8 Introduction
axis was orientated perpendicular to the maximum horizontal stress of
3.5-4.0 MPa which helped to stabilize the rock blocks in the roof. After
excavation of 140,000 m3 of rock and installation of the internal fittings,
the Gjenrik Olympiske Fjellhall can seat 5300 people. The impression one
has inside the cavern is the same as that in a building constructed above
ground.
41.4 Why do you think that the techniques used in rock mechanics
for site characterization, analysis and modelling are not the same
as those used in soil mechanics?
Af.4 Although there is a significant overlap between the two subjects,
for example both subjects make extensive use of stress analysis and
elasticity theory, soil particles are several orders of magnitude smaller
than the dimensions of the engineered structure, whereas rock blocks
can be of a similar size to the engineered structure. This means that the
discrete nature of the ground is more important in rock mechanics, and
techniques such as hemispherical projection and dedicated computer
modelling are required to assess the associated rock movements. Also,
some support methods such as the rockbolts and cable bolts mentioned
in A1.3 can only be used in rock masses.
In fact, the two main factors that cause the differences between rock
mechanics and soil mechanics are (a) the importance in rock mechanics
of the pre-existing in situ rock stress, and (b) the presence of the fractures
which govern the rock mass stiffness, strength, failure behaviour and
permeability. Understanding and modelling these two aspects alone
require a dedicated rock mechanics approach.
Q f .5 How can the subject of ‘engineering rock mechanics‘ be useful
to organizations outside the civil and mining engineering profes-
sions, e.g. to the petroleum industry, to insurance companies, to
environmental engineers?A1.5 The subject is potentially useful to any person or organization
concerned with the engineering behaviour of rock masses. In petroleum
engineering, the engineer wishes to be able to predict the stability of
wellbores and the conditions under which borehole breakout will occur
(damage caused by high rock stress at the borehole walls), in addition
to the overall rock mechanics behaviour of oil reservoirs. Similarly
insurance companies wish to evaluate the hazard to large structures
built on or in rock masses, and this requires a knowledge of engineering
rock mechanics. Environmental engineers need to understand processes
such as coastal cliff degradation, water flow through rock masses, and
the stability of disused mine workings.1.3 Additional points
In 2000, the largest underground excavation for civil engineering pur-
poses is in the Indian state of Himachal in the Himalayas. It is part