SINKHOLES, SOILS, FRACTURES, AND DRAINAGE - INTERSTATE-70 NEAR FREDERICK, MARYLAND

Numerous sinkholes have recently formed on both sides of Interstate 70 south of Frederick, Maryland, All the sinkholes are cover-collapse ty pes, which form when soil cavities grow upward from the bedrock surfac e until their roofs become unstable, Areas at greatest risk for sinkho le development lie within a network of dry swales, The roughly dendrit ic map pattern and presence of allochthonous siliciclastic alluvium su ggest that these swales are the vestiges of a vanished surface drainag e system. Sinkholes occur mainly along bedrock escarpments underlying the swales, which are located along an easterly-trending transverse fr acture and a series of strike-parallel fractures which intersect with it. Although the surface drainage appears to have Bowed east and north in the past, surface runoff in large quantities is infiltrating the g round or directly entering some of the sinkholes, then following subsu rface conduits which convey it southward under the highway. Compaction grouting has been employed to prevent collapse or further subsidence of the most threatened portions of the highway. Soil Survey maps can b e useful in locating cryptic intermittent or relict drainage pathways which may be at high risk for sinkhole formation when subjected to ant hropogenic concentrations of perched storm water.