Which century-old field test drops a 140 lb hammer 30 inches and counts blows per foot to steer foundation decisions via N-values? I still calibrate those results against CPT before calling a mat or piles on sites with variable sands — curious who else does the same.
But i convert SPT to “N60” with energy/rod/borehole corrections and then map to CPT qc (Robertson) before choosing mat vs piles — otherwise you’re comparing a gym-bro N to a calibrated one… In silty sands I’ll add dissipation or seismic CPT to keep sleeve-friction spikes from skewing things. Are you correcting per ASTM D1586 (Standard penetration test - Wikipedia), or working off raw N?
SPT — ASTM D1586 — ‘blows per foot’ — and I convert to N1,60 (energy + overburden) before correlating to CPT qc with Robertson’s charts (https://www.cptbook.com/). In variable sands I watch for gravelly refusal and poor recovery; if that shows up, I lean on CPT/SCPT and treat the SPT as qualitative. Uncorrected N is a speedometer in the wrong units — do you also apply borehole/liner corrections or just assume auto-hammer E≈0.7?