Philpott Dam is a concrete gravity structure which generates electrical power and reduces the risk of flooding for the town of Bassett, VA, from the Smith River. Due to heavy extended rainfall, the Philpott reservoir experienced a “pool of record” reaching 984.55 ft (NAVD88) on 22 May 2020, with water flowing over the uncontrolled ogee spillway for the first time since the dam’s construction in 1952. Although lake levels later receded to elevations below the spillway crest, the Philpott Lake area received additional rainfall which further saturated the already-wet abutment slopes. As a result, extensive landslides occurred on 24 May 2020 along the downstream slope of the right abutment, with debris entering the powerhouse and significantly damaging internal infrastructure. Specifically, material from one of the landslides inundated the powerhouse through forcible entry, blanketing much of the floor areas in mud. Slides also caused extensive damage to the switchgear building and blocked the driveway and parking area adjacent to the powerhouse.
The most extensive and damaging hillslope movement occurred by mobilization of saturated soil and saprolite along a steeply dipping subsurface bedrock contact. Conversely, a nearby less extensive slide appears to have been rotational and confined within the overlying soil mass. Regardless, these landslides appear to have been exacerbated by the improper disposal of excavated material on the right abutment slope during original dam construction activities.
Immediately following the 24 May 2020 landslide events, the Wilmington District consulted with USACE’s Geotechnical, Geology, and Materials Community of Practice’s Landslide Committee to develop short- and long-term response actions to evaluate slope failure origins and to mitigate physical risk to personnel and infrastructure. Many of these activities have already been implemented, such as life-safety operational measures, hillslope LiDAR surveying, debris clearing from dam toe access areas, and placement of concrete barriers to protect powerhouse ingress and egress. Additionally, long-term slope stability solutions have been developed through investigation and analysis by, and in consultation with, geotechnical contractors. Ultimately, the remedial design includes soil overburden removal, retention wall installation, and the construction of slope toe protective measures to enhance life safety and reduce the risk of future slope failures.
