A below grade foundation refers to the section of a building's foundation system that is situated beneath the natural surface level of the surrounding ground.
Based on terminology used in real estate and building analysis, "below grade" is a term that describes a space that is below ground level – usually referred to as a basement. Furthermore, in building analysis terms, it describes conditions where the envelope is in direct contact with the ground.
Connecting this concept to foundations, the "below grade" part of a foundation is the portion that lies underground. Foundations serve as the crucial link between a building and the earth, transferring structural loads and providing stability. Because foundations are inherently designed to anchor the building to the ground, a significant part of their structure is typically below the surface. This underground section is in direct contact with the earth, fulfilling the "below grade" description as a part of the building envelope interacting directly with the ground.
Key Aspects of Below Grade Foundations
Understanding the below grade portion of a foundation is important for several reasons:
- Structural Support: It's the primary part of the foundation that bears the weight of the building and distributes it evenly into the soil.
- Stability: Being embedded in the ground helps resist lateral forces like wind or seismic activity.
- Protection: It provides a barrier against ground moisture, soil movement, and frost heave.
Types of Below Grade Foundation Components
Different foundation types have distinct below grade components:
- Footings: Wide, shallow elements, usually concrete, that spread the load from the foundation walls or piers over a larger area of soil. These are entirely below grade.
- Foundation Walls: Vertical walls (often concrete or masonry) that extend from the footings up to the ground level or slightly above. A significant portion of these walls is below grade.
- Slabs-on-Grade: While primarily at ground level, the thickened edges or footings beneath a slab-on-grade extend below grade.
- Piers or Piles: Columns or shafts driven deep into the ground to bear loads on stable soil layers. The majority of their length is below grade.
Essentially, any part of the foundation structure that is positioned underground is considered "below grade foundation."
