Footing Excavation Testing 2

[psock]

I have a project where a small concrete strip footer is to be placed. The subgrade soil is a red clay and will be undercut approximately 2.5' to accomodate the footer. I was considering using a DCP to test the bottom of the excavtion, but I am not sure how to relate the DCP to psf.
Any ideas?

 

[jdonville]

psock,

Do you have anything against using a pocket penetrometer?

Jeff

 

[LKS93]

psock:

ASTM STP 99 (I think) is the standard for the DCP and gives you the two graphs and data you need to make the correlation.

I'm pretty certain you can google it and get the graphs.

 

[psock]

Not sure what a pocket penetrometer is, can you enlighten me?

 

[fattdad]

Step 1: Do a hand auger boring that extends at least 5 ft below the intended bearing grade.

Step 2: Determine the shear strength at depths below the intended bearing grade.

Step 3: Evaluate the design bearing pressure based on these data.

Step 4: Make sure during construction that the soil at the bearing grade is consistent with your earlier field program and the subgrade is free from disturbance.

I am against just going to the field during construction, looking at the bearing grade, doing some "penetrometer" test and signing off on bearing pressure. How do you know that there isn't a soft layer of peat 12 inches below the foundation excavation?

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

fattdad has good points - especially, as it appears that this is "new" to you - you don't have experience with the soils in the area. What is the footer to be used for? Is it a house foundation? or to hold up a deck attached to a house?? or . . . Was there a geotechnical report written for the site? If so, what does it show about the materials below the foundation level? If there was a geotechnical report written for the site, you really only need to confirm that the soil at the founding level is of the same ilk as that noted in the foundation report. Even in this case, if there is known locally to be "pockets" of poor soil, fattdad's point should be followed. In other words, you need to know your project, your geotechnical setting, etc. before you can rely on a pocket penetrometer or DCP or "kicking the dirt" as proof of the soils capability of providing suitable purchase.

 

[psock]

In reponse to BigH, its a pad for a garbage dumpster and I have been brought into this project because the last geotech was kicked off, so I am a little behind the curve for this site. Not sure if geo tech reports are going to be made available to me.
Thanks.

 

[fattdad]

Not sure if geo tech reports are going to be made available to me.

Earlier geotechnical engineering studies should have been provided to the owner/client (or whomever is paying the invoice) and as such, if the same money-man (money-person?) is hiring you, you should be able to get the reports. That is unless, somebody doesn't want you to have the same information that led to earlier unfavorable site recommendations. If earlier geotechnical information is not provided to you, I'd either redo what's needed for proper engineering or walk away from the client.

Not wanting to sound too suspicious, that is. . . .

f-d


¡papá gordo ain’t no madre flaca!

 

[oldestguy]

Fatdad is right.

This sounds fishy.

I'd bet there is crappy soil there and someone does not want to pay to fix it up. Hopefully you will be suckered into OKing a crummy site????

Go very slow, especially if you ae not really experienced with these checks.

 

[jeffcivil2]

Here in Atlanta in the Piedmont Region, we have similar cohesive soils. In between jobs, I once worked as a technician and we used blow count per edge of business card correlating to 500 psf per blow count, so on a residential with 2500 psf, you would need at least 5 blows per edge (2") of business card. You need to do this at 1' depth intervals down to a minimum of 5 feet depth.

Even if you get 6 blow counts up to 4 feet and you get 3 on the last one, what a technician will typically do is make them excavate the footing down more and then retest. They won't like it because they will use more concrete, but tell them tough luck.

-Jeff

http://www.civiltips.com

 

[jeffcivil2]

sorry my mistake... we first used the edge of the business card for the first mark and for the second mark on the penetrometer we used the long end and used 500 psf per count for the long end. We did this because the first few inches (side of card) is loose and gives innacurate results.

After the penetrometer has reached the first mark you start the blow count until the second mark has been reached.

Even if the contractor is stupid enough to do a pour the day after a rain (which they do), you may have to make them excavate until they reach dry soil and get the desired blow count.

 

[geocrete]

Dynamic cone penetrometer testing ASTM STP-399(EB) is used on piedmont soils (fine grain soils)as a construction control test when boring penetration data is known.

The test is performed by marking off 3 - 1.5" increments and counting the number of drops/increment of the 10lb weight it takes to drive the cone into the soil. the 1st 1.5" is used to seat the cone, and the 2nd and 3rd are averaged.

Typically used with a hand auger to estimate bearing, the 1st count should be started at 6" below footing bottom so there is enough soil to confine the cone, and tested approx. every 1' to a depth of 2 x width of base (2b).

The maximum depth of DCP testing is usually considered to be 10' below subgrade, because of the weight of the extension rods.

Most of the geotechnical /soils testing labs on the piedmont from VA to GA use this test method to verify/estimate soil bearing capacity during construction. ASTM still sells the book.

 

[DarthSoilsGuy]

side note..

avoid using the word "undercut" unless you're talking about unsuitable soils that must be undercut and then backfilled with structural soil. i believe you are talking about "excavation" for a 2.5' deep strip footing which is most likely designed for frost protection.

Ignoring the fact that we're only talking about a dumpster pad:

as far as the DCP blow count,

"DCP testing is performed to CONFIRM the presence of soils suitable for the XXXX design bearing pressure." The design bearing pressure i'm talking about is the allowable bearing pressure that the soils report gives, not what a structural/civil egr tells you they need.

That being said..

my numbers for testing at hand-augered 0', 2', and 4' holes were 5 blows = 500 to 2000 psf, 6 blows = 2500, 7 blows = 3000 psf. i would look more closely at >3000 psf projects so i won't throw out a general field rule here...

Our setup was 1-3/4" per increment, but since business cards were often used in the field... assume 2 inches and the rest of what geocrete said...

"The test is performed by marking off 3 - 2" increments and counting the number of drops/increment of the 10lb weight it takes to drive the cone into the soil. the 1st 2" is used to seat the cone, and the 2nd and 3rd are averaged."

i've worked around and seen reporting from many different testing firms... they all have similar but different ways of looking at this. i'm not about to say any one of them is more correct than the other.

another good practice... is to probe (with a fiberglass probe rod for OSHA reasons, although i sneak in the steel one) the entire surface of the exposed footing as well as DCP testing.