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densedds (Geotechnical) (OP)
20 Feb 13 11:31
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?

Helpful Member!  civilman72 (Civil/Environmental)
20 Feb 13 13:27
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.
Helpful Member!  cvg (Civil/Environmental)
20 Feb 13 14:07
given the cold temperatures and clay fill material, a post tensioned structural slab might have been recommended, not a slab on grade.
Helpful Member!  CarlB (Civil/Environmental)
20 Feb 13 15:58
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 (Geotechnical) (OP)
21 Feb 13 8:02
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 (Geotechnical)
21 Feb 13 8:28
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 (Geotechnical) (OP)
21 Feb 13 9:25
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 (Civil/Environmental)
21 Feb 13 9:42
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 (Geotechnical) (OP)
21 Feb 13 10:23
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 (Civil/Environmental)
21 Feb 13 10:25


Assuming the floor will not settle any further...

I would not make that assumption
Helpful Member!  fattdad (Geotechnical)
21 Feb 13 10:48
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. . .


¡papá gordo ain’t no madre flaca!

GeoPaveTraffic (Geotechnical)
21 Feb 13 12:10
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 (Geotechnical) (OP)
21 Feb 13 15:15
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 (Geotechnical)
21 Feb 13 15:39
I'd do at least three locations - the best, the worst and the average.


¡papá gordo ain’t no madre flaca!

GeoPaveTraffic (Geotechnical)
21 Feb 13 15:50
Agree with fattdad.

Mike Lambert

cvg (Civil/Environmental)
21 Feb 13 16:17
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 (Geotechnical) (OP)
21 Feb 13 16:42
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
Helpful Member!  GeoPaveTraffic (Geotechnical)
21 Feb 13 21:07
I don't believe that the dropped slab can make a significant contribution to the compaction.

Mike Lambert

fattdad (Geotechnical)
22 Feb 13 8:53
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).


¡papá gordo ain’t no madre flaca!

densedds (Geotechnical) (OP)
22 Feb 13 9:28
The voids would explain the hollow sounds produced by gently hitting the floor in some areas with a hammer.
fattdad (Geotechnical)
22 Feb 13 11:05
yes, there is actually a device that looks like spikes on a cylinder that you can drag across a slab to "listen" for voids. I just can't remember what it's called.


¡papá gordo ain’t no madre flaca!

miningman (Mining)
22 Feb 13 11:17
What is your immediate priority?? Is it to prove in court that the contractor is liable... which gets you absolutely nothing unless he agrees to fix the problem without additional litigation , or is it to remediate the building to a standard that it can be used... perhaps not to ideal standards, but usable.

If it were me, I would be willing to incurr the cost of coring thru the floor at a number of locations.... if the building is 40,000 sq feet, a 40 x 40 foot grid would require 25 corings. This would be much less disruptive than digging test pits in the slab, and would provide clear easily understandable data that a judge might consider in the event you continue into litigation. Depending on the results, I might consider pumping grout into the voids to reduce further settling
cvg (Civil/Environmental)
22 Feb 13 11:31
I'm not convinced that filling the voids with grout will stop future settlement. And slab jacking would be required to bring the slab back up to the correct elevation. I think this might be only a temporary fix.

Given the fact that you appear to have records that the fill was made using clay material in temperatures of 20 below zero, with arguably no moisture conditioning and no compaction test, all of which does not comply with the recommendations in the design report nor with any standard of care - in my opinion that is ample evidence to prove to a judge that huge mistakes were made during construction.
densedds (Geotechnical) (OP)
22 Feb 13 14:31
I've committed to the pit tests and the contracter is on his way over as I write this to look at the areas of slab removal. Tomorrow the testing people are comming over to take their samples. A report will be generated next week. This just keeps getting sader and sader as I was on a conference call earlier with the subs that poured the slab and my attorney and they we very open about the communications they had with my general. They expressed that on more than one occasion they mentioned their concern for the fozen soil and also weeks following the pour with the dropping slab.
densedds (Geotechnical) (OP)
25 Feb 13 11:07
Well Guys,

Went through with the cone/ Pit compaction test. Interesting, I learned alot. Attached are photo's of the 1+ inch gap under the slab. When making the final cut, the slab actually dropped and pinched the saw blade. I should get my report in a few days.

densedds (Geotechnical) (OP)
25 Feb 13 11:09
oldestguy (Geotechnical)
28 Feb 13 17:04
Then comes the fix up

It would appear that some form of compaction grouting will not only do some good at "compacting" but will fill voids and create a stable situation, even raising settled slabs. What about buried piping? That can be damaged by compaction grouting.

Not just any "slab jacking" contractor should be used, but one experienced in "compaction grouting".

You may even have to leave pipes for future lifting of slabs that still may settle.
densedds (Geotechnical) (OP)
25 Jun 13 22:14
They mentioned today doing some type of compaction grouting or "foam jacking". Can you tell me the prs and cons for such a procedure? Aslo, most of the research I have found on these procedures deals with outside applications. Is this suitable for a clinic type office where the floor is way out of wack and soil compaction is as low as 75% of proctor?
oldestguy (Geotechnical)
30 Jun 13 20:34
I did a Bing search on the Internet and the firm at the top of the "compaction grouting" list is a well known dependable firm. With any of them, be careful about all the possible problems as well as how payment is figured. Underground piping can be damaged.

I'd have an experienced engineer watching (that is one who has seen many of these jobs done) to watch for problems.
densedds (Geotechnical) (OP)
8 Jul 13 15:30
There is plenty of mechanical (plumbings,gas,heat) in the floor. It has been determined that the soil was not compacted properly. Is chemical or foam grouting a technique suitable for releveling and stabalizing a office floor for the long term on such soil?
cvg (Civil/Environmental)
8 Jul 13 18:23
compaction grouting is not suitable for clay soil. slab jacking with foam or other types of grout can raise and level the slab. however, it will not prevent future settlement. if the subgrade continues to settle than the slab might also continue to settle. no guarantees on eliminating the floor movement without improving the subgrade first.

If piping penetrates the slab and the slab moves, than the pipes will probably be damaged.
crcivil (Civil/Environmental)
9 Jul 13 10:12
In my opinion Fracture Grouting is the most suitable grouting method for clays. It would increase the density of the surrounding soil, fill voids, and could even raise and level the slab. Regards.
densedds (Geotechnical) (OP)
10 Jul 13 11:15
Can you tell me the negatives involved with Fracture grouting or any other type of grouting? I have an office with alot of piping under the slab.
fattdad (Geotechnical)
10 Jul 13 13:15
In my opinion, the fill has contracted (densified) from decreasing moisture content and it's in better condition now than it was from the onset. The $1M question is how do you intend on using the building? I mean if it's really a lightly-loaded slab on grade, then there's much less risk than if the slab is for heavy industry or warehousing.

I think I'd be more inclined to either reconstruct the slab and require subgrade compaction to 100% Proctor (relative compaction) or fill the underslab void with grout, floable fill or foam. The problem with foam is it may not offer the same slab support as the other options.

In all cases you are ending up with a compromized condition. Fracture grouting may be too risky with your utilities.


¡papá gordo ain’t no madre flaca!

EITGeotechnical (Geotechnical)
18 Jul 13 1:27
The 100% compaction on your silty clay materiel will not solve the issue. Before placing your concrete slab you should have 200 mm of Non-Frost Susceptible (NFS) compacted to minimume of 98% SPDD to prevent heaving on the winter.
crcivil (Civil/Environmental)
18 Jul 13 16:17

How much money is at stake? I mean, what's the size of the slab on grade? Replacing or trying to fix the subgrade and reconstructing the slab might be a bit expensive, unless we're talking about a small area in which case I would consider it as an alternative. Otherwise.. how about a cement-bentonite fill grouting tratment under the existing slab, followed by a series of small-diameter (3"-4") pin piles? Regards to all of you.
fattdad (Geotechnical)
18 Jul 13 17:37

Quote (EITgeotechnical)

The 100% compaction on your silty clay materiel will not solve the issue. Before placing your concrete slab you should have 200 mm of Non-Frost Susceptible (NFS) compacted to minimume of 98% SPDD to prevent heaving on the winter.

I'm assuming that the slab is independent from the foundation, the perimeter foundation is to proper depth for frost protection, and the building has conditioned air. I also understand the function of subbase aggregate below the slab on grade. My suggestion to "reconstruct" the slab, was intended to include the subbase aggregate, 'cause it's the subgrade soils that need recompaction. You'd have to harvest the stone for reuse (if possible) or haul in new prior to pouring concrete.

I did design work for a mausoleum and for that case, yes the entire slab on grade had to be protected from frost.

f-d, p.g., p.e.

¡papá gordo ain’t no madre flaca!

crcivil (Civil/Environmental)
18 Jul 13 18:20

What do you think of pin piling the slab? Regards.
densedds (Geotechnical) (OP)
22 Jul 13 11:00
One the the problems we are running into is that the floor was settling during the construction of the internal walls and such. Thus if someone comes in at this point and raises the floor to level, the walls, roof, doors, and such will all be raised as well and end up throwing everything else off. This is a dental office with vacuum, air, water, computer, electical, hydronic heat, and drains to all operatories. I tend to agree with the remediation plan that leaves the soil as is and replace the slab with a structural slab in hopes of not disturbing all the the said utilities. This would also allow leaving some of the interior walls that have been shimmed alone or replacing on a as needed basis. I have also been told that this remediation plan allows us to test all of the lines before we pour the new floor in contrast to chemical grouting or "jacking" where we would be fixing probles after the fact.
densedds (Geotechnical) (OP)
29 Jul 13 8:47
Exactly what risks do I run with a remediation that involves compaction grouting to stabalize the soil and chemical grouting to fill in the voids under the slab?
crcivil (Civil/Environmental)
29 Jul 13 23:18
To run out of money before completing the job?

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