Slope Stability Data Collection 



IN-SITU BOREHOLE SHEAR TESTS 



The borehole, direct shear test is a rapid method for obtaining 

 in-situ shear strength parameters. Borehole shear tests avoid 

 the need for recovering samples with all the attendant problems 

 of sample disturbance and representative sampling (Handy and 

 Fox 1967; Wineland 1975). 



The method basically consists of lowering a shear head down 

 a 3-inch (8-cm) diameter borehole to a desired depth in the soil 

 profile (fig. 15), expanding the shear head against the sides of 

 borehole with a known normal stress, and then recording the 

 maximum shear stress required to crank the shear head up the 

 hole (fig. 16). The test can be run at several depths in the 

 borehole to provide an idea of how the soil shear strength 

 parameters vary with depth. 



The boreholes were drilled to the bottom of the soil horizon or 

 disintegrated rock interface (fig. 2) using a 2-inch (5-cm) bucket 

 auger. The holes were then widened to 3 inches (8 cm) in 

 diameter using a thin-walled reamer. Occasionally undisturbed 

 samples were taken with the reamer for density and moisture 

 content determinations. These samples were also analyzed for 

 their grain size distribution. 



Figure 15. — Expandable shearing head of borehole device prepared 

 for lowering down borehole. 



Figure 1 6. — Borehole, direct shear test in progress in shallow, coarse- 

 textured soil developed on granitic rocks of the Idaho batholith. Pine 

 Creek study area, Boise National Forest. 



ROOT DISTRIBUTION AND 

 STRENGTH TESTS 



In order to make an assessment of the rooting contribution to 

 soil shear strength and slope stability, it was necessary to obtain 

 an estimate of both root distribution and root strength in the 

 granitic soil of the study watersheds. This estimate was 

 obtained from results of studies in granitic soils of other water- 

 sheds in the Idaho batholith (Curtis 1964; Burroughs and 

 Thomas 1977; and r\/legahan and others 1978). 



Burroughs and Thomas (1977) measured the concentration 

 of intermingled lateral roots (Douglas-fir) in a vertical plane or 

 trench midway between trees. Inclusion of all intermingled, 

 fresh roots up to 0.4 inch (1 cm) in diameter leads to a calculated 

 "root area ratio" of 0.045 percent. Most of the roots counted fell 

 into this size class. Roots in the size class to 0.4 inch (0 to 1 

 cm) comprised 96 percent of the roots counted in the sample 

 area. On the other hand, inclusion of fresh roots up to 3 inches 

 (8 cm) in diameter increased the root area ratio to 0. 1 74 percent 

 even though these larger roots were not numerous. Root area 

 ratios less than 1 percent may not seem significant with regard 

 to slope stability, but, as shown later, even a few tenths of a 

 percent of root section in the soil is sufficient to provide a 

 substantial and critical amount of shear strength increase in 

 shallow, coarse-textured soils. 



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