Subsoil category

Purpose

The subsoil category determines the expected seismic amplification for the site and governs the value of the parameter S (stratigraphic amplification coefficient) in the NTC 2018 response spectra.

Dynamic Probing NX automatically computes the category from the V_s values of the layers, derived from N_SPT → Vs correlations.

Supported standards

Standard	Reference parameter	Notes
NTC 2018 (D.M. 17/01/2018)	V_s,30	Main method for buildings and structures
NTC 2008 (D.M. 14/01/2008)	V_s,30	Compatibility with existing projects
Eurocode 8 (EN 1998-1)	V_s,30	European reference

How V_s,30 is computed

V_s,30 is the average shear wave velocity over the first 30 m of depth, weighted by layer thicknesses. For boring depths less than 30 m, the app uses a default seismic bedrock (rock) for the missing metres — this value is configurable in the Subsoil cat. tab.

How to read the result

In the Subsoil cat. tab of the Editor you find:

• V_s,eq: computed equivalent velocity (m/s)
• Category: letter A / B / C / D / E (NTC 2018) with text description
• Layer table: contribution of each layer to the V_s,30 computation

The calculation updates automatically every time you modify the stratigraphy or the V_s values of the layers.

Vs from laboratory tests

If you have direct V_s measurements (MASW, down-hole, cross-hole), you can override the V_s value of each layer in the dedicated column of the table — the V_s,30 calculation uses the manually entered values instead of those estimated from correlations.

NTC 2018 categories

Category	Description
A	Outcroping rock masses or very stiff soils (V_s,30 > 800 m/s)
B	Deposits of sands, compact gravels or stiff clays (360 < V_s,30 ≤ 800 m/s)
C	Deposits of medium-dense sands, gravels or medium-consistency clays (180 < V_s,30 ≤ 360 m/s)
D	Deposits of loose granular soils or soft cohesive soils (V_s,30 ≤ 180 m/s)
E	Profiles with surficial alluvial layers with V_s < 360 m/s and thickness within certain limits on a category A or B substratum