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Are you sinking ??

In our area we have basically two different soil conditions. The "flat" part of town is bay mud anywhere from a couple feet to a couple hundred feet deep. It's considered a zone of "moderate liquefaction" and will act like jello in an earthquake. Modern houses built on this typically have about 6 foot deep by 2-3 foot wide "slurry" poured prior to having a true foundation built with at least a 2 foot wide footing. The older houses that don't have an adequate footing are definitely sinking. Some a lot more than others, and some rather unevenly. I usually see about a dozen or so houses a year that have to have some kind of foundation work done. In the most extreme cases the house is lifted and a complete new foundation built under them. Others have had partial lifts, jacks, or helical piers added to stabilize/repair. The rest of town is built in/on/around the mountains that rise steeply out of the bay mud and the soil is a highly compacted mix of clay and sand/silt that is very stable and tends not to move at all. There are many 100+ year old houses on post and pier that haven't moved at all, even with the frequent, and sometimes large, earthquakes that we get here. We are a seismic zone D/E in northern CA.
 
I would guess clay, CO does not have a statewide code, has plenty of fracking areas, yet it has only a couple small mountain areas that report problems.
 
Our city had a builder that liked to bury stumps between the houses. What could go wrong? 50 years latter, now that the stumps are rotting, we are finding that the area of influence is in fact a 45 degree angle. Lots of foundations with portions that have no soil under the footing.
 
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