CSLB General Building (B)Planning and EstimatingHard
A contractor is evaluating the soil conditions for a new foundation. The geotechnical report indicates 'expansive clay soils'. What is the most significant concern for foundation design in these conditions?
- AIncreased risk of differential settlement due to soil swelling and shrinking.
- BHigher permeability leading to excessive water infiltration.
- CDifficulty in excavation due to high compaction.
- DLower bearing capacity requiring larger footings.
Show answer & explanationAnswer & explanation
Correct answer: A. Increased risk of differential settlement due to soil swelling and shrinking.
Expansive clay soils are characterized by their tendency to significantly swell when wet and shrink when dry. This cyclic movement can cause differential settlement, leading to severe structural damage to foundations and buildings. This is the primary concern for foundation design.
Why the other options are wrong
- B. Expansive clays typically have low permeability, which contributes to water retention and swelling, but high permeability is not the main issue.
- C. While some clays can be difficult to excavate, the primary concern with expansive clay is its volume change.
- D. While bearing capacity is always a factor, the more significant issue with expansive clays is their volume change, not necessarily inherently low bearing capacity, though it can be a factor.
Expansive Clay Soils
Soils that undergo significant volume changes (swell when wet, shrink when dry) due to their mineral composition, posing risks to foundations.
- Primary concern: differential settlement.
- Can cause cracks in foundations, walls, and slabs.
- Requires specialized foundation design (e.g., deep piers, structural slabs, moisture control).
Memory trick: Expansive clay, like a sponge it plays, swelling and shrinking, causing foundation woes and delays.