Leveraging PFMA to Address Unexpected Foundation Conditions during Active Rehabilitation on a Spillway Chute

The Stewarts Bridge Project consists of a 113-foot-high earthen embankment dam, radial-gated spillway, intake structure, and powerhouse. The project, owned and operated by Erie Boulevard Hydropower, L.P.(Erie) and regulated by the FERC, is located on the Sacandaga River in New York. The “High” hazard project generates hydroelectric power and provides flood control to the Hudson River Valley.
Considering the dam safety incident at Oroville Dam, the FERC mandated assessments of concrete chute spillways and unlined spillways at dams. In 2018, a focused spillway assessment and Potential Failure Mode Analysis (PFMA) was performed at Stewarts Bridge to identify concerns regarding the spillway chute, all of which were deemed Category II (not highlighted) or IV (non-credible). Key assumptions of the assessment and PFMA categorization were that the spillway crest structure, walls, and slabs were founded directly on massive bedrock with little susceptibility to erosion, and mass cyclopean concrete was placed on bedrock to provide a smooth working surface for slab construction, as indicated on Project drawings.
In 2020, repair work was undertaken to address minor deterioration of the spillway slabs. During construction, it was discovered that portions of the slabs were founded on fill rather than concrete or bedrock. A survey using ground penetrating radar was performed to further evaluate this unforeseen condition, and the study indicated that portions of the slab were underlain by fill with a maximum depth to top of bedrock of 9 feet. Using the PFMA as a guide, AECOM and Erie were able to identify loss of support to the spillway chute training walls retaining the embankment fill (making the wall more susceptible to failure during a high flow event) as a primary concern.
Based on this understanding, AECOM performed updated stability analyses on the training walls with the assumption that the slabs and material beneath it washed away during a flood event. The training walls were assumed to be constructed on bedrock, but without the resistance from the slabs and underlying fill material. The results of the stability analysis showed that the training walls remained stable without the presence of the slabs. This finding alleviated concerns with remediating the foundation conditions beneath the concrete slabs and allowed Erie to focus on confirming that the training walls are founded on bedrock. The PFMA-informed decision making after the discovery of the foundation issue led to a cost-effective solution that did not significantly affect operation.