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• Art. 17 Shearing Reriaem,ee of Cohaionlea So$ 107 ART. 17 SFiEARINC RESISTANCE OF COHESIONLESS SOII.S Sands and Inorganic S~11s The shear characteristics of sands and inorganic silts can, unless the soil is exceptionally loose, be represented reliably by Eq. 17.1. a = (p - u.) tan ~ = p tan ~ (17.1) Deposits of natural sands and silts may be encountered in any state between loose and dense. Depending principally upon the relative density, the value of ~ may range between Eairly wide limits; the grain- size distribution and the shape of the grains also have an influence on ~. Representative values for ~ under effective pressures p less than about 5 kg/cm' are given in Table 17.1. Table 17.1 Repreaenlatiue Values of 0 for Sanda and 574 Depreea .llaleria! Laoae Denac • Sa¢d, round grains, uniform 21.5 34 Sand, s¢gular grains, well graded 33 45 Sa¢dy gravels 35 50 Silty sa¢d 27-33 30.34 Inorganic silt 27-30 30-35 Since most of the shearing strength is caused by interlocking of the grains, the values of ~ are not appreciably different whether the soil is wet or dry. As the pressure p is increased from about 5 to about 50 kg/cm', the values of ~ decrease gradually by about 10°. The decrease is associated with an increase in the percentage of grains that are crushed as the state of failure is approached. Figures 15.3c and a pointed out the tendency of a loose sand to decrease in volume, and of a dense sand to dilate, during shear. The permeabi]ity of very fine saturated sand and silt is so low that the rapid application of a shearing stress is associated with a temporary increase of pore pressure u. (Eq. 17.1) if the soil is loose, or with a temporary decrease of u„ if the soil is dense. The strength of the soil is correspondingly temporarily decreased or increased. Hence, for ex- ample, if piles are being driven into one of these materials in a loose saturated state, the piles encounter only a slight resistance which is Terzaghi and Peck, 1967, "SdT Mechanics in Engineering Practice" Second Edition, 1967, John Wiley & Sons, Inc. New York, NY.