Land and site · Civil works on site · site-20

Prepare foundation ground

Get the ground ready for the foundation the design module specifies: excavated to firm, undisturbed soil, dry and free of organic material.

How

Excavate footings to the depth the engineer or code specifies, including frost depth in cold climates.
Remove topsoil, roots and soft pockets from under footings; do not build on organic or loose fill.
Keep excavations dry: pump out water and divert runoff away.
Have the bearing soil inspected where your building authority requires it before placing concrete or foundation material.
Example (US, IRC Table R401.4.1 as adopted in Seattle): where no soil test is required, the code presumes bearing values from 1,500 psf for clay and silt soils, 2,000 psf for sands, 3,000 psf for gravel, up to 12,000 psf for crystalline bedrock. The building official can demand a soil test where soils look questionable. These presumptive values apply only to code-prescribed foundations; anything else is for your engineer.
Install foundation drainage if the design calls for it.

Safety: Do not enter trenches deeper than your local safe limit without shoring or battered sides.

Site and Soil Evaluation for Onsite Wastewater Systems (MF2645). Kansas State University Research and Extension, accessed 2026-11
Onsite Wastewater Treatment Systems Manual, Chapter 5: Treatment System Selection. US Environmental Protection Agency (copy hosted by Iowa State University), accessed 2026-11
2021 Seattle Residential Code, Chapter 4: Foundations (IRC Table R401.4.1 as adopted). City of Seattle, Department of Construction and Inspections, accessed 2026-11