>- 



100 

 90 

 80 

 70 

 60 

 50 

 40 

 30 

 20 

 10 



3 in 



3/4 in 



No. 4 



U.S. STANDARD SIEVE SIZE 

 IMo. 10 No. 40 No. 200 



1 1 CT ' 1 — 



1 1 



- ' vk 1 



' l\ 1 

 1 1 ' \l 





1 ' 1 •> 



- 1 1 j j 



- 1 1 II 



II 

 II 



1 1 

 VI 1 



n 1 1 PINE CREEK STUDY 



II 



\ 1 1 Watershed No.1 



11 

 II 



\> 1 Composite Soil Sample 



II 

 II 



Depth = 24-36 m 



II 



1 1 



1 1 11 

 1 1 1 1 1 



1 1 1 



100 



10 



1. 0. 1 



GRAIN SIZE IN MILLIMETERS 



0. 01 



0. 001 



GRAVEL 



SAND 



SILT OR CLAY 



Coarse 



Fine 



Coarse 



Medium 



Fine 



UNIFIED SOIL CLASSIFICATION 

 Figure 24. — Grain-size distribution of composite soil sample from Watershed No. 1. 



place, the borehole shear test often yields low or nonexistent 

 values of cohesion (Wineland 1 975). Secondly, full or complete 

 saturation is required to eliminate apparent cohesion as a result 

 of capillary action. Lastly, some cohesion must be present to 

 explain the stability of what otherwise would often be a failed 

 slope, particularly in the case of cut slopes. 



Analysis of actual failures in both natural and cut slopes in the 

 Idaho batholith by Gonsior and Gardner (1971) suggests that 

 cohesion up to 0.9 Ib/in^ (6.2 kPa) may be mobilized. A similar 

 value was reported by Prellwitz (1 975) in his analysis of granitic 

 slopes in the batholith. Prellwitz (1975) suggests that a cohe- 

 sion of 0.76 Ib/in^ (5.2 kPa) is reasonable for soils above the 

 phreatic surface and 0.35 Ib/in^ (2.4 kPa) below for SW-SfVI 

 materials. 



Much higher cohesions have been reported in laboratory 

 triaxial tests on some granitic soils of the batholith (Hampton 

 and others 1974).^ Sample sites in this study were purposely 

 selected to provide a wide range in the weathering properties of 

 granitic rocks with the sampling heavier in the more weathered 

 rocks. Higher cohesion would be expected under these condi- 

 tions because more advanced chemical weathering of the bed- 

 rock has occurred, causing formation and accumulation of clay 

 colloids. Hydrolysis of mica and feldspars leads to formation of 

 clay minerals such as illite and kaolinite. The absence of strong 

 solution and eluviation leads to their accumulation. These fac- 

 tors may combine to produce a finer grained, more cohesive, 

 and correspondingly less frictional soil as shown by the results 

 of triaxial compression tests in figure 25. 



Highly weathered bedrock conditions are relatively rare in the 

 Idaho batholith because they are generally associated with 

 shear zones and zones of secondary hydrothermal alteration. 



Accordingly, high cohesion values such as those shown in 

 figure 25 probably occur on less than 5 percent of the upland 

 slopes of the batholith. Such soil conditions were not present in 

 the Pine Creek study watersheds as evidenced by the shear 

 strength test results reported in table 5. 



10 



10 



12 



COHESION (CI, (lb' in^ 



Figure 25. — Relationships between angle of internal friction and 

 cohesion tor various batholith soils (from Hampton and others 

 1974, footnote 1). 



16 



