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.
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.