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2008 annual meeting – Spruce Pine Mining District: Little Switzerland, North Carolina
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compositional layering and dips generally about 30-45 degrees to the southeast out of the
cut face. This surface forms the basal slip surface of the rock slide along thin schistose
layers within metagraywacke and schistose metagraywacke. Northwest-striking fracture
sets that dip steeply northeast form planar segments of the left-lateral release surfaces of
the rock blocks along the northeast side of the slide scar.
The degree of weathering of rock masses relates to their strength and permeability;
therefore, rock weathering classifications (in accordance with Williamson, 1984) are
provided. Most of the rock at this location is weathered to the stained state. Exposed
along the middle of the scar is a partly- to completely-decomposed graphitic, quartz,
muscovite schist interlayer that is remoldable with hand pressure. A line of vegetation
marks this zone of preferential weathering and seepage (Fig. 3). Seepage from this zone
probably contributed to destabilizing pore-water pressure along the basal failure surface
of the rock slide.
Acid-Producing Potential. Acid-producing rocks are common in the western and
central Blue Ridge. Bryant and others (2003) summarize many of the geotechnical
problems associated with acid-producing rocks. Acid runoff can occur when these rock
types are freshly exposed accelerating the weathering and hydrolysis of sulfide minerals
such as pyrite and pyrrhotite. Acid-producing rocks can also be a factor contributing to
slope instability. Schaeffer and Clawson (1996), and Douglas and others (2007) reported
means to identify and treat acid-producing rock types encountered in road construction.
Bogucki (1976) and Clark and Ryan (1987) reported debris slides and flows, and Wooten
and Latham (2004) reported embankment failures that mobilized into debris flows related
to acid-producing rocks.
NCGS studies along the Parkway included identification of sulfide-bearing lithologies
that are potentially acid producing. Selective sampling and testing of rock exposures
suspected of having acid-producing potential helped quantify the range of acid-producing
potential. Results of net neutralization potential (NNP) testing (Fig. 4) show lithologies
within the major map units identified as having acid producing potential. A
metagraywacke layer exposed at the northeast part of the outcrop at this stop (Fig. 5,
CER-002 in Fig. 2) was tested because the iron-oxide staining and gypsum blooms on the
weathering surface are characteristic of weathering sulfide minerals, chiefly pyrite and/or
pyrrhotite. The graphitic muscovite schist exposed in the slide scar was not tested
because of its advanced weathering state; however, it is likely that it contains significant
sulfide minerals in the stained and visually fresh weathering states as do several other
graphitic schist units exposed along the Parkway (Fig. 4). The advanced weathering, and
hence the permeability of the schist layer here, may be due in part to the presence of
sulfide minerals.
Summary. Here as in many rock slope failures along the Parkway, several factors
combine to result in rock slope instability: Ductile structural features like foliation and
compositional layering that define the slide surface dip toward the roadway and are
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