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114 Rydrmdlo and Nschaaicat Properties a/ Sofa ART. 18 SHEARING RESISTANCE OF COHESIVE SOIIS Normally Loaded Undisturbed Clays of Lotn to Moderate Senaitioity The results of drained triaxial tests on normally loaded cohesive soils can be expressed with satisfactory accuracy by Coulomb's equation in which c = 0. Thus a = p tan ¢ (18.1) The values of ~ for such materials, whether in a remolded or an un- disturbed state, are related to the plasticity index. Approximate values may be estimated with the aid of Fig. 18.1, although the scattering from the curve for most clays may be on the order of 5° (Bjerrum and Simons 1960). However, the exceptionally high value ~ = 47° was obtained (Lo 1962) for clay with a liquid limit of 426% from Mexico City. Hence, it is apparent that the statistical relation represented by Fig. 18.1 is not of general validity and should be used with caution. Under conditions usually encountered in the field, the low permea- bility of clays greatly retards the drainage; as a consequence the pore pressures u. associated with the forces tending to shear the clay may • not dissipate readily. Since the pore pressures associated with shear are positive (Fig. 15.5e), the strength indicated by Eq. 18.1 may not be developed for s very long tune; the time requ'ved for dissipation is governed by the consolidation characteristics and dimensions of the cohesive body (Articles 14 and 25). The conditions associated with complete lack of drainage may be approximated in consolidated-undrained triaxial tests (Article 15). The results of such a test, in which p, and ps are the effective principal w u e l a 4 9. 30 ~n 20 40 60 BO r00 P/osliCily /nder (y.J Fig. 18.1. Pelatioo 6etweeo 0 and plastidty index for days of moderate to low seruitivity under drained conditions. Terzaghi and Peck, 1967, "Soil Mechanics in Engineering Practice" Second Edition, 1967, John IJiley & Sons, Inc. New York, NY.