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What are the general conditions needed for a dam to be built?
1) The area needs to be geologically stable e.g. away from earthquakes, faulting and volcanic eruptions.
2) The valley sides need to be stable so that mass movement is unlikely.
3) The river catchment needs to be underlain by impermeable rocks and have sufficient rainfall.
4) There should be no exposed mineral veins containing toxic elements if the reservoir is for drinking water
What can happen if the bedding planes in valley sides dip towards the reservoir?
Land slips can occur, especially if incompetent impermeable rocks and competent permeable rocks are interbedded.
What qualities are needed in the underlying rocks of a dam?
Competent; high load-bearing strength to support both the dam and the water e.g. crystalline igneous or metamorphic; impermeable; the rock type should be uniform.
Why does the underlying rock need to be uniform?
Different rock types can experience differential subsidence, which could make the dam unstable. They may also weather differently and the depth of weathering is important in keeping the dam stable.
Impact of the attitude of the strata on dams?
Beds dipping upstream (dam into reservoir) or horizontally provide stable foundations, whereas beds dipping downstream (reservoir towards dam) give unstable foundations. There could be leakage or slippage along bedding planes and the dam could collapse as a result.
How do faults and joints impact dams?
They are zones of permeability and weakness. The reactivation of old faults can cause increased seismic activity, causing the dam to collapse.
Why are Anticlines bad for dams?
They make unstable foundations because slippage can occur along the bedding planes on the fold limbs, and tension joints on the crest of the fold could cause leakage of water out of the reservoir.
Impact of syncline folds on dams?
They’re more stable than anticline folds BUT can allow water to pass under the dam if they have permeable beds.
2 types of dam?
Masonry/concrete dams
Embankment/earth dams
3 types of concrete/masonry dam?
An arch dam
A gravity dam
An arch-gravity dam
Arch dams
Curved upstream so that hydrostatic pressure compresses the structure against the sides of the valley, strengthening it.
Thinner than other types of dam, so uses less construction material.
Suitable for narrow gorges with steep sides of strong rock, in remote areas.
Gravity dams
Held in place by the force of gravity due to the immense mass of the concrete/masonry.
When built on an impermeable, high load-bearing foundation, this is often the best type of dam.
Sometimes hollow - more economical to construct.
May be supported on the downstream side by a series of inflexible buttresses.
Arch-gravity dams
Combines rage strength of an arch with the force of gravity, and does not have to be so massive.
Useful in areas with a high flow of water but limited material for dam building.
Embankment/earth dams
Impermeable clay or concrete core held in place by piles of rock/earth/sand/clay, with an impervious covering.
Material binds itself together with friction between particles, so concrete is unnecessary.
Built in broad, shallow valleys, where the mass of the dam is spread over a wide areas so the foundations don’t need to be as strong.
Require large volumes of fill material.
3 techniques used to prevent leakage from reservoirs?
1) Grouting
2) Clay or plastic lining/geomembrane
3) Cut-off Curtain
Grouting
Holes are drilled into the rock and liquid cement is pumped in. This fills pore spaces, joints and fissures, reducing permeability and increasing rock strength.
Clay or plastic lining/geomembrane
Prior to filling, the reservoir is lined with an impermeable material such as clay or plastic to prevent leakage of water into the underlying rock. Clay is a good choice because if there is a local supply it will be cheap.
Cut-off curtain
An impermeable barrier, usually made of concrete, is constructed as an extension below the dam. This prevents leakage particularly from dams located on synclines. It strengthens the foundations and prevents slippage of beds dipping downstream.
