Repair process for an embankment dam affected by internal erosion

This article looks at the decision-making processes that went into determining the extent of problems and solutions needed to address dam safety issues at Chilhowee Hydropower Station. When seepage with intermittent cloudy water plumes exiting from the north embankment toe of Chilhowee Hydropower Station was identified, the dam owner brought on a design engineer to identify the extent of the problem and develop a solution. A Board of Consultants (BOC) of three very experienced embankment dam engineers was selected by the owner to provide overview and guidance, and the team of owner, engineer, and BOC worked successfully with the Federal Energy Regulatory Commission (FERC) during the entire process. Field subsurface investigations were performed, along with laboratory testing, to identify the problem, which consisted of a differential settlement induced crack in the upstream sloping clay core of the embankment near the contact with a concrete non-overflow (NOF) section. Various design alternatives were considered for repairing the embankment, and a final repair plan was developed. The reservoir water level was lowered about half way. The repair started with an excavation that allowed careful forensic observations and measurements of the clay core and filters in and near the zone of internal erosion and continued to top of rock at the bottom of the embankment behind a newly constructed secant pile wall. The wall provided a water barrier and earth support. Reconstruction of the embankment with nine material zones and refilling of the reservoir commenced immediately after the excavation was completed. Mitigation and coordination of environmental and cultural resources during this project were challenges whose lessons are noted in the article. © 2019, Association of State Dam Safety Officials. All rights reserved.