Design code and partial factors¶

Slope NX verifies stability against three references, selectable in the Analysis → Design code and verification panel.

Code	Approach
NTC 2018 (D.M. 17/01/2018)	Partial factors at limit states (ULS).
EC7-EC8 (EN 1997)	Design Approach with partial factors.
User	Free coefficients defined by the designer.

Partial factors¶

The factors are grouped into three groups.

Group M — materials. Reduce the design strength parameters:

\[ \tan\varphi'_d = \frac{\tan\varphi'_k}{\gamma_{\varphi'}}, \quad c'_d = \frac{c'_k}{\gamma_{c'}}, \quad c_{u,d} = \frac{c_{u,k}}{\gamma_{cu}} \]

Group R — global resistance. Divides the final FoS: FoS_d = FoS / γ_R.

Group A — actions. γ_G on permanent, γ_Q on variable.

Single-source principle¶

In slope stability the permanent actions derive from the soil self-weight: they are, at the same time, the destabilizing action and the origin of the resistance (friction). By the single-source principle the group A factors do not change the factor of safety. The verification therefore depends on the M and R factors.

How to read the FoS

Applying the partial factors (Limit States), the margin is already incorporated in γ_R: the verification is satisfied for FoS ≥ 1.0. With characteristic parameters (traditional method, no reduction) the reference values are FoS ≈ 1.3–1.5 in static and 1.1–1.3 in seismic conditions.

The design limit FoS set in the Analysis panel does not enter the calculation: it governs only the judgment (below threshold the FoS is flagged in red).

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