Assumed bearing capacity in area of miscellaneous fill

[ClarksonEng]

I'm looking to put in a footing or pier for an equipment support. Loads are very small (about 2k axial, 3 k*ft moment). I've got some old boring logs and the area I'd like to put the footer is in miscellaneous fill. I was hoping to use a 5'x5' (or smaller) footing or 3' pier setting at about 5' below grade. Does anyone have any recommendations on a bearing capacity? The consultant assumed 500 psf, which seams too conservative and leads to a much larger footing than is construct-able due to nearby utilities and risk of undermining nearby foundations during construction. Piles are also out of the question here due to lack of ability to get equipment to this area.

 

[BridgeSmith]

I'm sure a few of the geotech guys here can address this better, but I doubt anyone, on here or sitting in their office, is going to put a number on it without at least a classification of the material at the foundation location(s) (gradation, plasticity index, etc.). Getting a sample analyzed shouldn't be expensive. Getting the samples shouldn't involve much more than a shovel, a bucket, looking up the sampling procedure, and a little muscle. I think the geotech guys here can also likely point you to the sampling procedure.

 

[oldestguy]

Very "iffy" situation. Sure the blow counts may mean something, but the material types are full of questions. 500 usually is for this kind of site. Take the cinders. They can deteriorate forever! I'd build a footing of smaller size then bring in enough weights or use some sort of anchorage piles and overload the footing. Maybe go 1.5 times the expected load. Leave it on a few days if possible. Then your load should be capable of being adjusted in case of settlement.

 

[EireChch]

I wouldn’t accept a shallow foundation. Organic matter, timbers, silt clay.... looking for trouble

2k , Is that kip/sqft? If so that’s 100kpa. Not a “very small” load

You can get mini pilling rigs that are of similar width to your 5ft footing

 

[BridgeSmith]

Since the footing size is TBD, I presumed that the 2k is just 2 kips total (as the 3 k-ft is total).

My question is whether the 3 k-ft is in a specific direction, 2 directions, or any direction?

 

[ClarksonEng]

Right, 2k axial load total. 3k-ft is in either axis non-concurrently, it's a moment from wind loading.

 

[fattdad]

take the loaded area.
Determine the change in stress with depth.
Consider installing an engineered subbase below the footing.

I'm not sure why you have the footing at 5 ft? If it's for frost, well, then you need it that low. Otherwise, place the footing where it needs to go, then evaluate what would happen (foundation support, strength, settlement) if you had 4 ft, 6 ft, etc., of compacted aggregate below the bearing surface.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

A question for f-d. How do you evaluate strength of the "stuff" in the top 10 feet?. Take a log? Or some ashes?

 

[fattdad]

If I know the materials that will be used for newly-formed engineered subbase, I can assign a modulus value - strength too.

Not sure the basis for the question?

f-d

ípapß gordo ainÆt no madre flaca!

 

[PEinc]

If hoping to use a 5' x 5' footing, and ignoring the eccentricity from the 3 ft-kip moment, the average applied bearing pressure would be 2000#/(5' x 5') = 80 psf (a loooooong way from 500 psf).

http://www.PeirceEngineering.com

 

[EireChch]

Its never ideal to found in such crappy fill material. Originally I was totally against it as i thought it was 2kip/sf. My bad with the emperial to metric. The load is very low, as you said but still a long way from your 500psf as PEinc indicates.

What ever you do i think it would be prudent to undertake a handaguer borehole to 3-5m depth with some penetrometer testing to get some feel for whats directly beneath the footing. It would take one technician half a day to drill, log and test so a very small cost for the added information. I would not be placing all your confidence in that old borehole log. It looks to be at least 20 years old and i doubt there is an accurate ground level to a known datum on the log. For all you know the ground could have been filled again with even more crappy material.

I would also ensure that there is a net decrease in load too, you could achieve that with some lightweight fill beneath (or above) the footing. We have done this when encountering peat layers within the ZOI to ensure there is no (or negligible) settlement

 

[GeoPaveTraffic]

With fill of unknown quality, you are always going to be taking a risk. Conditions are likely to change, maybe very significantly, in very short distances.

I suggest you ask yourself the question: What happens if this new footing settles or rotates several inches?

If the answer is: No big deal, we can adjust the equipment and keep on going, then great.

If the answer is: We got big problems, then I would make sure to found the equipment foundation below the fill.

Mike Lambert

 

[bdbd]

Mostly the recommended bearing capacity for these types of fills is 25 kPa. Also, dont trust plate loadings etc. since the main problem is heterogeneity.