Area of influence and pressure diffusion

[Tom W]

The company for which I work suggested a Danby liner as remediation for a 96" culvert that was 30' deep, below an interstate highway, which needed replacement. The engineers refused our suggestion based on pressure diffusion, if that is the term. In a later discussion with one of the engineers I asserted that since the culvert was so deep the pressure from cars and trucks on the roadway would 'bridge' and be diffused a long way above the culvert leaving only ground pressure around the culvert which the old culvert, with the Danby grouted in place, could easily handle. That is to say that the pressure from the tires would diffuse from the roadway as in a pyramid, more pressure at the top and less pressure as the depth increased. The engineer told me that my thinking was wrong and that the pressure gradient would be more like an inverted pyramid with the pressure from the roadway at the top funneling down to the point of the pyramid at the bottom. In a subsequent discussion with the engineer I reitterated what he had said and asked if he had misspoken. He assured me that he was correct.

The engineer and I get along well. We laugh and joke and talk about a number of things in addition to jobsite discussions. But, when we discuss projects we are both serious. His assertion about the pressure gradient in terms of an inverted pyramid seems counterintuitive. Can anyone confirm that what this guy told me is correct or is he messing with me?

Thanks,
Tom

[CWSmith]

I'm NOT an engineer, and I agree with you that his "upside-down pyramid" analogy, appears to be counter-intuitive. That would illustrate to me that the design of any foundation, highway or otherwise, is to focus the load on a point, and not distribute it over as wide an area as possible.

I would be more inclined to reason that the load, stresses, etc. are such that they would transfer throughout the base, and as the base is spread, so then would the stresses, and would become less forceful as they are widely distributed.

However, it may be a case where continued variation of the load (various vehicle weights, speeds, etc.) may cause oscillations or waves of stress throughout the structure, and it's foundation (think of that as a stress distribution system). Because such stresses are distributed, doesn't necessarily mean that they are dissipated. Even many feet down, serious consideration has to be given to materials and structures. So, while I haven't a clue as to the items you specified, or their intended purpose, your civil engineer surely must.

I recall seeing a documentary on the Hoover Damn project and how they layed out and poured the concrete. The engineering of the lower layers, passages, etc. were extremely important. A joke was made about the "urban legends" of workers falling in and being sealed in the concrete. The comment was something to the effect that it never happened, because just one case would compromise the strength of the structure.

I hope this helps,

CWS