Failure of slopes on sides of reservoirs frequently during drawndown when the reservoir is nearly empty doesn't produce unmanageable problems. However, of more concern is when rock and earth slides into a full reservoir causing sudden destructive overflow of the dam.
Filling of a reservoir causes adjustments in the groundwater table in adjacent materials. Over a period of time when the reservoir is full a new groundwater table is established with coincides with the elevation of the water surface. The groundwater surface is a free surface in contact through unfilled pore spaces with the atmosphere, changes in atmospheric pressure are accompanied by changes in pore pressure in the saturated zone. Wave action also undercuts the slopes and oversteepens them.
(C) Wahlstrom, Ernest Dams, Dam Foundations and Reservoir Sites
Figure 1 - Glacial till deposits in a glaciated canyon in crystalline rocks
A reservoir is located in an extensively glaciated valley in crystalline rocks. Lateral morraines, consisting of a jumbled mixture of large and small boulders, gravel, and rock flour have been deposited by the glacier that occupied the valley high on its sides. Filling of the reservoir causes an elevation of the water table in the rill in the moraines, and because of the cross-sectional configuration of the valley, considerably increases the possibility for sudden downslope movement of the moraine material. Failure of the slopes might occur at any time.
(C) Wahlstrom, Ernest Dams, Dam Foundations and Reservoir Sites
Figure 2 - Tilted alteration of sandstone and shale
The bedrock profile of a canyon eroded by a stream in a tilted succession of sandstones and clay-rich shales. Unconsolidated materials in rock slides and stream deposits are not shown. Water from the reservoir, by seepage through the sandstones, comes into contact with the shale layers for a considerable distance into the canyon walls, and by slow penetration of the shales, greatly reduces their strength. Under these circumstances, a highly unstable condition is created, especially where the sedimentary layers dip into the reservoir.