Reservoir Leak Assessment & Repair Northstar Community Services District, Placer County, Ca

In 1972 and 1973, reservoir "A" was constructed with an approximately 520 foot long earth and rock embankment across the east branch of the West Martis Creek, about six miles southeast of Truckee, California. The capacity of the reservoir is 180 acre-feet and the water is used predominately for snow making at the adjacent ski resort in the winter and for emergency usage in the summer.
In early 1994, workers for the Northstar Community Services District (NCSD) observed for the fifth time in twenty-one years, leakage in the right abutment near the groin of the earth embankments and the natural ground in Reservoir "A". NCSD performed several previous repairs, with the latest repair being conducted in 1987. The repairs included filling the exposed holes with a water-reactive liquid grout, and excavating and backfilling with clay material. Some of these repairs appeared to work for several years; however, the leaks either reopened or tended to migrate to other locations in the immediate area.
An assessment of the site indicated the leakage was through voids in the native material. This material is a chaotic assemblage of angular to sib-round rocks, sand, silt, and clay from an ancient volcanic mudflow or "lahar" and silty sand beds of fluvial origin. This material was deposited on a steep slope. It appeared the voids observed in these deposits probably opened as a result of the slow downhill migration or "creep" of the surficial sediments. The depth of the creep affect material is at least twenty feet.
The affected area was repaired by excavating to several feet below the observable voids and backfilling with a two-foot wide layer of select import clay material against the native material. The remainder of the excavation was then backfilled with local clayey materials. 12 pp.