Original ground or Ground surface (Fig. 1) - The original ground surface at a dam site prior to construction.

Foundation of dam (Fig. 1) - The undisturbed material on which the dam structure is placed.

Lowest point of foundation - The lowest point of the dam foundation excluding cut off trenches less than 10 metres wide and isolated pockets of excavation.

Internal erosion - The formation of voids within soil or soft rock caused by the mechanical or chemical removal of material by seepage.

Leakage - Free flow loss of water through a hole or crack.

Seepage - The interstitial movement of water that may take place through a dam, its foundation or abutments.

Underseepage - The interstitial movement of water through a foundation.

Uplift - The upward pressure in the pores of a material (interstitial pressure) or on the base of a structure.

External pressure - When the pressure acts uniformly around the outside of a body, e.g. the water pressure on the outside of a tunnel lining, the term external pressure is used. The degree of external pressure causing complete structural failure is termed collapsing pressure.
An upward force on a structure caused by frost heave or by wind force.

Pore pressure - The interstitial pressure of fluid within a mass of soil, rock, or concrete.

Sand drain - A vertical hole formed in cohesive soil arid filled with sand for reduction of pore pressure.

Pressure relief pipes - Pipes used to relieve uplift or pore water pressure in a dam foundation or in the dam structure.

Well - A hole sunk into the earth to obtain water, oil, natural gas, etc., or into the fill or foundation of an embankment to collect water.

Drainage well - Vertical wells or boreholes downstream of. or in downstream shoulder of. an embankment dam to collect and control seepage through or under the dam and so reduce water pressure. A line of such wells forms a drainage curtain.

Relief wells - Vertical wells or boreholes downstream of. or in downstream shoulder of. an embankment dam to collect and control seepage through or under the dam and so reduce water pressure. A line of such wells forms a drainage curtain.

Vertical wells or boreholes downstream of. or in downstream shoulder of. an embankment dam to collect and control seepage through or under the dam and so reduce water pressure. A line of such wells forms a drainage curtain.