Frozen Clay fill 5

[densedds]

Hi all,

I am looking for advice on a office building I had built in 2009. The ground was actuallly scraped in October of 2008. Up here in the Dakotas, it turned very cold and due to the elevation of the existing lot, fill had to be hauled in to reach floor height. The building was scheduled for a four inch concrete slab floor. The elevation of the finished floor was to be 836.5 feet above sea level. The beginning elevation of the existing lot was 833. The elevation of the bottom of the footings was 830.2. The interior of the building was filled with lean clay that was in 20 degree below zero farienhiet. The exterior walls and roof was in place at the time of the floor pour. No windows were in and no heat in the building. They did use heat blankets in an attempt to thaw the ground, but no soil compaction tests were done prior to the pour. I now have a beautiful office building with a floor that has significant settleing Ff of 20 and Fl of 7.6. Their are areas that drop 2 3/4 of an inch in 3 feet. In short it was back filled with frozen clay. Is it commmon to do this type of backfill in -20 temperatures and get it compacted good enough to prevent settling?

densedds

 

[civilman72]

No. Most heat blankets will not thaw the ground to a sufficient depth at that temperature - they probbaly should have used hydronic ground heaters. But as the owner, the lack of compaction tests is a big problem.

 

[cvg]

given the cold temperatures and clay fill material, a post tensioned structural slab might have been recommended, not a slab on grade.

 

[CarlB]

No, fill should not be placed in below freezing teemperatures. You may be able to get decent results if using NFS material, heated fill, not too much below freezing. Some coarse gravels are placed frozen but some settlement is expected. It would be very unlikely proper compction could be achieved in those conditions when using silty or clayey material.

 

[densedds]

Looks like arbitration is inevitable. It has been reccomended to me to cut into the slab and take compaction tests now. Would this be wise?

 

[GeoPaveTraffic]

I agree with most of the other posts, frozen fill should NEVER be placed and fill should NEVER be placed on a frozen surface, period.

As for going forward, the only real option will be to remove the entire floor slab, remediate or replace the fill with properly compacted fill, and reconstruct the floor. In order to prove the problem and get someone to pay for it, the fill will need to be sampled and densities determined. This can most easily be done by removing a section of floor about 5 by 5 ft. and then testing and sampling the fill. Alternately, the floor could be cored and hand equipement used to sample the fill.

You have a very real and large problem. I assume you have an attorney and an engineer working either for you or your attorney. If you don't, you need to get one hired now. This is a problem that will only get worse with time. Total settlements could easily exceed 6 inches.

Good Luck, I'm affraid you are going to need it.

Mike Lambert

 

[densedds]

Yes I do have an attorney and we are learning together. The sad part of it all is there was a soil test done before we started and the report came back recommending engeneered granular fill. Of course I just learn this recently.

 

[civilman72]

You definitely need compaction tests. Assuming the results are substandard, it may be your best evidence to correlate the floor settlement/failure with contractor negligence.

But it's also possible that the soils beneath the floor slab have have been compacted over time, so a current compaction test may not represent the finished conditions prior to placement of the floor slab, possibly affecting the validity of your evidence.

Good Luck

 

[densedds]

I appreciate all of you taking time to respond. Assuming the floor will not settle any further (although a few internal sona tube footings are just recently starting to appear through the carpet and linoleum), is it possible to relevel the floor with some sort of leveling compound that bonds to the concrete?

 

[cvg]

Assuming the floor will not settle any further...

I would not make that assumption

 

[fattdad]

If you want to document the existing conditions and contrast those conditions to some specification requirement (i.e., subgrade for slab-on-grade construction prepared to 95 percent relative compaction), you'd need to do what's already been recommended. Remove a 3x3 ft square of the existing slab on grade and perform density testing.

A field density test is not enough though. For each field density test you should also obtain a bulk sample (i.e., at least 75#) and return that to the laboratory. You see it's in the laboratory that you test for the "maximum" density. It's the comparison of the field density to the maximum laboratory that yields the "percent relative compaction."

There are multiple ways to take the field density. Most engineers will use the nuclear gauge. I'm a fan of the nuclear gauge also. Problem is when you are working in a cut out of a concrete slab, you run the risk of some defendant saying the nuclear gauge results are influenced by the slab or work being done in a recessed area. This may be "BS," but it's something to consider. I'd certainly have the field engineer also perform a "sand-cone" test to gauge whether the results are the same. Either that or just use the sand cone method.

There are companies that specialize in concrete floor leveling. They use a grouting process to relevel the slab. I'm not sure that adding concrete to the top of the slab would be best.

Is this an office building? You see in the future, folks may move their cubicles, file boxes, desks, or relocate walls. Each of these changing loads run the risk of triggering more settlement.

There are acoustic methods to "see" map voids below slabs. I don't know this from first-hand experience, I've just seen such surveys done by others.

Sorry about this bad construction. Earthwork using moisture-sensitive soils should not be done in freezing weather. Granular fill would have been the right advice. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

I would not make the assumption that no additional settlement will occur, instead I would assume that significant additional settlement will occur.

Mike Lambert

 

[densedds]

Based on all of your advice, I have decided to go ahead with the compaction tests and scheduled it for this Saturday. In your opinions, what would be the best location for the pits? In an area of greatest settling, or would the soil in that area be compacted from the dropping slab.

 

[fattdad]

I'd do at least three locations - the best, the worst and the average.

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

Agree with fattdad.

Mike Lambert

 

[cvg]

I would recommend somewhat random locations for the testing and you may want to let your geotech determine the locations. later on, in court, the contractor may complain that you directed the tests be done in the worst locations. In fact, it might be also good to ask the contractor to witness this testing so there is no perceived foul play.

 

[densedds]

Do I have to worry about false compaction from the dropping slab? If I'm going to proceed with this and reroute traffic in the building, I would like to be able to rely on this test. Thanks Paul

 

[GeoPaveTraffic]

I don't believe that the dropped slab can make a significant contribution to the compaction.

Mike Lambert

 

[fattdad]

After the frozen fill was placed, it thawed. Thawing may have occurred after the slab was poured - don't know. I just know that with thawing and protection by the building, there'd be some reduction in moisture content over time. This reduction in moisture content would result in soil tension and that soil tension would result in increased "compaction." That in turn would result in further loss of subgrade support.

My suspecion is that there are voids below the slab in all sorts of random locations. My suspecion is that the areas with the greatest damage may not be the only areas with chronic problems. They may just be the areas with the greatest traffic or where the current loading is the greatest.

There are always alternate sampling stregigies. You can grid the site and use a random number generator if you want to. You can shoulder the responsibility to the engineer doing the forensic evaluation. You can collaborate with the engineer to reach some consensus on locations and budget. You can use an acoustic method to "screen" for potential voids and then target the test locations to those areas.

I like a minimum of three tests locations however.

You may need to return for greater sampling (after all at this point we don't know whether this is a 40,000 sf building or a 400,000 sf building).

f-d

¡papá gordo ain’t no madre flaca!

 

[densedds]

The voids would explain the hollow sounds produced by gently hitting the floor in some areas with a hammer.