A STATISTICAL APPROACH TO EVALUATE DEBRIS AVALANCHE ACTIVITY IN CENTRAL VIRGINIA

On August 19-20, 1969, the heavy rainfall associated with hurricane Camille triggered numerous debris avalanches in the Blue Ridge Province of central Virginia. Most slope movements occurred in Nelson County in an area that is predominantly underlain by Precambrian granites, gneisses, and schists. This study was undertaken to determine which geotechnical properties of soil and rock were important in promoting debris avalanche activity in central Virginia. Twenty-one first-order drainage basins in Nelson County were randomly selected and classified as unstable based on their values of horizontal curvature and average gradient as measured on topographic maps. Slope orientation, total soil thickness, and soil horizon thicknesses were measured in the field. Soils from the study basins were tested for geotechnical properties including natural water content, grain-size distribution, Atterberg limits, in situ dry density, and shear strength. These geomorphic and geotechnical properties were subsequently analyzed to ascertain what significant differences existed at the 99% confidence level between three failure classes of unstable basins. Where rock outcrops were present, information regarding discontinuity type, orientation, spacing, continuity, roughness, waviness, aperture, and water quantity was obtained. Values of hand sample hardness were also procured. The role of these bedrock characteristics in determining unstable basin failure class was statistically analyzed at the 95% confidence level. In general, the results indicate that basins classified as geomorphically unstable are particularly susceptible to debris avalanche activity if they contain coarse-grained soils with low values of plasticity index and moisture content. Geotechnical properties are also found to be significantly different when analyzed with respect to the underlying bedrock unit, which helps explain why basins underlain by certain bedrock units failed more often than basins underlain by other bedrock units. Differences in topography, due to the nature of the underlying bedrock unit, also explain the frequency of debris avalanche occurrences in a given bedrock unit. Characteristics of bedrock discontinuities do not appear to have influenced drainage basin failure.