Commercial Building Inspector B2Footings and FoundationsHard
A structural engineer has designed a shallow foundation system for a commercial building on a site with highly expansive clay soils. Which of the following foundation types is generally *least* suitable for this soil condition without significant soil treatment or specialized design?
- AContinuous strip footings with grade beams
- BConventional isolated spread footings
- CDrilled piers extending below the active zone
- DMat or raft foundation
Show answer & explanationAnswer & explanation
Correct answer: B. Conventional isolated spread footings
Expansive soils swell when wet and shrink when dry, causing significant differential movement. Conventional isolated spread footings are highly susceptible to this movement because they are typically shallow and each footing can experience different amounts of uplift or settlement, leading to structural damage. Mat foundations, drilled piers, and properly designed strip footings with grade beams are better equipped to handle or bypass expansive soil movements.
Why the other options are wrong
- A. Continuous strip footings with stiff grade beams can help bridge over localized soil movements better than isolated footings.
- C. Drilled piers extend below the active zone of moisture change, anchoring the structure to stable soil.
- D. Mat foundations distribute loads over a large area, bridging over localized soil expansion/contraction.
Expansive Soil Foundation
Foundation types and strategies to mitigate the effects of soil volume changes due to moisture content.
- Expansive soils swell when wet, shrink when dry.
- Causes differential movement and structural damage.
- Deep foundations or stiff shallow foundations are preferred.
Memory trick: Expansive soils need deep roots or a strong raft to ride the swelling waves.