Gravel or Sand Below a Foundation 6

[TBellP47]

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.

 

[jdonville]

TBell,

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

 

[csd72]

Depends on the foundation material:

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

 

[cvg]

also can help to reduce the chance of frost heave in colder climates

 

[SlideRuleEra]

During placement of concrete foundations, it gives a route for any excess water in the concrete to escape without causing problems.

In area with potentially high water table it provides stable support with water table fluctuations.

http://www.SlideRuleEra.net

 

[JAE]

Yes....BUT!!!!!

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.

 

[BigH]

I can tell you that I wouldn't want to put sand fill (especially river sand) directly under the footing. If you are wanting to provide a working platform to prevent disturbance due to steel placing, leveling course, to minimize precipitation disturbance, I would use a layer of crushed stone or a blinding mat (mud-mat) or a well graded sand and gravel (like a subbase or base course). On a job I had in India, they were "instructed" to use 6 inches of sand beneath a mud mat. Can you imagine how disturbed the sand got when the workers were walking around and dropping concrete from a chute? It was ridiculous but no one could "change the design" which, obviously, came out of some old "handbook". It would have been better to place the mud mad on the clay directly. I bring this up in that the same would happen if you put in sand below a footing - unless it was well graded with some gravel and perhaps a little binder material.

 

[oldestguy]

Of the numerous cases where I have seen this done, there generally is no problem. While uniform sand has weak shear strength, it does not compress noticeably.

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.

 

[Mccoy]

Another case for placing granular material under foundations, is when 2 different soils outcrop below the foundation structure. The horizontal oscillations occur then in the presence of homogeneous material and friction.

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.

 

[TDAA]

Not a good idea on expansive soils, unless you have provided a drain to remove excess water.

 

[brokenpile]

We just constructed a sheet pile wall in front of a exsiting concrete seawall on piles.There is six foot open space between under side of sea wall and grade.Engineer had us fill in with sand the lower three feet which then three feet of concrete will be poured to underside of seawall to prevent it from falling into the bay.Can the sand be washed through the steel sheet piling on the joints of the sheets overtime?After reading this topic it sounds like stone should of been used instaed of sand.

 

[Mccoy]

Not a good idea on expansive soils, unless you have provided a drain to remove excess water

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.