Understanding Soil Settlement for the Builder and How to Educate the Homeowner

With increases in housing starts and the high price of land, lots that once seemed unbuildable are now seeing homes and other structures. Some of these lots have been filled for better curb appeal and an easier sale.

Understanding soil structure and settlement has always been a critical component of excavation. Without it, problems such as settling and cracking of the foundation, slabs, stoops, and sidewalks can manifest months or years later.

Typically, in the design stages of commercial construction, a soil engineer prepares a subsurface exploration and foundation evaluation that would include a serious of soil borings. A soil bore can give you a glance at the subsurface, type of material, density, and moisture content, as well as possible water table at any given depth. This information also gives the excavator an idea of what kind of material they will be excavating and if it can be used as structural fill under slabs and walkways or if it would need to be removed from the site. Information from soil bores can help excavating contractors bid projects, but most residential construction projects don’t involve a soil engineer or soil borings.

Experienced contractors and inspectors make sure the bottom of the footing, once excavated, has no organic material (black dirt or topsoil) and virgin ground has been reached before pouring footings. The sides of the excavated walls can shed light about whether the site has been filled in the past. I believe there is an advantage in choosing a local excavator who has experience working near the build site. Clay, hardpan, silt, peat, and sand should be obvious to an experienced builder, but a local excavator may have knowledge about other materials and their structural integrity from working in that area over the years.

If questionable soil conditions exist, I recommend seeking out a geotech engineer for testing. If the soil is inadequate for building, there are a few possibilities an engineer may consider.  

1. The width of the footing can be redesigned to spread the weight of the building across more of the surface area in the trench, which is why footings are known as spread footings.
2. If you have to undercut a few inches, you may be able to add compacted fill back up to the planned subgrade, extend the height of the footings, or extend the height of the wall to the proper top of wall height.
3. Concrete slurry can be an option when undercutting. Concrete slurry is a controlled density fill that is self-compacting and used primarily as backfill in lieu of compacted fill. Trenches can be filled back to subgrade with slurry and then cured before forming of footings. Slurry can have a compressive straight of 1200 psi (pounds per square inch).
4. Larger undercuts can be filled with approved engineer fill and compacted in lifts. The size of the compact machine will determine specifications. Widening the trench may be part of this solution.
5. Pile driving is when poles are driven down into the bedrock to give support to the footings. This option is used when unacceptable soil cannot be removed, when too much must be removed, or for much heavier buildings.

Using the correct engineer fill and the proper installation of that fill under floor slabs and sidewalks is a must to prevent settling. Engineer fill, when placed correctly, will not settle. Some fill must be compacted in lifts, such as every twelve or eighteen inches, depending on the type of fill and the size of the compacting machine. The proper moisture content in the fill is critical for maximum compaction. You need enough moisture in the fill for the granules to slide into place while compacting—too much moisture and the fill will become soft, but too little moisture and the fill will not compact properly.

If poor soil conditions exist, the contractor should educate the homeowner when the proposal for the project is submitted. The proposal may include unit pricing such as:

1. Price per cubic yard for excavation and removal of unacceptable soils.
2. Price per cubic ton for additional compact fill material.

If the site is suspected of having an old foundation or rock underground, a unit price per cubic ton for excavation and removal may also need to be included in the proposal.

Typically, it costs more to remove unacceptable soil from urban locations because the materials must be trucked out of the city, whereas rural locations often have more places to dispose unacceptable soil. Sometimes, leftover fill can be used to build berms and enhance landscaping.