CSLB General Building (B)Site Work, Foundations and ConcreteHard
During excavation for a building foundation, the crew encounters groundwater seeping into the excavation, leaving the subgrade soil saturated and soft. What is the most appropriate action before proceeding with the footing pour?
- APour the concrete directly over the saturated soil to seal out further water intrusion
- BIncrease the footing depth without addressing the water intrusion
- CIncrease the cement content of the concrete mix to compensate for the wet subgrade
- DDewater the excavation and allow or improve the subgrade until it is firm and stable before placing footings
Show answer & explanationAnswer & explanation
Correct answer: D. Dewater the excavation and allow or improve the subgrade until it is firm and stable before placing footings
Saturated, soft subgrade soil has reduced bearing capacity and can lead to settlement or failure of the footing. The proper approach is to dewater the excavation using methods such as sump pumps or well points, and if necessary, remove and replace or stabilize the soft soil, ensuring a firm, dry subgrade before placing footings.
Why the other options are wrong
- A. Pouring over saturated soil traps water and weakens the bond between concrete and subgrade, risking future settlement.
- B. Simply going deeper without resolving the water issue does not guarantee stable bearing soil is reached.
- C. Adjusting cement content addresses concrete mix design, not the underlying problem of a weak wet subgrade.
Excavation Dewatering
The process of removing groundwater from an excavation before placing footings, ensuring the subgrade soil is firm and stable enough to provide adequate bearing capacity.
- Methods include sump pumps and well points
- Saturated soil has reduced bearing capacity
- Subgrade must be firm and dry before footing placement
Memory trick: 'Dry it before you pour it' — never build footings on saturated soil.