Interpreted stratigraphy¶
What it is¶
The interpreted stratigraphy is the subsoil model derived from the test readings. You define a number of layers — each with a lower depth, a soil type and unit weights γ and γ_sat — and the software computes for each the representative N_SPT value, from which the geotechnical correlations are then derived.
How to enter it¶
In the Interpreted stratigraphy tab of the Editor:
- The first layer starts at 0 m. Set its lower depth (e.g. 2.50 m).
- Click + Add layer to create the next layer.
- For each layer:
- Set the lower depth (m from ground level).
- Choose the soil type:
COES(cohesive),INCO(non-cohesive) or both if mixed — the type determines which correlations are applied. - Enter γ (dry or natural unit weight) and γ_sat (saturated unit weight) in kN/m³.
- Optionally enter clay (%) and the lithological description.
The 7 N_SPT aggregation methods¶
For each layer, the readings falling within its depth range are aggregated using the method chosen by the user. The available methods are:
| Method | When to use it |
|---|---|
| Mean | Homogeneous soil, limited variability |
| Minimum | Conservative approach — the worst value is used |
| Maximum | Upper bound estimate (e.g. for tip bearing capacity) |
| Mean − 1σ | Statistically conservative approach |
| Mean + 1σ | Statistically non-conservative approach |
| RNC (normal distribution) | Characteristic value in probability according to EC7 §2.4.5.2 |
| RC (log-normal distribution) | Like RNC but with log-normal distribution — preferable for N_SPT with low values |
The method is set in the N_SPT column header of the stratigraphy table, and applies to all layers simultaneously.
Which is the right method?
EC7 (and NTC §6.2.2) specifies using the characteristic value of the parameter, defined as the value with a 5% probability of being exceeded on the conservative side. The RNC and RC methods approach this statistical definition. For small samples (< 6 readings per layer) the mean remains the most robust reference.
N_DPM → N_SPT conversion¶
For continuous tests, the layer N_SPT is obtained from N_DPM by applying the β coefficient (see Instruments →). The product N_DPM × β is the equivalent N_SPT for each reading; the aggregation is then applied to these equivalent values.
Σ layers / test badge¶
At the bottom of the stratigraphy screen the badge shows:
- Green (✓): the sum of layer depths matches the test depth — the stratigraphy covers the entire test without gaps.
- Yellow (⚠): discrepancy between Σ layers and test depth — check that the last layer reaches the end of the test.