Characteristic values EC7 / NTC §6.2.2¶
Why characteristic values¶
European (EC7 §2.4.5.2) and Italian (NTC 2018 §6.2.2) geotechnical standards require designing on the characteristic values of parameters, not on mean values. The characteristic value accounts for the natural variability of the soil and sampling uncertainty.
How the calculation works¶
Dynamic Probing NX estimates the characteristic value of N_SPT for each layer by applying statistical methods to the series of N values collected in the layer. The available methods are:
| Method | Description |
|---|---|
| Normal | Estimate based on mean and standard deviation with Gaussian distribution |
| Lognormal | Useful when N_SPT has an asymmetric distribution (common with low values) |
| Student-t | Corrected for small samples (n < 30) — accounts for uncertainty in the mean |
The confidence level applied corresponds to that indicated by EC7 for the lower characteristic value (5th percentile, conservative side).
How to use it¶
In the Parameter estimation tab of the Editor:
- Select the statistical method (Normal / Lognormal / Student-t).
- For each layer the following are shown: mean, standard deviation, sample size and computed characteristic value.
- The characteristic N_SPT value is propagated to the correlations to obtain characteristic values of Cu, φ, Mo, Ey, etc.
Minimum samples
With fewer than 3 readings per layer the statistical calculation has low significance. The app flags cases with n < 3 with a warning — in these cases it is preferable to use a conservative engineering approach (e.g. minimum of observed values).
Characteristic values in the report¶
The exported Word report includes the table of characteristic values with the method used, sample size and resulting value — ready to be attached to the geotechnical report.