Field density testing by using a nuclear density gauge

[Good_Guy]

Hello

A contractor is backfilling with excavated soil against a concrete structure. The backfill matarial has been approved by the designer. The designer requirement is laying and compacting the backfill max in 300mm layers . The height of fill is approx 6 m

The backfill contractor wants to lay 3 layers of each 300 mm and then test it using a nuclear guage ( probably saving time and costs)

Can testing for all 3 layers or more together acceptable?

What is the limitation of testing(depths) of a nuclear guage.

Thank you all

 

[MTNClimber]

Depends on the length of the probe. Longest I've seen used on a typical site is 12-inches (305 mm). If that is the case, then no its not acceptable if you need a density for the 900mm fill layer. You can use the density gauge as a way to check the contractors means and methods. If they use 8 passes of a certain compactor and you get the desired minimum density, then one can just say "make a minimum of 8 passes on each layer". This is how a lot of compaction tests were done before nuclear density gauges were around.

But in the end the test only takes a few minutes depending on the length of the fill area. I don't see it as a cost savings unless the contractor is only putting 3 300mm lifts in a day. Then the tester can come out for an hour each day and test when they are done. Problem is though, if the test fails then would you make them rip out 900mm of fill? I probably would.

You could also make them dig small pits to test each layer and use a trench correction factor.

 

[oldestguy]

Has the equipment been calibrated recently or ever for deep testing?

 

[Good_Guy]

MTNClimber & Oldestguy

Your response is great. The nuclear gauge has a probe limitation of 300mm. The contractor makes a pit as you have stated and collects samples He is taking a risk with 2 layers (600mm) instead of testing each 300mm layer

The calibration certs for the nuclear gauge is upto date but I had no idea that there needs to be for dep testing

Deep testing:

is deep testing acceptable and once again limitations even if the probe is it acceptable fo

 

[MTNClimber]

I don't know the actual number (maybe someone else here does) but the radiation does penetrates a little past the probe depth. I would guess maybe 2" (50mm)? Not enough to get the value of the previous layer.

 

[oldestguy]

I ask my question because of my experience in the 1950's with the first prototypes of such instruments when developed at Cornell U. In particular doing this form of testing at depth, all sorts of questions can come up as related to possibly nearby variations in soil density and material type. The form of and type of material used in the"standards" likely are not the same as the on-site material found on site. Unless the standard test material is similar to the on-site material, one can question the accuracy of the nuc test. Going down there and doing the old standby physical density test can one be sure of the results. As to radiation penetration there is no nice limit. You can liken this test to sound levels. Sure near the source there is highest concentration, but it drops off with distance from the source. Much depends on the "stuff" through which it passes. In loose, low density stuff it goes farther at given levels as compared to dense stuff.

 

[Mad Mike]

Generally not acceptable- Contractor needs to provide results for each layer and has priced it into his contract.

It may be that Contractor doesn't have a nuclear gauge himself and is calling someone out to test for him...perhaps that person takes some time to mobilize? If this is the case I would understand him compacting 3 layers prior to testing- I would have him box out 2 small pits, one to -12 inches and the other to -24 inches...the test can be carried out reliably this way if necessary.

Final caveat, when testing this way, the test position is dictated by a random excavation point without visual inspection of the layer surface- there is no easy way to shift the test for a suspect result, or to position the test on a portion of the layer that looks suspect...if you have reason to believe the Contractor is crooked or incompetent, I wouldn't suggest allowing this.

All the best,
Mike

 

[fattdad]

Not a good contractor!

Not good practice.

We'd just say, "No!"

Regarding item 1, it's the contractor's requirement to install all elements in accordance to specifications. That's the goal, whether YOU test or not. Testing is in the contractor's best interest. So, short-cutting what actually in the contractor's best interest makes no sense, unless item 1 is true.

f-d

ípapß gordo ainÆt no madre flaca!

 

[jdonville]

I'll add to the list of opinions of "not acceptable". Nuc gauge only tests soils between the probe tip and the bottom of the unit. If multiple lifts are being placed per day, then the tester needs to be present all day test each lift - and not just where the contractor asks for the test.

Ideally, the test locations in plan should vary to ensure that compaction is being achieved everywhere and consistently. Pay special attention to the ends of the compactor passes to ensure that they actually get the number of passes required. Have the tester check suspect patches in addition to the scattershot locations. Make the contractor wait for the tester - no next lift placed until the previous one "passes".

 

[MTNClimber]

Let's find out what the fill is supporting? Roadway? Concrete sidewalk? Utilities? Grass? Toe of a steep slope? This plays into how much you want to hold their feet to the fire.

 

[darthsoilsguy2]

depends... if you tell me a 3rd party is present during backfill of a small area and they just don't want to make the work take twice as long, fine, whatever..... if you tell me the contractor wants to call in for testing periodically and tells the soil tech where to go check, not so much.

 

[Good_Guy]

Thank u all