Abstract:
In 1880, Water-power of the United States (published by U. S. Census Office) described Langley Dam, which was completed in the 1870’ s, as an earth and crib-work, 1, 000 feet long and 24 feet high structure impounding over 700 acres of water to a depth of 10 feet. In 1886, Langley Pond Dam failed as a result of the Charleston earthquake but was subsequently reconstructed to support local mills. Since the 1886 failure, the structure has experienced numerous extreme loading events and partial failures with the most recent occurring in 2014 when the County Engineer noted a well-defined boil at the downstream toe of the dam. Given high permeability of the soils, a cement bentonite cutoff was required to reduce seepage beneath and through the dam. The cutoff had a range in depth of 33 feet to 65 feet below the crest of the dam, and a width of 2.5 feet. Permeability tests on specimens prepared from grab samples of slurry were less than 1 x 10-7 ft/s. Unconfined compressive strength was specified as 50 psi at 3 days and 250 psi at 28 days. During construction, a 16 feet thick soft compressible alluvium layer was discovered beneath the temporary cofferdam. The layer needed stabilization in order to safeguard stability of the upstream slope of the dam. Cement bentonite barrettes were selected as the ground treatment method, given that the same equipment deployed for the cutoff could be used, thereby achieving economy during construction. Following geotechnical analyses for slope stability, lateral squeeze, bearing, and shear resistance of the barrettes, the slurry mix was designed to increase the minimum unconfined compressive strength to 300 psi. The barrettes had the following dimensions: 45 feet in length, 3 feet in width, 25 feet in depth and they were spaced at 20 feet on center.