compaction testing in a wet trench

[excavate]

i have a promblem in which this forum might be able to help. I am a contractor which installs infastructor for new subdivisions. i have run into a promblem that the compaction tests with a nucular gauge have failed do too too much moister(as much as 16%) when the pipe bedding was first put in it was dry and packed well. there was no compaction tests done at that time and now the municipality wanted that section dug up and tested .When tested it fails the nucular test do too high moister content. now the municipality has rejected the work and wants it dug up and redone. Is there another kind of test or a way around it?.(the rest of the job was tested in the dry and passed when dug up resently it also was too wet and failed)

 

[oldestguy]

You should give us the details of tghe specifications that you are under. Usually these specify a perceptual density with respect to a standard laboratory test, Assuming that is the type of specification the bedding has to meet, the moisture content is disregarded and only the dry density numbers sre used. If someone is complaining about moisture being "too high", I suspect they are not experts or trained inspectors. It may be necessary for you to higher a local professional engineer who is knowledgeable about that subject, possibly owning a testing laboratory. This sounds like a situation that may require going over the head of the inspector using a well qualified engineer to do it. There may be a lower density to begin with and, now being saturated, that material will be more difficult to densify to meet the spec. One other problkem may be present. Sometimes the nuc testing is affected by being in a trench.

 

[Ron]

For most soils, the moisture content is only important at the time of compaction. If you achieved compaction and it became wet later, that should not matter as compaction is compared to the dry density, not the wet density. There can be problems with doing nuclear testing in a trench and with high moisture contents. There are other methods of density testing that can be done. Assuming the soils are not gravelly, a drive sleeve can be used to determine the in-place density. A "Speedy" moisture meter could then be used to determine the moisture content or it could be oven dried.

There are soils that are affected by additional moisture after compaction...usually swelling clayey soils.

Also, keep in mind that compaction and stability are two different parameters. You can achieve compaction but still have stability issues, particularly if the soil is wet of optimum.

I agree with OG....get a local engineer out there who routinely does testing.

 

[oldestguy]

OG here again, thinking about this. Pipe bedding usually is single sized stone that easily compacts to meet specs. 16% moisture may not be a problem since it has so many voids. Another form of testing is the sand cone method. Assuming you are not finding the sample removed is not immediately leaving a hole full of water, it is a stand-by test used for many years. If in doubt, line the hole with plastic and fill that plastic with a known amount of water (for determining the volume). That will force out any water that would otherwise affect the volume determination. A careful use of the plastic and then the sand cone also should work.

 

[excavate]

the pipe bedding used is "A" gravel a sand and crushed stone combo. The lifts from the pipe bedding up is a sand and clay combo which is wet also and fails. the rest of the job was tested at the time of installation and easily passed now when dug up it is wet also and fails. The municipality is not concerned with this just the section that didn't have tests done during installation when it was dry. The local p-eng only reports the test results and not his opinion or recomdations and the municipal rep is only concerned with the numbers

 

[oldestguy]

Something sounds screwy. Once a placement and compaction meets the density test, getting saturated should make no difference in the density numbers which only is reported as the solids, not water content, unless there is some very unusual clay present causing swelling. To keep on good terms with the city maybe replacing is the best option, but it would seem that a different testing firm should be used from here on.

There may be some problem with the testing details. With the various materials used for backfill, it may be possible that the technicians do not compare the trench tests to the proper laboratory tested material for each individual field test. A thorough testing program would run one laboratory test for each field test. That usually is not possibly so some judgement is needed in selecting the proper lab result to use for the field work. That is where many an argument can come up on the job. A competent testing job is required for these circumstances. Also, knowing the failings of the nuc method is needed by the testing firm.

 

[mikeofBelAir]

I am a little confused....
Is the material actually wet, or is it merely testing wet?
Did the tech doing the nuke testing use the proper offset for trench moisture? Nuke moisture tests taken in a narrow trench will show higher than actual moisture due to the presence of the moisture in the side walls of the trench. The Instruction for the nuke gauge should cover this.

Do I correctly understand that the nuke tests are being done now, after the material was placed, and then compacted. Now it is being excavated again, and those are the tests that are failing? Would it not make more sense to use another method to determine the actual moisture--as in taking a sample to the lab for an overnight oven-dry moisture test? at least before re-doing all that work.