Required SPT N60 for a Target Bearing (Meyerhof)
The inverse of the SPT-bearing tile: the energy-corrected N60 the sand must show to carry a target allowable pressure at a 1 in settlement, N60 = qa_target / qa(N60=1). A 6 ft footing 2 ft deep needing 5 ksf wants N60 ~20 (round up for design). A serviceability check against the boring's N-value, not the ultimate bearing. A design aid; the geotechnical report governs.
Formula and source
N60 = qa_target / qa(N60=1), the Meyerhof SPT allowable qa = [N60/4 (B <= 4 ft) or (N60/6)((B+1)/B)^2] x Kd solved for the blow count; Kd = min(1 + 0.33 D/B, 1.33).
The Meyerhof SPT settlement-based allowable bearing on sand solved for the blow count, as compiled in Das, Principles of Foundation Engineering, by name.
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