Spt Cpt Correlations: SILT
SPT blow count (N-value) and CPT cone resistance (qc) correlations to engineering properties — friction angle, relative density, undrained shear strength, and elastic modulus by soil type and density class
| soil type | n value mid | density consistency | elastic modulus tsf max (tsf) | elastic modulus tsf min (tsf) | friction angle deg max (deg) | friction angle deg min (deg) | friction angle deg typical (deg) | n value max (blows/ft) | n value min (blows/ft) | notes | qc max tsf (tsf) | qc min tsf (tsf) | qc typical tsf (tsf) | su psf max (psf) | su psf min (psf) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| SILT | 1 | Very Soft | 8 | 2 | 22 | 20 | 21 | 2 | 0 | ML/MH silt; phi from Holtz & Kovacs 1981; Es ~2-4x Su for normally consolidated | 5 | 0 | 2.5 | 200 | 0 |
| SILT | 5 | Soft to Medium | 25 | 8 | 26 | 22 | 24 | 8 | 3 | Silts: SPT less reliable; use CPT qc or field vane; phi net of fines content | 20 | 5 | 12 | 800 | 200 |
| SILT | 14 | Stiff | 60 | 25 | 30 | 26 | 28 | 20 | 9 | Organic silt: reduce phi 3-5 deg; N60 often underestimates strength in clean inorganic silt | 60 | 20 | 40 | 2,000 | 800 |
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