Environmental Engineering Reference
In-Depth Information
3
Geotechnical questions associated with various
geological environments
As explained in Chapter 2 and later in Chapter 4, site investigations for a dam need to be
undertaken with a good understanding of the local and regional geological environment
and the investigations should be aimed at answering all questions known to be of relevance
to dam construction and operation in that environment. This chapter discusses twelve
common geological environments in which dams have been built and derives check lists of
geotechnical questions of specific relevance to each.
It is important that readers appreciate the limitations of these generalisations and check
lists. The lists refer simply to features that might be present because they have been found
during construction at many other sites in similar environments and because geological
reasoning suggests that they could be present. At any particular site the actual geological
conditions found will have been developed as a result of many geological processes acting
at different times over vast periods of geological time. If some of these processes have been
very different from those assumed in the “general” case, then some or even all of the gen-
eralisations may not be valid at that particular site.
For further accounts of regional and local geological enviromments, with derived pre-
dictions of site conditions, see Fookes (1997) and Fookes et al. (2000).
3.1
GRANITIC ROCKS
Included under this heading are granite and other medium or coarse grained igneous rocks.
Most rocks of these types have been formed by the cooling and solidification of large masses
of viscous magma, generally at depths of greater than 5 km below the ground surface.
3.1.1
Fresh granitic rocks, properties and uses
In unweathered (fresh) exposures, granitic rocks are usually highly durable, strong to
extremely strong (substances) and contain very widely spaced (greater than 2 m) tectonic
joints in a roughly rectangular pattern (
Figure 3.1
). Many of these joints are wholly or
partly healed by thin veins of quartz, or quartz/felspar mixtures. Sheet joints are common
but, as they are almost parallel to the ground surface, they may be difficult to detect dur-
ing surface mapping.
Fresh granitic rocks are commonly quarried for rip-rap, rockfill and concrete aggre-
gates, but mica-rich granites may be unsuitable for use as fine aggregates in concrete due
to excessive amounts of fine, platy particles in the crushed products.
3.1.2
Weathered granitic rocks, properties, uses and profiles
Chemical weathering of granitic substances usually causes cracking at the grain bound-
aries and decomposition of the felspars and ferromagnesian (dark) minerals, leaving
quartz grains essentially unaffected.
