Treatment of soil-wall friction angle |
  
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Treatment of soil-wall friction angle |
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The classical Coulomb method diagrams always assume that the retained soil can slide on the surface of the retaining wall. And they are all conveniently drawn on diagrams of what we would consider to be "gravity" retaining walls with negligible heel projections. It's easy to envision that this sliding effect could happen when there is no heel. So in those cases, the unmodified Coulomb equation makes sense.
Now introduce a significant heel into the picture. It's no longer quite as easy to envision the retained soil mass behaving the way the Coulomb theory diagrams depict. Very hard for the retained mass of soil to slide with respect to the wall when there is a heel in the way. Normally engineers would roll over to the Rankine formula for these conditions, because it is specifically intended to consider that the soil on the heel is inert, and that it is being pushed by the active soil on the other side of a vertical plane that extends up from the end of the heel. As of this writing, ENERCALC does not include a Rankine method option. So the solution for conditions with heels was to consider the Rankine-type behavior, and use the soil-on-soil friction factor along the vertical shear plane at the end of the heel.
