Could you take a look at this. We have had some people install rock inside a bullnose to minimize weed growth. I have concerns about too large of rock could prevent post rotation or trip a vehicle. Too small of rock could decelerate the vehicle too much and cause vehicle instability.
We are not overly concerned about the vehicle tripping as the aggregate size is not overly large and the slopes in the area are relatively flat.
There is some concern about the mat and rock surrounding the posts affecting the rotation and energy dissipation of the posts in the system. To alleviate that issue, we would recommend that you use leave outs around the posts consistent with our previous recommendations for posts in rock.
http://mwrsf.unl.edu/researchhub/files/Report246/TRP-03-119-03.pdf
Thanks
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