Permeability, Pore Pressure, and Uplift in Gravity Dams

Findings are presented of studies concerning the intensity of pore pressure in concrete dams and the resulting uplift force. The low permeability of concrete, even with as little as 2 bags of cement per cu yd, indicates that equilibrium pore pressure should never develop in a service dam. The effect of air entrainment is to reduce the penetration of pore pressure into a dam to a fraction of the penetration without air. An approximate solution for the determination of the percentage area subject to uplift pressure is offered; the area is expressed in terms of known voids and of the ratio of the elastic modulus of the solid constituents of the concrete to that of the composite concrete. The effective area is deduced to be approximately 100% of the gross area in the lean concretes investigated. However, in view of the low permeability, high intensity of pore pressure need never develop in the interior of a gravity dam if reasonable precautions are taken. Also, expansive strains caused by saturation and pore pressure are reported. These strains tend to increase the compressive stress near the upstream face and further improve the safety of a dam. Current (1956) design practice may, therefore, be overly conservative.