Weighted Average undrained shear strength for a footing with Vane Data

[tkassmel]

How do you come up with a weighted average undrained shear strenght value to be used in a bearing capacity analysis of a 10 foot square footing. I have the undrained shear strenght values from a vane shear test for increments of every 2 feet for 40 feet in depth.

How is it weighted?
Do you use a zone of influence of 1.5B?

 

[ishvaaag]

Maybe it should be taken along the assumed surface of failure. In practice most surely proportionally to the thicknesses of the layers in the depth of influence, maybe 2B. I will try to give a closer look at the question.

 

[ishvaaag]

More exactly -whilst I do further examination looking for example or so-, proportionally to the different areas along a surface of failure, since these are not 2D for footings, only for continuous strip foundations. This also makes clear the need of including any overburden layers, the shear strength of which needs be mobilized for sinking failure. Even it being common to use 1.5 times or 2 the width, this would be more consistent with a settlement issue than for a limit sinking load, since it is unlikely soil depths under 0.5 to 1 times the width be mobilized in shear. In any case, stand to anything you find in the technically reliable literature.