Underwater Intake Retaining Wall Stabilization

In 2000, plant operators at the Hodenpyl Hydroelectric Plant reported an apparent sinkhole developing on the upstream slope of the dam. An investigation determined that there was no sinkhole, but a gully was forming on the upstream sand slope of the dam due to a failing timber cribwall that served as the upstream portion of the intake structure retaining wall. The cribwall was installed and filled with sand during construction of the dam which was completed in 1925.

The repairs in 2001 included the installation of an independent cantilevered steel sheet piling wall, filling the area with new sand to bury the old timber cribwall and to match the 4H:1V upstream slope of the dam. In steeper areas of the embankment near the wall, the slope was covered with cabled articulated concrete block mats over filter fabric. All work was completed underwater in water depths of up to 62 feet. A vibratory hammer was used for the underwater driving of piles to minimize the vibrations at the site, as the foundation is very stiff clay and there were concerns about the effect of vibration on the sand embankment dam and the power plant. An inspection of the other intake retaining wall found that wall was in the same condition and it was repaired by the same methods the following year.

The Hodenpyl Hydroelectric project is on the Manistee River, approximately 23 miles northwest of Cadillac, Michigan. The project structures consist of an earth embankment intersected by a concrete and masonry powerhouse, and emergency spillway, and a tube spillway section beneath the powerhouse. The project powerhouse is a concrete structure with a masonry superstructure. It contains two 9,000 kilowatt (kW) Francis vertical-shaft turbine-generators and six spill tubes that function as the service spillway. The project's dam consists of a 4,165 foot-long earthen embankment intersected by the project powerhouse. The left section of the embankment is 105 feet long and has a reinforced concrete core wall. The right section of the embankment is 3,575 feet long and has a steel sheet pile core wall capped with concrete. The 72 feet high embankment was constructed of poorly graded sand. The upstream slope is 4H:1V and the downstream slope is approximately 2.5H:1V. 12 pp., 17 figures.