Commercial Building Inspector B2Footings and FoundationsHard
A commercial building is being constructed on a site with a high water table, requiring a permanent dewatering system during foundation excavation and construction. What is the primary concern for the long-term stability of the building's shallow foundations if the dewatering system is discontinued after construction?
- AIncreased seismic vulnerability due to liquefaction.
- BAccelerated corrosion of steel reinforcement in the footings.
- CReduction in soil bearing capacity and potential settlement.
- DIncreased risk of frost heave due to saturated soils.
Show answer & explanationAnswer & explanation
Correct answer: C. Reduction in soil bearing capacity and potential settlement.
When a dewatering system is discontinued, the water table will rebound to its natural level. If the foundation was designed assuming a lower water table (dewatered conditions), the re-saturation of soils can lead to a significant reduction in effective stress and thus a reduction in soil bearing capacity. This loss of bearing capacity can cause excessive settlement of the foundations.
Why the other options are wrong
- A. Liquefaction occurs in loose, saturated sands during seismic events; while a high water table is a factor, the direct consequence of discontinuing dewatering on foundation stability is typically settlement due to reduced bearing capacity, not necessarily liquefaction.
- B. Corrosion is a concern in wet environments, but the immediate and primary structural stability issue from discontinued dewatering is the loss of soil bearing capacity.
- D. While saturated soils contribute to frost heave, the primary concern with discontinued dewatering is loss of bearing capacity, especially if the foundation was designed for drier conditions.
Dewatering Discontinuation Impact
The potential negative effects on foundation stability when a temporary dewatering system is turned off and the groundwater table rises.
- Reduces soil effective stress.
- Decreases soil bearing capacity.
- Can cause excessive foundation settlement.
Memory trick: Water comes back, the soil gets soft, and the foundation might drop off.