Footing Excavation Testing
Footing Excavation Testing
(OP)
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?
Any ideas?





RE: Footing Excavation Testing
Do you have anything against using a pocket penetrometer?
Jeff
RE: Footing Excavation Testing
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.
RE: Footing Excavation Testing
RE: Footing Excavation Testing
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!
RE: Footing Excavation Testing
RE: Footing Excavation Testing
Thanks.
RE: Footing Excavation Testing
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!
RE: Footing Excavation Testing
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.
RE: Footing Excavation Testing
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
www.civiltips.com
RE: Footing Excavation Testing
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.
RE: Footing Excavation Testing
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.
RE: Footing Excavation Testing
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.