Engineering Rock Mechanics

Additional points 157

A9.7 0 In view of the complications described in A9.9, plus in this case
the limited crack length, a semi-empirical approach to the inclusion of
the water pressure is the most prudent approach. Haimson (1987) uses
the poroelastic equation P, - Po = (T + 3oh - OH - 2P0)/K, where Pc is
the critical pressure, Po is the pore pressure, K = 2 - a(l - 2u)/(l - u)
where a! is the Biot ‘constant’, and 1 1 a! 2 0, hence 2 1 K 2 1. Readers
are referred to the publications of Haimson for more detailed discussion
of hydraulic fracturing and to Cheng (1998)1° for an explanation of
poroelasticity.

9.3 Additional points

From the information in Section 9.1 and the answers in Section 9.2, we

have highlighted the fact that mechanisms in engineering rock mechan-

ics can be complex, on the small and large scales. It is easy to be critical

of semi-empirical approaches and rock mass classification methods, but

of necessity there are always some types of simplification and approx-

imation being made in engineering rock mechanics. Sometimes these

approximations are not obvious, as in the isotropic assumptions made

in the theory of elasticity which were discussed in Chapter 5; sometimes

they are obvious, as for the case of water flow as discussed in the current

chapter.

The subject of water flow in intact rock and rock masses is one of the

most intractable subjects in engineering rock mechanics. For this reason,

there are many aspects of the subject requiring further research studies.

In the US National Committee on Rock Mechanics book (see Footnote 3

in this chapter) on ’Rock Fractures and Fluid Flow’, eight research

recommendations are made. These are that the following should be

developed:

additional in situ research facilities;

improved conceptual models for fluid flow and transport;

improved understanding of the origin and development of fracture

improved fracture detection methods;

realistic numerical models;

understanding of stress, flow, temperature and chemistry coupling;

further research on waste isolation in fractured rock.

The last two recommendations are included because the subject of water

flow in fractured rocks is particularly important for radioactive waste

disposal. By definition, the waste will have been successfully isolated in

an underground repository only if radionuclides do not migrate in the

groundwater from the repository to the biosphere. However, to make

an engineering prediction for a long period into the future requires ad-

systems;

Haimson B. C. (1987) Measurement of in situ stress, in Geophysics, Methods of Exper-

imental Physics, Vol. 24B (C. G. Sammis and T. L. Henyey (eds), Academic Press, New

York, pp. 377-408.

’OCheng A. H.-D. (1998) On generalized plane strain poroelasticity. Int. 1. Rock Mech.

Min. Sci., 35’2,183-193.