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.

 

[dBasement]

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]

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]

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]

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]

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.

f-d

f-d

¡papá gordo ain’t no madre flaca!

 

[structuresguy]

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?