Existing Subgrade - no fill - Do you check for soil density as if it were fill.

[PT999]

After an area is excavated for footings, do you check for soil for density, or do you assume subgrade is suitable without testing. I have a project where Nuclear Gauage was used with failing tests, no fill was used.

 

[mike20793]

They will often do a field test, like checking compaction, to ensure the subgrade meets the recommendations of the specifications and/or geotechnical report.

 

[dik]

Are your footings to be bearing on undisturbed soil?

Dik

 

[oldestguy]

Interesting subject. A common situation in my experience, especially for natural (not fill) silty clays in the northern states where frost action is common, the natural density of the soil is usually not as high as a 95 percent compaction density might be. However, usually the criteria that is applied to "checking" the support is that the unconfined compression value is such that the footing pressure,so tied to, is not exceeded. It may or not be the Qu value. Density is not usually checked. Many times the split spoon blow count is reviewed.

 

[BigH]

I've never used a "check" on the bearing soil by density test under a footing. The geotechnical report will (or better well had) determine the correct location for the footing support. Basically, the material is verified. I have heard of using miniature cones as a check - or miniature vanes. The biggest problem is that during excavation, the base may be disturbed from the excavator's teeth - and this disturbed material needs to be removed. For clayey soils, the density test is rather dubious in my view; would be more appropriate if you do one for cohesionless.

 

[yaschief]

Use of a Nuclear Gauge to determine the density of undisturbed native soil has no value, even if they took a sample and ran a 5 point Proctor test on it.

 

[cvg]

well it really depends on what you are building. for the dam work I do, all foundations are inspected, ripped, moisture conditioned and compacted and tested with nuke/sand cones and then inspected again before anything is placed to cover them up.

 

[emmgjld]

After an area is excavated for footings, do you check for soil for density, or do you assume subgrade is suitable without testing. I have a project where Nuclear Gauage was used with failing tests, no fill was used.

Given my situation in the semi-arid western Colorado, I am continually using the nuc gauge to confirm in-place conditions. I have been doing this for virtually all open-excavtion observations since about 1992. I wish I had started this when working for my Father back in the mid 1960's.
Given my local geology of Expansive Bedrock materials, potential collapsible soils from debris fan deposits, potential collapsible soils from alkali (sulfate) cementation, very 'movable' soils (dispersive-like characteristics) when subjected to the infiltrating groundwater and enormous soils density differences in former agriculture areas (the low-density plow layer vs. the compacter zone immediately beneath the plow layer).
I have a fairly specific set of criteria for a number of soil conditions. Most of these are to confirm the actual design conditions of the Geotechnical Reports are met. The actual criteria is NOT % compaction, but rather a minimum/maximum unit weight which corresponds with the conditions of the Geotechnical Reports. I have not figured out the logic of some of the local 'others' who are always trying to obtain a standard % compation..

Are your footings to be bearing on undisturbed soil?
The undisturbed soil needs to be properly defined. In some areas, This criteria leads to an enormous number of small problems which can plague local foundations.

Use of a Nuclear Gauge to determine the density of undisturbed native soil has no value, even if they took a sample and ran a 5 point Proctor test on it.
Come to my area. I have a number of attorneys to introduce you to. I also have a soil rock/profile which I would love to hear your superior opinion.

 

[BigH]

Good point emmgjld . . . saprolite comes to mind which, as I have discovered in our residual soils in Malaysia has a very low bulk density due to leaching . . .

 

[PT999]

Soil is theoretically undisturbed, although the excavator may have disturbed it a little at the top.

The consensus is that density testing has no value. But I still have spongy spots, so I have asked that they be removed and replaced with RCA.

 

[cvg]

The consensus is that density testing has no value.

that is definitely not the consensus. I think a more accurate consensus is that many contractors (and also owners and some engineers) don't want to spend money on additional QC testing or field inspection.

 

[Ron]

Agree with cvg....not the consensus.

Density is density. If you need it in the fill, you need in "undisturbed" soil as well. Undisturbed does not mean that the soil is competent. You have to check this with a variety of tests, including in-place density.

In my opinion it is a false assumption that undisturbed soil needs no further evaluation or treatment.

 

[PT999]

The situation is this, As special inspector (not engineer of record), I arranged for nuclear gauge density testing, and some locations failed. The Geotech engineer came out and stated that the testing was not required for non-fill situations, and that the conditions observed (even though density was less than 95% in some locations) are suitable for the new building to be constructed.

So the question remains, is density testing (with passing results) for bottom of footings of non-disturbed soils required under ICC International Building Code (2015) requirements, or not.

 

[Ron]

PT999....that was not your original question.

No. The 2015 version of the IBC does not specifically require density testing in subgrade soils that are not fill; however, it does require that you verify that proper bearing capacity in the subgrade will be achieved under shallow foundations. See Table 1705.6

Keep in mind that prescriptive methods of the code are MINIMUM requirements. The geotechnical engineer can require more; however, in your case, apparently he/she has not done so and does not see the importance of it. I would disagree and reiterate my previous point.

 

[wannabeSE]

Ron,
Why do you disagree with the Geotech? From what I gather so far is that the soil (unknown classification) is less than 95% (of what: standard proctor, modified?) And, the Geotech has visited the excavations and stated that the code does not require density testing and it is suitable for the new foundations.

We don't know the loads on the new foundation, whether the <95% is standard or proctor, nor how far below 95% the density test show. Since the results are given in a percentage, should we assume the soil was tested in a lab to determine maximum?

Since I am not a geotechnical engineer and have incomplete information, I would assume that the geotech has given the proper standard of care for this project.

 

[oldestguy]

OG chiming in here again. I think how you check for suitable bearing capacity depends a lot on two things: typical soil conditions in the area that you work with a lot and the experience of the person doing the testing or checking, When I first heard about the GOW sampling method by Raymond Concrete pile about 1948, the engineers then (somehow geotech didn't become a moniker until later) and any form of penetration tests somewhat related to the GOW test (N value) were used for all sites. Some old timers even resorted to the tip of an umbrella or a simple "Heel" indent to OK the material as OK. As I got more into it a common device gained popularity in the New York state and Wisconsin soils was an off shoot of the umbrella, a 1/2" diameter rod about 3 feet long fitted with a cross handle on top. If it shoved in no more than a few inches with full body weight, the site was OK. It even supplemented density tests by the sand cone method. In the 50's the pocket spectrometers came along and the list goes on. As to adopting the nuclear testing methods, that never was here until about 1954 at Cornell U. where I used the first nuc "probes" for my research project. However crude the "tests" might have been, the experience of the tech was important, since density could be checked, if really needed, but usually not on natural soil. Summing up, this OG can't recall a single failed footing on either compacted fill or natural ground that I checked or supervised checking, running well into the thousands of jobs. Take it or leave it.

 

[Ron]

@wannabeSE.....I disagree with the assumption that "undisturbed" soil is necessarily competent. I have also done many, many footing inspections to determine the competence of the bearing soils, but that assessment goes beyond the assumption of undisturbed soil being competent. If the geotech visited the site and did his own assessment of the bearing soils and determined them to be suitable for the foundation, I'm good with that. My only issue is the competence assumption without further evaluation, whether it is density testing, a probe rod, a hand-held penetrometer or some other method.

@OG....I agree with your assessment. I've done many myself; however, that process is an engineering evaluation based on experience and backed by some level of test evaluation done in the past whether you did plate load tests, penetrometers, density testing, probe rods or jumping up and down in the footing. All are evaluative.

With all due respect to PT999 I think he did the right thing. Perhaps with his level of experience he wanted a greater comfort level with the subgrade. Nothing wrong with that. If the geotech was able to come out to the site and make a further engineering assessment of the conditions to override PT999's need for further information, that's great. PT999 was not the geotech of record and that decision, absent relevant test data, was not his to make and he made the right call.

 

[Terratek]

The answer is, as is so common in geotechnical engineering, "it depends". It's pretty common to have a low percent compaction when running density checks on native subgrade since soil can be less dense in the native condition than the compacted condition. Yet many soils are stronger in an undisturbed native condition, even at that lower density - eg most clays. It's always better to have more information than less. The nuke gauge results shouldn't be the controlling factor, just one of several factors. Proofing the excavation by prodding with a backhoe would be a good way to evaluate suitability. The recommended bearing pressure in the geotechnical report may also give you an idea of what to expect in regards to subgrade reaction to proofing.