Rockmasses 15temples at the World Heritage site at Ellora in SW India. This temple
has been created in the Deccan Traps by simple hand excavation of the
volcanic basalt. The pillars that can be seen at the entrance are part
of the in situ rock mass. Above the temple, natural rock fractures’ are
visible; such fractures are encountered in almost all rock masses and can
lead to instability of engineered structures. Most of these temples have,
however, remained stable well beyond a civil engineering design life of
120 years, the figure that we often use today for design purposes.
In Fig. 2.2, a road has been severely damaged by the sliding of a large
block of rock on which the road had been built (to the right of the upper
picture). The rock block was able to slide because there was a large-scale
natural weakness, a shear zone, in the limestone formation as shown in
the lower picture. The coefficient of friction on the limestone bedding
planes was low because they were clay-filled, and this enabled the
limestone block to slide and damage the road. For all rock engineering
projects, it is crucial to be able to locate such sigruficant geological
features.
Fig. 2.3 shows two slopes. In Fig. 2.3a, a pre-split rock slope at the
side of the A82 road in Scotland is shown. The pre-splitting technique
is a rock-blasting process whereby the final rock slope is created as a
fracture plane first by blasting in a row of parallel blastholes, with the
rock subsequently being excavated up to this fracture plane. The fact
that the blasting has been successful is evidenced by the visible traces of
the half boreholes left on the rock surface as the whitish parallel lines.
However, the rock already contained fractures formed long ago when
the rock was subjected to high stresses caused by tectonic activity. Because
the fractures were formed as a result of the applied stresses, they tend to
occur in sets of sub-parallel fractures with specific orientations. The sets
of fractures can occur at several orientations because there were different
phases of tectonic activity during the history of the rock mass. In Fig. 2.3a,
two fractures from different sets have formed a rock wedge which has
slid out of the excavation (and was removed during slope construction).
The engineer standing on the top of the slope indicates the scale.
These natural fractures are an inherent feature of rock masses. En-
gineers cannot speclfy that the rock mass should be unfractured: the
properties of the rock mass have to be established by site investigation
and the design adjusted accordingly.
In the case of this road, the location of the road and hence the slope
were determined by the overall topographic features, and there was little
- During the development of rock mechanics, the word ‘discontinuity’ was used to
denote natural faults, joints, fissures, etc., because they are discontinuities in the rock
continuum. The word ‘fracture’ was previously used mainly to denote man-made discon-
tinuities. Nowadays, and especially in the USA, the word ’fracture’ is used in place of
’discontinuity’. We have adopted this usage in this book.
Figure 2.2 Road instability in Spain. The displacement of the road, shown in the top
photograph (a), was caused by movement of a large limestone block released by the
shear zone, in the lower photograph (b), with sliding on clay-filled bedding planes. Note
the engineer standing on the lip of the shear zone, in the black square.