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Re: Protective Slab Design

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My comment in navy.

Thor A. Tandy P.Eng
Victoria BC
email: vicpeng(--nospam--at)
I have been asked to provide the structural details of a concrete cap
protecting an existing 33" irrigation line.


The sketch was drawn by a civil/civil, so don't be too critical.

The design criteria includes a fire truck with a double axle load of 60k. I *think* that's the total load for both axles. I'm trying to get more detail (spacing of axles, tire size, etc).

My concerns are as follows:

Distribution of wheel load? IIRC from my bridge days, it's 0.8 x span of the slab. If someone references ASHTO, my copy is so old, I'm afraid to tell you what version it is :o).

Soil pressure, especially when the wheel is right over the vertical wall. I think I can turn the vertical wall into a footing and distribute the load much further than the distribution width I use for the slab design.
As you note, I, also, would try to have the wheel load evenly distributed over a "footing".  As it shows now, there will be a net moment on the base, which may cause a "depressed hat" profile.

Surcharge, when the wheel rolls off the slab turning the vertical wall into a retaining wall. I think I need to take this load out via frame action of the upside down "U" frame and resisted by passive pressure on the other side.
Makes sense, however, if you rely on passive pressure as the restraining force, the repetitive nature of the loading may require investigation of the ground so that a bump doesn't occur over the lid.
Also, I don't think it's a good idea to lay the road immediately on the concrete if there is any chance of settlement around.  A cushion of 6" of basecourse ...?

At first glance, it appears to me that the bottom, outward legs of the "hat" (2'-6" wide) are not necessary if the vertical legs are sized to accommodate the soil pressure and the Styrofoam is extended to line the inside face of the vertical walls. But, maybe I just haven't thought about it long enough.
In line with my first comments?

Other issues which I have not considered.

Anyone wishing to share how they would address the above issues and solve the problem would be welcome.


T. William (Bill) Allen, S.E. (CA #2607)