Key geological issues - Geotechnical Engineering of Dams - page 10

Soil substance - a homogeneous aggregate of mineral grains, either non-cohesive, or
cohesive but which disintegrates or can be remoulded by hand, when immersed in water.
Defect - a discontinuity, or break in continuity of a body of rock or soil substance.
Mass - any body or rock or soil which is not effectively homogeneous; it can comprise
two or more substances without defects, or one or more substances with one or more
defects.
In most situations the engineering behaviour of rock masses is dominated by the num-
ber, type and configuration of defects within them. The behaviour of soil masses is usually
influenced, and sometimes dominated, by defects present in them.

2.2 TYPES OF ANISOTROPIC FABRICS

Any of the following types of fabric, developed uniformly within a body of rock or soil
substance, will cause that substance to be anisotropic with respect to its engineering prop-
erties - strength, stiffness and permeability.
Bedding - layered or parallel arrangement of grains, developed during deposition as
sediment
Foliation - layered or parallel arrangement of grains (often tabular or flakey in shape)
developed either by viscous flow (in igneous rocks) or by pressure and heat (in
metamorphic rocks)
Cleavage - foliation in which many surfaces have developed along which the substance
splits readily
Lineation - linear arrangement of (often elongated) grains, developed by viscous flow
(in igneous rocks) or pressure with or without heat (in metamorphic rocks);
the lines of grains may or may not lie within surfaces or layers of foliation.
If a body has any of these fabrics developed in a non-uniform way, that body is mass,
by definition.

2.3 DEFECTS IN ROCK MASSES

Figure 2.1 illustrates and defines the most common and important types of defect met in rock
foundations for dams. It also relates characteristics of the defect types to ways in which they
were formed. The joints and two types of faults have developed directly from the stress his-
tories of the rocks. Soil infill and extremely weathered seams usually occur along or within
the above defect types, and hence most of them also result (indirectly, at least) from the stress
histories. As a consequence of this, the defects invariably occur in more or less regular pat-
terns, which can usually be distinguished at each site by the use of structural geology.
Further discussion of these defects and their significance in dam engineering is given in
Sections 2.3.1 to 2.3.5. The characteristics of defects which occur in particular rock types or
geological environments are discussed in Chapter 3.

2.3.1 Joints

Joints generally cause a rock mass to be less strong, less stiff and more permeable than an
equivalent body of the rock substance. Where joints have been acting as conduits for
ground waters, the rock next to them is often weakened by weathering (see Section 2.6 ).
As joints are typically of limited extent, they do not usually present serious problems in
embankment dam foundations but may be more significant in concrete gravity and arch
dam foundations. Treatments for jointed rock usually include excavation, grouting and
drainage (see Chapters 17 and 18).