Load Spread Under Footings

In routine consolidation analysis, the load spread under a shallow foundation can be taken as varying from 30 to 45 degrees. This is used to compute additional stresses induced within the clay due to the superstructure.
I have been asked why 30 to 45?..I feel this question might have been addressed before (which unfortunately I have been unable to locate).
In relation to the above, does the angle depend upon the type of soil or just the footing? Loose the soil, less is the angle of dispersion of the load?
Would someone also guide me to the relevant literature?


Depending on the type of footing and the soils you will get varied distributions. 30 to 45 degrees is a good rule of thumb. Get a hold of any Foundation Engineering Book by either Das, Cernica, or Bowles. You need to look at the pressure distributions isobars that occur under footings. On excavation details I have always seen the following: With crushed stone under the footings the distribution angle is 26.5 degrees + or -. With compacted fill its about 45 degrees. This is just my experience and I cannot explain the technical differences. Not following these basic values may lead to a soil failure, as the bearing pressure will push out the soil sideways.

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