Page - 165 - in Book of Full Papers - Symposium Hydro Engineering

compromised anchor resistance and potentially promoted the formation of shallow voids beneath the slab. Although the slab cracking and resulting large drain flows were recognized during the first year of project operation, they apparently were judged as non- threatening, and subsequently became accepted as normal. These conditions were reported in many inspections with no further comment. After completion of the project, the detailed geologic information concerning the extent of strongly weathered rock in the right abutment and beneath the service spillway chute was relegated to project records, and the abutment was incorrectly assumed and portrayed as being composed of generally non-erodible rock below a few feet of soil cover. The seriousness of the weak as-constructed conditions and the lack of repair durability was not recognized during numerous inspections and review processes over the almost 50-year history of the project, including three Potential Failure Mode Analyses (PFMAs) completed between 2004 and 2014. The first two PFMA efforts did not identify any significant potential failure modes (PFMs) related to the service spillway chute. The 2014 PFMA identified the service spillway PFM that initiated in February 2017, but it was judged extremely unlikely and ruled out of further consideration, principally because it was believed that the rock foundation would not headcut enough to fail the headgate structure and result in uncontrolled release of the reservoir. There is no record of the original design and construction of the service spillway chute having been subsequently reviewed for consistency with more modern design and construction practices or for potential chute failure modes. Chute slab repairs were neither effective nor durable. Over time, chute flows and temperature variations led to progressive deterioration of the concrete and the corrosion of steel reinforcing bars and anchors, with likely loss of slab strength and anchor capacity. 165