A building site has soil with a standard penetration test (SPT) N-value of 8. According to CBC, this soil is classified as which site class?
Correct Answer
D) Site Class E
CBC classifies soils with SPT N-values less than 15 as Site Class E (soft clay soil), which affects seismic design parameters and foundation requirements.
Why This Is the Correct Answer
CBC (aligned with ASCE 7) classifies soils with average SPT N-values less than 15 blows per foot as Site Class E (soft soils). Site Class E indicates weak, compressible soils that significantly amplify seismic ground motion, triggering more conservative seismic design parameters and scrutiny for foundation design.
Why the Other Options Are Wrong
Option A: Site Class D
Site Class D corresponds to 'stiff soil' with SPT N-values between 15 and 50 blows per foot. An N-value of 8 is well below this range and does not qualify for Site Class D.
Option B: Site Class C
Site Class C corresponds to 'very dense soil and soft rock' with N-values greater than 50 blows per foot or shear wave velocities in a specific higher range. An N-value of 8 is far too low for Site Class C.
Option C: Site Class F
Site Class F is reserved for soils requiring site-specific hazard evaluation (e.g., liquefiable soils, sensitive clays, peats). While an N-value of 8 indicates weak soil, the classification alone based on N-value places it in Site Class E, not F. Site Class F requires additional specific failure-mode criteria beyond just low N-values.
Memory Technique
N < 15 = Site Class E. Remember: 'Extremely soft soil = E.' The lower the N, the worse the soil, and E comes after D alphabetically just as SPT values drop below 15.
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