Engineering Rock Mechanics

386 Design and analysis of underground excavations

Figure 20.28 shows a wide range of principles associated with the ground
response curve, as illustrated in the following points.

The ground response curve for different locations on the periphery of

the excavation is different.

The ground response curves indicate that at, some locations, support

is not necessary (because the displacements equilibriate at zero support

pressure), and in other locations support is essential (because the

ground response curve does not intersect the zero support pressure

axis).

Attempting to achieve zero radial displacement is impractial: to do so

would require extremely high support pressures and support stiffnesses.

The support cannot be installed at zero radial displacement, because the

elastic response of the ground, on excavation, is instantaneous.

Different types of support have different stiffnesses, and these result

from the material of construction, the geometry of the support system,

and the quality of construction: as a result, these different supports will

offer different degrees of support to the excavation, and will support the

rock with different support pressures.

It is possible for supports to attain their peak strength (including some

degree of yielding) and still be effective in supporting the excavation.

In conjunction with a given ground response curve, three variables

determine the mechanical efficacy of a given support scheme: its time of

emplacement, its stiffness, and its peak strength. The ground response

curve itself can also be a function of construction techniques. Hence, the

engineer has to optimize the overall interaction between the ground

1: Steel sets with good blocking.

2: Steel sets with poor blocking.

3: 50mm thick shotcrete.

4: Early installed mechanically anchored rock bolts

5: Late installed mechanically anchored rock bolts.

0.4

h

k 0.3

6-

2

8

*

c O.*

5

a a

m

0.1

0 25 50 75 100 125

Radial displacement, si (mm)

Figure 20.28 Available support lines and ground response curves (from Brady and

Brown, 1985, and Hoek and Brown, 1980).