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Marine Clay - This soil type occurs on hilltops and sideslopes in the Coastal Plain. Highly plastic silts and clays of variable thickness underlie surface silts to gravels a few inches to a few feet thick. The clay deposits consist of highly fractured and broken clays and silty clays which have a high shrink-swell potential. The depth of the seasonal high water table is variable depending on soil stratification, landscape position, and local hydrologic features such as seepage areas. Water is often perched above slowly permeable subsurface strata. Depth to hard bedrock is generally greater than 50 feet.
Slope instability and poor foundation support are major problems requiring intensive geotechnical analysis. Numerous structures have sustained damage from inadequate construction practices. Septic drainfields and infiltration trenches are poorly suited because of plastic clays, perched water table, and unstable slopes. Source: Fairfax County Soil Map Unit Descriptions |
