Table 6. — Summary of stability calculations at different stations 

 for granitic soil In the Pine Creek study watershed, 

 Idaho 



Station number 



Stability calculations 



1 



2 



3 



4 



5 



6 



Factor of safety'' (F) 



1.2 



1.04 



1.0 



1.1 



1.04 



0.9 



Critical piezometric level^ 



6.5 



2.1 







3.2 



1.7 



N/A 



(Her), incfi 















Required cofiesion for 















stability, lb/inch^ 















H„/H = (dry) 























0.12 



= 0.5 



0.14 



0.24 



0.23 



0.20 



0.20 



0.53 



= 1 .0 (saturated) 



0.40 



0.52 



0.46 



0.45 



0.46 



0.93 



'Based on infinite slope model (equation 2.) 



^Piezometric height (above failure surface) at which F = 1 .0 for slope and soil 

 parameters given in table 5. 



(H), local slope angle (P), soil density (y or yo), friction angle (4)), 

 and soil cohesion (Cs) . The calculated factors of safety are close 

 to unity and suggest the slope is only marginally secure. The 

 calculations are based on zero cohesion (Cs = 0) and dry 

 slopes (Hw = 0). Critical piezometric levels (still assuming zero 

 cohesion) are also shown. The slope should theoretically fail at 

 the critical piezometric level (F = 1.0). These critical 

 piezometric levels are on the order of a few inches. The fact that 

 the entire slope did not fail when pieometric levels in excess of 

 these critical values developed in the slope (figs. 22 and 23) 

 means that some cohesion must be present. Required cohe- 

 sion to prevent failure at various piezometric levels is also 

 calculated and tabulated. Values range from 0.4 to 0.9 Ib/in^ (2.8 

 to 6.2 kPa) at full saturation (Hw = H). These residual cohe- 

 sions are consistent with values reported by Gonsior and Gard- 

 ner (1 971 ) and Prellwitz (1 975). These observations are based, 

 of course, on the assumption that the infinite slope model ade- 

 quately represents stability conditions in granitic slopes of the 

 Idaho batholith. Limitations of the infinite slope theory in this 

 regard are discussed by Hartsog and Martin (1974), but do not 

 appear to apply in this case. 



The influence of both friction and cohesion on the factor of 

 safety of a typical granitic slope with a shallow soil mantle is 

 shown in figures 28 and 29. A slope thickness of 36 inches (92 

 cm), slope gradient of 35°, and soil densities of 88 and 1 1 7 Ib/f1^ 

 (1.4 and 1.9 g/cm^), dry and saturated, respectively, were 

 selected for the analysis. Factor of safety is plotted against 

 cohesion for various values of friction and piezometric elevation 

 in the slope. As shown in the figures, stability is far more 

 sensitive to soil cohesion than to friction angle, particularly 

 when slope becomes fully saturated (Hw = H). It is also clear 

 from this analysis that some cohesion must exist in steep slopes 

 in order to provide the critical margin for stability when 

 piezometric levels rise in the slope. Little cohesion is required to 

 maintain a stable slope. Only 0.66 lb/in' (4.6 kPa) is needed at a 

 friction angle of 1 9° and a slope angle of 35° (tanc^/tanp = 0.50) 

 when Hw/H = 0.5 (fig. 28). For these same conditions of slope 

 and friction angle, at full saturation, (Hw/H = 1 .0), the required 

 cohesion is 0.88 lb/in' (6.1 kPa) (fig. 29). 



The influence of a rise in piezometric surface on the factor of 

 safety (all other factors held constant) can also be determined. 



/9 =35° 

 H =36" 



t.O 2.0 



COHESION (Cs),lb/in2 

 Figure 28. — Influence of cohesion on the stability of a sandy residual 

 soil resting on an inclined bedrock contact. Hw = 0.5. 



H 



1.0 2.0 



COHESION (Cs),lb/in2 

 Figure 29, — Influence of cohesion on the stability of a sandy residual 

 soil resting on an inclined bedrock contact. Hw = 1 ■ 



H 



18 



