CONCLUSIONS 



The following main conclusions can be drawn from the results 

 of the Pine Creel< study in particular and about the role of forest 

 vegetation on stability of slopes in the Idaho batholith in general: 



1. Soils that develop on granitic rocks of the batholith are 

 typically shallow, coarse-textured soils (loamy sands to sandy 

 loams) that are found on steep slopes that average 60 percent 

 or more in many drainages. 



2. Batholith soils tend to be highly erodible and prone to 

 mass soil movement particularly when disturbed by road con- 

 struction and timber harvesting. 



3. Forest vegetation on the batholith helps to maintain more 

 secure slopes by a series of stabilizing mechanisms. These 

 include mechanical reinforcement by root systems; soil mois- 

 ture depletion by interception, transpiration, and regulation of 

 snow accumulation and melting; and by buttressing and soil 

 arching action behind embedded tree trunks. 



4. Removal of forest vegetation without regard to slope 

 stability can result in loss of the stabilizing influences of forest 

 vegetation. Results of the Pine Creek study show that vegeta- 

 tion often provides the margin of safety between a secure and 

 failed slope. 



5. The factor of safety against sliding in slopes of the batho- 

 lith is very sensitive to cohesion. Almost all, or at least a signifi- 

 cant fraction of this cohesion, can be provided by root reinforce- 

 ment in batholith soils. 



6. Several measures are recommended to mitigate the im- 

 pact of vegetation removal on slope stability. These measures 

 include selection logging in preference to clearcutting, limitation 

 of size of clearcut units, establishment of vegetation leave areas 

 in critical areas, and careful integration of diversions and drain- 

 age measures with vegetation leave areas. 



7. Live barriers of trees should be left when reasonably 

 feasible below the toe of fill slopes and above cut slopes. Buffer 

 zones of vegetation should also be left along the margin of 

 streams. 



8. Critical areas or slopes of high landslide potential can be 

 identified by calculating a probability of failure that takes into 

 account the uncertainty and variability in the input variables in a 

 geotechnical model on which the assessment is based. 



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