Horizontal Loads
Headwater (H1) - For the basic calculation of stability the level in the reservoir will be assumed at or above the level required for the passage of the design flood. In many instances the dam is designed for the highest level of watertightness, e.g. a concrete parapet.
Silt (H2) - A changed land usage as a result of a dam may well result in increased erosion, causing a deposition of silt. Unless very deep deposits of silt are likely it is adequate to assume a triangular load allotting an appropiate relative density to the fluid. This would have a maximum value of 1.4.
Reservoir Behaviour (H3) - Wind and other natural causes will induce movement in the reservoir as waves, reservoir set-up or seiche effect.
Ice Loading (H4) - It is assumed that ice will not form and exert pressure on the dam at times of maximum flood. The slope of the upstream face of the dam as well as the slope and roughness of the valley walls will influence the magnitude of ice loading. Even wind blowing down the reservoir at 50km/hr may increase the ice loading by 4-5 t/m of exposed face.
Tailwater (H5) - In some cases water is ponded downstream from the dam. Assistance from this may be assumed but it must not be overlooked that, in the case of an overflow dam, flood waters passing over the dam might well evacuate such water from the face of the dam.
Seismic Force (H6) - Force acting on dam in horizontal plane.
Seiche effect (H7) - Is an undulation in the reservoir water due to natural causes, intermittent wind, variation in atmospheric pressure, earthquake and motion of the Earth. Usually less than 0.5m, though levels of 2m have been reported in Lake Geneva.
Vertical Loads
Weight of Dam (V1)- The unit weight of material in the dam should be determined as accurately as possible. An underestimation by 1% will represented a considerable additional cost on the dam.
Vertical Water Loading (V2) - Imposed on any sloping surface of the dam, usually the upstream face, but also on the downstream for overflow dams.
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Uplift (V3)- Hydrostatic forces acting within a dam and its foundations including interstitial or pore pressures. Some Engineers rely on drainage to prevent occurence of uplift, assuming the drainage will be effective for the entire life on the dam, therefore some inclusion for uplift must be included in the design. See diagram for distribution of pressure. [k values vary between 0.25 to 0.50 depending on conditions.] Seismic Force (V4) - Force acting on dam in vertical plane. |
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Water Density - Some rivers carry very heavy silt load in seasons which changes the density of the reservoir.
Reservoir set-up - The result of continuing wind causing one end of the reservoir to be at a higher level. Calculations for a large reservoir in which the fetch is 38km would indicate the following values:
| Return Period | Wind Speed | Set-up |
|---|---|---|
| (years) | (km/h) | (m) |
| 1000 | 160 | 0.75 |
| 100 | 125 | 0.45 |
| 10 | 95 | 0.26 |
| 5 | 88 | 0.22 |
| 2 | 77 | 0.17 |
Thermal Effects - Concrete dams will be subject to loading from temperature variation within the dam caused by hydration of the cement and due to seasonal variations. Water as depth doesn't vary, but towards the surface it varies with season. A skew loading is used to describe solar and air temperature effects.
Construction Loads - Concrete dams of cupola and buttress shape offer good resistance to water loading when complete but during construction it is necessary to control the rate of construction and to include reinforcement in overhanging sections.
Direction of Forces - At certain locations it may be appropiate to increase the radius of an arch dam and accept higher stresses within the dam in order to ensure better angle of incidence of the resultant thrust with the abutment. The direction of resultant forces is important for gravity and buttress dams - especially on stratified rock.
Hydrostatic Loading within the Foundation or Abutment - Faults, cracks and joints are present in most damsites. Forces due to a large dam may cause cracks to appear in the rock upstream from the dam, this may cause jacking loads that could cause failure. To avoid this, careful surveys should be made of the orientation and inclination of faults, joints and cracks.
Tectonic Forces - Besides seismic effects, there may be significant tectonic forces on the Earth's crust at the site and these may be upset by deep excavation or saturation due to the filling of the reservoir.
