Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 21

2 Engineering geology aspects for design and classification

2.1 General

Underground cavities can be constructed down to depths of more than 2,000 m below the
surface. The range of variation of the ground around the structure, which is structurally
or hydraulically affected during and after the formation of the cavity, or which affects
the structural stability and serviceability of the structure and the construction processes
required to construct it, is thus practically unlimited. This fact requires that the tunnel
engineer has a good understanding of geology, geological engineering and geomechanics
and demands close cooperation between the different professions. The following section
deals with the essential knowledge and factors, which are required for the estimation of the
ground conditions as they affect the construction of an underground structure.

2.2 Origin, properties and categorisation of rocks

2.2.1 General basics
Starting comment . Rocks are naturally formed combinations of various minerals (min-
eral aggregates) or of just one mineral type [50]. They can be solid or loose. Rocks are the
basic material that forms the rock mass, although the properties of the rock mass are not
identical with the properties of the rock.
Loose ground or soil is a combination of minerals and/or fractured pieces of rock and/or
organic components without mineral bonding. It is possible to grade the mineral content
according to grain size. Predominant contact between the particles at points is a character-
istic. Loose ground consists of several phases: solid-liquid, solid-gas or solid-liquid-gas.
Solid rock is a combination of minerals and/or fractured pieces of rock and/or organic com-
ponents with mineral bonding, which gives the rock a certain strength. Predominant contact
between the particles at surfaces and surface bonding of the components are characteristics.
Each type of rock has been formed by geological processes, which can be assigned to
a cycle held in motion by the balancing of energy equilibrium inside the Earth's crust.
(Fig. 2-1). In this cycle, the rocks are moved by the effects of endogenous (caused by
forces from inside the Earth) and exogenous (caused by forces acting from outside the
surface of the Earth) processes into areas, which do not correspond to their original forma-
tion conditions. The rocks are changed by the resulting adaptation to new conditions of
stability. The processes of formation and alteration of rocks illustrated in Fig. 2-1 are still
continuing today and lead to continuous changes at the Earth's surface. Since the duration