Serious design and construction flaws led to water flows under Oroville Dam’s main spillway which caused it to crack, according to engineers’ reports.
Two reports were released by the California Department of Water Resources (DWR) this week following accusations of state secrecy surrounding the Oroville Dam repairs.
Both have been written by engineers on a Board of Consultants (BOC). The main report blames the failure of the spillway, which carries excess water flows from the dam, on water flows under the concrete. The DWR has awarded a £215M spillway repair contract to American construction and engineering firm Kiewit Infrastructure West.
Following weeks of heavy rain major erosion damage was discovered on the dam’s main concrete spillway in February. Further erosion damage was found on the second, emergency, spillway, prompting an evacuation of 180,000 residents.
“The failure likely occurred as a result of high velocity flow…penetrating under the slab, causing a strong uplift force and causing the slab to lift, eventually causing all or part of the slab to break away. Subsequent erosion of foundation material caused progressive failure both upstream and downstream,” said the report. It adds that the restored spillway should be “properly prepared with suitable drainage.”
Uneven thickness in the concrete also contributed to the spillway cracking. Repairs had been carried out on spalls and cracks in the concrete several times since the spillway was constructed in 1968, with the most recent repair taking place in 2009.
As a result the board recommends completely replacing the concrete chute’s floor, and the walls in the upper section of the spillway.
“The construction schedule might be impacted by doing so, but there are a number of flaws beneath and on the top of the floor slab that should be corrected that it would be wise to choose total removal and reconstruction,” says the report, parts of which have been redacted due to fears over terrorist attacks.
Although repair work will not finish on the lower section of the chute by the start of California’s rainy season in November, the BOC has stressed it is “vital” that the upper section is completed by then to avoid further damage.
An official report on the cause of the spillway failure will be released by a forensics team in the autumn.
Engineers on the BOC also use the report to urge state officials to draw on their knowledge.
“We got to this point because of a crisis, but while we are here use this as an opportunity to learn as much about the engineering geology as possible,” states the report.
In February erosion damage was discovered on the dam’s main spillway, causing engineers to shut off water flows while the damage was assessed. Water levels in the reservoir crept up and ran over the emergency spillway for the first time in the dam’s 48-year history.
Erosion damage was then discovered at the head of the emergency spillway, causing California governor Jerry Brown to issue evacuation orders for around 180,000 residents in the area.
Immediate repairs have been ongoing since the event, along with the removal of a 764,555m³ debris pile at the bottom of the main spillway.