Cracking dams intro

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Fontana Problem

  • Patterned cracking was first observed in 1949 as the upstream movement began.
  • Next cracking was observed in the walls of the foundation at the drainage gallery in the curved portion of the dam in 1972. Two hypotheses were developed: (1)the foundation was deficient under the curved portion of the dam or (2) the main portion of the dam was expanding due to heat and/or other causes and this was pushing on curved portion so that it began to bend.
  • The crack growth was monitored; in 1973 the crack was opening. Authorities suspected solar heat as the culprit.
  • Two-dimensional analyses were performed to determine the causes of cracking. The analyses confirmed that the foundation was not the problem; the rest of the dam was expanding and causing cracking in the curved portion.
  • A petrographic exam revealed an alkali-aggregate reaction -- a chemical reaction was occurring between the cement and the aggregate components of the concrete.
  • The chemical reaction and the heat had caused the cracking near the curved portion of the dam. Cracking in the main part of the dam was resisted by the foundation.
  • Temporary solutions included spraying cold water on the downstream face to lower the temperature; crack opening diminished as a result.
    Also, the area around the crack was post-tensioned, which means steel wire was stretched across the area and attached on either side. Then the steel wire contracts and pulls the crack closed.
  • More permanent solutions:
    grouting the crack (filling it with a cement-like material)
    cutting expansion slots across the upper portion of the dam.
  • More computer analyses were used to determine how many expansion slots and their positioning; results suggested cutting one slot.
  • A slot was cut in 1976. It was almost closed by 1983.
  • The slot was recut in 1983.

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