Engineering Rock Mechanics

Additional points 281

From this we find that the area of steel bars required per square metre
of pillar face is 11.3/210 = 0.054 m2. In terms of bars whose diameter
is 25 mm, this represents 110 bars, which could only be obtained by
inserting the bars at a spacing of about 95 mm! However, although the
quantity of bars is large, the stress induced in them is only p/Ab, or 46.3
MPa. As high tensile steel bars are capable of working at 300 MPa, this
shows that this form of passive reinforcement is inefficient.
To improve the efficiency of the reinforcement, it would be appropriate
to apply a tension to the bars during installation. This would generate
the required support pressure directly, without requiring the pillar to
deform elastically. This is known as active reinforcement.

16.3 Additional points

The book by Hoek et al. (1995) explains many of the principles of rock
reinforcement and support and associated subjects.
There is specific reference material available on rock reinforcement.
For example, the Schlumberger Lecture Award paper by Windsor (1997)
summarizes the approach to rock reinforcement systems and highlights
the components of the structural analysis, remembering that the main
function of rock reinforcement is to enhance the strength of the dis-
continuous rock mass. Windsor provides a complete methodology for
studying rock reinforcement, starting with the elements in the table be-
low and including the development of safety factors.

Rock joint system components Reinforcement system components

(in parallel) fin series)

1. Areas of intact rock


2. Areas of asperities


3. Areas of infill


4. Areas of planar friction
   
   
   1. Rock
   
   
   
   


5. Internal fixture


6. Element


7. External fixture

A publication by Villaescusa et al. (1999) contains papers by Brown
on the evolution of support and reinforcement philosophy, and by
Hoek on support for very weak rock associated with faults and shear
zones.
Our main emphasis in this book is on rock mechanics applied to civil
and mining engineering, but the mechanical principles apply to all rock
engineering. In petroleum engineering, the occurrence of unstable bore-
holes is a major problem - not only in inclined and sub-horizontal wells
but also in vertical wells. The mechanical understanding of borehole in-

Hoek E., Bawden W. E and Kaiser I? K. (1995) Support of Underground Excavations in

* Windsor C. R. (1997) Rock Reinforcement Systems. Int. 1. Rock Mech. Min. Sci. 34, 6,

gVillaescusa E., Windsor C. R. and Thompson A. G. (4s) (1999) Rock Support and

Reinforcement Practice in Mining. Proceedings of the International Symposium on Ground

Support, Kalgoorlie, Western Australia. Balkema, Rotterdam, 437pp.

Hard Rock. Balkema, Rotterdam, 215pp.

919-951.