The principal objective of overburden analysis is to delineate the spatial occurrence of potentially acid or toxic material that can adversely affect water quality or vegetation. Once identified, this material can be given special handling during mining and reclamation to minimize impacts to the hydrologic balance. Strata capable of neutralizing acid- and toxic-forming material can also be identified through overburden analysis.
Collection of overburden samples is accomplished most efficiently during initial exploration activities or during geologic and hydrologic baseline data collection. Continuous rotary-drilled cores provide maximum information for consolidated rock sequences. Unconsolidated materials are typically sampled by split barrel (split spoon) or Shelby tube methods. Weakly consolidated rocks may require special soft sediment core barrels or other methods. Samples collected from drill cuttings or un-weathered samples obtained from highwalls and outcrops may also be suitable for overburden analysis.
Spacing of overburden sample locations is determined largely by site-specific conditions. Complex or variable geochemical and stratigraphic conditions necessitate more intensive sampling than where overburden properties are consistent over a large area. Regulatory authorities may be consulted for recommendations on overburden drill hole spacing based on their knowledge of local geologic conditions and previously collected geologic data that may be useful.
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