Slope stability problems associated with timber harvesting in mountainous regions of the western United States

Natural soil-mass-movements on forested slopes in the WesternUnited States can be divided into two major groups of closely relatedlandslide types. These include, in order of decreasing importance andregional frequency of occurrence: (1) debris slides, debris avalanches,debris flows, and debris torrents; and (2) creep, slumps, and earth flows.Each type requires the presence of steep slopes, frequently in excessof the angle of soil stability. All characteristically occur under highsoil moisture conditions and usually develop or are accelerated duringperiods of abnormally high rainfall. Further, all are encouraged oraccelerated by destruction of the natural mechanical support on theslopes.
As forest operations shift to steeper slopes, they play an increas-ing role in initiation and acceleration of soil mass movements. Thelogging operation itself is a major contributor through (1) destructionof roots, the natural mechanical support of slope soils, (2) disruptionof surface vegetation cover which alters soil water distribution, and(3) obstruction of main drainage channels by logging debris. Roadbuilding stands out at the present time as the most damaging operationwith soil failures resulting largely from slope loading (from road filland sidecasting), oversteepened bank cuts, and inadequate provisionfor slope and road drainage.
At the present time attempts at prevention and control are limitedto identification and avoidance of highly unstable areas and developmentand implementation of timber harvesting techniques least damaging tonatural slope stability.
KEYWORDS: Soil stability, soil erosion.