Engineering Rock Mechanics

280 Stabilization principles

7

(a) Stresses on excavation

boundary

There are three basic concepts that permit consideration of the potential

for interlayer slip and establishing the extent of the regions thus affected:

(a) when rock is excavated and an opening formed, the excavation surface

becomes a principal stress plane, with the result that the components

of normal stress in the rock rotate to become parallel to the excavation

surface (see Fig. 16.11(a));

(b) slip on a discontinuity can occur when the major principal stress (in

the plane of the excavation surface) applied across the discontinuity

acts at an angle greater than qj to the discontinuity normal (see Fig.

16.11(b));

(c) a convenient geometrical construction, utilizing (a) and (b) above, can

be used for establishing the extent of the boundary of the opening over

which the potential for interlayer slip exists (see Fig. 16.11(c)).

In Fig. 16.11(c), the geometrical construction used involves drawing a
construction line normal to the single discontinuity set under considera-
tion, followed by two further construction lines inclined at an angle @j to
the normal. Tangents to the excavation boundary, which are parallel to
these two construction lines, are then drawn. The key to this construction
is that between these two tangent points, criterion (b) above is fulfilled, and
hence this is the region for potential inter-layer slip. This construction is
valid for the entire excavation boundary.
The construction applies to any concave shape of excavation. In Fig. 16.12, the
construction is shown for a much more irregularly shaped excavation. It is
not necessary to know the geometrical centre of the excavation shape;
keeping the normal line of the construction and the discontinuity normals
parallel, all three lines of the geometrical construction are moved until the
outer lines touch the excavation boundary on both sides. In Fig. 16.12(a),
the construction is shown for @j=200, whereas in Fig. 16.12(b) the construc-
tion is shown for @j=500. This not only demonstrates the construction
method, but also shows that there is potential for inter-layer slip in the walls
for the lower friction angle, but not for the higher friction angle. Such a
reduction in the friction angle could result from deterioration of the
discontinuity surfaces with time, with the result that previously stable
zones of an excavation boundary become liable to instability.

Discontinuity

(b) Stress-based criterion

for slip

Slip when uniaxial stress applied

I I at orientations within this zone

(c) Geometrical construction