For a gravity dam the weight of the structure is the main stabilizing force, and hence the construction material should be as heavy as possible.
Structure self weight is accounted for in terms of the resultant, W, which acts through the centroid (center of gravity) of the cress-sectional area. The weight of the structure per unit length is
W = c * A
Where:c is the unit weight of concrete
A is the cross-sectional area of the structure
The unit weight of concrete may be assumed to be 24 kN/m3 in the absence specific data from laboratory test trials. For final designs the specific weights shall be based on actual test data. Where crest gates and other ancillary structures or equipments of significant weigh are present they must also be accounted for in determining the weight of the structure.
It is essential to make sure that the actual specific weight obtained for the construction material is more than or at least equal to that assumed in the design.
Earth and Silt Pressure
The gradual accumulation of significant deposits of fine sediment, notably silt, against the face of the dam generates a resultant horizontal force, Fs. The magnitude of this force in additional to water load, FWH, is a function of the sediment depth, hs, the submerged unit weight, ss, and the active pressure coefficient, Ka, and is determined according to Rankine‟s formula.
Fs = ½ Ka ss hs2
Where Ka = (1-sina) / (1+sina)
a = angle of internal friction of material.
When the dam is full, wind will act only on the downstream face, thus contributing to stability. When the dam is empty, wind can act on the upstream face, but the pressure is small compared to the hydraulic pressure of the water. Hence for gravity dams wind is not considered. For buttress dams, wind load on the exposed buttresses has to be considered.