After presenting its second Interim Status Memorandum report regarding the Oroville Dam spillway incident to the California Department of Water Resources (DWR), the Independent Forensic Team (IFT) leader, John France, asked that the IFT report be made public.
Although the IFT’s work will not be completed and its final report will not be issued until later this fall, the IFT wanted to share some of its findings to date, so that they may inform the ongoing spillway evaluations at other projects.
The September 5, 2017 IFT report, available here, discusses the events leading up to the Orville Dam spillway chute failure on February 7, 2017.
Initially, a section of the concrete chute lifted and moved, exposing highly weathered rock and soil material of the high-velocity spillway flow of water. As the water rapidly eroded the foundation materials at the first slab location, the water rapidly created a larger erosion hole.
IFT said the slab couldn't resist the uplift force of water as it flowed down the chute and the uplift force was magnified by multiple physical factors that significantly affected the spillway chute's eventual failure:
- Deterioration of concrete slab repairs on the chute such as unsealed cracks, spalled concrete at previous repair locations.
- Underdrains that intruded into the chute slab section, reducing the thickness of concrete above the drains to 7 inches or less compared to a design minimum thickness of 15 inches elsewhere. Cracks in the thinner concrete allowed water to pass through the slab.
- Erosion under the chute slabs, creating shallow voids under the slabs. Up to 50 percent or more of the foundation in some areas was not properly treated by removal of weathered materials and cleaning of soil-like materials from the surface.
- Corrosion of the rebar across the concrete cracks/joints.
- Anchor corrosion where anchors were not properly encapsulated with grout.
- Absence of waterstops at contraction joints, and a less than optimal shear key configuration.
- A drainage system with no filtering, possibly broken or disconnected pipes caused by the method of placement, and likely inadequate collector drain capacity for the flow that occurred through the slab.
- Single top layer of nominal reinforcement bars.
- Use of a relatively large concrete aggregate size, resulting in a propensity for cracking and spalling at keys and over drains, and damage to drain pipes.
The IFT report also found deficiencies in design testing, organizational culture and communications, and adherence to operation and maintenance standards.