Required SPT N60 for a Target Bearing (Meyerhof)

The energy-corrected N60 the sand must show to carry a target allowable pressure at a 1 in settlement.

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No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.

Details, formula, and sources

ScopeThe 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 or tableN60 = 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).

Source and editionThe Meyerhof (1956) SPT settlement-based allowable bearing on sand (N60/4 and (N60/6)((B+1)/B)^2 ksf; Das, Principles of Foundation Engineering, 8th ed., Eqs. 7.32-7.33, which carry no depth factor), solved for the blow count, with the depth factor Kd = 1 + 0.33 D/B (capped at 1.33) applied as in spt-bearing-capacity. Das prints that depth factor only with Bowles' (1977) modified form, N60/2.5 and N60/4 (Eqs. 7.34-7.35), which runs about 50% higher; this tile keeps the conservative original constants.

Free accessThe Meyerhof N-value allowable-bearing correlations are standard published foundation-engineering results.

What governsEstimate. AHJ and licensed professional govern.

API field namesqa_target_ksf, b_ft, d_ft, n60, kd

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