Technical Documentation

CPT Calculation
Methodology

A transparent breakdown of the mathematical models and correlations used by Borebase to process Cone Penetration Test (CPT) data.

Input Parameters

The core analysis relies on standard raw data channels recorded during penetration.

Depth below ground surface (m)
Cone area ratio (typically 0.8)
Unit weight of water (9.81 kN/m³)

q_t = q_c + (1 - a) \cdot u_2

Derived Basic Parameters

Initial corrections are applied to account for pore pressure effects and geometric ratios.

R_f = \left( \frac{f_s}{q_t} \right) \cdot 100 \%

TIP

*Note: Borebase uses qt in the denominator for Rf when available, falling back to qc if u2 is missing.

u_0 = \gamma_w \cdot (z - z_w)

\Delta u = u_2 - u_0

Stress Normalization

To compare soil behavior at different depths, parameters are normalized by vertical stress. Borebase uses an iterative approach where soil unit weight (γ) is estimated based on soil behavior type.

\sigma_{v0} = \sum_{i=0}^{z} (\gamma_i \cdot \Delta z)

Borebase calculates stress cumulatively from the surface down. γᵢ is estimated dynamically using the Robertson classification chart zones for each depth increment.

\sigma'_{v0} = \sigma_{v0} - u_0

Q_t = \frac{q_t - \sigma_{v0}}{\sigma'_{v0}}

F_r = \left( \frac{f_s}{q_t - \sigma_{v0}} \right) \cdot 100 \%

Soil Classification

Classification follows the Soil Behavior Type (SBTn) methodology defined by Robertson (1990, 2009).

A single index parameter that defines the radius in the Qt - Fr chart.

I_c = \sqrt{(3.47 - \log Q_t)^2 + (\log F_r + 1.22)^2}

Zone	Soil Behavior Type	Ic Range
1	Sensitive, fine grained
2	Organic soils - peats	I_c > 3.60
3	Clays - silty clay to clay	2.95 < I_c \leq 3.60
4	Silt mixtures - clayey silt to silty clay	2.60 < I_c \leq 2.95
5	Sand mixtures - silty sand to sandy silt	2.05 < I_c \leq 2.60
6	Sands - clean sand to silty sand	1.31 < I_c \leq 2.05
7	Gravelly sand to dense sand	I_c \leq 1.31

Engineering Parameters

Calculated for clays (Ic > 2.05).

S_u = \frac{q_t - \sigma_{v0}}{N_k}

TIP

Where Nk is the cone factor (default 15, configurable per project).

Calculated for sands (Ic ≤ 2.60).

N_{60} = \frac{q_c}{k_{factor}}

TIP

k_factor varies based on mean grain size (D50) correlations with Ic.

M = \alpha_M \cdot (q_t - \sigma_{v0})

INFO

αM is determined by Ic: If Ic > 2.2 then αM = Qt (for Qt < 14). Otherwise αM is derived from specific soil behavior type constants.

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