Hearthstone Lake Dam, Upper North River No. 77

As part of the NRCS Watershed Rehabilitation Program, VA NRCS developed a 95% Design to upgrade Hearthstone Lake Dam, a high hazard dam, to meet NRCS and Virginia DCR Dam Safety criteria. VA NRCS determined that the dam and spillway system met dam safety criteria for spillway capacity, integrity, and stability. However, several additional modifications were required, including addressing the seismic stability of the riser.

The NRCS National Design Center (NDCSMC) reviewed the NRCS design and identified potential issues with filter compatibility between and internal instability of multiple embankment zones, raising concerns about internal erosion. The NDCSMC recommended that a full-height chimney drain system be installed to collect seepage and mitigate the potential progression of internal erosion. However, VA NRCS did not have the capacity to further evaluate this issue and design the proposed modifications. Schnabel was selected to provide these services.

Schnabel performed an extensive subsurface investigation to characterize the 100-foot-tall embankment to analyze the filter compatibility between zones. Sonic drilling, large test pits, and field gradation testing of 2,000 lb bulk samples were required to characterize the embankment oversize materials. Through this effort, Schnabel demonstrated that the existing embankment zones generally met NRCS filter compatibility criteria, resulting in over $2M of savings. Instead of a full height chimney drain, Schnabel developed a rehabilitation design to replace the rockfill toe with a graded filter toe drain installed into rock to control seepage through the foundation. Construction of the proposed improvements was completed, under Schnabel’s observation, in May 2020.