DIYing DCP or simple loading of a probe for basic PSI

[Boots14127]

Hi All,

Non Geo-engineer here, hoping someone can help me with a DIY-able approach to soil testing. The places I've checked want $600 to do testing, which is an appreciable percentage of my deck pier budget. I understand that is for significant projects like a house, however I'm just trying to get an idea of whether my soil is 1500/2000/2500 PSF.

From researching, it appears CPT is more accurate than SPT in my clay soil.
Would any of the following yield any useful data?

I would auger to 5' depth/frost line, clear dirt to undisturbed soil from several locations. Heck, I could do an initial 10" auger to all 18 pier locations.


Option 1: DIY a piece of 1" black iron with a 1" 60-degree tip.
Weld a bracket on it such that I can add barbell weights to it.
Record how much weight it requires to sink 12".
Multiply weight x144 to find PSI which my pier can not support.
I guess this method doesn't help much except finding my overload limit?

Option 2: Do the above, but instead of a point, place an 6" x 6" plate on the
end and load until it starts to sink. Take that weight x 4, and that is
slightly over my PSF rating

Option 3: Get some steel tube or rod and make something like

https://www.researchgate.net/profil...6@1523368580682/Dynamic-Cone-Penetrometer.png

Option 4: Get a 48" T-probe, and.. ?


I've read about people taking a piece of rebar and dropping a sledge hammer from x inches as a rough field test-like DCP. Only every account is different, and I don't know if rebar will flex and seriously distort data.

I could of course just upsize my piers, however since its still not quite Spring, I have the time to throw a few bucks to get some actual data, even if it would not pass muster on actual projects.

Appreciate any insights.

 

[EireChch]

If this is for a deck for your house why do you need to need to know what the PSF is. Just extend the piers for a 2ft below frost line?

Regardless of the above, the issue with your proposals is that you are only attempting to test approximately 6 inches to a foot below your hole. Also you are kind of doing a load test, which is again too small to give an accurate reflection on the performance of a pier. A pier will get some capacity from end bearing and side friction. You are only "testing" end bearing.

If I were you i would attempt to recreate a dynamic penetrometer, so your rod and 60 deg cone with a 5 or 7kg weight from a specified drop height. Correlate this to an SPT and use that. Its all a bit crude and probably no where near accurate but its a bit of fun eh.

 

[Boots14127]

Thanks for replying Eriechch,

"If this is for a deck for your house why do you need to need to know what the PSF is. Just extend the piers for a 2ft below frost line?"

So, I should have mentioned my deck is 46x16 (736sqf) x2' high.
At just 60psf (Dead & Snow) I'm looking at 45,000# divided by 15 piers, with largest tributary @4000#+.

I know thats not much in general construction, however depending upon the soil type I might be able to suffice with a 16" pier, or need a 20-22" one depending upon the tables you look at. I am going with long 10' beam spans also, which is where the general rule of thumb of 16" pier every 6' is not working for me.


I liked the DCP idea, however with material from HD, anything I'd make would be so off dimensionally alone as to probably make any correlation with known tables useless.

Which leaves me with the load testing idea.

I'm not fully following why it would not work.

A 6x6 plate (1/4 sqf) loaded to 375# should be equal to 1sqf @ 1500#, yes?
If I can raise that load to 500#, without sinking, then I can assume 2000psf bearing soil. Of course, that could be an outlier, but if I do this across 25% of my pier locations, it would seem to up the confidence level, right?

If I add still more weight, say 550#/2200psf and it starts to sink/subside, then I can reasonably assume my upper limit.

My line of thinking is find the load limit before subsiding
Knock 20% off that figure
figure out the sqf of the footing at revised psf, say 16"
Upsize foot diamter from say 16" to 18"


I realize this is sketchy science.
The more I think of it, I'm wondering if I should just go with 24" piers all around, as at 3sqf they should support 1500x3=4500#/ea.
Maybe 28" Bigfoots for the middle beam as it has the highest tributorys.

Appreciate your time!

 

[dirtsqueezer]

I would say if your deck is big enough to require 18 piers, it is worth the $600. The firm I used to work for that I would perform DCP or a t-probe would have been at least double for that consultation. If you're thinking safety, longevity, and ultimately peace of mind for the entire life of your structure, that is quite a small price to pay.

 

[Boots14127]

Thanks dirtsqueezer, I've been thinking about that.
I have so many piers because I've opted for a floating deck as the previous one was not installed properly and required several K$ of repairs.
With a ledger install, I'd be able to get that down to 12.
Talked with the owner of the local tool rental, and I think the smart thing to do would be to rent their small backhoe and just dig my holes to 24" and probably be able to remove any existing concrete from previous deck, instead of augering first and then having to go back to rent another machine. I think its actually cheaper this way. Once I dig down to depth, I can scrape away to somewhat undisturbed ground and use one of those $30 T-style soil probes. If it looks iffy, or if the inspector thinks it looks iffy, I can drop the money on a soil test.
Either way, I'll just upsize the footer a fair bit at the $100-200 cost of extra concrete.

Appreciate everyone's help!