Laserfiche WebLink
<br />6 <br /> <br />LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COLORADO <br /> <br /> <br />FIGURE 3.-Lawn Lake dam and Reservoir, prior to failure (July 11. 1958) (aerial). Photo courtesy of the National Park Service. <br /> <br />that the dam's interior probably was not extensively <br />burrowed enough to cause failure of the dam. <br />Earth fill in the dam was sufficiently fine grained to <br />be frost susceptible. Soil-property changes induced by <br />. numerous freeze-thaw cycles could have been a possi- <br />ble cause of failure. However, no ice lenses nor frozen <br />ground were found during investigations at the dam- <br />site in July 1982; any changed soil characteristics would <br />be reflected in the geotechnical investigations of the fill <br />material. <br />Results of an undrained embankment-stability <br />analysis by the Office of the State Engineer (1983) in- <br />dicated a safety factor of only 0.6 existed in the dam <br />for the assumed phreatic conditions. Assumption of an <br />undrained condition, based on the results of permeabil- <br />ity tests, is unrealistic. Some type of steady-state <br />drainage must have existed in the dam which kept pore <br />pressures below critical values and prevented the dam <br />from failing. . <br />During investigations at the damsite, numerous <br />pieces of lead caulking that was used to form the seal <br /> <br />between the outlet pipe and the gate valve (fig. 6) were <br />found. This lead cau1king was corroded and deteriorated <br />to the point that it would be unlikely to form a water- <br />tight seal. The use of lead caulking to secure the pipe <br />and valve was not in compliance with original plans and <br />specifications approved by the Colorado State Engineer <br />in 1902, which indicated that the valve would be encased <br />in concrete. <br /> <br />CAUSE OF FAILURE <br /> <br />Data gathered by the Office of the State Engineer <br />(1983) indicated that the most likely cause of the Lawn <br />Lake dam failure was deterioration of the lead caulk- <br />ing used for the connection between the outlet pipe and <br />the gate valve. The resulting leak eroded a pipe-shaped <br />cavity in the earthfill (called piping), creating a void <br />along the outlet pipe. The leak continued to remove easi- <br />ly eroded earthfill and may have reduced the strength <br />and stability of the dam embankment. Such a scenario <br />of leakage, piping, and resulting embankment failure <br />