A geotechnical report recommends drilled piers extending 25 feet below grade for a commercial building. The report cites 'potential liquefaction' as the primary concern. What soil condition is most likely present?
Correct Answer
C) Saturated loose sand
Liquefaction typically occurs in saturated, loose to medium-dense granular soils (sands and silts) during seismic events, requiring deep foundations to reach stable bearing strata.
Why This Is the Correct Answer
Liquefaction is a phenomenon that occurs when saturated, loose to medium-dense granular soils (primarily loose sand, sometimes silty sand) temporarily lose their shear strength during seismic shaking. The water pressure in the saturated voids increases to the point where soil particles behave like a liquid. Deep drilled piers are specified to bypass the liquefiable layer and anchor into stable bearing strata below.
Why the Other Options Are Wrong
Option A: Expansive clay
Expansive clay swells and shrinks with moisture changes, causing heave and settlement — but this is not liquefaction. Clays are cohesive and do not liquefy during earthquakes.
Option B: Bedrock at shallow depth
Bedrock at shallow depth is actually a desirable foundation condition providing excellent bearing capacity. It is the opposite of a liquefaction concern and would not require deep piers for seismic reasons.
Option D: Organic peat
Organic peat is a problematic soil due to its compressibility and long-term consolidation settlement, but peat does not liquefy. Peat is organic, highly compressible, and unsuitable for direct bearing, but the mechanism of failure is settlement, not liquefaction.
Memory Technique
Liquefaction = 'Liquid Sand' — saturated loose sand acts like liquid during an earthquake. The word 'liquefaction' contains 'liquid,' which reminds you of water-saturated, granular (sandy) soil.
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