Trench adjacent to sign pole

[haynewp]

I have a billboard sign structure with a pole embedded about 15 feet. There will be an adjacent trench dug to periodically service a utility line. There will be a trench box used so the soil should not be significantly laid back toward the pole.

I am concerned about undermining the pole for overturning (there is moment due to the cantilevered sign weight in this direction) with the adjacent trench. I had thought keeping a 45 degree angle from the ground line to the trench would be sufficient, and the only reference I could find is below which matches the 45 degree angle that I had an initially thought to use. But I can't find any other justification for the 45 degree angle or how to arrive at the required distance otherwise. A geotech friend of mine felt the 45 degrees would be sufficient under a short term case but had not justified with any calcs. Page 19 of the below document has the exact case I am looking at. Is the 45 degrees somewhat standard for this scenario as the document would lead to believe or should a geotech be officially involved?

http://www.essentialenergy.com.au/asset/cms/pdf/contestableWorks/CEOP8041.pdf

 

[oldestguy]

We routinely use that 45 degree limit and generally no problems. However, the trench might better be protected by braced sheeting, since a trench box is loose and soil can shift some.

 

[Ron]

45 degrees is a relatively conservative estimate based on a generalized soil condition, mostly looser materials. That value keeps you out of trouble for the angle of repose and for the phi angle, such that when either of them is that high, the lateral load attenuation in the soil will be considerable by the time it gets to the point if intersection.

 

[haynewp]

Thanks. I wasn't really sure the 45 deg angle could be applied as a zone to avoid for a foundation resisting overturning similar to how we normally apply it as a zone to avoid when making sure to not undermine a building footing in bearing.