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• 2.0 Probability Based Upon the Factor of Safety <br />Analyses <br />Probabilities based upon the factor of safety analysis use the coefficient of variation in the soils <br />sheaz strength to assign corresponding variations in factor of safety. The factor of safety is used to <br />predict probable landslide outcomes. <br />2.1 Pre-Correction Shear Strength of the Soil <br />The shear strength of the Busting landslide (prior to correction) was determined by a back- <br />calculation of the landslide, assuming a factor of safety of 1.0 and the Busting water pressures. <br />The shear strengths were determined for both the effective stress analysis and the total stress <br />analysis. As shown on Figure 1, the effective stress analysis determined the shear strength to be <br />12.1 degrees with a cohesion of 200 psf, compazed to the measured value of 13.1 degrees with a <br />cohesion of 200 psf, and this was judged to be a reasonable value. The total stress analysis <br />determined the "undrained" shear strength to be 720 psf, as shown on Figure 2, which was also <br />judged to be a reasonab]e value. (As a note, undrained shear strength of 720 psf closely matches <br />• the calculated value.) <br />A reanalysis of the landslide was made with slightly different groundwater conditions and the <br />back-calculated friction angle was 11± degrees. For purposes of this report, the 12± degree friction <br />angle was used. <br />2.2 Shear Strength of the Pins (Micropiles) <br />The shear strength of the 30-inch diameter steel pipe within the pins is 0.67 ksi. The pins were <br />installed on 12-foot centers in 3 rows. Each row was 12 feet apart and the pins in the middle row <br />were staggered to be midway between the pins on each of the outer rows. The contribution of the <br />pins increase in shear strength was determined to be 8312 psf using a weighted average of the <br />area of the pins and the soil between the 3 rows of pins and then assuming that the soil had no <br />sheaz strength in this zone. For purposes of modeling only, the average weighted shear strength of <br />8312 psf within the 3 rows of pins was distributed along a 50-foot section of the shear plane at the <br />pin location having an average shear strength of 3900 psf. <br /> <br />0626024\60558-LCET 2 <br />