Last reviewed 4 Nov 2026 · 3 min read
1. Why geology decides the site
Most dam and tunnel failures and cost overruns come from geology: weak or fractured foundations, leakage paths, faults and unexpected water. A civil engineer therefore needs a geological model of the site before design.
2. Dams — geological requirements
| Requirement | Why |
|---|---|
| Strong, sound rock at shallow depth (granite, gneiss, quartzite, massive basalt, well-cemented sandstone) | To carry the dam's weight and water thrust without excessive settlement |
| Impervious foundation and abutments | To limit seepage and uplift |
| No active faults or shear zones across the dam axis | Movement or weak gouge can cause failure |
| Favourable dip of beds — beds dipping upstream are best | Downstream-dipping beds can allow sliding and leakage under the dam |
| Stable abutments and valley slopes | Landslides into the reservoir or at the abutments are dangerous |
| Low seismicity or design for it | Earthquake loads and reservoir-induced seismicity |
| Availability of construction materials | Aggregates, impervious clay (for earth dams) |
Problem rocks
- Limestone and dolomite — solution cavities (karst) cause leakage.
- Shale and clay-rich rocks — weak, swell, slake when wet; slide along bedding.
- Highly jointed or weathered rock — low strength, high permeability.
- Gypsum and rock salt — soluble.
Treatment measures
| Measure | Purpose |
|---|---|
| Consolidation grouting (shallow) | Strengthens and seals jointed rock near the foundation surface |
| Curtain grouting (deep line) | Forms an impervious curtain to cut seepage under the dam |
| Cut-off trench / diaphragm wall | Stops seepage through pervious overburden |
| Drainage galleries and relief wells | Reduce uplift pressure |
| Dental treatment | Removes and backfills weak seams and fault gouge with concrete |
| Rock bolts and anchors | Stabilise abutments |