# Compressive strength of gravel itself



## ront02769 (Nov 28, 2008)

Depends on the composition of the gravel including size of the stone and type of soil making up the mix. Ron


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## jomama45 (Nov 13, 2008)

Most minimum allowable soil bearing values come in at 1500 to 2500 PSF, or 10.5 to 17.5 PSI. I would expect gravel to be slightly higher, to much higher than that, depending on a number of variables, including compaction, moisture content, thickness, etc... That said, it's still going to be a fraction the strength of concrete.......


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## Daniel Holzman (Mar 10, 2009)

This is a surprisingly difficult question. The individual particles making up "gravel" are usually some type of stone, often crushed limestone. The word "gravel" in the geotechnical world refers to individual particles that are larger than about 1/4 inch in diameter. However, the term "gravel" is often used loosely to refer to soil mixtures that include a percentage of large particles.

In any case, the compressive strength of an individual gravel particle, if it is stone, is very high, often 20,000 pounds per square foot or higher. You are probably not talking about the compressive strength of a single particle, rather you are probably interested in the compressive strength of a layer of gravel. In the laboratory, we usually measure the shear strength of a material, or in the case of gravel we often measure the friction angle of the gravel, which can be related to shear strength. For soils, including gravel layers, compressive strength is difficult to measure, and even difficult to define.

Typically when you load soil, gravel included, the soil compresses under load, and if it compresses too much, or compresses unevenly, you run into potentially serious problems like cracking of a slab, or differential settlement of a foundation. The compression of the soil occurs because the individual particles shift around, and the soil becomes denser. In normal practice, you will not see an individual particle fail, the particles just shift around, the soil becomes denser, and compacts as a result. Typically you can place widely variable loads on gravel, and all that happens is that as the load increases, the soil compacts (compresses). So it is not very meaningful to ask what the compressive strength is for gravel, as there is not one value at which the soil fails.


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## iLikeDirt (Apr 27, 2014)

Thanks Daniel, that's exactly what I was after.

My next question is the same, but for scoria (lava rock). I am hoping that the answer will similarly be something like "so high that something else will fail before it does."

Before anyone asks, I'm wondering about the feasibility of creating a rubble trench foundation, except that the "rubble" would be scoria instead of gravel, which is a far better insulator. The idea is to preserve the desirable features of superior drainage and lower cost compared to a conventional slab with concrete footers, but additionally add some insulating power. Scoria is only about R-2, which isn't great, but 24" of it ain't bad!


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## stadry (Jun 20, 2009)

i believe lava rock has a lower compressive strength than most typical crushed stone,,, ambient earth temp is approx 68f +/- year round,,, is the addl cost of scoria worth it in nm ?

just askin'


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## iLikeDirt (Apr 27, 2014)

Actually, the ambient earth temperature varies wildly based on soil depth and season:










See: http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm


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## landfillwizard (Feb 21, 2014)

I would contact a local Geotechnical Engineer and ask him if he can get a sieve analysis and a modified proctor for the material you want to use. When you have the scoria placed, a technician can use a density meter to measure how much the material is compacted as compared to a theoretical maximum density. 

This would eliminate any differential settlement and increase the shear strength of the material used for a base.


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