The Applicability of Filter Compatibility Evaluation Procedures for Existing Embankment Dams – Hearthstone Lake Dam Case Study and General Evaluation and Design Considerations

Hearthstone Lake Dam was constructed in the mid-1960s in Augusta County, Virginia, under the administration of the NRCS. The dam, which is classified as a high hazard structure by the VA DCR and the NRCS, is located on US Forest Service property but is owned and maintained by the Headwaters SWCD and Augusta County. The Virginia NRCS developed a 95% design between 2012 and 2016 to upgrade the dam to meet current NRCS seismic stability criteria and address several O&M needs. The NRCS National Design, Construction, and Soil Mechanics Center performed an independent review of the design. Based on the results of a filter compatibility evaluation performed using available original investigation and design information, they identified potential filter compatibility issues between embankment zones and recommended that a full-height chimney drain be installed in the dam to mitigate potential internal erosion issues. The chimney drain was estimated to increase the rehabilitation construction costs from about $1.2M to $5.8M.

Schnabel Engineering was retained by the County to perform a subsurface exploration to collect additional data, characterize the embankment and foundation soils, evaluate filter compatibility of the various embankment zones, and design the modifications. Sonic drilling techniques, large test pits excavated on the slope of the dam, and field gradation testing were required to adequately characterize the materials in the embankment. Based on the data collected during the subsurface exploration, the filter compatibility analyses demonstrated that the embankment zones generally met NRCS filter compatibility requirements. As a result, a full-height chimney drain was determined to be unnecessary. A robust toe drain was instead installed to provide an adequate filter for the foundation soils. The resulting construction cost savings for the toe drain design compared to the full-height chimney drain concept exceeded $2M.

The rehabilitation design was completed in mid-2018, and the project was bid in late 2018. Howdyshell Excavating, Inc. of Mount Solon, VA was the selected contractor, and construction of the dam rehabilitation began in November 2018. The project was completed in May 2020.

This presentation highlights an existing zoned embankment that was determined to have satisfactory filter compatibility between embankment zones despite not meeting NRCS requirements for filter design, as well as lessons learned from construction.