Security from Under-Seepage-Masonry Dams on Earth Foundations

This paper gives the results of, an investigation of more than two hundred masonry dams with various kinds of earth foundation, to determine the length of the percolation path necessary to prevent failure from under-seepage or piping. Based on this study, a new method of analysis of such structures has been developed, which generally permits the use of smaller seepage distances than are ordinarily considered to be safe.

When a masonry dam is founded on earth, some of the water from the reservoir percolates beneath it and appears on the down-stream side. If the velocity of the flow where it emerges is sufficient, particles of the foundation material will be carried away by the water, and thus decrease the resistance to the percolation. The result is an increased velocity and greater erosion, ultimately causing the formation of a channel or “pipe” beneath the dam, which may enlarge rapidly and cause the failure of the structure. This process is known as piping. To prevent piping it is necessary to design the dam so that the velocity of the seepage water as it emerges on the downstream side is insufficient to remove the foundation material. This is accomplished by making the route along which the water may percolate of such a length that the velocity.of flow is reduced to a safe value. The investigation described herein was undertaken to determine from data on actual dams the length of the percolation path necessary to insure safety for dams founded on various kinds of material.

A more complete report, including drawings that show dimensions and typical sections of all the dams, as well as a comprehensive bibliography has been filed for research purposes in Engineering Societies Library, New York, N. Y.