Rippable versus non-rippable earth material

Rippable Triassic material - Chapel Hill, NC (photograph by NCGS)	Non-rippable Carolina terrane material - Chapel Hill, NC (Photograph by NCGS)

The rippability of earth material is a measure of the ability to excavate and/or remove the material with typical excavation equipment (ex. backhoe, dozer, etc).  Encountering unanticipated non-rippable earth material at a construction site can cause delays and cost overruns.  Geologic maps can provide some insight as to the possible presence of non-rippable material. 

Unexpected non-rippable material can occur in almost any location in the Mountains and the Piedmont. Some areas of the State tend to have an elevated occurrence of unanticipated non-rippable material due to the geology.  These areas include the Triassic basins and the Carolina terrane. 

Map showing approximate extent of Triassic basins and Carolina terrane in North Carolina.

Triassic basin non-rippable rock types

Rock material close to the surface in the Triassic basins is often considered rippable.  However, local variations in rock type may cause areas to have significant non-rippable rock.  Sandstones layers and diabase are two rock types, that if under a site, may be non-rippable. Detailed geologic maps are available for some areas of the Triassic basin in the Durham and Sanford area.

Carolina terrane non-rippable rock types

The Carolina terrane occupies a significant portion of the Piedmont of North Carolina.  The Carolina terrane is composed of metamorphosed volcanic rocks in which some of the rocks can be very hard and resistant to erosion.  Outcrops of the resistant varieties of rock are often non-rippable.  Detailed geologic maps are available for some areas of the Carolina terrane.