Finding soil bearing capacity?

[IHDiesel73L]

Can anyone tell me how I can find the bearing capacity of my soil short of having an engineer actually test it? I need to know how many piers/what size I will need to support my new building. I pulled an NRCS soil report on my area and the soil taken from test holes I dug at the building site matches the description:

AnoC2 - Annandale Gravelly Loam
Depth to restrictive features: 18-30" (I have dug to 48" without hitting large rocks)
Seasonal high water table: >60" (it's about at the surface right now, hence concrete piers rather than posts in the ground)
Potential frost action: Moderate
Drainage class: Well

What I can't seem to find out is what this soil will support. 2000lbs PSF? 3000? etc... NRCS is checking for me but their engineer is out today. I thought about trying to do my own test by testing how much weight PSI it will support by rigging up a way to place a controlled amount of weight on a 1" x 1" stake. Soil that can support 3000lbs PSF should also be able to support about 21lbs (3000/144) PSI right?

 

[tcartwri]

Google "Cone Penetration Test"

Or call your local building inspector.

 

[BeezFun]

If you use 2000psf you'll be fine. Really crappy soil can be as bad as 1000, the best you'll get out of rocky/gravelly drained soil is about 6000psf.

Your test with a 1" stake won't work because it's too small. You might land on a rock or you might end up where some dinosaur took a crap. Also what matters is the soil at the depth of the bottom of the hole. So if you have a 40" frost line, you have to do the sample at 40" and below.

 

[IHDiesel73L]

If you use 2000psf you'll be fine. Really crappy soil can be as bad as 1000, the best you'll get out of rocky/gravelly drained soil is about 6000psf.

After doing some reading on the topic I think 2000 might be a good estimate given the conditions. There is gravel present, but there is also a fair amount of clay and silt mixed in, and it's fairly wet due to the high water table. I plan on using Bigfoot footings as the base of 8" piers. 10 piers with 24" bases will give me 31 SF of footprint/62,000lbs of support in 2000lb PSF soil. I suppose it could be better than that, in which case I have a pretty good safety factor :thumbsup:

 

[Tractor Seabee]

Your loam is a compressible material, not suitable for supporting a building or even a driveway slab. You have to strip all that off first. There is no substitute for a density test except a SWAG (S is for selective not scientific) which may or may not be accurate. Reputable builders will do the test and get the engineered solution.

Ron

 

[CurlyDave]

Can anyone tell me how I can find the bearing capacity of my soil short of having an engineer actually test it? I need to know how many piers/what size I will need to support my new building...

Typically, piers do not support a load by bearing on the bottom end of the pier. The support is from friction on the sides of the pier.

You need to have a soils engineer look at the soil and an architect or a mechanical engineer design the building and the piers.

You may think this is expensive and unnecessary, but it will be money well spent. They will save you the expense and problems of an unstable building, and the architect will be able to design something you can build yourself and save that way, if you tell him this ahead of time.

The test you propose ignores the friction on the sides of the stake, which may well dominate its load bearing capacity.

 

[EddieWalker]

The biggest reason foundation issues are so common in my part of the country, East Texas, is because so many builders failed to identify the type of soil they where building on and just went with the same slab foundation design everywhere they built. Knowing what type of soil you have is very important to knowing how the foundation must be designed. There are dozens of types of clay, and all sorts of mixtures of soils out there. Once you identify the soil, you can look up on a chart to see what it's rated for. You can also buy the tool that measures the soil and test it at every corner and along the lines of the footings. If it's poor soil, you usually just go deeper and wider. Sometimes you have to add additional load bearing beams into the ground to tie it all together and help support the weight of the house. An average house weighs something like 300,000 pounds.

Eddie

 

[IHDiesel73L]

Your loam is a compressible material, not suitable for supporting a building or even a driveway slab. You have to strip all that off first.

I got a call back from NRCS who said they referred the question to their soils engineer who can presumably give me a better answer. I will say this though-the subdivision I live in is built on a former 300 acre farm. According to the soil survey, the loam is more than 10' deep. My basement is maybe 8' deep. There are also homes and other outbuildings on slabs and plenty of driveways all in fine shape. I get that it's not bedrock, but it would surprise me to learn that its that poor?

Typically, piers do not support a load by bearing on the bottom end of the pier. The support is from friction on the sides of the pier.

I think you're thinking of pilings? Everything I've read about pouring piers refers to footprint as it relates to the bearing capacity of the soil-bigger footprint (area) means more bearing capacity. Pilings are driven (usually until refusal) and are often tapered, which creates friction against the ground. Piers are just concrete columns that are cast and then backfilled. Even if the backfill is well tamped I can't see it creating as near as much friction as a post hammered into the ground.

The biggest reason foundation issues are so common in my part of the country, East Texas, is because so many builders failed to identify the type of soil they where building on and just went with the same slab foundation design everywhere they built. Knowing what type of soil you have is very important to knowing how the foundation must be designed. There are dozens of types of clay, and all sorts of mixtures of soils out there. Once you identify the soil, you can look up on a chart to see what it's rated for.

I know what type of soil I have, I just haven't been able to find any type of rating. The NRCS report does describe it as "suitable for building," but that's it. I will wait to hear from the soils engineer.

 

[metalsmith]

You don’t say where you are, is it somewhere that doesn’t have standard building codes to use as guidelines? Also check to see what spec’s other foundations near you have been built to. One place to not skimp on are foundations, is darn hard and expensive to come in later and beef up. Pay close attention to if your soil is prone to shifting ( not related to frost line) , sections of Texas for example, which require unique foundations. Bigfoot /bell footings work very well over plain straight poles/ cassions as long as their base ground remains stable throughout the building life. You do figure in sidewall friction on deeper cassions but it really doesn’t factor in on shallow footings, so you may need to go deeper to find better load bearing soil, larger bell footings or more footings.

 

[IHDiesel73L]

You don’t say where you are, is it somewhere that doesn’t have standard building codes to use as guidelines? Also check to see what spec’s other foundations near you have been built to.

I'm in NJ-I don't think we have anything too exotic here in terms of soil. I've heard lots of horror stories about expansive clay soils in the Midwest/West, but I don't think that's case here. Homes are either slab on grade or plain old foundation walls and footings. I've gotten my zoning permit and am in the process of preparing drawings for the local inspector so he'll of course chime in, but the other purpose of preparing the drawings is to also put together a material take off sheet so that I can shop it around at the local lumberyards. The difference in the number/size of piers will affect my bottom line in terms of how much concrete, how many tubes/footing forms, and how much digging I have to do.