FERIC conducted a synoptic survey of 102 landslides on the Queen Charlotte Islands, 97 of which originated within logged areas, to supplement the Fish/Forestry Interaction Program's air photo landslide survey and to provide forest engineering input into the interpretation of probable causes and possible preventative measures. Ninety landslides were selected from Rood's (1984) population and 12 more recent failures were added to provide information on fresh landslides. Sample landslides were not chosen randomly so statistical inferences were not made from the survey data.
Thirty-one landslides initiated from logging roads, almost all in road fill slopes. The principal factors in road-related landslides appeared to be overloading of steep slopes with fill or sidecast material and inadequate control of road drainage, usually in combination. Road engineering and construction practices contributed to stability problems to some extent, but insufficient maintenance of road drainage systems, particularly on inactive logging roads, was considered to be the most significant factor in road- related failures. In the opinion of the authors, the frequency of road-related failures could be substantially reduced by improving the level of road maintenance, including putting roads to bed.
Specific causes for landslides that initiated within clearcuts but away from roads could rarely be identified. In particular, the role of yarding disturbance (gouging soils and uprooting stumps) as a failure mechanism could not be reliably distinguished from indirect logging factors (tree removal and root deterioration) or natural factors (storm and seismic activity). However, the yarding process probably accelerated landslide activity at critical points where poor deflection generated severe yarding disturbance on sensitive slopes. Reducing the frequency of this type of landslide would require maximizing available deflection through careful field layout and the use of mobile yarding systems, and minimizing the occurrence of difficult yarding situations such as that through or over major gullies.
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Updated December 16, 2008