4 r- 



X Watershed no. 1 (piezometer no. 5) 

 • Watershed no. 4 ipiezometer no. 31 



10 20 



APRIL 1st WATER EQUIVALENT FOR SILVER CREEK RIDGE (IN) 



Figure 23. — Annual peak piezometric response for peak sample 

 points on Watersheds No. 1 and No. 4 versus annual April 1 snow 

 water content in inches. 



passes through the average for the two pretreatment years 

 ((dashed line, fig. 23), we obtain estimates of changes in soil 

 nnoisture levels of 140, 134, an<j 140 percent for 1973, 1974, 

 and 1975, respectively, for Watershed No. 1. 



Estimated increases in maximum piezometric levels range 

 from 65 to 1 40 percent following clearcut timber harvest and/or 

 wildfire. We want to emphasize the fact that these increases are 

 not statistically significant. The increases, however, are rela- 

 tively consistent for both watersheds for all postdisturbance 

 years over a wide range of climatic conditions. Moreover, the 

 documented onsite hydrologic responses of increased soil 

 moisture carryover and increased snow accumulation and melt 

 rates following vegetation removal would tend to cause in- 

 creased piezometric levels. Everything considered, we feel that 

 peak piezometric levels were increased about 100 percent as 

 the result of clearcutting and relatively intense wildfire on the 

 study area. 



Stability Relationships 



SOIL SHEAR STRENGTH 



The results of borehole shear tests and field density 

 measurements in Watershed No. 1 are summarized in table 5. 

 Soil friction angles (<!)) not only tended to vary with location, but 

 with depth as well. Friction angles varied from 29 to 39 degrees, 

 and generally increased with depth to a limiting value of 38 to 

 39 degrees at the contact between soil (decomposed granitics) 

 and fractured, disintegrated bedrock. The friction values re- 

 ported in table 5 are station averages for the surface soil alone. 

 No cohesion was detected in any of the borehole tests. The 

 gradation of a composite sample taken from the 24- to 36-inch 

 (61- to 91 -cm) depth of the soil horizon in Watershed No. 1 is 

 shown in figure 24. The soil consists of 10 percent fine gravel, 

 86 percent sand, and 4 percent by weight silt size material; it 

 would be classified as a well-graded sand (SW), according to 

 the Unified Classification system. In-situ densities ranged from 

 92 to 101 Ib/ft^ (1 .47 to 1 .62 g/cm^) with an average value of 96 



lb/ft= ( 1 .54 g/cm^). Densities tended to increase with depth as did 

 the friction angle of the soil. Only the mean density (96 Ib ft^) is 

 tabulated in table 5. Subsequent sensitivity analyses showed 

 that such small variations in soil density from station to station 

 had a negligible influence on calculated slope stability; hence 

 the reason for tabulating only the mean. 



Table 5. — Summary of soil-slope data for granitic soil in the Pine 

 Creek study watershed 



Station number 



Slope or soil parameters 



1 



2 



3 



4 



5 



6 



Friction angle,^ degrees 



34 



29 



29 



34 



32 



37 



Slope angle, degrees 



30 



28 



29 



32 



31 



40 



Soil depth, inches 



30 



36 



30 



30 



30 



48 



Soil density,^ Ib/ft^ 

 Dry density,^ Ib/ft^ 



96 



96 



96 



96 



96 



96 



88 



88 



88 



88 



88 



88 



Saturated density,^ Ib/ft^ 



117 



117 



117 



117 



117 



117 



Cohesion, lb/inch^ 



























'In-situ borehole shear test. No cohesion intercept detected. 

 ^Mean value based on several measurements of field density and water content 

 at different stations. 



The soil shear strength parameters, densities, void ratios, 

 and gradations measured in the Pine Creek study watersheds 

 are comparable to those reported by Gonsior and Gardner 

 (1971) during extensive field and laboratory tests of granitic 

 soils in the Zena Creek timber sale area in the Payette National 

 Forest. Most of the soils investigated by them had slightly higher 

 silt size fractions and so were classified as well-graded silty 

 sands, or SW-SM materials, according to the Unified Classifica- 

 tion. 



Gonsior and Gardner (1971) conducted numerous direct 

 shear and triaxial compression tests to investigate the effect of 

 unit weight and moisture content upon the shear strength of 

 granitic soils. Their test results showed that strengths based 

 upon effective stress parameters exhibited negligible variation 

 over a wide range of conditions. For design purposes, they 

 recommended 35 and as reasonable values for the angle of 

 internal friction (4)) and soil cohesion (Cs), respectively. Shear 

 strength envelopes measured in triaxial compression indicated 

 slight residual cohesive strength (on the order of 2 lb in-' [14 

 kPa]) in most tests on saturated specimens. Gonsior and Gard- 

 ner (1971) advise, however, that some of the cohesion mea- 

 sured in triaxial tests on saturated specimens could be 

 accounted for by partial saturation, a curved failure envelope, or 

 by membrane resistance; no corrections were made for this 

 factor. 



These findings are also corroborated by Lumb (1962) who 

 examined the influence of variation in cohesion and friction 

 angle with void ratio and degree of saturation. His tests were run 

 on both undisturbed and remolded samples of decomposed 

 granite with fine, medium, and coarse gradation. Considerable 

 "apparent cohesion" was manifest in partially saturated speci- 

 mens as a result of capillary forces. As expected, apparent 

 cohesion tended towards zero at full saturation (that is, below a 

 piezometric surface) in all soils tested, fine or coarse grained. 



Undoubtedly, some residual soil cohesion does exist in grani- 

 tic soils; borehole shear test results notwithstanding. In the first 



15 



