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Geotechnical Problem Solving by John C. Lommler

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2.2

Soil Stresses and Strains

2.2.1 Introduction to Soil Stresses

I have heard the following lament from fellow engineers so often that I cannot attribute it to one person or group of engineers. It goes something like this:

When I was in school, the professor told us that soils are not linearly elastic like steel. Soil doesn’t have a straight line stress–strain curve; in fact, it doesn’t even follow the same stress–strain curve during loading and unloading. Soils are not homogeneous and they aren’t isotropic like the engineering materials we studied in sophomore year strength of materials class. Just about the time I wrapped my mind around the anisotropy of soils thing, my damn professor turned to the next chapter in our soils textbook and started calculating soils stresses using the Boussinesq theory. Almost matter-of-factly while calculating soil stresses caused by surface loadings my professor mentions that the Boussinesq theory assumes that soils are elastic, homogeneous, isotropic materials. Now wait one darn minute, first he says soils are anisotropic, then with the wave of a hand he changes his mind and says that it is OK to use the elastic material Boussinesq theory. What’s up with that? He never answered my question. Why not use an analysis that matches the material?

I’m still confused 10 years after graduation from college. First, I’m told that soils are not linearly elastic; they are not homogeneous or isotropic. Then I’m told to go ahead and use equations that are ...

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