Wing Wall/Retaining Wall Analysis (varying retained backfill height) 2

[EastEng1012]

Hey All,

I am curious on how others approach the design and/or analysis of wing walls (e.g., for culvert headwall) or any type of retaining wall with the height of the wall sloping downward to match the grading (varying retained back fill height). Generally what approach do you take:

1) Analyze it for the section with maximum retained height at the start of the wall;
2) Use an average retained height across the wing wall length; or
3) Analyze at both the start and end (based on different retained height) and maybe taper footing heel length from start to end

My senior engineers have always told me to use the maximum retained height at the start of wall for the 1-ft strip design. This make sense in certain applications but could be conservative for certain applications (longer walls and/or large difference in retained height from start to finish). Anyways, it was a thought I have had in my head and curious what others have to say about it. I am assuming both 1 and 3 are appropriate depending on the specific situation.

 

[SlideRuleEra]

3) Analyze at both the start and end (based on different retained height) and maybe taper footing heel length from start to end.

I am assuming both 1 and 3 are appropriate depending on the specific situation.

Suggest taking the Senior Engineer's advice and forget about #3.
Consider:
A tapered footing will require each piece of rebar to be a different length, placed at a specific location.
Concrete forms will be somewhat more complex.
Footing construction is below surface of the ground, possibly in wet conditions (this is for a culvert). Time working below ground (until the next rain) is of the essence.

... and all to "save" a token amount of concrete, while spending for "excess" labor and taking unnecessary risk with weather.

There is much to be said in favor of simplicity and constructability.

http://www.SlideRuleEra.net

 

[BridgeSmith]

We've typically done the structural design for the maximum height, although we did some FEA that indicated we could use the average of at least a 5' long section without issues.

For stability (overturning, sliding and bearing) of culvert wingwalls and such (where the length less than 4 times the height), we use the height at 1/3 the length from the high end, i.e. for a wall that tapers from 12' high to 6' high, we'd size the footing for a 10' retained soil height.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[EDub24]

I've always designed for the worst case and carried that design throughout even if the wall and retained soil height reduces. Yeah you could get slick and have a x-section that changes as the soil height decreases but as Slide Rule says the extra cost associated with formwork and labor would almost make it a moot point. Just imagine the rebar guys having to place the rebar in a varying length footing. It would be a nightmare.

I'm actually working on a project that involves a fairly steep slope (11.4%) over about 80 feet. To help save some costs what I ended up doing is designing two separate retaining walls. A larger one for the 1st half the of wall with the higher retained soil height and a smaller one for the 2nd half with an expansion joint between them. I kept the top of footing elevations the same so there wouldn't be any additional excavation.

 

[EastEng1012]

Thank you all for your responses!

It seems the consensus is to use the worst case condition which would be the starting height section and carry the design throughout the length of the wall. I agree that this is the best way to keep it simple both from a design standpoint and constructability. I was curious though because I have evaluated existing headwalls that follow typical DOT standards and you will find some with footings where the heel length changes along the length.

I like BridgeSmith's approach for specific situations (For stability - where the length less than 4 times the height, use the height at 1/3 the length from the high end).