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 Topic: Geotechnical design and analysis Subject: difference between drained and undrained shear strength of clay
 Author: lee 2002-08-25 what's the difference between drained and undrained shear strength of clay or silt? Why sometimes we use undrained strength instead of drained strength in geotechnical design?If we carry out undrained unconsolidatd triaxial tests on silty clay, normally we'll get a C value and occassionally small phi values. CU and CD tests are more expensive and take a lot of time to perform. Is there a correlation of drained parameters from the UU tests? how does drained and undrained shear strength of clay affect the design of temporary or permanent sheetpile? which type of parameter is to be used? Does it mean a clay having a Cu value of 20 kN/m2 with phi value = 0 from a UU triaxial test carries the meaning that the clay is in undrained mode? and say if I need the drained shear strength of the same type of material without performing CU or CD triaxial test, all I need is to find a correlation between plasticity index and the phi value? say the material is having a PI of 50%, so I look up the chart and obtain the phi value of, say 24 degree. so the conclusion is that the drained shear strength of clay is C = 20kPa and phi = 24 degree?glad to have comments and answers to my problems. thanks a lot Reply to this posting
Follow-up:
 Author: Paul Mayne 2002-08-29 Hi Lee,This is best handled through critical-state soil mechanics. The UU test should be abandoned - Yes, it is simple, butmisleading. UU = useless & unreliable. All soils are frictional materials characterized by an effective stress friction angle (Phi' to most of us; It canbe represented by other parameters, as in q-p space). Thecomplication comes when we have a low permeability material(like a clay or silt, or mixed soil) and low the soil quicklysuch that excess porewater pressures develop. Then the combination of delta u with the effective phi' gives you an"undrained" strength, although other possible modes may occur(different stress paths, "semi-drained" and "partially undrained")If you not measure the porewater pressures, you can only seepart of the picture (e.g., Unconfined compression and UU). Thusthe total stress strength (such as su or cu) will give the results you describe). This is confusing & misleading too.If you use a simple model like Cam-clay (or NorSand), you canexplain all of these very elegantly: normally consolidated,overconsolidated, drained, undrained (or any drainage), porewater pressures (positive and negative), dilatancy,contractive, and much more. Check out David Muir Wood's book on critical state soil mechanics.RegardsPaul