Engineering Rock Mechanics

290 Surface excavation instability mechanisms

Isotropic material

If the ground is reasonably

isotropic, the surface tends

to be circular in section.

Almost circular

g highly jointed rock,

oken rock, weathered

ck, tailings or soil

Anisotropic material

If the ground has a bedded or

laminated structure, or some

other characteristic which

makes it anisotropic, then the

slip surface tends to be elongat

in a direction parallel to the

structural feature laminated soils

g well laminated rocks

slates, mudstones, schists)

egulary jointed rock,

Major structural features present

It their is a major discontinuity,

fault or clay seam in the region

of the instability, the qlip surface

will tend to follow this feature

as far as possible.

ows major structural

E g bedding planes, joints, faults,

shear zones

Low cohesion, granular materials

If the ground has a granular

nature, with a low cohesive

strength, the curvature of the

slip surface is less marked (1.e.

the surface tends to be planar)

and the tension crack is

small or non-existant some soils

Inhomogeneous material

For example, the presence of an

underlying bed of hard, strong

material can limit the extent of

failure

very nearly planar

E g heavily broken rock, tailings and

materd E g changes in lithology, igneous

intrusion, mineralization

Figure 17.4 Development of curvilinear slips.

surface. In practice, the factor of safety is determined for assumed slip
surface locations. In the margin sketch, the slip surface is shown discretized
into four elements, each of which has normal and shear forces applied to
it. Each element has three unknowns associated with it: the normal (N) and
shear (S) forces, and the location of the line of action of the normal force
relative to the element itself (n).
For the case shown, therefore, there is a total of 12 unknown parameters
in the problem. However, there are only three equations of static equilibrium
available to solve this problem: XFx = 0, XFy = 0 and CM = 0, where F, are
components of forces in the x-direction, Fy are components of forces in the
y-direction and M are moments in the x-y plane. There are insufficient equa-
tions to determine the unknowns: i.e. the problem is statically indeterminate.
To solve the problem, we have to make assumptions which reduce the