Geology topics
Publications and source records attributed to M. A. Sabol.
The failure potential of the debris-avalanche dam at Castle Lake near Mount St. Helens, Washington, by three processes of seepage erosion (1) Heave; (2) piping; and (3) internal erosion, is examined. Results indicated that the dam is stable against piping but potentially locally unstable against heave. -from Authors
The debris avalanche that occurred during the May 19, 1980, eruption of Mount St. Helens blocked South Fork Castle Creek and created Castle Lake. Stability of the blockage was of concern, and a digital model that simulates three-dimensional groundwater movement in the blockage was constructed as part of the analysis used in a follow-up study that assessed the blockage 's stability. Slug test results in the debris avalanche deposits and model results indicate that the average horizontal hydraulic conductivity of the blockage material is approximately 2.5 ft/day, whereas the ratio of horizontal to vertical hydraulic conductivity is approximately 10 to 1. The model was calibrated to seasonally high groundwater levels and groundwater discharge. Model-predicted recharge rates for this time period were 0.97 cu ft/sec. Most of the recharge (81%) results from the infiltration of precipitation, whereas discharge by seeps through the blockage accounts for 81% of the total discharge. Because water levels under the crest of the blockage are higher than lake level, the movement of groundwater is toward the lake and the toe of the blockage. The model allows the water levels to be estimated at any location in the blockage. This information is required for making estimates of the stability of the blockage against failure by gravitational-induced or earthquake-induced slope failure, liquefaction, the process of seepage erosion, or by erosion.
Gas Works Park, in Seattle, Washington, is located at the site of an abandon gasification plant on Lake Union. Wastes deposited during 50 years of plant operations (1906-1956) have extended the shore line 100 ft and left the park soil contaminated with a number of hazardous material. Soil contaminants include polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls, pesticides, volatile organic compounds, cyanide, and metals. PAHs and metals have been detected in Lake Union sediments. Maximum total PAH concentrations exceeded 100 million micrograms/kilogram in some places in the soils of the park at 6-inch depths and in some lake sediments. Other contaminants present are much lower in concentrations. The park is on glacial drift overlain by gasification waste materials and clean fill. Waste materials include sand and gravels, mixed with lampblack, oil, bricks, and other industrial wastes. Groundwater flows through the soils and waste toward Lake Union. Vertical groundwater movement is uncertain, but is assumed to be upward near Lake Union. Concentrations of most soil contaminants are probably low in the groundwater and in Lake Union due to the low solubilities and high sorptive characteristics of these contaminants. However, no water quality data are available to confirm this premise. (USGS)
A slope stability analysis on the South Fork Castle Creek debris avalanche blockage, near Mount St. Helens, Washington, was conducted to determine the likelihood of mass failure of the blockage and resultant breakout of South Fork Castle Creek Lake. On the basis of material properties, groundwater levels, and seismic history of the blockage, slope stability with and without earthquake-induced forces was determined. Results indicated that the blockage will not fail from gravitational forces at September 1983 groundwater levels. An increase of 25 feet or more in water levels could cause local failures, but massive failure of the blockage is improbable. Blockage slopes are potentially unstable for present and higher water levels if an earthquake with magnitude greater than 6.0 should occur. Retrogressive slope failures are possible, but lowering of the blockage crest below lake level and consequent lake breakout are considered remote. Significant earthquake shaking could cause cracks in the blockage that might facilitate piping.