Geology in dam site selection

A dam must be founded on ground that is strong, stable and watertight.

Geological factors

1. Foundation rock: strong, massive, sound, with low weathering. Granite, gneiss, quartzite and massive sandstone are good; weak shale, phyllite, schist and soft clay are poor. The type of dam (concrete gravity, arch, earth fill) is chosen from the foundation.
2. Geological structure: avoid faults, shear zones, folds with weak planes, closely jointed rock; bedding should be favourable (beds dipping upstream are best; beds dipping downstream, especially at low angles, are dangerous because the dam can slide along them).
3. Watertightness: avoid highly permeable rock, solution cavities in limestone, and open joints; check with Lugeon tests; plan grouting and cut-off.
4. Abutments: stable, no landslides or rock fall; adequate rock cover.
5. Valley shape: narrow, with sound rock at shallow depth, so smaller dam volume.
6. Seismicity: avoid active faults near the site; design for earthquakes.
7. Reservoir: rim and basin should be watertight and stable (see reservoir site).
8. Construction materials: aggregate, sand, clay and rock fill near the site.
9. Groundwater and seepage conditions around the dam.
10. Sedimentation rate of the catchment.