Engineering Rock Mechanics

Additional points 397

The vertical stress is

Overtical = y. z = 0.029 x 800 = 23.2 MPa

and hence the ratio of horizontal to vertical stress is
37.0
23.2

k = - = 1.59.

The maximal height of a stope such that the compressive strength of
the rock in the crown is not exceeded is given by
2
,,=-(-+MI w gcrown =
4k2 overtical
The maximal height of a stope such that the tensile strength of the rock
in the sidewall is not exceeded is given by

2x2

• = 10.7 m.
  2w
  h= -
  ussidewall
  Gertical

+k-1 -5 23.2 + 1.59 - 1

Thus we see that sidewall failure is the limiting condition if no
stress-induced failure is allowed. If backfill is used to prevent degrad-
ation of the sidewalls, then the height can be increased to about 15 m
without failure occurring in the crown.

20.3 Additional points

In the last four chapters we have been able to highlight some of the
main rock mechanics techniques used in the design of surface and un-
derground excavations. However, it is not possible to cover all aspects
in a book because in a real design case there will be additional factors
related to the specific nature of the rock mass, the site and the engineer-
ing objective. This means that not only must the engineer be competent
in the techniques we have presented but also be able to approach new
projects with a creative eye and brain.
Much can be achieved in understanding a new site and the require-
ments for design by thinking through the subjects covered in the book
chapters. For example, some of the most important questions stimulated
by the content of the chapters are

Where can information on previous similar work be obtained?

Is the structural geology of the area understood? Are there any

major faults or shear zones present?

Is the regional pattern of in situ stress understood for this area?

Are local factors influencing the in situ stress distribution at the site?

Is it likely that the rock mass can be modelled as an elastic material?

Is the deformability, strength and failure of the intact rock under-

stood?

Are the geometrical and mechanical properties of the fractures

known?

What are the special rock mechanics features of the rock mass at the

site?