Bearing Capacity

[LIGWY]

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.

 

[Ron]

Bearing capacity is only one parameter that you have to consider. If your fdn bearing pressure is 4ksf and you only have 1ksf, you'll have to overexcavate a lot of soil (theoretically to a depth of at least twice the foundation width) and then how do you know you won't run into a settlement issue? What does your Geotech consultant say about it? What is your boring profile? Was there a geotechnical investigation done for this? If not, get one.

 

[msquared48]

With this difference in bearing, augercast piles should be considered, particularly considering any long term settlement.

What do the borings show for the material 5 to 20 feet down?

Mike McCann
MMC Engineering

 

[BigH]

What type of structure do you have? What type of foundation are you envisaging. As msquared48 asked, what is the nature of the soil you are encountered, the relative density (very loose, loose) or consistency (soft, firm)? Once this has been established, one can maybe provide some assistance on what toys you can play with.

 

[pozze]

i think you should considering pile foundation for that problem.because if u choose not to use piles than you have to go very deep on ground and you have to use gigant dimensions of foundations.

 

[msquared48]

Welcome to the forums pozze.

Mike McCann
MMC Engineering

 

[oldestguy]

It would help if you say what part of what country. Who determined the 1,000psf?

 

[cvg]

there are many ways to deal with this, all of them require you to work directly with a geotechnical engineer familiar with your site.

 

[Riggly]

Ligwy,
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.

 

[fattdad]

Dear OP: We really need to know more information to properly help you. Can you provide the following:

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!

 

[LIGWY]

I was explaining to the client the very same points that all of you have brought up that piles should be used for his project and why. I was getting a HUGE number of over excavating and explaining this to him. Yes I do have a geotechincal engineer involved and he acturally determined that the bearing was 1500 psf (not much better). I was explaining to the client that as per the bore holes and the geotechnical report that piles are necessary to reach to stable soil and would be more cost effective. I explained to him that bearing was not his only issue. Settlement and other factors are very important to address as well. I was just making sure I was on the right track and telling my client the facts. Thank you for all your input.

 

[BigH]

As I first indicated - and as others have pointed out - what is your soil stratigraphy? The soil strata engineering properties? There are other means to improve soil bearing than simply piles (while, yes, they may be the most appropriate here). You have a geotech - what is he recommending - 1500 psf and leaving it at that? I'd get another geotech who would work with you to come up with the most suitable foundation support system. Seems strange that he would not be offering you recommendations/advice on use of piles, etc. Weird.

 

[LIGWY]

The geotech has recommended piles the client felt it was extreme and I was explaining this to him. The geotech did his job I am dealing with a client that has not worked in this area.

 

[fattdad]

something doesn't make sense. You have a structure that has some total weight. It's sits on foundations. The foundations have to be sized for some bearing pressure. The client wants 4,000 psf. The geotech says, nope, only 1,500 psf. The soils down in the ground (let's say 10 ft deep) will likely feel the same stresses either way (or close). The distinction between 1,500 psf and 4,000 psf is likely a distinction that affects the soil stresses in the upper interval of soil. So, how do piles factor into this? If the soils at a depth of 10 ft are relativelly firm, then you'd likely have an option for a soil raft or an engineered subbase.

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!