---
title: "Dam site evaluation factors; gravity dam failure modes"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:gravity-dam-site-evaluation-failure-modes"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/gravity-dam-site-evaluation-failure-modes
site: BCT Engineering
---

# Dam site evaluation factors; gravity dam failure modes

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2072 Kartik, 4+4 marks
- Video: [Dam Site Selection || What Are The Points To Be Considered While Selecting Dam Sites ||](https://www.youtube.com/watch?v=4BCWRPcxMN4) — Nitu Creator
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/gravity-dam-site-evaluation-failure-modes
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

What are the factors to be considered in dam site evaluation? Describe the different failure modes of a gravity dam.

## Short answer

Dam site evaluation factors: topography, geology, hydrology, reservoir, materials, seismicity, access, environment, economy. Failure modes: overturning when resultant outside base, sliding along base or weak plane, crushing at toe, tension cracking at heel, foundation failure, overtopping, and seismic failure.

## Answer

#### 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which topographic condition is most favourable for a dam site?
   A. A wide valley at the dam site and narrow upstream
   B. A narrow gorge with a wide valley upstream
   C. A flat plain
   D. A steep valley with weak abutments
   Answer: B. A narrow gorge with a wide upstream valley gives large storage with least dam length and volume. (Key point: Ideal topography)

2. For a gravity dam foundation, what is needed?
   A. Soft alluvium with high permeability
   B. Any soil with a cut-off
   C. Strong, sound, low-permeability rock without major faults or weak seams
   D. Rock with many shear zones
   Answer: C. Gravity dams need sound rock with low permeability and stable abutments. (Key point: Foundation requirement)

3. Why is overturning rarely the first mode of failure?
   A. Because dams are very light
   B. Because uplift acts upward only
   C. Because overturning moment is always small
   D. Because crushing at the toe or tension at the heel occurs first
   Answer: D. Before the resultant falls outside the base, the toe crushes or the heel cracks in tension. (Key point: Overturning)

4. What is the consequence of tension cracks at the heel?
   A. Water enters the cracks and increases uplift
   B. The weight of the dam increases
   C. Uplift is eliminated
   D. The toe stress drops to zero
   Answer: A. Cracks let reservoir water in, raising uplift, and this reduces stability further. (Key point: Cracking effect)

## Related in Hydropower Engineering

- [Gravity dam stability with seismic load](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-seismic-stability-2073) — Hydropower Engineering, chapter 4
- [Middle third rule for gravity dam](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Amiddle-third-rule-derivation) — Hydropower Engineering, chapter 4, asked 2×
- [Gravity dam stability (SG 2.4, RL 0-70)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-stability-2082-bhadra) — Hydropower Engineering, chapter 4
- [Gravity dam section: shear stress near toe (2082)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-stability-2082-baishakh) — Hydropower Engineering, chapter 4
- [Gravity dam stability and shear stress at toe (2081)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-stability-2081-bhadra) — Hydropower Engineering, chapter 4
- [Gravity dam forces and stability checks (2081 Baishakh)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-forces-stresses-2081-baishakh) — Hydropower Engineering, chapter 4

## Related in other subjects

- [Failure modes and stability criteria of gravity dams](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Agravity-dam-failure-modes) — Hydro Power, chapter 4, asked 2×
- [Geology in dam site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Adam-site-selection) — Engineering Geology II, chapter 4
- [Pavement evaluation; flexible failure types and causes](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Apavement-evaluation-flexible-failure-causes) — Transportation Engineering II, chapter 4
- [Interaction diagram and failure modes](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Ainteraction-diagram-and-failure-modes) — Design of RCC Structure, chapter 10