Compaction- Soil Density and Moisture

[shaggy650]

I am working on a taxiway project at a regional airport. The spec calls for 19" 2A sub-base, 8" FAA spec sub-base, and 6" asphalt. We dug the area out and have to compact the virgin dirt to 95%. The material is clay and is very tight but moist. The first half we dug out was able to air out for a weekend longer than the second half. We were able to get to the 95% compaction but the optimum moisture is 15%, we are getting anywhere from 17-25% moisture. All tests are with a nuclear gauge. They wouldn't approve the grade because of the moisture. We used a disc to help dry it out, and the first half we dug out the disc barely broke the surface it was that hard. We had rain in the forecast and wanted to get it covered, so graded it up and rolled it. The roller was bouncing on the grade, you literally couldn't drive straight the drum was bouncing so much. So we drove a loaded tri-axle on it and no movement. The moisture was still high but they approved and we were allowed to put on the sub-base.
-So today I put a 4" lift of stone on it. We need 100% compaction and optimum moisture is 6.9-8.3%. The material was coming in dry so we wet it down with the water truck and roll it. We got the 100% compaction but the moisture was low. It was at anywhere from 2%-4%. We wet it down some more, the water is puddling on top of the stone, the stone then starts to get mushy from all the water and the moisture percentage barely moved, but they are reluctant to allow another lift because we do not have the optimum moisture.
The tester (from a 3rd party but hired by us) says the moisture number is to allow optimum compaction kinda like a lubricant to allow material achieve optimum compaction, but we achieved the specified compaction without the moisture, but the job engineers say we need to achieve the moisture level to meet the spec.
-If we got the compaction why would we need to wet the material down just to meet the spec? Are we wetting it down just to make it wet? Won't it dry out eventually anyway? My brain tells me the water is going to go somewhere, and gravity will make it find the clay underneath, that was already to wet for their liking.
Yesterday we were trying to dry out because the dirt was way to wet, today we are spraying hundreds of gallons on the stone that is 4" above the the dirt.
Anyone been in this predicament before, or know enough to explain the logic to me? Or is it the normal situation where the engineers seem to be in out in left field?
Thanks for any input.

 

[mowingman]

I have worked as a soils engineer on numerous airports and dams and foundations over a 20 year timespan.The airport work usually calls for 98% compaction using an ASTM modified proctor test. Last time I looked, the ASTM regs did not say anything about meeting moisture %'s.
As long as the compaction test met the spec, we did not care what the moisture was. I have never heard of a spec being written where the moisture % had to be met. Normally if you get the compaction right, you are good to go.
I suggest having someone in your organization go read the spec as it is written. The inspector or owner's engineering reps., may not know what they are doing. It sure would not be the first time an inspector or some yuppy engineer got the info wrong.

 

[shaggy650]

That's kinda what I thought, Thanks

 

[redneckracin]

The optimum moisture is an idealized number. If you hit 100% compaction using your nuke gauge, you are at 100% compaction. I would make sure that the nuke gauge has been calibrated and that its set correctly for the material being testing. Also, is the source in the correct position for testing?

 

[shaggy650]

The gauge is calibrated. We are getting the 100+% compaction without the optimum moisture, but the engineers are saying if the moisture is not within 2% of optimum it is not good enough and to wet it down. So we are drenching the grade to the point it is starting to get mushy under the trucks, and puddling on the surface. The geo-tech testers are pissed that they are almost ruining the grade just to see a number. So the last lift we did today we compacted and wet it down (soaked) to optimum moisture. My guess is on Monday morning they will make us soak it again before we start another lift. Oh well they are the experts.

 

[Hoppsxc140]

Also just started on an airport project as well with the +/- 2% moisture spec and ran into the same subgrade issues. We question the engineer about a soils report before bid date and none had been done. the typical build was .75ft of P-209 in the shoulders and .25ft HMA, there are also areas of full depth builds of 1.5 ft P-209 and .5 HMA. After award we dug test holes and optimum was around 19% and native in place was 30-48%. It is the same clay hard material. Change order was written for separation fabric and 1ft of FAA sub base. Timelines were also a driving factor in our case. First if an engineer is telling you to apply water I would be specific with the documentation of that and when it destroys your native grade I would get them to buy separation fabric. Second if possible get the water added in the stock pile so it comes out with a higher moisture. Third our FAA spec calls out for even lifts with no lift greater than 6 inches, if possible build thicker lifts. If your density gauge it trying to test 4 inches it will bubble at the tip and get into the clay material below skewing the results.

 

[Shimmy1]

I haven't done a great amount of spec work, but have done some. I have been told a couple times that if the moisture isn't in spec, the compaction number will not be completely accurate. Something else to consider is some guys on here have also said the moisture numbers are important for the fact that if your material is too wet, but in spec with compaction, if it dries after the test it could shrink. Same as if it's too dry. If it picks up moisture after the test, it could swell. Just my $0.02 from what I've read on here.

 

[Raildudes dad]

Ive been in the highway business as an engineer for the local county highway department for 44 years. It's been a long time since I actually helped our techs do density in the field, long before nuke's came in use. We used the Reinhart test. It was a bad day when the balloon broke and you had antifreeze all over

Ive never heard of trying to lower or raise the moisture content of the native subgade especially clay. If it's too soft, we undercut the material as needed either replacing it with re-compacted dryer native material or good sand if we can use under drains to drain it. Add water to clay material? Never.

As for the aggregate layers we pound the aggregate into the cone and weighed it to determine the maximum unit weight of the material. You do a Speedy moisture test to determine the insitu moisture content. The Michigan DOT had and still have charts that you used to adjust the maximum weight with the insitu moisture compared to the optimum moisture. The 95%, 98% and 100% density is achieved with the existing moisture content. As far as I know ,this method is still used to determine the aggregate unit weight.

I will admit I've never work worked with FAA specs.

The only time we have added water to aggregate is if the top several inches gets bone dry if it sets too long uncompacted. That very rarely happens if you roll it tight after the initial grading.

It sounds like a couple of you are working with folks in charge that don't know what they are doing.

The nuke certainly speeds up the density testing over the old Reinhart's. Plus, you can't use the Reinharts on asphalt.

I can't get real excited with the nukes especially on asphalt - too much voodoo adjusting for my liking. Not getting density, lets adjust the factor. The roller isn't doing anything different

 

[buddy605]

Try and add the water while it is loose. I did airport work and looked for the 100% modified spec but never required the contractor to get within moisture. I would have concern with a hidden layer of loamy material, if water soaks below and into a possible hidden layer and with vibration it could destroy the under layers. And who would pay for that. That is a story I would tell them and see if they would bite. That alway makes them pull in there horns. If things get screwed up who pays or liable. The by the books guys are afraid of responsibility.

 

[mowingman]

Quote: " We used the Reinhart test. It was a bad day when the balloon broke and you had antifreeze all over "
Boy, do I remember those days. I prayed every day the balloon would not break on one of my tests. I much preferred the old "sand cone" test method.

 

[shaggy650]

I think the clay subgrade has been holding up. We were adding water before initial rolling of the lift, we would get the compaction numbers at around 3% moisture but had to hit at least 4.7% We would spray it again and static roll, small puddles of water on the surface. Tech would notice as moisture came up compaction would go down slightly (He thought proctor was wrong). By the time we got the numbers right the grade would be mushy under the trucks and even mushy under your feet in spots. 19 inches of 2A later and the same thing, the stone was completely saturated and moving under trucks. The tech documented everything in reports that we were told to add water and it caused the grade to move. The P209 was coming in wet but not up to specs, but we refused to add water, 8 inches of that and still a soggy mushy grade. They wanted me to fine grade but I can make the stone under my feet so I don't know how I can fine grade, let alone pave. If it can sit for while it will end up rock hard when the water settles out, but they are on a quick time line to finish. Oh, the engineer on site is new and is his first job in this field, so all decisions are made over the phone with his boss.

 

[mjt]

Hello We had the same problem when doing freeway work in Victoria Aust.
Subgrade had to be put in at optimum, a lot of the time it failed a test roll because the material had positive pore pressure in it. Water is holding the particles apart as a lubrication.
If the material is wetter the nuclear meter will normally give a better reading than if it is dry.
The density of the material is measured from the source ( probe in the ground.)
The moisture is not measured from the probe but is measured from a different area of the device ( back scatter ) so the guage can get the wrong moisture content. It measures to a different depth than the probe. To get the right moisture you would have to take samples each time to the same depth and weigh it wet and dry to work out the moisture content.
Nuclear gauges can be add or minus 20kg on a calibration block ( - or + 0.9% of the reading approx.)

( We wet it down some more, the water is puddling on top of the stone, the stone then starts to get mushy from all the water and the moisture percentage barely moved, but they are reluctant to allow another lift because we do not have the optimum moisture.
The tester (from a 3rd party but hired by us) says the moisture number is to allow optimum compaction kinda like a lubricant to allow material achieve optimum compaction, but we achieved the specified compaction without the moisture, but the job engineers say we need to achieve the moisture level to meet the spec.)

The gauge would be reading drier, because it would be mearsuring the dry material underneath the top wet material.

On your job did they take samples and oven dry them to get better results or did they rely on the nuc gauge to get the moisture results for each test.
Nuc gauges are not as good as everyone thinks they are.
If you tested in the same hole at 90 degs rotation I have seen a variation up to 3%.
I have added 2 attachments to have a look.

mjt