Optimum Location of Drains in Concrete Dams

Abstract

Hydrostatic pressures from reservoir and tail water act on the dam and occur within the dam and foundation as internal pressures in the pores, cracks, joints, and seams. Measurements in existing dams confirm that uplift pressures undoubtedly exist, not only at the foundation junction plane, but on other planes within the dam to which water has gained access in the course of time. The distribution of pressure through a horizontal section of the dam is assumed to vary linearly from a full hydrostatic head at the upstream face to zero, or tail water, at the downstream face, provided the dam has no drains. When drains are constructed, the internal pressure within the dam should be modified in accordance with the size, location, and spacing of the drains. The internal pressure distribution through the foundation is dependent on size, depth, location, and spacing of drains, on rock porosity, jointing, faulting, and to some extent the grout curtain. The pore pressures act over the entire area of any section through the concrete. Because of possible penetration of water along construction joints, cracks, and the foundation contact, internal pressure should be considered to act throughout the dam. Internal hydrostatic pressures should be used for analyses of the dam and the overall stability of the dam at its contact with the foundation. In this study, an analytical solution based on seepage theory is presented to determine the average uplift pressure across the section of a gravity dam having a system of equally spaced drains of uniform diameter. The optimal location of the drains for the minimum uplift is also obtained.