Two factors directly affect the design of a dam, the intensity of hydrostatic pressure at various points within or under the dam and the area upon which pressure acts.

It is now accepted for design purposes that uplift pressures act on the full area of the section. The intensity of pressure may be represented by the diagram showing the ideal case of underflow conditions for an impermeable dam with a straight base on a homogeneous isotropic foundation of unlimited depth and horizontal extent.

Headwater and tailwater depth are represented by h1 and h2, respectively. The concentric semi-ellipses represent lines of flow of water passing through the foundation. The hyperbolas, drawn normal to the lines of flow at all points, represent lines of equal hydrostatic pressure within the foundation and at the base of the dam. This network of flow lines and pressure lines is called a flow net. The diagram indicates an almost linear distribution of pressure on the base, and this is the distribution for which the stability of the dam should be checked if no drainage is provided. (or all drainage is blocked.)

Drainage is in the form of curtain of cored vertical holes 150mm or more in diameter at 3-5m spacing and located 304m from the upstream face. A gallery runs from one end to the other of the dam, above the tailwater level. Drainage from the holes is led away via open gutters, with measuring weirs installed to record the flow.

It is now general to adopt a distribution of uplift pressure as above, the value of factor k being decided having due regard to the porosity of foundation rock and the existence of joints and cracks therein. It is important to expend effort and money on a drainage system to ensure satisfactory function over the entire life of the dam.

Variation in contraction joints and uplift.

(C) Wahlstrom, Ernest Dams, Dam Foundations and Reservoir Sites
(C) Thomas, Henry H. The Engineering of Large Dams