Environmental Engineering Reference
In-Depth Information
and finding the answers to such questions, and of relating them to other site factors such
as climate and topography and the proposed development.
4.1.2.1
Questions relating to rock and soil types, climate and topography
The relative importance of any one of the engineering questions on
Table 4.1
and the amount
and kind of site investigations needed to get the answer to it will depend on the topographic,
geological and climatic environments in which the project is to be located. For example,
consider the effects of first filling of a water supply dam in an arid region, in a steep sided
valley underlain by a very weak sandstone. The water table is likely to be very low or absent,
and the sandstone may owe a large part of its strength and stiffness to cementation by a
water-soluble minerals such as gypsum. Filling of the reservoir has the potential to cause
dramatic changes to this site - significant raising of the water table, solution of water sol-
uble mineral with resulting weakening and possible increase in permeability of the foun-
dations, and possible instability in the storage area sides. Hence specific geological
questions to be answered during investigations of this site would include the following:
-
What are the cementing agents in the sandstone?
-
How much reduction in strength and stiffness will occur in the sandstone when satu-
rated for long periods?
-
Could solution effects during dam operations result in increase in permeability of the
foundation?
-
Could solution/strength reduction or water table rise result in instability in (a) the
foundation or (b) the reservoir sides?
At the other extreme, a site in a high rainfall area with gentle slopes underlain by very
strong quartzite, and with a high water table, is likely to be almost unaffected by inundation.
It can be seen from these two very simple examples that certain generalizations can be
made about geotechnical conditions likely to be met at a site, when its broad geological
setting is known, and this is considered together with the site climate and topography.
4.1.2.2
Questions relating to geological processes, i.e. to the history of development of
the site
It is not enough, during the design and construction of a major dam, to know simply what
rock or soil types are present, their engineering properties and their approximate distribu-
tion. Understanding the site environment implies also understanding the geological processes
which developed the region and the site. The most important processes are usually the
youngest, commonly those relating to the near-surface. This is partly because they will
have had a major influence on the strength and stability of the valley slopes, and also
because they may still be active, or may be reactivated by the construction or operation of
the dam. Table 4.2 is a list of such processes, some of which have been discussed in more
detail in Chapters 2 and 3.
Table 4.2.
Processes which may be active enough to affect a dam project.
- Destressing
- Freezing
- Chemical weathering of rocks and/or soils
- Burrowing by animals
- Solution
- Growth of vegetation
- Deposition of cement
- Growth of vegetation
-Erosion by wind or water
- Rotting of roots of vegetation, or buried timber
- Deposition of sediment
- Seismicity, i.e. shaking, or displacement on a fault
-Creep, landsliding
- Vulcanism
- Subsidence
- Glaciation
-Pressure by groundwater
