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<br />HISTORY OF THE DAMS <br /> <br />Two dams in Rocky Mountain National Park failed on of July 15, 1982--the <br />Lawn Lake dam and the Cascade Lake dam. A discussion of their histories <br />f 011 ows , <br /> <br />Lawn Lake Dam <br /> <br />Lawn Lake dam was an earthen dam constructed in 1903 by the Farmers <br />Irrigation Ditch and Reservoir Company to impound additional water in an <br />existing mountain lake for irrigation storage (fig. 3). Originally Lawn Lake <br />was a natural moraine-dammed lake with a surface area of 16,4 acres, based on <br />the ori gi na 1 map and impoundment plans fi 1 ed wi th the Colorado State Engi- <br />neer's Office (Office of the State Engineer, 1983). The irrigation company's <br />plans called for an earthen dam to be 24 ft high to the spillway, which would <br />create a reservoir of 47,1 acres surface area and 8ll acre-ft of storage, <br />However, based on several capacity surveys and records of the private irriga- <br />t i on company, the hei ght of the Lawn Lake dam was 20.35 ft to the spi 11 way <br />with a reservoi r capac i ty of 611. 96 acre- ft, in September of 1907. I n Septem- <br />ber 1931, probably in response to below-average precipitation and dust-bowl <br />conditions on the Colorado High Plains, the dam was enlarged to a height of <br />24 ft at the spi llway and a reservoi r capacity of 817,2 acre-ft. Thi s en- <br />1 argement was never approved by the Colorado State Engi neer (Offi ce of the <br />State Engineer, 1983), Postfailure surveys (discussed later) indicated that <br />the dam was 26 ft high (water was 24 ft deep), had a capacity of 674 acre-ft, <br />and was about 560 ft long at the time of failure (figs, 4A and 48), <br /> <br />Lawn Lake dam was constructed of heterogeneous local earthfill from the <br />surrounding ground moraine. These glacial. deposits have a maximum thickness <br />of about 25 ft in the immediate vicinity of the dam. The underlying <br />Precambrian igneous and metamorphic bedrock is highly fractured and jointed, <br />but fresh. About 4 to 5 ft below the crest of the dam, a 12-in. layer of <br />dark, organic material appeared to cover the entire dam (fig, 5), The origin <br />of this organic horizon is uncertain, but it probably represents a top layer <br />of humus on the 1903 embankment prior to the height enlargement of 4 to <br />5 ft in 1931. <br /> <br />Earthfill material for the dam is composed entirely of silty and poorly <br />graded sands with varying amounts of fine gravels and considerable amounts of <br />organic material. Geotechnical testing of 20 samples taken from the damfill <br />materi a 1 fo 11 owi ng the fail ure was conducted by the Offi ce of the State <br />Engineer (1983), The tested samples contained 7 to 26 percent nonplastic to <br />low-plasticity fines; they were classified SP-SM and SM in the Unified Soil <br />Classification (U,S. Army Corps of Engineers, 1953). Permeability testing on <br />three samples gave results of 0, 0.7, and 6,6 ft/d. Results of triaxial shear <br />tests on remolded samples are given in table 1. The remolded samples are <br />probably not representative of in situ strength properties, <br /> <br />8 <br />