How do you estimate friction angle from SPT N?

First standardize measured N: N₆₀ = Nₘ(ER/60)CbCrCs, then calculate (N₁)₆₀ = CnN₆₀ with explicit stress inputs. The enabled property method reads the Caltrans Chart 1 low, average and high friction-angle curves for sand ((N₁)₆₀ 0–70); it rejects N₆₀ substitution and values beyond the chart.

What is the correlation between SPT N and undrained shear strength?

PileCalc deliberately does not offer a universal scalar SPT-to-undrained-strength conversion. Such correlations are highly dependent on geology, sampling, energy, and local calibration, so the calculator asks for measured project parameters for cohesive soils instead of guessing.

Are SPT correlations reliable enough for design?

They are estimates, not measurements. Even after energy/equipment and overburden corrections, SPT property correlations have broad scatter and method-specific applicability. Use them for preliminary sizing and sanity checks, then confirm governing parameters with project-specific laboratory or in-situ testing.

What is ε50 and why does it matter?

ε₅₀ is the strain at 50% of the maximum deviator stress in a triaxial test on clay. It sets the displacement scale of clay p-y curves (Matlock, Reese) — softer clays have larger ε₅₀ (0.02 for soft, 0.004 for very stiff), which makes the pile deflect more before the soil resistance mobilizes.

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Free SPT Correction Calculator — N₆₀, (N₁)₆₀, φ, Unit Weight

Free SPT calculator with explicit FHWA N₆₀ and (N₁)₆₀ corrections and the public Caltrans friction-angle and unit-weight charts. No hidden equipment factors.

SPT blow count N(blows/ft)

Soil type

FHWA SPT standardization

N₆₀ = Nₘ·(ER/60)·Cᵇ·Cᵣ·Cₛ and (N₁)₆₀ = Cₙ·N₆₀. Enter factors from the actual test record; PileCalc does not infer equipment corrections.

Measured energy ratio ER(%)

Borehole factor Cᵇ

Rod-length factor Cᵣ

Sampler factor Cₛ

Effective overburden σ′v(kPa)

Atmospheric pressure Pₐ(kPa)

Overburden exponent n

Explicit Cₙ cap

Index supplied to correlations

How to use it

1
Enter the measured test record
Type measured N, hammer energy ratio, and the borehole, rod-length and sampler corrections for the actual equipment.
2
Set the soil type
The property method applies to cohesionless soil and reports the moist unit weight that Caltrans Chart 2 publishes (no submerged value). Cohesive-soil scalar estimates are intentionally not offered — use measured project parameters instead.
3
Set the normalization convention
Enter effective overburden and atmospheric pressure in the same units, the stress exponent and Cn cap. The Caltrans property method requires (N₁)₆₀ and rejects N₆₀ substitution.
4
Read the parameters
Review measured N, N₆₀, Cn and (N₁)₆₀, then use the Chart 1 φ and Chart 2 γ curves. The average curve is labeled as the representative value; pick within the low–high range by soil type.

Where the numbers come from

The Standard Penetration Test is usually the only strength data on a boring log, but a raw blow count depends on the rig that produced it. Following FHWA GEC 5/GEC 12, this tool first computes N₆₀ = Nₘ(ER/60)CᵇCᵣCₛ and then the optional overburden normalization (N₁)₆₀ = CₙN₆₀. The default overburden factor Cₙ = (pₐ/σ′ᵥ)^0.5 is the Liao & Whitman (1986) form, which FHWA GEC 5 (FHWA-NHI-16-072, Eq. 5.10, p. 5-31) prints without an upper bound; the default cap Cₙ ≤ 1.7 is the Youd et al. (2001) limit. Every equipment factor, stress, exponent and Cₙ cap is an explicit input you can see and adjust to match your equipment and the correlation you apply next. See the FHWA site-characterization manual and FHWA GEC 12.

Property estimates use the public Caltrans Geotechnical Manual, Soil Correlations (March 2021), which charts low, average and high curves of friction angle (Chart 1, (N₁)₆₀ 0–70) and moist unit weight of granular soil (Chart 2, (N₁)₆₀ 0–50) after Bowles (1977). PileCalc reads those curves and reports the low–high pair plus the average curve, which Caltrans uses for GM and SP soils. The source gives no submerged unit weight. The same correlations are built into PileCalc's full analysis app, where they prefill sand γ and φ soil-layer fields directly from (N₁)₆₀.

Use them as estimates, not measurements

SPT blow counts scatter with hammer energy, rod length, borehole disturbance and operator, and each correlation was fit to a broad soil family. Because FHWA finds no universal SPT-to-undrained-strength relationship for cohesive soils, this tool estimates sand parameters only — clay strength, ε₅₀ and subgrade modulus belong to laboratory or in-situ data. Treat these values as a starting point for preliminary design and sanity checks, adjust them to local experience, and confirm whatever governs your design with laboratory or in-situ testing. Once you have parameters, feed them into the p-y curve generator or the pile capacity calculator to see what they mean for the foundation.

Common questions

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