Stress Below Footings & Pile Capacity

Worked

• A 6 ft × 6 ft footing carries 180 kips. At 6 ft below its base, Δσ = 180,000/(12 × 12) = 1,250 psf — a quarter of the 5,000 psf contact pressure.
• A 14 in square precast pile 40 ft long in clay with s_u = 1,000 psf and α = 0.8: perimeter 4.67 ft, so Q_s = 0.8 × 1,000 × 4.67 × 40 = 149 kips. Tip area 1.36 ft², so Q_p = 9 × 1,000 × 1.36 = 12 kips. Q_ult = 162 kips; at FS = 2.5, 65 kips allowable.