Hoover Dam and the Evolution of Uplift Theory

The earliest technical article by an American engineer on hydraulic uplift was the ASCE Presidential Address by James B. Francis in 1888. He suggested applying full hydrostatic pressure at the upstream heel of a dam, diminishing to zero pressure at the downstream toe. Thoughts on the potentially destabilizing role of uplift were mentioned by John R. Freeman in 1911 in his comments on the failure of Bayless Dam near Austin, PA. Between 1911–1918 Arnold C. Koenig, J. B. T. Coleman, and James B. Hays discussed how to account for uplift in masonry dams in the ASCE Proceedings and Transactions. Up until 1927–28 most engineers assumed that uplift pressure was tied to the permeability of the foundation and that of the dam structure. Most assumed that concrete was impervious and incapable of transmitting meaningful pore pressure. The textbooks cited by the engineers who drafted the plans for the St. Francis Dam cited examples that appeared in textbooks published between 1908–18. In 1918 Edward Wegmann asserted that it was impossible to accurately estimate the uplift that might develop beneath a dam and that engineers should rely on their own judgment. Others suggested that an uplift pressure diagram in the shape of a trapezoid be employed, assuming development of two-thirds the theoretical uplift, varying lineally to zero at the toe, unless uplift relief wells were employed. All the various theories were called into question when Hoover Dam was filled to capacity in 1934–41 and excessive uplift pressures developed beneath the dam’s downstream face. This led to tripling the depth of the grout curtain, which took nine years complete. In 1936 Karl Terzaghi introduced his theory of effective stress, which established a distinction between total stress and those ascribable to hydrostatic (pore water) pressure. In 1939 ASCE formed a Subcommittee on Uplift in Masonry Dams. In 1945 Terzaghi summarized the results of experimental work that suggested water was able to transmit pore pressures in concrete. The ASCE committee released their final report in 1952, which included uplift measurements of a modest number of high-head dams and commented on what they felt constituted reasonable bounds on how much uplift could develop if relief wells were installed immediately downstream of grout curtains.