A geotechnical report indicates the presence of liquefiable soils at a depth of 15 feet. What is the primary concern for foundation design in this condition?
Correct Answer
D) Loss of soil support during seismic events
Liquefiable soils lose their bearing capacity and behave like liquid during seismic events, potentially causing foundation settlement or failure.
Why This Is the Correct Answer
Soil liquefaction occurs when saturated, loosely packed soil loses its shear strength during seismic shaking — behaving temporarily like a liquid. A foundation bearing on or above liquefiable soil at 15 feet can experience sudden loss of support, causing differential settlement, tilting, or catastrophic structural failure. This is the definitive primary concern in foundation design when liquefiable soils are identified.
Why the Other Options Are Wrong
Option A: Increased bearing capacity
Liquefaction decreases bearing capacity, not increases it. The fundamental problem is that soil that normally supports load becomes unable to do so during an earthquake. 'Increased bearing capacity' is the opposite of what occurs.
Option B: Reduced excavation costs
Liquefaction has no meaningful positive effect on excavation costs. If anything, the presence of liquefiable soils typically increases costs because mitigation measures (deep foundations, ground improvement, compaction grouting) are required.
Option C: Improved drainage characteristics
Liquefiable soils are saturated — they contain excess water. Drainage characteristics are a separate geotechnical concern unrelated to liquefaction risk. Improved drainage actually helps prevent liquefaction by reducing pore water pressure, but liquefaction itself does not improve drainage.
Memory Technique
Visualize liquefaction literally: the soil 'liquefies' — it becomes like quicksand. A building sitting on quicksand loses all support. That's the concern: what was solid ground during normal conditions becomes a liquid-like mass during an earthquake, pulling support out from under your foundation.
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