dgillette:
There's a way to do it. If you have Koerner's book "Designing with Geosynthetics," there's a figure used to show how to distribute tire loads. Unfortunately, I don't have the page number to share with you (the book is at work), and I'm sure you have access to the book. I have the...
In the absense of additional data, I would agree with Geo69, but I would also use the guidance from DM 7.2-194, regarding piles and soil friction.
The friction angle (delta) between timber piles and soil is 3/4 of phi. So, Using 26 degrees as the delta, phi would be 4*26/3, or about 35...
My suggestion would be to look through AASHTO's manual for highway design, as everything is parametrically listed (in Chapter 3 or 4, I believe), which deals with retaining structures and foundations. Depending on your loads, and the spacing between soldier piles, you can better determine what...