Investigating the potential and mechanism of soil piping causing water-level drops in Mountain Lake, Giles County, Virginia

• Piping is responsible for periodic water level fluctuations at Mountain Lake.
• Piping occurs through four sinkhole-like depressions at lake-bottom.
• The piped sediment consists of fine sand and silt at varying densities.
• Lab studies indicate that piping occurs under hydraulic gradients of 1 to 10.

Mountain Lake, located in Giles County, southwestern Virginia, is one of only two natural lakes in the state of Virginia. The lake exhibits rare self-draining behavior, with recent episodes in 2008, 2011, and 2012 that drained the lake almost completely and revealed the presence of four sinkhole-like depressions, with piping holes at their bottoms and sides, near the northeastern and northwestern margins of the lake. This phenomenon has raised concern over declining tourism to the Mountain Lake Lodge. The purpose of this study was to investigate the piping potential of lake-bottom sediment and its role in seepage and lake-level fluctuations.Sixteen grab samples of lake-bottom sediment were collected from the vicinity of the depressions. Additionally, five Shelby tube samples were obtained to determine the in-situ density of the sediment. Grain size distribution, Atterberg limits, and compaction-mold permeameter tests were used to evaluate lake sediment's susceptibility to piping. Grain size distribution and Atterberg limits test results reveal that the lake-bottom sediment consists predominantly of fine sand and silt, which are well known for their susceptibility to piping. Results of the compaction-mold permeameter test show that the critical hydraulic gradient (ic), the hydraulic gradient at which the sediment starts to pipe, ranges between 1 and 10, depending on the density, grain size distribution, and plasticity characteristics of the sediment. These results suggest that piping of lake sediment is the primary mechanism responsible for the formation of lake-bed depressions and lake level fluctuations.