Geology and Dams
On a large dam construction project the engineering geologist is concerned with:
- the geology of the dam site including the foundation for the dam itself and the sites for other structures such as spillway, diversion tunnel and outlet works. Questions that need an answer include whether the dam foundation has sufficient strength and durability to support the type of dam proposed, whether the foundation is watertight and if not how much grouting will be required and whether the spillway will require concrete lining;
- the geology of the area to be occupied by the reservoir once the dam is completed. Questions often asked here include whether the storage area is watertight or are there areas of cavernous limestone which might lead to the dam not retaining water and whether landslides into the reservoir are possible which might cause a wave of water to be pushed over the top of the dam;
- finding sources of the construction materials which will be needed to build the dam.
Extensive site investigations are usually required to answer these questions. No two dam sites are identical as far as geology is concerned so each new dam construction project must be investigated individually. Some dam sites may be relatively uniform in their geology ie one rock type with a simple structure and a regular pattern of surface weathering. More often though the geology will be complex with several different rock types with different physical properties such as strength, durability and susceptibility to weathering. The geological structure may also be complex with geological units folded and faulted into a complicated, difficult to interpret pattern. Degree of surface weathering may vary suddenly from one geological unit to another further complicating the task of the engineering geologist.
The following two examples are of dams where the site geology was a very significant factor in the design and overall layout of the entire project:
Glennies Creek Dam: A 10 metre thick layer of completely weathered, non-welded tuff (a soil type material) at the dam site had a controlling influence on the choice of type of dam and the siting of the dam, diversion tunnel and the spillway; in fact, the whole project layout was determined by the outcrop and weathering pattern of the non-welded tuff.
Windamere Dam: The embankment dam was built on a weathered, sedimentary rock foundation. The rock fill construction material to build the dam was obtained from an unlined rock cut spillway in unweathered andesite about 1 km from the dam site. If a spillway had been built adjacent to the dam in the weathered sedimentary rocks it would have had to have been lined with concrete to prevent erosion, at a greatly increased cost.
In the design of embankment dams there are two major decisions which have to be taken, both of which depend on geological factors:
The geology of the dam spillway is thus important to the overall design and layout of the whole dam construction project.
- The extent to which it will be necessary to provide concrete lining and/or energy dissipation structures in the dam spillway.
- The extent to which the spillway excavation will be able to supply fill for use in the construction of the dam embankment.
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