Uplift in gravity dams and associated problems

Abstract

The complex nature of a gravity dam including its galleries, drain - holes, roads and ancillary buildings, the variable nature of concrete, the heterogeneous foundation, the indeterminacy of the forces acting on the dam including the restraint of the foundation and abutments, all lead to a three -dimensional problem as complex and insolvable in exact terms as could possibly be met in engineering design. The design of gravity dams is then, of necessity, based upon a number of simplifying assumptions, perhaps the most important of which is to consider the problem in two dimensions and limit calculations to finding the stress distribution on a cross -section at right angles to the longitudinal axis of the dam. If the dam and foundation are considered to be made of one homogeneous, elastic material, an exact solution is then theoretically possible if the applied forces are known. It is not generally possible to determine exactly the magnitude of the applied forces which consist mainly of the weight of the material, the internal and external action of the impounded water, and in some locations ice pressure, silt pressure and earthquake shocks. In addition the uneven cooling and shrinkage of concrete set up forces which are inherent in massive structures. The dependence of most of these forces on the elements of nature makes them subject to uncertain variation, but in most cases a good degree of approximation can be reached. This is the result of many years of exhaustive research which has reduced enormously the approximation and guesswork in early dam designs.

It is paradoxical that the action of water pressure, which the dam, primarily, is meant to withstand, should have been misunderstood through centuries of dam building and even to -day eludes complete understanding. The early dams were built like walls, with broad rectangular profiles intended to resist the horizontal thrust of water in their impounded reservoirs. These structures, "designed" by purely empirical methods, generally possessed factors of safety far in excess of those permitted by present day economy or demanded by present day practice. With the advent of competitive industry and the development of mechanical sciences, dam design became more scientific.

Because the horizontal thrust of water on a structure increases linearly with depth, it was soon realised that the ideal profile for a dam was a triangle with a vertical upstream face. Dam profiles, therefore, became gradually more slender, tending towards the ideal shape, and in the latter part of the 19th century this reduction reached a significant stage.

Around the turn of the century a number of large dams in Europe and in the United States of America failed because of the movement downstream of one of their sections and the subsequent collapse of the remaining structure under the onrushing water. The resulting loss of life and property was catastrophic and serious investigations had to be made into the causes.

It was gradually realised, as postulated by a few advanced thinkers of that time, notably Levy25 in France and Intze 3 in Germany, that water under pressure could not only push horizontally on the structure but could also penetrate into the concrete or masonry of the dam and its foundation probably, it was thought, through cracks and fissures, and so cause an upward as well as a forward overturning force. This upward or "uplift" force reduced the effective weight of the dam and diminished the shear resistance at its base.

The uplift theory was too revolutionary to be immediately and universally accepted. Dams had been built successfully for hundreds of years assuming that the entry of water into a properly constructed dam or its foundation was impossible, or at least confined to local fissures and negligible in overall effect. It was not long, however, before evidence established that water under pressure did exist within "solid" dams, at least near the foundation.

Hollow pipes were built into new structures, connecting inspection galleries with holes drilled through the joint between dam and foundation. Some time after impounding the reservoir these holes were found to fill with water which rose up the pipesto heights which indicated considerable water pressure at the foundation. The evidence of these measurements firmly established the reality of water under pressure beneath a structure and the resultant uplift force.