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# Ultimate lateral force for P-Y Curve for clay

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 Antrox (Structural) 12 Sep 12 3:25
 Hi there, Could you explain why in the criteria to obtain the ultimate lateral bearing capacity for soft clay for API p-y curve a effective unit weight and not a total unit weight is needed? Indeed, the criteria is: pu=3*Cu+gamma*z+J*Cu*z/D where: Cu = undrained shear strength gamma=effective unit weight Those terms seem to run in a contradiction. What do you think about? Thank you
 GeoPaveTraffic (Geotechnical) 12 Sep 12 15:14
 I'll start by saying I'm not familure with the equation you posted. However, I assume that effective unit weight is used since water pressure would potentially act on all sides of the foundation element and only the boyant (effective) unit weight of the soil can be counted on to resist the net lateral force. Think of it this way: If the foundation were above the water table the effective unit weight = the total unit weight. As the water table rises, the water pressure starts to act on all sides of the foundation. At the same time the effective unit weight of the soil drops. I assume that the source of your contradiction is the use of undrained strength and effective unit weight. There are many occations where the strength of the clay may be undrained regaurdelss of whether the soil is above or below the water table. Hope this helps. Mike Lambert
 Antrox (Structural) 13 Sep 12 10:39
 Thank you for your reply. On your second point, I agree with you. A clay might be saturated due to the suction by capillarity. I have some doubts about your first point. The undrained shear strength is defined in the total stress approach. Accordingly , the submerged unit weight should be used and the pressure of the water should not be considered. In the similar case of a surface foundation founded on the seafloor, in the "short term condition" the pressure of water (the archimede force) should not be taken into account. Am I wrong?
 fattdad (Geotechnical) 17 Sep 12 12:48
 ask yourself whether to use total or effective strength parameters in evaluating the horizontal stresses acting on a retaining wall. Hopefully, you'll get the answer, "Both!" In a phi=0 (total stress approach), you have nothing to use but total unit weight and undrained shear strength. I would not use any value for Cp in total strength analyses. For effective strength analyses, I'd use the effective friction angle, ignore cohesion and take a value of phi/10 for Cp. Somewhere in this mix though you'd need to look at deformations. There's no substitute for a p-y relationship! f-d ¡papá gordo ain’t no madre flaca!

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