Rebuilding an Illusion

Barr, Douglas W.; Bogart, Jonathan K.; ,

Rebuilding an Illusion

by Douglas W. Barr, (F.ASCE), Founder and Senior Vice President; Barr Engineering Co., Minneapolis, MN,
Jonathan K. Bogart, (M.ASCE), Engrg. Mgr.; Enviroscience Engineers and Land Surveyors, Inc., Minneapolis, MN,

Serial Information: Civil Engineering—ASCE, 1991, Vol. 61, Issue 1, Pg. 54-57

Document Type: Feature article

Abstract:

A dam that controls the level of Fall Lake, a major entry to the Boundary Waters Canoe Area Wilderness of northern Minnesota had to be rebuilt, but federal agents barred constructors from working on the land or leaving behind a visible structure. The original, a timber crib dam built by loggers in 1901, had deteriorated. The designers specified a rockfill dam with a 1 ft wide concrete core wall. To prevent washouts, boulders on the downstream face were to be tethered to the wall. The contractor elected to work all winter, bringing in men, machines and material over the frozen lake. The contractor completed half the new dam at a time, building a cofferdam for each side and incorporating the old dam to minimize new materials. The concrete mix was brought to the site dry from 70 miles away, water added for a 2-3.5 inch slump, poured, housed in a plastic tent and warmed to 80F for curing. Toe rocks, weighing up to 3,500 lbs, were drilled and heated before a � inch stainless steel cable was attached with grout. These were placed to match the natural-appearing surface of the rapids spillway. At the crest, 2-5 ft boulders were placed in apparently random fashion to further the illusion that no dam exists.

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