2007-08-10_PERMIT FILE - C1980007 (3)

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4 Project Components The modifications proposed at Monument Dam include components that will improve stability of the dam and the marginally stable landslide located on the left (south) abutment. The following describes in detail the components of the proposed modifications to the dam and landslide. The details are presented graphically in the project drawings included in "Construction Drawings for Monument Dam Preventive Measures" (Barr, 2007). 4.1 Dam The proposed modifications to the dam include: • A buttress on the downstream face of the dam. Buttresses typically improve slope stability because they: (1) increase the resisting forces by adding weight to the toe, (2) increase the strength of underlying soils by increasing effective stresses, and (3) lengthen the potential failure surface, which increases the forces resisting slope movement. The buttress will be constructed of compacted colluvium from the borrow area south of the access road. • A drainage blanket that will be placed underneath the buttress material to prevent high porewater pressure buildup in the buttress material. The drainage blanket will be constructed of clean, granular material. The drainage blanket is also intended to control seepage observed in the past on the downstream slope of the dam. • An upstream clay blanket across the entire dam with the purpose of inducing head loss as water flows from the reservoir into the dam embankment. The loss of head across the upstream blanket will reduce porewater pressures throughout the embankment, thus improving overall dam stability. • Trench drains will be installed in the bench that makes up the downstream portion of the left abutment. The trenches, spaced every 30 feet, will be excavated to 25 feet deep (the maximum reach of a backhoe) and backfilled with freely draining, granular material. The high-permeability material will draw down groundwater levels within the dam and the left abutment. This will improve stability of the dam. • A modified intake structure that will include a hydraulically controlled slide gate and an intake trash rack. Controlling the valve on the upstream side of the dam instead of its current position on the downstream face will reduce the likelihood of leaks in the outlet pipe causing high porewater pressures throughout the embankment. ' A drop structure on the downstream side of the dam to dissipate energy of water flowing through the outlet pipe. P:\Mpls\06 CO\26\0626067\WorlcFiles\DesignReport\FINAL\DesignReportFINAL.doc 5