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re: Las Vegas Settlement

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I have no knowledge on the behavior of the soil types out west, but in Florida we have mostly sand with a lime rock base that varies from 50-200+ feet down (very generalized). Throw in some pockets of organics and clay for good measure... We normally get about 2000-2500 psf bearing capacity without a large deal of compaction and soil prep. We do have significant sinkhole activity in the Central FL area. These are caused by subterraenous erosion and leaching of the limerock layer that causes a collapse of the sandy layer on top of the limerock caves/aquifers/general voids.. Fluctuations in yearly mean water tables as well as unusually large amounts of rainfall can also have a large influence on this action, which can cause settlement as well as sinkholes.
 
After the hurricanes of 2004 I did a lot of forensic investigations of houses with cracks in masonry and wood framed with stucco walls, some with obvious settlement, others not so obvious. These structures ranged in age from 1 year to 130+ years old. The oldest one had the most severe settlement. Most of these structures were one and two story residential structures, so the gravity loads were not that large. The age of the building settling would not surprise me after what I have seen, even though it seems to be a generally accepted rule of thumb that most structures settle within 3-5 years of construction, which in most cases would make sense to me.
 
According to the Encyclopedia definition, gypsum and limestone are similar:
Gypsum, common mineral consisting of hydrated calcium sulfate (CaSO4·2H2O). It is a widely distributed form of sedimentary rock, formed by the precipitation of calcium sulfate from seawater, and is frequently associated with other saline deposits, such as halite and anhydrite, as well as with limestone and shale.
 
Out west there are some significant gypsum deposits as I understand.. For example in Utah, White Sands in NM, and a little town outside of Vail, CO is named Gypsum, so there must be plenty of gypsum out there. If gypsum and limerock are similar in their chemical makeup and properties, then I would look for recent large amounts of rain or snowfall that may have caused the soil to become wet or saturated, or raised the water table, and now it is drying out and settling. This can take place several months after the rainfall depending on the water table, or maybe months after the spring snow melt. Or as with Kipp's experience the gypsum was leaching out due to groundwater...
 
HTH,
Andrew Kester, PE
Lake Mary, FL

 
 
 
 
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