Commercial Building Inspector B2Structural SystemsMedium
A building inspector is examining the foundation plans for a new commercial warehouse. The plans specify a mat foundation over expansive soils. To mitigate the effects of soil movement, the design includes void-forming materials beneath the mat. According to the IBC, what is the primary purpose of these void-forming materials?
- ATo create an unconfined space allowing soil expansion without lifting the slab.
- BTo insulate the foundation from temperature fluctuations.
- CTo increase the bearing capacity of the soil.
- DTo provide drainage for hydrostatic pressure relief.
Show answer & explanationAnswer & explanation
Correct answer: A. To create an unconfined space allowing soil expansion without lifting the slab.
Void-forming materials, often referred to as 'void forms' or 'honeycomb forms,' are placed beneath foundations on expansive soils. Their primary purpose is to create an unconfined space into which the expansive soil can swell without exerting uplift pressure on the overlying foundation slab. This prevents structural damage from soil heave.
Why the other options are wrong
- B. Insulation is typically achieved with rigid foam boards, not void-forming materials, and addresses thermal bridging, not soil expansion.
- C. Void-forming materials do not increase bearing capacity; they address expansive soil issues.
- D. While some foundations include drainage, void forms' primary role is not hydrostatic pressure relief.
Void-Forming Materials
Lightweight, crushable materials placed beneath foundations on expansive soils to create a space for soil heave without exerting uplift on the structure.
- Used to mitigate damage from expansive soils.
- Creates a compressible zone beneath the foundation.
- Protects against uplift forces caused by soil swelling.
Memory trick: Expanding soil needs a void to avoid a fight with the slab.