You're building on a hillside lot with a 15% slope. The soil report indicates potential liquefaction. What foundation approach does CBC typically require?
Correct Answer
B) Deep foundations extending below liquefiable layers
CBC requires deep foundations (piles or caissons) that extend through liquefiable soils to competent bearing material to prevent foundation failure during seismic liquefaction events.
Why This Is the Correct Answer
Deep foundations extending below liquefiable layers are required by CBC when a soil report identifies liquefaction potential. Liquefaction causes soil to lose its bearing capacity during seismic shaking, essentially behaving like a liquid. Deep foundations (piles, drilled piers/caissons) bypass the liquefiable stratum and bear on competent material below, preventing catastrophic foundation failure.
Why the Other Options Are Wrong
Option A: Standard shallow footings with extra reinforcement
Standard shallow footings with extra reinforcement are insufficient when liquefaction is identified. Adding rebar to a footing sitting in liquefiable soil does not solve the problem — the soil itself loses bearing capacity during an earthquake, and no amount of footing reinforcement compensates for soil that can no longer support load.
Option C: Thickened edge slab with post-tensioning
Thickened edge slabs with post-tensioning address differential settlement and expansive soils, not liquefaction. While PT slabs are excellent for certain soil conditions, they remain at shallow depth and would still be founded in liquefiable material, making them equally vulnerable to failure during seismic liquefaction.
Option D: Stepped footings following the slope
Stepped footings following the slope address grade changes on hillsides but do not address liquefaction. Stepped footings are a common solution for sloped terrain to maintain minimum embedment depth, but if the soil is liquefiable, the footing depth relative to slope is irrelevant — the soil will still lose capacity regardless of how well the footing follows grade.
Memory Technique
Liquefaction = soil turns to liquid. You cannot float a building on liquid soil with a shallow footing. Deep foundations are like stilts that reach past the liquid layer to solid ground below. Picture a building on stilts driven through water to the lake bed — that is the concept of deep foundations bypassing liquefiable soils.
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