Stability Evaluation of Monolithic Headwall with Wingwall

[ahardy89]

Hello all,

I am relatively new engineer still learning the most practical and efficient ways of analyzing certain problems. I have been asked to do an initial check of the stability of a proposed design for storm water headwall with wingwalls. I was told to evaluate the headwall and wingwalls as one monolithic structure vs evaluating them independently as separate retaining walls. The wingwalls are at 45 degrees and taper in height to match the proposed backfill grading. They share a slab between the wingwalls as shown in the plan sketch below. I have experience analyzing cantilever retaining walls as a unit strip but have not done a monolithic structure such as this with both the headwall and wingwalls combined.

I have a few questions in regards to how to y'all would approach this:
1. I assume the critical forces to check are driving forces (sliding and overturning) in the longitudinal direction (in direction of pipe culvert
alignment)?
2. Is it safe to assume the forces in the transverse direction will balance out since the wing walls are symmetrical?
3. Is there a simple way to compute the thrust on the wing walls in the longitudinal direction if the backfill height varies or should i check at
increments along the length and use the tributary area between the strips for the check (multiplying the resultant by sin 45)?
4. Is the configuration of the apron slab between wingwalls ok to terminate at the end of the wingwall length?

After the geometry is checked and modified to meet stability criteria, the strength will be evaluated.

Really appreciate everyone's insight and recommendations.

 

[r13]

1. Use earth pressures normal to the wall segment.
2. Don't cancel any force, it prone to cause mistake.
3. Divide the wing walls to 2 or 3 segment, and use average soil pressure to simplify te calculation.
4. Yes.

You shall locate the geometry center of the base, and perform 3D analysis.

 

[ahardy89]

Thank you for response.

To clarify no. 1 - the earth pressures will act normal to the headwall and wingwall segments both. So for the wingwalls.

For no. 2 I should have not used the wording cancel out. I will still check the forces but I meant that sliding should be ok in that direction because the vector of the components will balance out either in that direction but you will still have a thrust for the other component.

By 3D analysis you mean like finite element model or just referring to check the forces about the centroid in each global axis (x, y, and z)?

 

[r13]

2. Correct, sliding will only occur in direction normal to the main head wall, as the lateral sliding forces cancel each other. (Same as the direction of global rotation)

The 3D refers to the x, y and z axes. You should be effectively use hand calculation.