Engineering Rock Mechanics

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(a)

Rock reinforcement and rock support

Figure 16.3 (a) Isochromatics in photoelastic model indicating shear stresses around a
square excavation, (b) using a slotting technique to move the shear stresses away from
the excavation, thus reducing rock damage at the periphery of the excavation (from
Roberts, 1977).

stability and a discussion of methods of reducing instability is included
in Fjaer et al. (1992). lo
The most elegant stabilization option is, if possible, to develop an
excavation geometry such that no stabilization is necessary. In terms
of rock fractures, an excavation can be oriented such that minimal
rock block instability occurs, and, in terms of stress concentration (the
amount by which an applied stress component is concentrated by the
excavation), tunnels oriented parallel to the maximal principal stress
experience the least stress concentrations on their boundaries. In an
isotropic stress field, a circular tunnel cross-section reduces the maximal
stress concentration to a factor of 2. In an anisotropic stress field,
with the horizontal to vertical stress ratio being k, the maximal stress
concentration is minimized by having an elliptical cross-section in which
the ratio of width to height is also k.
Deviatoric stresses (i.e. the difference between the major and minor
principal stresses) produce shear stresses which in turn cause failure.
When the excavation is made, the stress normal to the excavation surface
is reduced to zero, creating high shear stresses in the rock adjacent to
the opening. Another stabilization option considered in the early days
of rock mechanics was to move these shear stresses further back into
the rock using a rock slotting technique (Fig. 16.3, from Roberts, 1977) ll.
Although it is an elegant solution, practical problems outweigh its use-
fulness, except in special circumstances where the slots can be easily cut.

lo Fjaer E., Holt R. M., Horsrud P., Raaen A. M. and Risnes R. (1992) Petroleum Related

l1 Roberts A. (1977) Geotechnology. Pergamon Press, Oxford, 347pp.

Rock Mechanics. Elsevier, Oxford, 338pp.