Sandy writes: We were told the lot we were buying had “bad dirt” which didn’t compact properly. So the builder is supposedly removing all the bad dirt and replacing it so that a home on a slab foundation can be built. How common is this practice and what can we do to ensure everything was done properly? Also, my husband saw them removing dirt and dumping it on the lot next door which needs building up.
Steve Bliss, of BuildingAdvisor.com, responds: There are some types of soils that do not compact well – most commonly soils with a lot of organic materials, such as topsoil, and some types of heavy clay soils. Less common “problem” soils are things like peat and expansive clays. It is also possible that the site was filled sometime in the past with poor quality soil or that the was not properly compacted.
It is common, and good building practice, to remove organic or other problem soils, before pouring a slab. If the soil does not have sufficient strength (bearing capacity), then the slab foundation will crack.
The removed soil is typically replaced with a granular soil (sand and gravel), which compacts easily and has high bearing capacity. Any soil used as fill under a foundation should be compacted.
What type of “bad dirt” was removed and where it is going is another question. It may be that the soil is being stored there temporarily or that it is being used on a part of the property that will not be built on.
I would suggest asking the building for a more technical description of the “bad dirt” as you want a better understanding of the issues. Also ask if the native soil remaining needs further compaction before pouring the slab. If the soil is native, undisturbed soil (as opposed to fill), and is a strong enough soil type, then it may not need any compaction prior to pouring the slab. The specific concrete mixture, foundation design and reinforcement, and weight of the building are all factors that determine if the soil strength is sufficient.
If you still have concerns after talking to the builder, I’d suggest hiring a soils, geotechnical, or civil engineer to spend an hour at the site to tell you if you are likely to have foundation problems.