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SoonerSoilGuy (Geotechnical) (OP)
20 Aug 09 13:48
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 (Geotechnical)
20 Aug 09 21:01
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?
Helpful Member!  Ron (Structural)
20 Aug 09 21:54
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 (Civil/Environmental)
23 Aug 09 23:20
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 (Geotechnical)
26 Aug 09 18:26
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.  
Helpful Member!(2)  fattdad (Geotechnical)
27 Aug 09 9:31
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.


¡papá gordo ain't no madre flaca!

Riggly (Geotechnical)
13 Sep 09 4:06
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 (Geotechnical)
13 Sep 09 22:41
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 (Structural)
13 Sep 09 23:44
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 (Geotechnical)
14 Sep 09 2:02
blush hokie - I've been in Indonesia too long where they, for some untold reason, use cm.  My 5 to 10 are cm, not mm! blush again.  I KNOW BETTER!
hokie66 (Structural)
14 Sep 09 6:28
I'm sure you do.  One of the reason I hate centimetres.
DRC1 (Civil/Environmental)
23 Sep 09 15:01
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 (Geotechnical)
23 Sep 09 15:06


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.


¡papá gordo ain't no madre flaca!

DRC1 (Civil/Environmental)
23 Sep 09 15:37
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 (Geotechnical)
23 Sep 09 16:20
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.


¡papá gordo ain't no madre flaca!

DRC1 (Civil/Environmental)
23 Sep 09 16:34
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 (Structural)
30 Sep 09 14:31
"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 (Civil/Environmental)
1 Oct 09 15:34
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 (Geotechnical)
1 Oct 09 22:46
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 (Geotechnical)
2 Oct 09 12:58
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. . .


¡papá gordo ain't no madre flaca!

dBasement (Geotechnical)
16 Dec 09 10:55
One concern I've had with this procedure is the potential for fines migration.  In a wet area with sandy soils, fines can wash from adjacent materials into the coarse rock fill resulting in voids and increased settlement potential, generally within the backfill, but also potentially from the bearing soils.  Undermining is a possibility.
cntw1953 (Civil/Environmental)
16 Dec 09 11:42
I share dBasement's concerns. I once saw rock filled muddy road (mainly clay) sinking into wave form, the ground besides the low points obviously have been pushed higher.
Just an observation over a construction site.
cvg (Civil/Environmental)
16 Dec 09 11:59
if fines migration is a concern, a dense graded aggregate base material (road base) should be used. In fact, open graded material should be used for drainage purposes only, not for stabilizing a foundation.
Patgeotech (Geotechnical)
17 Dec 09 1:34
Interesting practical question.  I would like to add something regarding the use of a stone layer and long term settlement. On flat terrain, and depending on the soil type underneath the stone layer, we have investigated many cracked houses where sand/stone layers were used - the groundwater migrates to these sand/stone layers and if drainage is restricted, then the water tends to pond (stone layer acts as a drain) and cause softening of the underlying soils. So while the settlement in the short term would probably be alright/minimal with the load being distributed within the stone, the long term settlement as a result of softening could be a problem. We tend to go for the concrete blinding layer rather than sand/stone layer for this reason.  
fattdad (Geotechnical)
17 Dec 09 8:31
The presence of water does not soften soil.  Heck, every embankment dam would fail when it went into service if this was the case!  Additionally, the trixial test would not be a very good standard if this was true (all that water and such).  

There are dynamics where water can have an affect, but there is no universal truth to the "water softens soil" statement.



¡papá gordo ain't no madre flaca!

structuresguy (Structural)
17 Dec 09 13:59
Let me add a perspective from a structural engineer to your discussion.  I am in Florida, where we routinely have near surface ground water.  Water is encountered almost all the time when digging holes for foundations.  Many times the contractor can't get the required compaction of the native sands, since they are too saturated.  The response from the geotech is inevitably over excavated a foot and fill with crushed stone.  Now, being someone who is used to dealing with hard materials (concrete, steel, etc), I really can't see how adding a layer of stone over a sponge will do anything to strenghthen that sponge.  So I just don't see how this works.  If they can't get compaction at the original surface, then why assume that if they dig a bit deeper (but not any wider) that they will be good for compaction.  

I have questioned this but never gotten a really solid reason for doing it.  Of course, my level of responsibility starts at the top of soil, and goes up.  so if the building settles due to poor compaction, and the geotech engineer let it happen, then it is now my problem.  I can show that my foundation is fine, and that the soils were the failure.  So can someone here explain how this works?

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