Engineering Rock Mechanics

1 02 htact rock

Increasing

confining pressure

Ductile

____

Brittle

Figure 6.15 The effect of confining pressure in the triaxial test and the
brittle-ductile transition.

An idea of the variability in the confining pressure associated with the

transition is given in the table below (after Goodman, 1989).

Rock type Confining pressure

(MW

Rock salt 0

Chalk < 10

Limestone 20-100

Sandstone > 100

Granite +loo

It is emphasized that these are representative values, chosen to illustrate

the general trend.

One would expect different post-peak behaviour around caverns

excavated in a soft rock salt and a hard granite. On construction, the rock

at the excavation periphery ceases to be under triaxial compression, because

the normal and shear stresses on the rock wall have been reduced to zero.

So, the rock will tend to behave in a more brittle fashion. In a rock salt

cavern, we could have brittle failure visibly occuring in the walls, with more

ductile behaviour occuring out of sight further into the rock. Similarly, in

a chalk possessing a 10 MPa brittle-ductile transition, and because the

overburden stress is approximately 1 MPa for every 40 m of depth, we

might expect ductile behaviour in deep civil engineering in soft rocks,

depending of course on a whole host of other factors.

Finally, perhaps the most important aspect of this phenomenon is to

understand the reason why the stress-strain curves take the form they do.

As we have mentioned earlier, in compression the rock tends to fracture

perpendicular to the least principal stress, i.e. parallel to the major principal

stress. Consequently, the application of even a small confining pressure has

a significant effect on inhibiting the development of these cracks, and

indeed, the mechanism of crack formation, which gradually changes to

shearing as the confining pressure is increased.