United States 

 Department of 

 Agriculture 



Forest Service 



Intermountain 

 Forest and Range 

 Experiment Station 



Research Paper 

 INT-271 



May 1981 



Forest Vegetation 

 Removal 



and Slope Stability in 

 the Idaho Batholith 



Donald H. Gray 

 Walter F. Megahan 



INTRODUCTION 



Slope Stability Problems 

 in the Batholith 



DESCRIPTION OF THE BATHOLITH 



The Idaho batholith is a 16.000 mile' (41 000 km') moun- 

 tainous area of granitic rocks located in central Idaho (see 

 location map in fig. 1). Shallow, coarse-textured soils (loamy 

 sands to sandy loams) typically develop on slopes that average 

 60 percent or more in many drainages. Soils of this type have 

 been shown to be extremely erodible (Anderson 1954: Andre 

 and Anderson 1961 ). 



The granitic bedrock exhibits various degrees of weathering 

 and fracturing. An idealized subsurface profile in a slope in the 

 natural, undisturbed state, is shown in figure 2. The soil profile 

 can be divided roughly into three major zones according to 

 degree of weathering, indicated by surface soil (decomposed 

 granitics and organic matter), fractured, disintegrated rock, and 

 relatively competent, partly weathered bedrock. These zones 

 vary in thickness, and changes in composition or texture are 

 transitional rather than abrupt. A more precise delineation can 

 be made by using a classification for granitic rocks developed 

 by Clayton and Arnold (1972). Their classification criteria in- 

 clude apparent degree of mineral alteration and rock competen- 

 cy with respect to angulanty of joint sets and response to a 

 hammer blow. 



The surface soil is almost without stones and is derived from 

 granitic rock in a fairly advanced state of physical, if not chemi- 

 cal, weathering. This horizon usually consists of a sandy, 

 coarse-textured soil with little or no intrinsic cohesion. In general 

 it would be classified as an A-1-b soil under the American 

 Association of State Highway Transportation Officials 

 (AASHTO) Classification and as a SW-SM soil under the Uni- 

 fied Classification (Gonsior and Gardner 1971). Such relict 

 features or vestiges of the original rock structure, as joint planes 

 may persist in the soil. It is this part of the profile that is generally 

 the most prone or vulnerable to both surficial and mass erosion 

 (Durgin 1977). 



Cyclonic storms and/or deep snowpacks that release large 

 volumes of water to the soil within short periods are annual 

 occurrences in the batholith region. Water supplied by these 



climatic events rapidly infiltrates the soil surface and continues 

 downward until it reaches the zones of reduced hydraulic con- 

 ductivity at the interface with the fractured, disintegrated bed- 

 rock. Continued inflow of water creates a saturated layer at this 

 level, which in turn causes both buildup of a piezometric head 

 and subsurface flow along the weathered bedrock surface 

 (Megahan 1972). The subsurface flow zone is shown on figure 

 2. Depth of the soil piezometric surface is one of the most 

 important factors influencing stability of steep mountain slopes 

 mantled with shallow, noncohesive soils. 



Moscow 



IDAHO 



^BATHOLITH 



• Missoula 



Lewiston 



Figure 1 . — Location map showing the Idaho batholith and study area. 



1 



