Gravel or Sand Below a Foundation
Gravel or Sand Below a Foundation
(OP)
Can someone tell me the true significance of placing "x inches" of gravel or sand below a foundation? and should it always be used?
Also, is there a reference made to this procedure in ACI? If so, what is the ref. number? (Trying to find additional literature on this subject; i.e.: engineering manuals, textbooks, etc.)
Thank you in advance.
Tom P.
Also, is there a reference made to this procedure in ACI? If so, what is the ref. number? (Trying to find additional literature on this subject; i.e.: engineering manuals, textbooks, etc.)
Thank you in advance.
Tom P.





RE: Gravel or Sand Below a Foundation
The granular soil acts as a "bridge" over any local relatively weak spots in a foundation soil, providing a more uniform reaction force on the underside of the foundation or slab. The granular layer provides structural support also by reducing the applied pressure at the bottom of the granular layer due to applied loads.
The 2:1 method is a well-known rule of thumb for estimating the stress decrease with depth of granular layer below a rectangular foundation, although the method can be cautiously used with strip loads as well.
You may find articles/texts discussing "modulus of subgrade reaction" and "vertical sress increase in a soil mass due to foundation load" helpful to your understanding of the action of the granular layer.
Jeff
RE: Gravel or Sand Below a Foundation
In clay or silt - helps prevent reinforcement support chairs from sinking into the soft wet surface clay. Also stops wet clay/silt mixing with the concrete.
For rock foundations - provides a level surface and minimises the chance of restraint from interlock between the concrete footing and the rock.
There may be other reasons as well.
csd
RE: Gravel or Sand Below a Foundation
RE: Gravel or Sand Below a Foundation
In area with potentially high water table it provides stable support with water table fluctuations.
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RE: Gravel or Sand Below a Foundation
In areas where you have loess soils or other soils that are collapse susceptible - the LAST thing you want to do is put any sort of granular material under or nearby your footings. This is because you create an opportunity for the fines to drain/erode out from under the footing and into the small voids in the granular matrix....thus causing settlement.
Be careful with this.
RE: Gravel or Sand Below a Foundation
RE: Gravel or Sand Below a Foundation
However, one case with saturated sand that replaced a silt undercut, followed by a saturation of that sand in a "bath tub", the sand flowed out from under due to the weight of the footing, column and roof steel only, but also due to running of plate compactors nearby, helping the sand to flow. With no nearby vibration, it did not flow.
The cure was interesting, in this industrial building with the footing measuring 8 x 8 ft. Roof was not on, but steel was. A crane picked up the column and roof steel a little, leaving the footing free. A dozer shoved the footing off to one side.
Saturated sand was dug out and replaced with damp sand, compacted.
Footing was shoved back in, at proper grade, wiggled back and forth to "seat it", and column re set. All worked out fine.
You don't always have to discard compelted work.
Oh yes, on the same site, dry sand did flow out from under the foundation of a large air compressor. However, that compressor did so much shaking that nearby precision testing of machines also was affected by the shake. This was a most unusual situation, not common.
RE: Gravel or Sand Below a Foundation
That was a solution I suggested, after consultation in this forum a few years ago, when rock and OC clay outcropped just below the design structure, 50% each, tracing a perfect diagonal
In that case it's also advisable to replace w/ the same granular material a good slice of the weaker material.
Using a layer of crushed stone, never sand, below foundations is a very common practice over here.
Apparently it works out good.
RE: Gravel or Sand Below a Foundation
RE: Gravel or Sand Below a Foundation
RE: Gravel or Sand Below a Foundation
TDAA, good point
As a matter of fact that kind of stone layer is often used above expansive soils in my area. Usually grade is excavated with a slight inclination and outlet drains are installed. I do not enter in such details though, usually only pointing at the dangers of possible water stagnation at the stone-soil interface + general suggestions.