone has been met on the project to repair the stricken Oroville dam in California.
The Californian Department for Water Resources (DWR) said at the end of last week that engineers had successfully placed all concrete on the main spillway a week early, meeting a November 1 deadline for a substantial part of the repair work. Additional work including dry finishing, concrete curing, joint sealing, completing sidewall backfill and site clean-up will now take place.
The repair work is set to cost around $1.1bn (£850M) according to local media.
The dam is the US’s tallest at 275m high.
The main spillway was initially closed on 7 February 2017 after engineers found severe erosion damage to the concrete structure. However, the closure caused more water to be diverted down the dam’s, auxiliary primary spillway increasing the risk of flooding elsewhere. Around 180,000 people were evacuated because of the increased risk.
Because of the increased water flow due to heavy rains, erosion damage soon appeared at the auxiliary spillway’s head, undermining the concrete weir.
In an attempt to temporarily fix the problem, engineers filled the emergency spillway with large rocks and gravel using trucks and helicopters.
A damning report into why it failed found the dam’s main spillway was designed by a graduate engineer with no prior experience, was knowingly built on weak rock and was hit by cost pressures during construction.
In December last year, cracks which appeared in the newly repaired spillway were dismissed after remedial actions including adjusting the concrete mix design and contacting concrete experts were carried out.
Footage from the repair work was captured in a timelapse video in October last year.