This document describes the implementation of the Robertson (1990) Soil Behaviour Type classification chart in Atlaned, including the normalised parameter definitions, the zone polygons, and the assumptions used when ancillary inputs (unit weight, water table) are not supplied.

Scope

The classification covers the nine SBT zones defined by Robertson (1990) using normalised cone resistance Qₜ and friction ratio Fᵣ. Pore-pressure-based variants are documented separately.

Normalised parameters

Atlaned computes the normalised cone resistance and friction ratio as:

• Qₜ = (qₜ − σᵥ₀) / σ′ᵥ₀
• Fᵣ = fₛ / (qₜ − σᵥ₀) × 100%

where:

• qₜ is the corrected cone resistance, computed from qc, u2, and the cone area ratio a
• σᵥ₀ is the total vertical stress at the cone tip
• σ′ᵥ₀ is the effective vertical stress at the cone tip
• fₛ is the sleeve friction

When unit weight is not measured directly, Atlaned falls back to Robertson & Cabal (2010):

γ / γw = 0.27 · log(Rf) + 0.36 · log(qt / pa) + 1.236

Zone boundaries

The nine zones are bounded by the polygonal regions specified in Robertson (1990) Figure 4. Atlaned stores the polygon vertices as immutable constants in src/cpt/sbt/robertson1990.ts and performs point-in-polygon testing on each depth sample.

Edge cases

• Negative friction ratio. If fₛ ≤ 0, the sample is flagged and assigned no zone. The flag is surfaced in the UI but does not block the analysis.
• Above water table. σ′ᵥ₀ is computed from total stress without pore-pressure correction.

Validation

The implementation is validated against the 1,200-sounding reference dataset bundled at verification/datasets/robertson-1990/. Tolerance: zone match ≥ 99.5%, Qₜ within 0.5%, Fᵣ within 1.0%.