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112 Rydraalk and tareAanied PropeAlea o1 SosL <br />ART. 18 SHEAAIIYC RESISTANCE OF COHESIVE SOILS <br />Normally Loaded Undisturbed Cloys of Low to Moderate Senaititnty <br />The results of drained triaxial tests on normally loaded cohesive soils <br />can be expressed with satisfactory accuracy by Coulomb's equation in <br />which c = 0. Thus <br />a=fStanm (18.1) <br />The values of ~ for such materials, whether in a remolded or an un- <br />disturbed state, are related to the plasticity index. Approximate values <br />may be estimated with the aid of Fig. 18.1, although the scattering <br />from the curve for most clays may be on the order of 5° (Bjerrum and <br />Simons 1960). However, the exceptionally high value b = 47° was <br />obtained (Lo 1962) for clay with a liquid limit of 426% from Mexico <br />City. Hence, it is apparent that the statistical relation represented <br />by Fig. 18.1 is not of general validity and should be used with <br />caution. <br />Under conditions usually encountered in the field, the low permea- <br />bility of clays greatly retards the drainage; ae a consequence the pore <br />pressures u. associated with the forces tending to shear the clay may <br />• not dissipate readily. Since the pore pressures associated with shear are <br />positive (Fig. I5.5e), the strength indicated by Fq. 18.1 may not be <br />developed for a very long time; the time required [or dissipation is <br />governed by the consolidation chsracteristics and dimensions of the <br />cohesive body (Articles 14 and 25). <br />The conditions associated with complete lack of drainage may be <br />approximated in consolidated-undrained triaxial tests (Article 15). <br />The results of such a test, in which ps and p, are the effective principal <br />h <br />a <br />4 <br />a <br />u <br />O <br />s <br />0 20 40 60 BO l00 i <br />P/ostitily /nde+ (%) <br />i <br />Fig. I8.1. Relation between m and plastidty rode: (or days of moderate to low <br />sensitivity under drained condition. <br />• <br />Terzaghi and Peck, 1967, "Soil Plechanics in Engineering Practice" Second Edition, <br />1967, John Wiley & Sons, Inc. New York, NY. <br />