Engineering Rock Mechanics

362 Design and analysis of underground excavations

If possible,

design with

the rock

Figure 20.1 Assessing roof and sidewall structurally-controlled instability from
visual observations.

part of Fig. 20.1, the stippled area of rock blocks is likely to be unstable, and,
if acceptable, changing the designed outline of the excavation to that shown
would harmonize the structure with the rock and reduce the degree of
instability.
Another example is shown in Fig. 20.2, in which the excavation orien-
tation is considered. As a general principle, and as is clear from the
figure, tunnels parallel to the strike of discontinuity sets are likely to have
more instability problems. Similarly, with reference to Fig. 20.3, a large
excavation will be more unstable than a small excavation in the same rock
mass.
Much can be readily estimated by careful observation. It is apparent by
direct observation whether a rock type is susceptible to weathering.
Similarly, visual assessment of rock discontinuity surfaces can provide
guidance on their mechanical behaviour. This is not to say that we advocate
abandoning more sophisticated forms of analysis: it is just that a great deal
can be learnt from observation, especially once the rock mechanics
principles are understood.

20.1.2 Elastic anulysis applied to stratified rock
Beam analysis. Following the pragmatic design approach, more analyti-
cal approaches are now considered. One of the oldest problems that
has been considered in rock engineering is that of roof beam flexure-
because the main design factor for many excavations in stratified rock
is the maximum acceptable unsupported roof span. In Fig. 20.4, there
is an example of the flexure of a beam above an excavation. With this
geometry we can calculate both the maximum tensile stress induced in such
beams by their self-weight, and also the maximum deflection of the beams.
In the case where the beams thin upwards, the strata will not separate
directly above the opening-providing the elastic properties of the beams
are equal. Conversely, if the beams thin downwards, strata separation will