4.8 Minimal Excavation Foundations

Grading and excavation during construction can degrade the infiltration and storage capacity of native soils. A minimal excavation foundation is a building BMP that minimizes mass grading and site disturbance by distributing a building’s structural load onto piles or limited excavation perimeter walls.

As noted in the Low Impact Development Technical Guidance Manual for Puget Sound (2012), “[m]inimal excavation foundation systems take many forms, but in essence are a combination of driven piles and a connecting component at, or above, grade. The piles allow the foundation system to reach or engage deeper load-bearing soils without having to dig out and disrupt upper soil layers, which convey, infiltrate, store, and filter stormwater flows” (WSU-PSP, 2012).

Piles are a less disruptive approach to site development. The piles may be vertical, screw-augured, or angled pairs that can be made of corrosion-protected steel, wood, or concrete. The connection component handles the transfer of loads from the above structure to the piles and is most often made of concrete. Cement connection components may be pre-cast or poured on site in continuous perimeter wall or isolated pier configurations.

Although not as widely used as other LID practices, minimal excavation foundations hold an important place within LID guidance. Minimal excavation foundations can make sites developable that would be otherwise undevelopable. Sites with shallow depth to bedrock, high water tables, or challenging soils such as the lithosols and caliche soils that occur in various areas of eastern Washington can be made buildable through the use of minimal excavation foundations.