Quote:
Originally Posted by bomberjet
Houses really shouldn't be built on any type of backfill. That's asking for problems. The ground should be excavated down to underside of basement, in situ material compacted in place, then built up from there. Or something like that. Piled foundations are better, but must be done properly and are more expensive.
Compacting of the fill below said houses is key. And from what I see of people in construction, they will do the bare bones minimum. In grading works, they test for compaction. Contractors will literally compact the same area 10 times, then test it and be like " we all good son. we out." Meanwhile the rest of the area is under compacted. This is bad for differential settlement. If it's going to settle, which most likely it will, you want it to settle evenly. Having one area more compacted then another leads to uneven settlement, where you get the issues.
Localizing this to a house foundation, there should be no issues with quality control.
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Lots of incorrect info.
During the grading design of new neighborhoods, we generally design to balance the site. Material cut from the storm pond is used else where on site. Additionally, we try to bury as much topsoil as possible in back of lot trenches and parks to avoid the cost of hauling topsoil off site. These borrow pits also generate large amounts of common material that can be used in compaction elsewhere on site.
Generally fills below a certain elevation relative to front of lots is considered "engineered fill". This depends on many factors, but the intent is to ensure that basements are cut into material that has been graded and compacted by the developer not the home builder (builders and their trades are too dumb). It's closely monitored by geotechnical consultants and each lift (generally 1ft) needs to pass before they can move on to the next. the top couple feet of the lot doesn't need to be compacted the same way.
Grading has so many factors...material balance, depth of cover of utilities, major overland flow toward the storm pond...etc...large engineered fills are far easier to deal with by comparison and as a result we don't really think twice about it.