Questions and answers: geological setting 2 1slopes in a road cutting, the working rock slopes in a quarry are not
permanent. So long as the fault does not affect excavation too much, the
associated instability is acceptable.42.3 The picture below shows tooth marks from the bucket of a
mechanical excavator in the Carboniferous rocks of a near-surface
slope in an opencast coal mine. What evidence is there here of
geological disruption to the rock strata?A2.3 The excavator tooth marks show that the rock is soft, but a much
more important aspect is the evidence of glacial deformation. Note
the irregular marker bands passing across the slope and below the
hammer head. Such irregular near-surface strata are evidence of glacial
perturbations and the possibility of slope instability problems.
42.4 A site investigation was conducted in a granitic rock mass
(see picture on next page). One side of fracture #300 in the core is
shown. What does this fracture indicate about the rock mass history
and what significance does this have for rock mechanics design of
slopes and tunnels in the rock mass?A2.4 The alteration ring around the fracture (the thin and lighter zone at
the base of the fracture in the photograph) indicates that some alteration
has occurred because of circulating water or other fluids. The texture
on the fracture, running from top left to bottom right, represents a
'slickensided' surface which occurs when the rock surfaces have moved
over one another. Thus, fluid has travelled through this fracture and
there has been shear movement on the fracture. These features indicate
a connected rock fracture system in which the rock blocks have been