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Old 09-01-2010, 08:13 PM   #1
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Soil under slab - No crushed stone used


My apologies if I am in the wrong forum place for this discussion, but looking for some answers and hoping someone out there can help. We are having a 16x24 two-story addition built onto our existing house. We do have a GC for the project. Our contract states that they would put crushed stone down before laying the slab foundation. However, after some recent water issues in this new basement, it came to light that they NEVER used the crushed stone and poured the foundation directly over soil only. The building inspector never caught the mistake (and neither did I) at the time. The project is 90% complete at this time and I don't believe there is anything they can do to fix that mistake at this time. Just wondering if anyone has any insight as to what issues we could face due to them not using the crushed stone. Is our house gonna sink? Foundation going to separate, leak? Or will we be ok. Our house is placed somewhat on a hilly road, so the water flows downward during rainstorms...so we do have soil that will move over time. Very concerned and of course the contractors just keep telling me they did the right thing and that what they did was better. I know they are covering their butts and not sure what to do, if there is anything I can do at this stage. Thanks in advance for any help you can offer.

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Old 09-01-2010, 08:28 PM   #2
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Soil under slab - No crushed stone used


http://www.buildingscience.com/docum...ms?full_view=1

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Old 09-01-2010, 10:23 PM   #3
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Soil under slab - No crushed stone used


Thanks for the article. I'd also like to mention The signed contract specifically states that crushed stone would be dumped into the foundation area prior to pouring the concrete. They have acknowledged they did not do this, but defend themselves by saying the soil is better (which we seem to all know it is not). Furthermore, I am awaiting confrimation from them regarding a Vapor barrier, but I believe there was absolutely no vapor barrier used (as the contractor has stated that "the only thing under the concrete is virgin ground"). Any suggestions as to what actions I should take regarding this issue? Unfortunately the building inspector didn't realize they didn't use crushed stone (it is not required by NH code..but only recommended) so he did sign off on the foundation aspect of it at the time (but agrees with me they should have used stone).
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Old 09-02-2010, 01:07 PM   #4
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Soil under slab - No crushed stone used


If the soil composition is such to drain, fine. Yet why was it called out for gravel, ask the engineer. A vapor barrier is essential to stop the moisture (now for sure with a drainage plane) from rising through the slab. At least they used rigid foam at the slab perimeter, right?

""the only thing under the concrete is virgin ground")" ------- usually they excavate down to the footing depth on the whole basement, then back-fill dirt (or gravel) to bring the sub-grade up so the slab is sitting on the footing (with the v.b. under and covering the end of the slab). Hopefully they mechanically compacted the soil as it was installed (not with the excavator's bucket). I would keep an eye on the rest of construction or hire someone knowledgeable. To screw up at the beginning is a taste of things to come.
http://www.nrmca.org/aboutconcrete/cips/29p.pdf

http://www.100khouse.com/2009/06/24/...b-air-sealing/

http://www.energysavers.gov/your_hom.../mytopic=11490

http://www.servicemagic.com/article....age.13702.html

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Old 09-02-2010, 02:15 PM   #5
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Soil under slab - No crushed stone used


Normally crushed stone is used under the slab for three reasons:

1. It is easily compacted, hence will form a very strong surface to place concrete on, minimizing the potential for settlement of the concrete.

2. It is highly permeable, hence allows moisture under the concrete to drain away from beneath the slab.

3. Normally (as noted by others) the excavation extends below the slab, hence backfilling to the appropriate grade is required, and crushed stone is a cost effective product for filling to grade.

If the naturally occurring soil is the equivalent of crushed stone in terms of strength and drainage, there may not be a need to place crushed stone under the slab. The perimeter walls generally extend to frost line, which is typically four feet in NH, and of course may go deeper if you have a full basement and grade requires. If the naturally occurring soil below the footers (the perimeter foundation) is adequate, it is not necessary to place crushed stone underneath the footers. However, this can only be determined by actual examination by a qualified individual at the time of excavation.

In your case, it is essential that you determine the characteristics of the soil beneath the slab and the footers in order to determine if there is the potential for long term trouble, meaning settlement, drainage issues, or moisture penetration through the slab. This may require excavation near the slab using a backhoe. No doubt the contractor does not wish to do this, however you may need to insist, as it will be impossible to evaluate your situation until it is done.
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Old 09-02-2010, 08:19 PM   #6
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Soil under slab - No crushed stone used


Thanks again for all the insight. I am a stay at home mom of two toddlers, with minimal construction knowledge (prior to this project anyways)...so this site has really been a big help to me in confirming that they should not have done what they did. In response to GBR, the contractor's answer to "why did you state you were going to use crushed stone under the foundation if you believe that sand is better" was "We put that in the contract because that's what we use when we don't have the soil available." We do have a perimeter drain (pvc piping) around the structure that ties into the perimeter drain that was surrounding our existing house (which is a 1 story ranch built in 1990 w/ a full walk out basement that has never seen a drop of water). The area we live in (and the house next door, even) does tend to have issues with flooding (and flooded basements) during severe rain storms..but we have never had problems (and would like to keep it that way). I was able to confirm with the contractor today that there was absolutely NO vapor barrier used. His response was "A vapor barrier was not needed. If you decide to finish the basement off later, you would put a vapor barrier on top of the concrete and then put your flooring down over that." To my knowledge..the excavating crew dug out the area...and the guy just used a "hoe" looking tool to stamp the soil down before they poured the concrete. There is nothing else there except the existing soil that they dug up. The project is already at the "interior painting stage" so there is no turning back to hire someone else at this point. Only came to light because of the recent water issue (which made me start thinking every possible scenario through...and made me realize I never saw crushed stone delivered..and brought me to start questioning them). Otherwise, they never would have mentioned it...and it would have been completed w/o me ever knowing it wasn't used. The water problem issue, they believe is at the corner where the new foundation is supposed to adhere to the old foundation. However, they believe the leaking was occuring above ground (above the waterproofing area) and sealed it yesterday with "flexible caulking made for concrete" from the outside. They say this will solve my problem...but even if it does, it still leaves me with the issue of them not constructing the foundation properly and as contracted. Daniel, Thanks for the information and the suggestion you made requiring the soil being tested. That may be an option we offer to the contractor at the end of all this.
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Old 09-02-2010, 08:54 PM   #7
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Soil under slab - No crushed stone used


Oh, and we also have a hairline crack in the foundation floor extending from the "previous house foundation" wall to the center support pole, to the outside "new house foundation" wall. The contractor says this crack is an expansion joint that was placed in the slab and that it is supposed to crack there so it does not crack anywhere else. I have heard from other people that this is right. Would love to confirm w/ all of you?! Thanks.
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Old 09-02-2010, 09:06 PM   #8
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Soil under slab - No crushed stone used


Read “e”, f, and “h”: http://www.nrmca.org/aboutconcrete/cips/06p.pdf


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Old 09-04-2010, 10:59 AM   #9
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Soil under slab - No crushed stone used


curious; is radon a concern in your area? if so, i hope pipes were installed for it. j
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Old 09-04-2010, 02:01 PM   #10
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Soil under slab - No crushed stone used


http://energy.cr.usgs.gov/radon/rnus.html

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Old 09-05-2010, 12:33 AM   #11
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Soil under slab - No crushed stone used


good link, g. glad we are in a moderate area. j

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