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  • Building on previously disturbed soil

    When reading about foundation footings, I often see the note that footings need to extend to undisturbed soil, and I understand that.

    However (in my limited experience) it seems that I see footings being dug only to meet locally specified frost depth. This is on previously disturbed soil. And it seems that inspectors don't question this.

    On a room addition project, or a teardown & in-fill new construction project, it is obviously unlikely that the specified frost depth is deep enough to assure you are sitting on undisturbed soil.

    I know there are a lot of guys making a living doing room additions and infill housing, could you please explain your typical real world process for making sure your footings are on solid, undisturbed soil?

  • #2
    Re: Building on previously disturbed soil

    The proper cya way to do it would be to get a soil compaction test. Most guys will just go with their gut though. If the soil hasn't been dug up in recent years most will assume all of the settling that is going to occur has with the passing of time. The majority of people I've worked with will just visually look at the hole and if it looks solid that's good enough for them. When I was pm for a home builder the majority of our foundations were hammered out of rock so it wasn't an issue. We only did a compaction test once because the inspector in the area knew that the lot was filled in 20 years ago and the developer was suspected of burying trash in his fill at the time.
    As far as the local inspectors go all of the ones ive dealt with in the past are pretty cluless with the exception of one in regards to the things that matter. They would all have their niche codes they would check like max stair height and proper ground fault outlets but would never question the beam over their head that was about to explode because it was severely undersized.
    The inspectors I dealt with during my time with the home builder were an absolute joke.
    Darrel Hunter

    "You can't build a reputation on what you are going to do." - Henry Ford

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    • #3
      Re: Building on previously disturbed soil

      In residential construction the inspector does a rod probe test. If the soil is loose you will be made to go to solid material in the judgement of the BI. You may have to go the engineered fill route but that's above a BI's pay grade, it's then in the hands of an engineer

      On commercial jobs an engineering firm is hired to do the same inspections. They have the borehole logs and the soils report so they know what is assumed to be down there. They do a rod test and a soil composition test and if it fails to meet what's specified then in most cases you dig to good bearing soil and use engineered fill or pour the extra concrete. Unless it is a couple of inches the engineered fill is the only practical solution. The other option is to redesign the foundation and that's usually an expensive route.
      Last edited by dave_k; 09-21-2012, 04:56 PM.

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      • #4
        Re: Building on previously disturbed soil

        Thanks dave_k and dlhunter for the replies, it sheds some light on the process. So, now I'm trying to get a feel for what is normal for residential projects.

        Is a soil composition test (and possible use of engineered fill) generally considered overkill for something like a room addition project? Or, are these things that are often needed but "conveniently left out" to keep prices down and win a bid?

        How about for tear-down and in-fill building?

        Thanks guys, this is not in regard to a specific project but it is something I have been wondering about for a while.

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        • #5
          Re: Building on previously disturbed soil

          Like Dave said its up to the inspector but unless there is a reason to suspect unstable soil I don't see a reason for any soil testing. I think the soil composition and compaction test cost about $800 when we did it. Like I said above the norm around me was to dig to frost level or to "solid" ground. Obviously you wouldn't be good on topsoil.
          Darrel Hunter

          "You can't build a reputation on what you are going to do." - Henry Ford

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          • #6
            Re: Building on previously disturbed soil

            As an engineer, I would say that its not real easy to evaluate soil without expensive testing. For most small projects like additions its typical to look at the house and see if any settlement or heaving has occurred. That will give an idea if there are problem soils on the site. When excavating for the footings I would also look at the house footings to see what was done. In my area footings are placed a minimum of 3 feet deep, which is typically below disturbed soil, but not always. You also have to be careful when building an addition with a slab or a crawl space next to a basement. The overdig is disturbed earth. Pushing a rod into the ground can give a reasonably good idea of soil compaction, with some experience. Some Geotechnical testing firms will do a quick test like that for a small amount of money. If there is any question about the soil a couple hundred bucks is worth getting an experts opinion. If you are talking about a house and there is any question about the soils I would hire a Geotech firm to evaluate.

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            • #7
              Re: Building on previously disturbed soil

              Originally posted by c_kott View Post

              Is a soil composition test (and possible use of engineered fill) generally considered overkill for something like a room addition project?
              If the soil you are building on can't support what you are building it's going to cause problems no matter what. Unless you're municipality requires it I wouldn't do any sort of subsoil testing. I would get some education on soils so you know a problem when you see it. Even if the inspector doesn't do a soil inspection if you build on soil that can't bear the weight of the structure it's going to come back to bite you.

              That being said, I've never had to involve an engineer for subsoil investigation or to design a foundation for soil conditions building single family residential. Our prescriptive code covers most soil conditions and most residential neighbourhoods that I've built in have been problem free.

              I found an old cistern once and have found dumps where the builder buried his scrap but the footing was below these in each case.


              Originally posted by c_kott View Post
              How about for tear-down and in-fill building?
              As long as you're going below what was there before you're probably fine. If you're building for someone I would make sure they are responsible for any unforeseen conditions. No one can reasonably predict what the conditions are under the foundation of a house.

              Thanks guys, this is not in regard to a specific project but it is something I have been wondering about for a while.[/QUOTE]

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              • #8
                Re: Building on previously disturbed soil

                Dave:

                It depends upon your area, ever since the 1998 UBC we've had to get geotechnical studies on everything we build, even a small addition. Since the geotechs always require pier and grade beam foundations we have to get Chapter 17 Special inspections on all piers. In fact in most areas, including Silicon Valley where I am now building, we have to get Geological studies as well as geotechnical studies, that means a trench large enough for the County Geologist to walk through and examine the soils layers.

                I see that the OP is in the St. Louis area, if you remember several years ago we had several women from organizations (like HADD) dedicated to getting back at tract builders who had built their defective homes, one was from the St. Louis area, the tract builder had only done one study in the entire subdivision, then applied the soils values from that one study to the entire subdivision, they were suing the AHJ for malfeasance for not requiring studies on each lot.
                You will ask what goal the U.S. is pursuing? .... their external debt is huge, and ruining other countries is their customary method. Even ownership of the global 'printing press' is no longer helping. Nor is full control over NATO, None of that if enough for the 21st century colonizers. They don't just need to preserve the dollar as the only global currency but also to get their hands on the economic wealth of other large powers and regions. - Sergei Naryshkin

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                • #9
                  Re: Building on previously disturbed soil

                  Originally posted by mjr6550 View Post
                  Some Geotechnical testing firms will do a quick test like that for a small amount of money. If there is any question about the soil a couple hundred bucks is worth getting an experts opinion. If you are talking about a house and there is any question about the soils I would hire a Geotech firm to evaluate.
                  There is something called a "Chance soil probe". It is turned into the soil to determine the bearing capacity / consistency of the soil for helical piers. I am sure geo firms could utilize this to aid in traditional foundations.

                  Next time I talk to my soils guy here I'll ask if that can be used outside of the world of helical piers.... He does not charge that much to do some tests.
                  “Racism is man's gravest threat to man - the maximum of hatred for a minimum of reason.”
                  Abraham J. Heschel (Jewish theologian and philosopher, 1907-1972)

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                  • #10
                    Re: Building on previously disturbed soil

                    A few general comments...Many footing assumptions for residential construction are based on 16" wide footings and too many contractor want to keep it small therefore saving a yard of concrete. When soil conditions appear questionable it's ok to build bigger(wider).

                    Say you have typical 2500psf soils, pretty much toward the lower end of the capacity charts. A 1' section of 16"w footing is 192 sq. inches of bearing ~ 3341#. Upsizing to a 20"w footing is 240 sq. inches ~ 4176# or a 25% increase in bearing capacity.

                    Quick example, a 14' x 20' addition, is 48+/- LF of footing. A 10"d 16"w footing takes ~2cy of concrete.....a 20"w footing takes ~2.5cy BUT THE SAME LABOR to erect and pour! It's a no brainer to me to upsize when in doubt!
                    Take Care

                    Jim

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                    • #11
                      Re: Building on previously disturbed soil

                      Originally posted by Dick Seibert View Post
                      Dave:

                      It depends upon your area, ever since the 1998 UBC we've had to get geotechnical studies on everything we build, even a small addition. Since the geotechs always require pier and grade beam foundations we have to get Chapter 17 Special inspections on all piers. In fact in most areas, including Silicon Valley where I am now building, we have to get Geological studies as well as geotechnical studies, that means a trench large enough for the County Geologist to walk through and examine the soils layers.
                      This is the thing. We all live in different geological areas with conditions unique to each area. I think the detailed soils investigation in your area would relate to the active fault lines and landslide dangers. We live on an ancient lake bed in an area that's never experienced earthquakes. With the exception of glacial till deposits there is nothing here but clays, silt and sand. Even within a forty or fifty miles of here there are varying local conditions that we don't have to deal with here and in our municipality there are areas that are built on played out gravel pits or on landfilled and reclaimed wetlands that would require subsurface investigation.

                      Originally posted by Dick Seibert View Post
                      the tract builder had only done one study in the entire subdivision, then applied the soils values from that one study to the entire subdivision, they were suing the AHJ for malfeasance for not requiring studies on each lot.
                      That's always going to be the risk and even if you do drill boreholes how can you guarantee that the boreholes will find hidden local conditions. On commercial jobs where we always have a borehole log it's pretty common to run into old foundations, cisterns, buried tanks and sewer lines, cars, garbage dumps and even graves. Unless you drill right on top of them the borehole won't find it and if you dig near some of these buries items without uncovering them you can hit pockets of disturbed soil and unless you know what to look for you won't recognize it.

                      If a soils report isn't required and your competition isn't providing one I guess it comes down to your tolerance for risk.

                      I learned about 10 years ago, quite by accident that I can dowse subsurface objects. If i'm doing an excavation I do a quick check and I have found abandoned drains that the engineers have sworn can't be there because they are not in any drawings. (in this case the engineers and the owner got quite upset when I told them there was something down there. They, like myself don't believe dowsing is superstitious bunk however for some reason I can't explain it works for me)

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                      • #12
                        Re: Building on previously disturbed soil

                        It is pretty simple here, new homes and additions require a building permit and engineered frames and foundations. Engineers require soils testing and will not design foundations without soils test.
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                        • #13
                          Re: Building on previously disturbed soil

                          I always do a soils test and engineer my foundations, even when I could get by with following prescriptive code. (I work in 3 different jurisdictions)

                          All construction represents compromise between economy and reliability. Nothing is designed to withstand every conceivable event or unknown factor--soil tests and engineered foundations included. However, soil tests and engineered foundations, IMO, are a relatively small cost for the value and risk management they provide.

                          I don't ever want to be on the witness stand with an attorney asking me why I didn't spring for a $500 test that could have prevented the failure of a multi-hundred thousand dollar structure. "Well sir, I poked the ground with my steel rod and it seemed good" isn't going to make you look very smart. Paying a state-certified expert to test and design goes a long way to reduce liability.
                          Bill

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                          • #14
                            Re: Building on previously disturbed soil

                            Our attorney advises us to let owner order and pay for soils testing as well as engineering.
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                            • #15
                              Re: Building on previously disturbed soil

                              I've only rebuilt on infill lots, so am pretty sure they will provide adequate bearing. As well, the building inspectors see and test the footing area before we pour. Despite that, if anything looks out of the ordinary I have a geotech firm inspect and give me their opinion. One that surprised me was when I went several feet below the pre-existing 1948 bungalow basement and it looked (and was) the ten-thousand year old shorefront of the predecessor to Lake Ontario. Beachfront with water-washed rocks. Fully capable of supporting a six-storey office building I was told. Just widen the footings.
                              "The fatal flaw of all revolutionaries is that they know how to tear things down but don’t have a f**king clue about how to build anything." Jim Goad

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