← Bearing Calculator
Direct CPT/CPTu su correlation tool. Enter measured cone data; all su methods computed and compared simultaneously. Best-estimate highlighted.
CPT Measurements
qc — cone resistance (MPa)
u2 — pore pressure tip (kPa)
fs — sleeve friction (kPa)
an — net area ratio
In-situ Stress State
σv0 — total vertical stress (kPa)
u0 — hydrostatic pore pressure (kPa)
σ'v0 — effective vertical stress (kPa)
Depth z (m)
Plasticity index Ip (%)
Corrected & Derived Parameters
Robertson (1990) Soil Behaviour Type
SBT Zone
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Normalised parameters (Robertson 1990)
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su by Method — All Correlations
MethodEquationsu (kPa)ApplicabilityReference
su vs Nkt — Sensitivity of cone factor choice (Nkt = 10 to 20)
Pore pressure ratio Bq — drainage behaviour indicator
Method notes & references

Nkt — cone factor (CPTu)

su = (qt − σv0) / Nkt. Nkt calibrated against lab su; 10–20 typical, 12–15 most common for stiff clays and tills. Must be site-specific.

Lunne, Robertson & Powell (1997); Aas et al. (1986) NGI

Nke — effective cone factor

su = (qt − u2) / Nke. Less sensitive to u2 measurement error at shallow depth. Nke ≈ 8–12.

Lunne et al. (1997) Chapter 5

ΔU method (NΔu)

su = (u2 − u0) / NΔu. Uses excess pore pressure directly. NΔu ≈ 6–10. Reliable only in soft–firm clays with good pore pressure response.

Robertson et al. (1986) CanGeoJ 23(2)

Friction sleeve (fs)

su ≈ fs / αs where αs ≈ 0.7–1.0. Very approximate; fs sensitive to surface roughness and equipment. Use only for cross-check.

Schmertmann (1978); Lunne et al. (1997)

Robertson (1990) SBT

Normalised Qt and Fr classify soil behaviour type. Zones 1–9. Zone 3 = clays, Zone 4 = silty clay/clay. Identifies drained vs undrained zones in profile.

Robertson, P.K. (1990) Soil classification using CPT. CanGeoJ 27(1), 151–158

Bq pore pressure ratio

Bq = (u2 − u0) / (qt − σv0). Bq > 0.4 indicates undrained behaviour. Bq ≈ 0 = drained. Negative Bq = dilative, overconsolidated or sensitive.

Robertson et al. (1986); Lunne et al. (1997)