Crushed Stone Under a Spread Footing 4

[SoonerSoilGuy]

Hey all, we all have seen recommendations like remove a foot or two underneath the proposed footing level and replace it with crushed stone bridge lift. Can someone explain me as to why we do that when we run the settlement analysis with the original soil profile. Does anyone think that the stone is heavier than the native soil and cause the settlement of the soil underneath.
May be I am thinking wild. but let me know your thoughts.

 

[oldestguy]

I don't know about you, but I put in the compression characteristics of that undercut fill in the settlement calculation.

What would you do if you did a 4 foot undercut?

 

[Ron]

SSG...that's usually done to provide a stable placement platform for placing the concrete. It is sometimes necessary in high groundwater conditions or when the soil is slow to drain and the footing bottom gets mushy. Overexcavating a foot of loose or muddy soil and backfilling with gravel is much better than placing concrete on such soil.

Do the numbers. The amount of settlement you're going to get from a foot of gravel as compared to a foot of muddy soil will be less. Further, any settlement you get will happen immediately upon load application. Can long term settlement result from this? Yes. If you have a fluctuating water table that impinges the bottom of the footing, the fine sand and fines below the gravel can work their way into the gravel voids; however, that should not be significant.

 

[DRC1]

I have seen it done for the reasons Ron mentions. I have also seen it done in large footing in the dry because there is a lot of traffic anticipated by the form installers, the rebar insallers and during concrete placement. Without the stone the top layer of sugrade can get quite torn up.

 

[darthsoilsguy2]

i stay away from bridge lifts in foundations bearing areas. i think the OP may be talking about repairing a deep uncontrolled fill by minor soil replacement.

on the other side, undercutting to something hard however and backfilling with something hard is almost always acceptable* *=with conditions.

 

[fattdad]

If you take a soil with an elastic soil modulus of 75 tsf and replace it with an aggregate with a soil modulus of 300 tsf, you will reduce the anticipated settlement from that replaced soil layer by 75 percent. Considering that the stresses below a footing attenuate fairly rapidly, this can have a profound effect in controlling/limiting anticipated settlements.

f-d

¡papá gordo ain’t no madre flaca!

 

[Riggly]

Soonersoilguy,
Your point seems very academical (which is good), but on a practical perspective, I believe the effect is insignificant for the depth of material you mentioned. First of all you are replacing the soil with stones (poor graded, high void ratio material). That materials are not necessarily excessively heavier than natural soil. In the case of well graded aggregate, your might be able to get densities in the 140s pcf range. But for just say #57 stones, it is very unlikely to be that high. Since the soil has already been overburden with the natural soil, the increase weight that will cause settlement(due to the stones only) is how much the stone is heavier than the natural soil. Therefore, we are talking very small loads. Additionally, as mention in previous replies, the elastic modulus of the stone is likely higher than the natural soil, which further reduce elastic settlement.

As Ron mentioned above, I would always recommend placing a stone layer. If you have been at a construction site when it is raining you will appreciate that even more.

 

[BigH]

Riggly hit the point on the fallacy of only a 1 to 2 ft replacement to "reduce" settlement. I agree with others that a well compacted crushed stone base gives you 1) a good platform and leveling platform to place your steel, 2) protection against rain and 3) protection against construction disturbance as foundations are formed and poured. However, a mud mat (blinding concrete) will also do the same thing - about 5 to 10 mm of 5 to 10 MPa concrete.

 

[hokie66]

Agree with BigH, except I normally like to see the mud mat of lean concrete at least 50mm thick. That way it takes up some of the irregularities in the excavation. I wouldn't consider either crushed stone or blinding concrete to improve the settlement situation.

 

[BigH]

hokie - I've been in Indonesia too long where they, for some untold reason, use cm. My 5 to 10 are cm, not mm! again. I KNOW BETTER!

 

[hokie66]

I'm sure you do. One of the reason I hate centimetres.

 

[DRC1]

I would say it does and it doesn't reduce settlement. It does not reduce settlement in terms of conventional soil analysis. Since the stone layer is so much stiffer than the soil, it in effect simply moves the bottom of the footing to the bottom of the stone. However, during the course of construction, the top of the subgrade can get torn up and become loose, especially if the site is muddy. The reconsoldation of this loose layer under the concrete can produce settlement. Using the stone prevents that problem.

 

[fattdad]

Since the stone layer is so much stiffer than the soil, it in effect simply moves the bottom of the footing to the bottom of the stone.

I don't agree with this point. The stone acts as an elastic media and there will be load spread within this layer. Schmertman's widely-recognized method for calculating settlement below a footing has been well tested and to me recognized as appropriate.

f-d

¡papá gordo ain’t no madre flaca!

 

[DRC1]

I dont disagree with the concept, but for spead footings typically the footing is undercut to approximate the same size as the footing itself and only undercut a foot or so, so the amount of dispation is not worth the effort. Make the undercut 3 foot thick say and a good bit wider than the footing, and yes I agree the dispation of load through the stone will significantly lower contact pressure on the natural material.

 

[fattdad]

The dissipation of load is independent of soil type. Boussinesq equations' only assumptions are an elastic material and a Poisson's ratio of 0.5.

Not trying to be argumentative, that is. . .

Interesting discussion.

f-d

¡papá gordo ain’t no madre flaca!

 

[DRC1]

No, I don't find it aurgumentative, just makes you consider problems mor carefully and look at them from different angles. Got to think about that and I am all thought out for today. I'll check back in later. Have an enjoyable evening.

 

[seny]

"If you take a soil with an elastic soil modulus of 75 tsf and replace it with an aggregate with a soil modulus of 300 tsf, you will reduce the anticipated settlement from that replaced soil layer by 75 percent. Considering that the stresses below a footing attenuate fairly rapidly, this can have a profound effect in controlling/limiting anticipated settlements" is good point, just necessary to understand from what thickness of crashed stone or mud slab it can be taken into consideration. where i am practicing it is never more than ~3" for mud slab (i prefer) or ~8" of gravel. code allows to increase soil bearing capacity w/ depth of excavation but not so rapidly. sometimes i use pre-excavation with following compacted back-fill by crashed stone to raise bottom elevation of foundation, but it's different story.

 

[DRC1]

Fattdad,
Sorry for not getting back I have a couple night projects running as well as daytime work, so I am a little short on sleep. To answer your response, it still seems that the stone will behave differently than the underlying soil. With out looking it up (often a fatal flaw), I would assume that Boussinessq requires homogeneous material. It would be interesting if someone could set up a FEM model and run it to see what distribution you get from that.

 

[BigH]

Yes - Boussinessq does require "same" material - but you can model differing materials to give the equivalent "same". Then you can use the Bousinessq - or you can look it up in Poluos and Davis' Elastic Solutions to Soil and Rock Mechanics. This is now downloadable (I had originally scanned my copy page by page!)

 

[fattdad]

I'm standing corrected: Boussinesq does also assume the same material. Westergaard is for layered systems and will give you greater stress attenuation. So, I use Boussinesq to be conservative (or so I think).

Further insight welcome - just reporting what I do. . .

f-d


¡papá gordo ain’t no madre flaca!