Engineering Rock Mechanics

The influence of geological factors on rocks and rock masses 25

Induced I

Figure 2.22 Rock engineering construction of an unsupported tunnel in which no
loads are applied.

engineering defensive strategy is to support with respect to horizontal
stresses rather than vertical stresses. Hence, we need to understand the
concept of stress and its generation by natural mechanisms.
Basically, the vertical stress component is caused by the weight of the
overlying strata, whereas, the high horizontal stress is mainly due to
tectonic forces. In some cases, the horizontal stress can be very large, as is
illustrated in Fig. 2.24 showing the subduction zone between the Nazca and
Pacific tectonic plates in South America. In Fig. 2.24(a) the location of the
Chuquicamata open pit and the El Teniente underground mines in Chile
are shown. Both are very significantly affected by the high horizontal stress
which acts in a west-east direction: this stress component is perpendicular
to the long axis of the essentially elliptical Chuquicamata open pit and has
caused problems of stability. Such stress-associated problems have been
dramatically manifested underground in the El Teniente mine in the
Andes. In 1987, a major rockburst occurred during block caving develop-
ment at a height of 2700 m above sea level. Without a knowledge of the

In situ

1 1 1 stress

Figure 2.23 Pressurized water tunnel involving load application to a rock mass in

which the pre-existing stresses have been redistributed by excavation.