Existing Concrete Headwall Analysis (and Increasing Height)

[EastEng1012]

Hey All,

I am working on a project in the early phases that involves slope stabilization of an embankment. There is a head wall for a twin barrel (84-inch diameter) culverts on the downstream slope of the embankment. The re-grading to stabilize the slope requires increasing the height of the existing headwall. The owner wants to avoid replacing the headwall completely if possible. The existing headwall is a standard detail from the state DOT. It is 10' high and 20'-8" length with wing walls at 45 degrees on both sides.

We are trying to quickly do some calculations to determine if it is feasible to increase the height. I am trying to figure out the best way to analyze the existing structure (stability and strength). I have talked to a couple of our senior structural engineers and they mentioned doing a FEM analysis but I figured this could get complicated with meshing and the circular opening. I was thinking of breaking up the wall in to segments and calculating the lateral forces. The segments would exclude the area where the pipes are located and no soil pressure exists (to simplify I would assume a square section equal to diameter). I would then calculate the FS against OVT and Sliding.

I was curious how some of you would approach this evaluation. Again, we want to just see if it is feasible at this stage so we are trying to do some quick calcs (I am trying to avoid FEM if possible). But any recommendations or suggestions for approach based on previous experiences would be great!

Thank you!

 

[dhengr]

Rwalker8202:
There is a 2.5’ – 3’ high horiz. beam over the top of the two culverts, unknown thickness and reinforcing, which spans 21’ to the wing walls. It has a distributed lateral load from the soil above the top of the culverts, and some discrete lateral loading from wall sections around the sides of the culverts. And, depending upon the detailing at the headwall/wingwall joint this may be a simple beam, with little fixity at its two supports. Can the wing walls and their found. support this? I would be surprised that they over designed this headwall by a factor of two in the first place, just in case. So, you have to make this first cut analysis to see what extra might be there, or whether it will handle several more feet of headwall height. If you need to add wall height and spanning strength on the 21’ span, consider this: An added horiz. beam atop and tied into the existing headwall top; on top of the existing wing walls and in their plane, place new beams/kickers/struts. They slope down atop the wingwalls to new footings which now must take significant vert. loading, plus thrusts (down stream) from the kickers/struts which carry the new embankment and the new top beam on the headwall. You are saving the existing structure if it is in good condition and adding the needed new structure over the top where it is needed.

 

[EastEng1012]

Thanks dhengr for the response!

I like the idea of constructing a horizontal beam to span across the top of the head wall for the added height and also above the wing wall for the load path.

When you say slopes down to a new footing are you referring to extending the wing walls downstream in plane with a stem and footing for sufficient length as required for stability?

Also, for the wing walls, I need to check the out of plane bending (as a typical cantilever retaining wall still. Since the wing wall stem height is varying due to slope, is it safe to assume the average height?

 

[dhengr]

Rwalker8202:
The struts/kickers, over the wing walls, which I am talking about are in the same plane as the wing wall. They may be thicker than the wing wall, and at the top they are as high as your new 21’ horiz. beam. That depth (top of strut) may slope down at the same slope as the top of the existing wing wall, which forms the bot. form for the new strut. The strut also provides the increased height you need w.r.t. the new embankment slope. At the down stream end of the existing wing wall, the strut may be some considerable distance above any new footing elev., so that ftg. will have vert. load, thrusts down stream and cross stream, and a large moment from the axial thrust (from the new horiz. bm. reaction, maybe the wing wall takes part of this through dowels, etc.). That new wingwall ftg. will need some considerable design attention. It may be/go longer down stream, with the strut following down, and thus a lower ftg. moment. The existing walls may not be able to withstand the new canti. retaining wall actions. All of these walls, and new horiz. beam and struts could have tie-backs into the embankments if needed, I would think. You didn’t cause the problem, the original designer didn’t design for it, you are just trying to save and use good existing structure, if you can, with good clean new design and structure, at a reasonable cost.

 

[EastEng1012]

I struggling with exactly where the new footing would be located in your suggestion. Correct me if I am wrong, but the new footing would be located at the downstream end of the wing walls. So the kicker/strut run on top of the wing wall and where the existing wingwall terminates, there would be an new vertical wall section down to the new footing (that would be designed for the new vertical load, thrusts, and thrust moment).

 

[dhengr]

Rwalker8202:
You’ve pretty well described what I had in mind, of course, the devil’s in the details. I would expect that the new and the old are all tied together, so they pretty much act in unison and share and distribute the various loads, and potential variations, you do not want significant differential movements or deflections. The existing wing walls might (will) take some of the axial thrust from the new struts, depending upon your design details. The added (new) wall and ftg. must take the remainder. The existing wing walls are already loaded, so how much they will take in addition is part of your design problem, your engineering judgement and experience, and based on your analysis of the existing structure and conditions. You haven’t really given us many of the existing conditions and details, so you have to determine exactly how to do it. All I was doing was offering a new structural system which would seem to cause the least disruption to the existing conditions, and otherwise seem to solve your basic problem.