210_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011

210_CEG 4012 Notes Fall 2011 CEG 4012 Notes Fall 2011 - and...

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SL36 Total vs. Effective Stresses for Saturated Clay (a review) o Recall that clays have different drained and undrained Mohr-Coulomb failure envelopes. o When tesls are performed on a saturated clay consolidated at an effective confining stress, 0'3, then the deviatoric stress (its strength), .6.o d = (cr'l - cr'3), increases with increasing consolidation stress. The effective stress Mohr-Coulomb failure envelope with c' and \$' is shown below. o If an undrained test is performed on a saturated clay without consolidation to the confining pressure at the start of the test, O"J, then the deviatoric stress .6.crd continues to reflect the strength at the confining stress cr'3 to which the clay has been previously consolidated, Le. the strength of a soil is a function of the effective stress to which it has been consolidated. So 0'\ = 0'3 + dad.
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Unformatted text preview: and a', = 0"3 + dad. Therefore any increase in 0"3 to a new confining pressure 0'3 is necessarily followed by an equal increase of a', to a,. (Since the clay is saturated and undrained, any change in confining pressure 0'3 must increase the pore pressure since water is incompressible, and the pore pressure will increase a' as well.) The result is the total stress Mohr·Coulomb failure envelope shown below with Co and \$ = o. Shear Stress, , . _ ...-. , ... ... , , \ \-, .-. / 1\ I I \ Normal Stress, ~"'-----'--t--'------+---~--'-------4~" t."d ,,', I-----------oj 0', 0'31------------1 '" t."d o l\Iole that fOFNC clay, c' = 0 o For short term analys-es7V/e use C u and 4lu (",,_Odor clay) and total stresses o For loog term analy:ses, we use c' and \$.' and efJectil,le stresses...
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