Page - 162 - in Book of Full Papers - Symposium Hydro Engineering

There are a number of possibilities for changes that could have occurred during that time period. The IFT believes that some combination of the following factors most likely was involved: • New chute slab damage and/or deterioration of previous slab repairs, which could have created stagnation pressure and increased flow into the foundation in areas that were already vulnerable because of weak foundation conditions • Expansion of relatively shallow void(s) under the slab, through foundation soil erosion and/or shrinkage of clayey soils, which would have increased uplift force by increasing the area over which uplift pressure was applied • Corrosion of the steel reinforcing bars or dowels across concrete cracks or joints, which then failed in tension, resulting in reduced uplift capacity • Reduction in foundation anchor capacity due to corrosion or reduction in bond length due to erosion, which would have reduced uplift capacity All of these factors, as well as other less prominent factors and factors eliminated from consideration, are discussed in detail in the full forensic investigation report [1]. Cavitation, groundwater flow/pressure, and seismic loading are physical factors that were suggested by others but were judged by the IFT to not have been significant contributory factors. The IFT completed calculations that indicated that, for the original service chute, cavitation would be expected only downstream of Sta. 31+00, and there only for extended periods of discharge greater than 2,830 m3/s (100,000 ft3/s). The historical discharges greater than 2,830 m3/s (100,000 ft3/s) were not of sufficient duration to cause significant cavitation damage. This conclusion was supported by observation of chute slab sections remaining in place downstream of Sta. 31+00 after the incident, which did not reveal telltale indicators of incipient cavitation. The IFT reviewed available information concerning groundwater and geologic conditions in the area of the service spillway. This review led to the conclusion that there was no evidence of large amounts of groundwater flow in the spillway location, and such groundwater flow would not have been expected for the geologic conditions at that location. Therefore, although groundwater flows may have contributed in a very small way to uplift pressures that developed under the chute slabs, it was the IFT’s opinion that groundwater was not a significant contributor when compared to water pressure injected through slab joints and cracks when the service spillway was operating. The IFT completed a review of seismic activity in the vicinity of Oroville Dam over the last 20 years, with a conclusion that no earthquakes stronger than M 4.0 occurred within 160 km (100 mi) of the site in that time period. No ground motions 162