A building site has been classified as having liquefiable soils. What foundation system is typically recommended by CBC for this condition?
Correct Answer
A) Deep foundations extending below liquefiable layer
Deep foundations that extend below the liquefiable soil layer are required to transfer loads to stable soil or rock.
Why This Is the Correct Answer
Deep foundations — such as driven piles or drilled piers — are required when a site has liquefiable soils because they transfer structural loads through the unstable liquefaction zone down to firm, stable soil or bedrock beneath. Shallow foundation systems cannot provide adequate support when the upper soil layer loses strength and behaves like a liquid during seismic shaking.
Why the Other Options Are Wrong
Option B: Shallow spread footings with grade beams
Shallow spread footings with grade beams sit within or near the liquefiable layer. During an earthquake, the soil beneath them could liquefy, causing uneven settlement, bearing capacity failure, and structural collapse. Grade beams tie footings together but do not solve the root problem of unstable bearing material.
Option C: Floating slab on aggregate base
A floating slab on an aggregate base provides no deep anchorage. The aggregate base itself may be within the liquefiable zone. This approach is suited for uniform, stable soils — not for sites with known liquefaction potential.
Option D: Mat foundation with post-tensioning
A mat foundation distributes loads over a large area, and post-tensioning strengthens the slab itself, but neither measure addresses the fundamental problem: the soil beneath the mat can still liquefy. Without extending past the unstable layer, a mat foundation remains vulnerable.
Memory Technique
Think 'Liquefiable = Go Deep.' Picture soil turning to liquid — you need to 'dive' your foundation past the danger zone to solid ground below. Deep foundations are like anchors dropping past the soft mud to hit the rocky bottom.
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