Figure 1 0. — Slipout in slope above road cut, Watershed No. 1 , Pine 

 Creek study area, Boise National Forest, Idaho. (Photo taken July 

 1977.) 



Figure 11. — Large gully in slope. Watershed No. 1 , Pine Creek study 

 area, Boise National Forest, Idaho. Gully was formed by subsur- 

 face flow emerging at cut face followed by piping and "spring 

 sapping," which triggered localized slope failure. (Photo taken July 

 1977.) 



STUDY DESIGN AND METHODS 

 Overall Experimental Design 



As noted previously, the original purpose of the Pine Creek 

 study was to evaluate subsurface flow occurrence on repre- 

 sentative granitic slopes and the effect of clearcutting and road 

 construction on subsurface flow. Hydrologic data collection on 

 the study watersheds included water inflow, change in water 

 storage, and outflow of water occurring as subsurface flow. 



Subsurface flow was measured with collection troughs in- 

 stalled on the road at the base of the cut slope across the entire 

 width of the watersheds. Estimates of water inflow into the 

 watersheds and changes in water storage were obtained from a 

 recording rain gage and snow lysimeter. In addition, 45 snow 

 stakes and 23 neutron access tubes for soil moisture deter- 

 mination were located in a grid pattern on the study watersheds. 

 Finally, 25 piezometers were located in suspected water- 

 accumulation areas within the watersheds. All data collection 

 sites or stations are shown in figure 8. 



Some additional data were collected following the wildfire on 

 the study area to evaluate the effects of burning on surface 

 erosion rates. Data compilation included rill surveys, a network 

 of erosion pins, and collection of eroded material in splash pans 

 and in the subsurface flow collection troughs. 



Piezometric levels or ground water conditions are also critical 

 determinants of the mass stability of slopes. Accordingly, a 

 conjunctive slope stability investigation was undertaken in the 

 experimental watersheds in the summer of 1977. Estimates of 

 soil shear strength (required for stability analyses) were 

 obtained from in-situ borehole shear tests (see fig. 8 for loca- 

 tions) and from results of laboratory triaxial tests on granitic soils 

 reported by other investigators (Lumb 1962; Gonsior and 

 Gardner 1971; Hampton and others 1974^; Prellwitz 1975; 

 USDA Forest Service 1977). The slope stability investigation 

 also included an analysis of the contribution of forest vegetation 

 to slope stability from root reinforcement, surcharging, and 

 buttressing or soil arching action. 



Piezometer Installation and 

 Data Collection 



A total of 25 piezometers were installed in the study water- 

 sheds: 14 in Watershed No. 1 and 1 1 in Watershed No. 4. The 

 precise location of these piezometers is shown in figures 1 2 and 

 13. It should be noted that piezometer Nos. 10-14 and 9-11 in 

 Watersheds 1 and 4, respectively, were lost in slope failures 

 that occurred in 1 974 and 1 975 (see figs. 1 2 and 1 3 for location). 



The piezometers were installed vertically by hand augering a 

 2-inch (5-cm) hole through the loamy sand soil to the surface of 

 the underlying fractured, disintegrated rock. A typical piezo- 

 meter installation is illustrated schematically in figure 14. The 

 piezometers were read frequently dunng the active snowmelt 

 period in late spring and at monthly intervals throughout the 

 following summer and fall. Both the present and the maximum 

 water depths were measured at each piezometer. The max- 

 imum reading between measurements was determined by not- 

 ing the position of powdered cork on the aluminum rod. The 

 powdered cork floats up as piezometric levels rise and adheres 

 to the aluminum rod at the point of highest rise. This technique 

 was employed by Swanston (1967) in his study of soil water 

 piezometry in southeast Alaska. 



'Hampton, D , W, F, Megahan.and J L. Clayton. 1974. Soil and rock properties 

 research in the Idaho batholith. USDA For. Sci. Lab. Rep., Boise, Idaho 121 p. 



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