Dam site evaluation factors; gravity dam failure modes

Factors in dam site evaluation

• Topography: A narrow gorge with a wide valley upstream gives a large reservoir with the least dam length and volume; sound abutments with good height.
• Geology and foundation: Strong, sound rock with low permeability for a gravity dam; no major faults, shear zones, folds or weak seams; stable abutments free from landslides.
• Hydrology: Sufficient and dependable inflow; floods for the spillway design; sediment yield (to estimate the loss of storage).
• Reservoir: Capacity and area-capacity curve, watertightness (no leakage through limestone, faults), submergence of land, houses and roads, and evaporation losses.
• Construction materials: Quarry for aggregate, sand, cement and water near the site; availability of borrow areas for an earth dam.
• Seismicity: Distance from active faults and expected ground motion.
• Access and facilities: Road and rail access, power and labour, space for camps and diversion arrangement.
• Environment and society: Resettlement, forest and wildlife, downstream flow, and cultural sites.
• Economy: Cost per MW or per unit storage; comparison of alternative sites; a possible multipurpose benefit.

Failure modes of a gravity dam

1. Overturning (rotation about the toe): When the overturning moment of the water thrust and uplift exceeds the resisting moment of the weight. It happens when the resultant falls outside the base; it is rare, because crushing at the toe or tension at the heel occurs first.
2. Sliding: The dam slides along the base or a weak plane in the foundation when the horizontal thrust exceeds the shear resistance. $FS=(\mu\Sigma V+\tau B)/\Sigma H$.
3. Crushing (compression failure): The stress at the toe exceeds the strength of the concrete or the foundation rock.
4. Tension (cracking): When the resultant lies outside the middle third, tension at the heel (reservoir full) opens cracks. Water enters the cracks and increases uplift.
5. Foundation failure: Differential settlement, shear failure in a weak layer, or piping/seepage under the dam.
6. Overtopping and scour: Inadequate spillway, so that flood water flows over the crest and erodes the toe.
7. Seismic failure: Cracking and sliding due to earthquake inertia and hydrodynamic forces.

All the failure modes are checked in the design, with the factors of safety for overturning ($\ge1.5$), sliding ($\ge1$ friction, or $\ge3$-5 shear-friction), and the stress limits.