CPT Soil Behavior Type Robertson Calculator
Geotechnical Site Characterization: Determine normalized Soil Behavior Type ($SBT_n$) and index ($I_c$) from Cone Penetration Test (CPT) measurements according to Robertson (1990, 2009).
CPT In-Situ Measurements
Overburden Stress Profile
Robertson SBTn Classification Results
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Robertson CPT Soil Behavior Classification Theory
The Cone Penetration Test (CPT/CPTu) is widely regarded as the gold standard in geotechnical site investigation due to its repeatability and continuous vertical logging resolution.
1. Normalized Robertson Parameters
Raw cone tip resistance $q_c$ and sleeve friction $f_s$ are normalized against in-situ vertical effective and total overburden stresses:
q_t = q_c + u_2 · (1 - a) Q_tn = [ (q_t - σ_v0) / p_a ] · (p_a / σ'_v0)^n F_r = [ f_s / (q_t - σ_v0) ] × 100%
where $p_a$ is reference atmospheric pressure (100 kPa), and $n$ is the stress exponent varying from 0.5 in clean sands to 1.0 in intact clays.
2. Soil Classification Index Formulation
The Robertson $I_c$ index represents the radial distance from the coordinate $(3.47, -1.22)$ on the $\log Q_{tn} - \log F_r$ chart:
I_c = √[ (3.47 - log10 Q_tn)² + (log10 F_r + 1.22)² ]
Frequently Asked Questions
What is the Robertson Soil Behavior Type index (Ic)?
The Soil Behavior Type index ($I_c$) is a dimensionless geotechnical parameter developed by P.K. Robertson (1990, 2009) that correlates Cone Penetration Test (CPT) measurements ($q_t$, $f_s$, $\sigma_{v0}$) directly to soil classification categories (from sensitive fine-grained clay to dense sand and gravel) without requiring continuous laboratory grain-size sieve analysis.
Why is pore water pressure u2 subtracted from or added to cone tip resistance qc?
Water pressure acts on the unequal shoulder area behind the cone tip, reducing the measured thrust $q_c$. The corrected total cone resistance $q_t = q_c + u_2(1 - a)$, where $a$ is the net cone area ratio (typically 0.70 to 0.85). In soft saturated cohesive clays, failure to correct for $u_2$ can underestimate tip resistance by 20% to 40%.
What are the Ic boundary ranges for common soil types?
According to Robertson (2009): $I_c > 3.60$ is Zone 2 (Organic clays/peat); $2.95 < I_c \le 3.60$ is Zone 3 (Clays); $2.60 < I_c \le 2.95$ is Zone 4 (Silt mixtures); $2.05 < I_c \le 2.60$ is Zone 5 (Sand mixtures); $1.31 < I_c \le 2.05$ is Zone 6 (Clean sand to silty sand); and $I_c \le 1.31$ is Zone 7 (Dense gravelly sand).