One of the highest dams in Georgia is a compacted earth structure that serves as a water supply source and supports recreational activities for the Big Canoe residential community in North Georgia. During a routine dam safety inspection by the Georgia Safe Dams Program (GSDP), potentially excessive seepage was reported near the toe of the dam. A resulting detailed evaluation of the existing site conditions led to the development (by others) of a rehabilitation program involving the construction of a large toe buttress to minimize the adverse impacts of the seepage and to provide the requisite calculated factor of safety regarding static and seismic slope stability. The goal of the dam owners (Big Canoe Property Owners Association) was to evaluate alternatives to the toe buttress remedy that could achieve the required factor of safety and be more cost-effective and less environmentally intrusive. Geosyntec was retained by the dam owners to perform an independent third party review of the recommended remedy and assist in the identification of alternatives.
Geosyntec's Scope of Services
Geosyntec's approach was to validate the original analysis by physical inspection and by conducting additional analyses of the dam's structural integrity. Geosyntec conducted a site visit and confirmed the seepage conditions near the toe. With the exception of this localized area, the remainder of the dam appeared to be in excellent condition. To confirm the subsurface conditions and the phreatic water levels in the dam, Geosyntec developed and implemented field investigation, performance monitoring, and laboratory testing programs to obtain site-specific soil strength, seepage, and seismic response information. These data were used to re-evaluate the existing conditions of the dam. The results of the field investigation program, coupled with seepage and detailed seismic response analyses indicated that alternative rehabilitation options were available that were significantly less costly and much less environmentally intrusive. Geosyntec developed potential remedies that included supplemental drainage to address seepage at the toe. Once the locally excessive water levels were controlled, the requisite static and seismic stability could be readily demonstrated. The technical approach and the analysis results were provided in a technical report to the GSDP. Once the recommended strategy was implemented, the toe seepage was controlled and to date there have been no indications of inadequate performance.
By conducting a thorough site investigation, it was possible to confidently design a seepage control system that fit the specific project. Detailed seismic response analyses were not routinely performed in Georgia as part of dam safety assessments, due to the anticipated low ground accelerations from the design earthquake. However, Geosyntec's experience in the performance of these analyses, in lieu of the overly conservative pseudo static analyses, definitively confirmed the seismic stability of the Lake Petit Dam. The result of these technically sound engineering solutions allowed the POA to save more than $1 million in construction costs that would have otherwise been expended for the toe buttress and the restoration of the roads within the community.