Retaining wall freeboard height design

[reggeo]

We are designing a 26' high retaining wall. We have been required to substantiate the freeboard height and impact load we recommended (5.0', 125 pcf), based on possible future debris flow.

The ascending slope is about 1.5:1.0; the max. depth of sloughing has been postulated at 3.0'. I am assuming that at most only about 1/3 of the length of the upslope could slough down at any one time. It appears that a graphical solution would be most practical, but that's just a guess.

Any and all help appreciated.

 

[reggeo]

You are correct; assume a natural angle of repose of 2.75:1.00 H:V for collected material starting at the top of the 5' freeboard and project upslope. Use geometry to determine the volume per foot of wall. That "1/3 length of slope" won't work for very long or tall slopes; get your project geologist to provide input re: size/depth of sloughs and slumps typical to the project area and/or slope.

The 125 pcf is derived by assuming the modeled sloughed material against the freeboard essentially has no shear strength; that's about what you get when you plug C=0 and PHI=0 into the failure wedge analysis.

Hope this helps