1/3 increase in toe pressure

[inaz]

Retaining wall design parameters.

In several geotechnical reports I've seen an allowable foundation toe pressure of 1/3 x allowable foundation pressure for eccentric or lateral loading. Why is this recommended? and is this an appropriate recommendation? It seems to me all unrestrained retaining walls are under lateral loading, so why the specific increased pressure?

Thanks for you thoughts.

 

[g7mann]

Dear Inaz:

In my area of practice this 1/3 increase in allowable soil bearing is typically allowed for "transitory" wind and seismic loadings - possibly similar to a short, periodic "rocking" of your retaiting wall. However, this increase is allowed only if the design factor of safety is 3.0 and/or the subgrade has been redensified or comprises a compacted [and tested] structural fill. To the best of my knowledge there have not been any significant failures as a result of using this increase. Unfortunately, in spite of its widespread use, I don't know how this modification came about.

Hope this helps.