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Soil arching action

Soil arching action

(OP)
Is there some particular depth of a soil where there is no increase in vertical load at that depth due to the weight of the soil? In other words, is soil able to bridge itself or is there arching action at some given depth? As an extreme example, say I am interested in the soil pressure 300' below ground. If someone were to place an additional 2' of soil at the ground surface, it seems like the load due to the additional 2' of soil would not be "felt" at a depth of 300'.

RE: Soil arching action

Not a geotech here, but the load spreads as it goes deeper. This is due to a combination of the soil properties, friction, and fluid pressure if present. So the deeper you go, the more that load is spread over a larger area. That makes it feel like increased loads have very little impact and great depths.

RE: Soil arching action

(OP)
I agree, typically for a surface load we assume that the load spreads out anywhere from a 1:1 to a 2:1 distribution through the soil. Seems like this would also apply to the load due to the soil itself. If you have a buried structure, at some depth, you would no longer account for the weight of the soil above the buried structure. In my mind, I would simply draw a load triangle above the buried structure with sides sloped at 1:1 or 2:1. Any soil above that triangle would not be seen by the buried structure. Hence, you would get some arching effect similar to that in masonry lintel design. I guess what I was getting at is, is there another way of rationalizing or looking at the arching effects of soil?

RE: Soil arching action

This deals with two possible alternatives. One is simply the pressure from that material above and it increases with depth. Explains why diamonds were formed, etc. The second situation is some form of structure that can "give" or deflect due to some of that pressure and then there is some form of arching above it, as in coal mines. A common "rule of thumb for a tunnel could be the roof only has to carry about two diameters height of that stuff above it.

RE: Soil arching action

Elastic solutions are your friend. If the areal extent of the 2-ft of soil was hundreds of acres, then yes, there would be a stress response at 300 ft. Heck, maybe even a thousand feet?

If the loaded area was just a few feet by a few feet, you could calculate some stress response at 300 ft, but it'd be trivial. Typically, if the change in stress is less than 10 percent of the overburden stress, we'd ignore further integration.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Soil arching action

There are two different effects here.

Yes, a point load, or a load over a small area, will distribute so that it has negligible effect through deep fill.

But no, you can't rely on arching to transfer uniform load away from a structure, unless it is very flexible. A typical reinforced concrete culvert or arch structure will be stiffer than the surrounding soil, so it will attract load rather than shed it. If the depth of fill is increased then the design vertical load should be increased in proportion.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Soil arching action

I agree with fatdad.

This is an elastic problem.

It all depends on the width of what you plan to place on top.

Usually, after a depth of 2 to 3 times the width of the base, the soils underneath will not feel anything you place on the ground surface.

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