Crushed Stone Under a Spread Footing
Crushed Stone Under a Spread Footing
(OP)
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.
May be I am thinking wild. but let me know your thoughts.





RE: Crushed Stone Under a Spread Footing
What would you do if you did a 4 foot undercut?
RE: Crushed Stone Under a Spread Footing
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.
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
on the other side, undercutting to something hard however and backfilling with something hard is almost always acceptable* *=with conditions.
RE: Crushed Stone Under a Spread Footing
f-d
¡papá gordo ain't no madre flaca!
RE: Crushed Stone Under a Spread Footing
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.
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
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!
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
Not trying to be argumentative, that is. . .
Interesting discussion.
f-d
¡papá gordo ain't no madre flaca!
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
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.
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
Further insight welcome - just reporting what I do. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
Just an observation over a construction site.
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
RE: Crushed Stone Under a Spread Footing
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!
RE: Crushed Stone Under a Spread Footing
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?