Bearing Capacity
Bearing Capacity
(OP)
I am working on a project with a foundation that MUST have 4,000psf bearing capacity. The existing soil is only at 1,000 psf. What is the formula to help determine the amount that needs to sub excavated to achieve the required bearing capacity. Everything I have deals with a point load. I also am looking at placing a fabric down to help stabalize the ground prior to placing structural fill.





RE: Bearing Capacity
RE: Bearing Capacity
What do the borings show for the material 5 to 20 feet down?
Mike McCann
MMC Engineering
RE: Bearing Capacity
RE: Bearing Capacity
RE: Bearing Capacity
Mike McCann
MMC Engineering
RE: Bearing Capacity
RE: Bearing Capacity
RE: Bearing Capacity
If you require 4000psf and you only have 1000, you have a serious problem. Was this obtained from a geotech engineer? If not, I strongly recommend you consult one. But you were asking about the total excavation to obtain the bearing capacity. In a highly simplied way you can basically remove the amount of overburden pressure equivalent to the 4000psf. Theoretically, the soil will have already experience this overburn, and should be able to take the load with no problem. For example a 4000psf would require excavation of 33ft(provide you are above the water table), if the soil unit weight is 120pcf! But that is a very impractical approach, that is total ignoring the contribution by the depth factor "D" in the bearing capacity equation. You might have to input a depth factor in the bearing capacity equation and determine want unknown depth will give the bearing capacity of 4000 (a little algebra). Additionally, you should consider deep foundation, or soil improvement, vcc, soil mixing etc, as appropriate.
Good luck.
RE: Bearing Capacity
soil profile;
Anticipated wall and column loads;
The source of your 1,000 psf net allowable bearing pressure recommendation;
The type of structure (i.e., masonry, steel, cast-in-place concrete, precast, nuclear reactor, hospital, pumping station); and
Why the requirement for 4,000 psf?
Here's the deal; Bearing design bearing pressure is a moving target. If you have a layered system with alternating layers two feet thick and one is very dense sand and the other soft clay, you can't just dig to the dense sand and then use some big bearing pressure. Heck, the next soft clay layer will feel some of the load and will compress. So, all the layers can be affected, not just the one that you are first in contact with. To properly design for this condition, you'd have to consider the depth of increased soil loads, which is dependent on the wall or column load and the design bearing pressure. It won't take you long to realize that there is a "what came first, the chicken or the egg" problem here.
The other dimension to this whole problem is what is considered "tolerable settlement?" If the soil layers will support 8,000 psf, but the resulting settlement will be 4 inches, who cares? Nobody may want to let their foundation elements (and the column that it's supporting) settle 4 inches! So, you have to temper the "allowable" bearing pressure to something that the client can accept (i.e., 1 in).
There are clearly opportunities to use an engineered subbase or a soil raft to increase the allowable bearing pressure and keep settlements in check. The extent that these opportunities may be helpful for your project would depend on the overall soil profile and the nature of the proposed construction.
It would be incorrect for you to get any advice from this group of folks unless you first provide the requested information. Then again, you could also ask whomever gave you the 1,000 psf, what site improvement measures would be appropriate to increase the bearing pressure to 4,000 psf.
f-d
¡papá gordo ain't no madre flaca!
RE: Bearing Capacity
RE: Bearing Capacity
RE: Bearing Capacity
RE: Bearing Capacity
I'm assuming this is not for a ground supported tank farm or some other such big structure.
Is there some reason, you don't want to give us the soil profile (i.e., soil classification and N-values with depth)? Can you provide some sense of the column and wall loads?
f-d
¡papá gordo ain't no madre flaca!