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<br />, <br /> <br />, . <br /> <br />.[awn T,akC', OC-3'1 <br />July 15, 19!J2 <br /> <br />l'a'.l" '1 <br /> <br />Summary of Findinqs <br /> <br />1. Lawn Lake Dam did I!Ql fail by overtop!-Jlnlj. At the lLme of failure, the water <br />surface was about three feet uelow the Cr<.::;l of the dum. <br /> <br />2. There was evidence of seepage on the downstream slope of the dam as high as 5 <br />feet 7 inches below crest elevation. <br /> <br />3. Soils exposed by the breach 5 feet below the crest of the dam were found to be <br />totally saturated near the centerline of the embankment. <br /> <br />4. At the right side of the breach, an organic clay/silt layer runs through the em- <br />bankment 5 feet below the crest. A clean sand lense runs through the dam <br />approximately 10 feet below the crest, and a peat layer with tree roots was <br />observed approximately 12 feet below crest. The foundation of the dam consists <br />of angular glacial till. <br /> <br />S. Two large rodent holes were noted in the crest of the dam at approximately station <br />1+75. <br /> <br />6. The breach was centered 16 feet to the nght of the outlet section. <br /> <br />Conclusions -- <br /> <br />A. The dam pcobably failed by piping failure as the result of: <br /> <br />1. Pervious seams withln the dam, or <br />2. Progressive sloughing of the downstream slope, or <br />3. Rodent actl vity, or <br />4. Failure of the outlet conduit upstream of the control valve, or,. /." ~, '"' <br />S. A combination of the above. /.,'. ~ <br /> <br />B. Failure may also have occurred as a result of embankment slide , followed oy <br />collapse at the fa iled section. (Z:-6', A'om/lOVA/ ..:///oC c.e .....EO<i'C "c",4'/(//.,'E,) <br /> <br />Further <br /> <br />study is required to more concisely determine the cause of failure. <br /> <br />Recommendations -- <br /> <br />1. Staqe 2 data qatherinq: <br /> <br />a. Perform a geologic/geotechnica I investigation of both sides of the breach <br />and of the foundation. This 'MJuld include mapping of the various soil <br />la yers, obta ining soil samples for later study, and provid ing 5 ite photo- <br />graphs of soil types. Some excavation may be necessary with hand tools. <br />Thi,; function should be performed cy members of the geotechnical branch <br />of this office. <br /> <br />b. Survey the embankment <.:ross-seClLon both to the right and left of the breach. <br />