Environmental Engineering Reference
In-Depth Information
2
Key geological issues
Most dams and reservoirs are located at or near the bases of river valleys. The topogra-
phy and geological situation at every site will have developed by the interaction of many
geological and related processes during vast periods of time. The resulting geological
structure at dam and reservoir sites can be complex, and no two sites will be the same.
Some of the processes which formed the site may still be active, or may be reactivated by
the project and capable of influencing the feasibility of its construction or operation. It is
vital therefore, that sites are investigated using all appropriate knowledge and methods of
classical geology. These are essential for good engineering geology (Baynes, 1999; Fookes,
1997; Fookes et al., 2000; Hutchinson, 2001).
The engineering team responsible for construction of a dam at each site must be able to
make reliable predictions about suitable construction methods and how the dam and its
foundation will interact and perform, under every envisaged operating condition. The
predictions will usually involve both judgements and quantitative analyses, based on data
provided by the site investigation team. This data is provided as a geotechnical model,
which consists of the following:
-a sufficiently detailed three-dimensional picture or model of the site geological situa-
tion, using geotechnical descriptive terms, and an assessment of the history of the site
and the effects of any processes which are still active, and
-
adopted values of parameters for critical parts of the model, as required for the analyses.
This chapter introduces geotechnical descriptive terms, and uses them while discussing
processes found to be important in dam engineering. Defects or discontinuities and their
modes of formation are defined and discussed first, because understanding of them is
needed throughout the topic. Some aspects of the valley-forming processes of weathering
(breakdown) and erosion (removal) of rocks and soils are then covered in detail because
they are the youngest and most active processes at most sites. For further study of these
processes the reader is referred to Selby (1982), Bell (1983b), Hunt (1984), Fookes and
Vaughan (1986) and Fookes (1997).
2.1
BASIC DEFINITIONS
The following definitions and descriptive terms are recommended, and are used throughout
this topic.
Material
- a non-specific general term to describe any sample or body of soil or rock.
Substance
- material which is effectively homogeneous in its engineering properties, but
may be isotropic or anisotropic. The term “effectively” here means within the tolerances
applicable to the project in hand.
Rock substance
- a homogeneous cohesive aggregate of mineral grains which has such
strength that it cannot be broken up or remoulded by hand when dry, or when immersed
in water.
