Rockslide of February 17-18, 2001
(courtesy of Richard M. Wooten & Timothy W. Clark, NCGS)

2. Geologic conditions

Rockslide scarp along the quarryface.
F1 = foliation plane 1 and failure plane
F3 = foliation plane 3 (shear surface)
J1 = fracture plane 1
J2 = fracture plane 2
F1 and F3 delineate a dislodged boulder in the foreground away from the rockslide. The long dimension of the boulder is about 9 feet. Examples of F3 shear surfaces in-plane are labeled in black.
Exposures in the main scarp reveal the geologic conditions that caused the rockslide. The promontory where the rockslide originated is comprised of foliated and fractured quartz-sericite phyllite. In this, as in most cases, planar discontinuities (e.g., foliations) within the rock determine the stability of a rock mass. Here the foliation (F1) that acted as the main slip surface dips 65°-70° toward the NW and intersects the quarry face. The parallel alignment of light-colored micaceous minerals (sericite) defines this smooth-surfaced foliation. Other foliation planes (F2 and F3) intersect F1 resulting in the splinter-like appearance of the rock. Brittle fracturing of the rock produced joints (J1 and J2) that are oblique to, and crosscut the earlier foliations.  These fractures acted as planes of separation (release surfaces) within the rock mass. 

In the main scarp, extensive oxide staining on the slip surfaces along foliation planes is conspicuously absent. This contrasts with the nearly ubiquitous staining and iron oxide mineralization along joint (fracture) surfaces. Staining and mineralization along these surfaces is evidence of infiltrating water; therefore, its absence suggests that the buildup of pore water pressure along the slip surfaces (foliations) over the long term was not a major contributing factor in triggering the rockslide. In contrast, precipitation infiltrating along the fractures followed by successive freeze and thaw cycles over the long term dilated the fracture openings and contributed to the instability.

The primary factor leading to the instability of the rock mass was the oversteepened slope that resulted from the quarry excavation. The triggering mechanisms for the February 17-18, 2001 rockslide are more difficult to infer because they operate over both the long and short term. Slow, gravitational creep can progress until the frictional resistance along weaker foliation planes decreases to the point where the rock mass fails catastrophically. Rock creep was observed in the upper two feet of the rock mass in the main scarp immediately below the soil zone as evidenced by foliation planes deformed in the down slope direction. Although not directly related to the rockslide, the near-surface rock creep does show the long-term gravitational effect on the weaker phyllite and schist units.

Copyright © Jean-Michel Margot
Date: 2001/09/23