Commonsense Geotechnical

Now that’s a mouthful for a Wednesday morning, geotechnical, or soils mostly has to do with the first line of defense keeping buildings against gravity. Know your geotech and build your house upon the rock, sometimes literally. Ignore it, or assume, and you end up with the Leaning Tower of Pisa, or worse. For instance, in many areas of the country, the underlying clay soils expand (a lot) when wet: foundation irrigation under the building is the only way to build, and better get them away from the exterior walls or they will be cracked like a lobster. And where are they? Check out this map, pretty random pattern.












So what common sense can we apply:

  1. Know what you are building upon, I have seen buildings on old landfills that had sunk 4″ in the width of an apartment. I have seen a six by six jacked 18′ down into an old stump dump. I have found shingles 6′ down on a sandy point. So it’s not just Love Canal anymore, if it has a questionable past don’t ignore it. I have seen a warehouse built on a peat bog where the slab, at the time, was 18″ thick, because every time it sank, they placed another layer on top- kind of like looking at the sedimentary rocks along the interstate highway cuts.
  2. Test. Test. Test. But remember, until the entire excavation is done, you can not breathe easily. I say a quarter acre lot where they dug all four corners and the middle to eight feet with no refusal (our way of saying we didn’t hit a big rock or ledge- a.k.a. mother earth), and yet there was a huge ledge cap with a six foot crack in the middle of the lot!
  3. Water changes everything- what feels firm underfoot when dry can turn to soup or worse when wet.
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