When encountering liquefiable soils during site preparation, which foundation approach does CBC typically require?
Correct Answer
B) Install deep foundations to stable soil
CBC requires deep foundations (piles or caissons) extending through liquefiable layers to reach stable, non-liquefiable soil or rock for adequate seismic performance.
Why This Is the Correct Answer
CBC requires that foundations on liquefiable soils extend through the liquefiable layer to reach stable, non-liquefiable soil or rock via deep foundation elements such as driven piles or drilled caissons. During an earthquake, liquefiable soils lose bearing capacity entirely, so any foundation element relying on them for support will fail. Deep foundations bypass the problem zone.
Why the Other Options Are Wrong
Option A: Increase footing width by 50%
Increasing footing width by 50% addresses bearing capacity under static loads but does nothing to solve liquefaction. Wider footings still sit in or above the liquefiable layer and will lose support during seismic shaking.
Option C: Apply soil stabilization chemicals
Soil stabilization chemicals (such as grouting or permeation) can be used in some cases, but CBC does not identify chemical stabilization as the primary or typical required approach. The code-preferred solution is bypassing the layer with deep foundations.
Option D: Use post-tensioned slabs exclusively
Mat (raft) foundations spread load across a large area but remain at or near the surface. They cannot transfer load below the liquefiable zone and will settle or tilt catastrophically if the soil liquefies beneath them.
Memory Technique
Liquefiable soil = 'quicksand' — you need to reach solid ground beneath it. Deep piles or caissons punch through the quicksand to the stable layer below. Go DEEP to get past the liquid.
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