A gravity dam is a massive structure built from concrete or masonry that holds back water using its own weight. Where arch dams and buttress dams redirect loads into the abutments or foundations, a gravity dam resists the push of the reservoir with the mass of its cross-section, which is why the stability evaluation of gravity concrete structures starts with geometry and weight rather than complex framing.
Gravity dams are straight in plan and are usually built where the foundation is sound rock. They are among the most durable water-retaining structures ever built and need very little maintenance over service lives measured in decades.
What Is a Gravity Dam?
A gravity dam is designed so that the weight of the dam itself resists the external forces acting on it. The section is typically triangular, widest at the base where the water pressure is greatest. The elementary profile is the theoretical minimum section that just satisfies stability, and practical dams are built thicker to add safety margins and to carry roadways and crest works.
Materials and Scale
Concrete gravity dams are preferred over masonry for most modern projects because concrete can be placed quickly in any weather and because its quality is easier to control. The placed concrete is checked throughout the pour, and gravity water absorption tests give a quick field measure of how dense and durable the material is.
Two of the best-known examples show the scale involved. The Grand Dixence Dam in Switzerland is 284 meters high and is one of the highest gravity dams in the world. The Bhakra Nangal Dam in India stands 226 meters high and is the highest and biggest gravity dam in the country.
The foundation decides whether a gravity dam is feasible. A gravity dam is most suitable when the foundation is sound rock capable of carrying the huge base pressures without settlement, and the site must be investigated and treated before any concrete is placed.
Forces Acting on a Gravity Dam
Every gravity dam is analyzed for the forces that push it over, slide it, or lift it, and for the forces that hold it down. The forces are expressed in kilograms per meter run of the dam, so the analysis is done on a unit-width slice of the section.
Primary Hydraulic Forces
Water Pressure on the Upstream Face
Water pressure is the major external pressure and the main overturning force acting on the dam. For a vertical upstream face it is P = wh2/2, where w is the specific weight of water and h is the depth of water, and the resultant acts at h/3 above the base.
Weight of the Downstream Wedge
When water stands on the downstream side, the weight of that wedge, P = whd2/2, acts downward at the center of gravity of the wedge and helps stabilize the dam, where hd is the depth of water on the downstream side.
Secondary Forces
Beyond water pressure, the designer accounts for the following:
- Weight of the dam, the main stabilizing force, acting at the center of gravity of the cross-section.
- Upstream silt pressure from sediment deposited against the face, which can also exert vertical forces where the face is sloped.
- Uplift force from water seeping under the base, reduced by grout curtains and drainage galleries.
- Seismic forces in earthquake zones, which add horizontal and vertical accelerations to every other load.
- Ice pressure in cold climates, acting on the upstream face at the waterline.
- Wind pressure and wave action on the exposed face.
The same force logic applies to smaller gravity structures. Civil engineering texts classify gravity and non-gravity retaining structures by how they resist overturning, and a gravity dam is simply the largest member of that family.
| Force | Direction | Effect on stability | Typical expression |
|---|---|---|---|
| Water pressure (upstream) | Horizontal, toward downstream | Overturning | P = wh2/2, acting at h/3 above the base |
| Weight of the dam | Vertical, downward | Stabilizing | Area of section x unit weight |
| Downstream water wedge | Vertical, downward | Stabilizing | P = whd2/2 |
| Uplift | Vertical, upward | Destabilizing | Reduced by grouting and drainage |
| Silt pressure | Horizontal, toward downstream | Overturning | Depends on sediment depth and density |
| Seismic forces | Horizontal and vertical | Destabilizing | Code-defined accelerations |
| Ice and wind | Horizontal | Overturning | Local climate values |
Construction of Gravity Dams
Building a gravity dam is a large-scale earthworks and mass-concrete operation that runs in a fixed order, from river diversion to the finished crest works.
Foundation Preparation
The foundation is excavated to sound rock, cleaned, and treated. A grout curtain is injected into the rock beneath the upstream heel to cut seepage and control uplift, and drainage holes or a gallery collect the water that does get through.
Placing Mass Concrete
The dam is built in lifts, with each layer compacted and cured before the next. Mass concrete for gravity dams uses large aggregate and a low cement content to limit the heat of hydration, and low-heat or blended cements, cooling pipes, and carefully timed lifts keep internal temperatures under control.
Roller-Compacted Concrete Option
Many modern gravity dams are built with roller-compacted concrete, which is placed with earthmoving equipment and compacted by vibratory rollers. RCC dams build faster and cost less than conventional mass concrete while delivering the same stability, provided the mix and layer joints are controlled.
Cost follows volume. A large gravity dam consumes hundreds of thousands to millions of cubic meters of concrete, and the placed unit cost, including formwork, cooling, and quality testing, is the biggest single item in the project budget.
The construction sequence runs as follows:
- Divert the river and excavate the foundation to sound rock.
- Treat the rock with grouting and install the drainage system.
- Place mass concrete in lifts with contraction joints and cooling control.
- Build the overflow section, spillway gates, and stilling basin.
- Install instrumentation for monitoring movement, seepage, and uplift.
- Complete the crest works, roadways, and appurtenant structures.
The full sequence, from river diversion to crest works, is described in the water-resources guide to gravity dam construction, which covers foundation treatment, lift placement, and quality control in more depth.
Stability Analysis and Design Specifications
The stability of a gravity dam is checked against overturning, sliding, and overstress at the base, for each load combination the dam will face over its life.
Stability Criteria
Codes such as IS 6512 in India and USBR guidance in the United States require minimum factors of safety against overturning and sliding for normal, unusual, and extreme load combinations. For normal operating conditions the factor of safety against overturning is commonly 1.5 or higher, and the dam must also resist sliding along the base and along any weak plane in the foundation.
The Middle-Third Rule
The resultant of all forces must fall within the middle third of the base for the dam to stay in compression across its full width under normal loads. If the resultant moves toward the heel, tension develops, and concrete in tension cracks and lets water in.
Design Specifications
For a typical concrete gravity dam the base width is roughly 0.7 to 0.8 times the height, and the upstream face is nearly vertical or slightly battered. The top width is set by the crest roadway and freeboard requirements, and the downstream slope is chosen so the resultant stays inside the middle third at full reservoir.
The detailed design specifications for gravity dams, including section proportions, load cases, and material requirements, are covered in the companion guide to gravity dam design on this site.
Advantages and Disadvantages of Gravity Dams
Advantages
- Very durable and solid, with low maintenance over a long service life.
- Safe against overturning by weight alone, with no reliance on water pressure against the face.
- Suitable for high dams on sound rock, including large spillway discharges over the crest.
- Simple, well-understood analysis and construction methods.
- More tolerant of overtopping than embankment dams.
Disadvantages
- Requires a strong rock foundation, which limits site selection.
- Uses much more concrete or masonry than arch or embankment dams.
- High construction cost and a long construction period.
- Uplift and seepage must be controlled with grouting and drainage.
- Thermal cracking must be managed during mass concrete placement.
Gravity does the same quiet work in buildings that it does in dams; the anatomy of how gravity-flow and pressure-assisted toilets work shows how a small column of water produces enough force to flush a fixture cleanly.
Applications and Crack Control
Gravity dams serve irrigation, water supply, hydropower, flood control, and navigation. They are chosen when the foundation is strong and the site can accept a massive concrete structure, and they are often built with the spillway incorporated into the main section so flood water passes over the crest.
Typical Applications
- Irrigation storage and regulated releases.
- Municipal and industrial water supply.
- Hydroelectric power generation.
- Flood control and flow regulation.
- River navigation and water-level maintenance.
Measures to Control Cracks
Cracking is controlled at the design and construction stages. Contraction joints divide the dam into blocks that shrink without tearing, cooling pipes and low-heat cement limit thermal gradients, and reinforcement is placed at stress concentrations such as openings and galleries.
The concrete is cured carefully, and instruments monitor temperature, movement, and uplift so problems are caught before they become cracks. Where gravity plumbing is not available, such as a basement bathroom below the sewer line, an up-flush toilet system pumps waste up to the drain instead, which is the same problem of moving water against gravity that dam designers solve on a much larger scale.