Frequently asked questions¶
The calculation says "no valid surface". Why?¶
The typical causes, explained by the actionable panel that appears:
- Grid of centres off position relative to the slope → use Auto-place grid.
- Undrained analysis without cu: if the materials have no undrained cohesion, the strength is zero → switch to Drained or set cu.
- Profile too short: surfaces emerge beyond the end → Extend profile.
What's the difference between Drained and Undrained?¶
Drained (effective stress) uses c′ and φ′ and subtracts the pore pressure at the base: it is the long-term condition. Undrained (total stress) uses cu with φ = 0 and does not subtract u: it is the short-term condition in fine soils. New projects start in Drained.
Which method should I choose?¶
For circular surfaces start with Bishop simplified. For general (non-circular) surfaces use Janbu, Spencer or Morgenstern-Price. Fellenius is the most conservative, useful as a reference.
Why must the FoS be ≥ 1 and not ≥ 1.3?¶
It depends on the verification method. With partial factors (NTC/EC at limit states) the margin is already in γR, so the verification is satisfied for FoS ≥ 1.0. With characteristic parameters (traditional method) the reference values are FoS ≈ 1.3–1.5 (static) and 1.1–1.3 (seismic). See Design code.
How does the calculation treat the backfill behind the wall?¶
With connect on, the backfill is modeled as real soil: its weight and strength enter the calculation, with the material you assign by double-clicking the wedge. If instead you drew the profile already at the finished grade, leave connect OFF. See Retaining structures.
Can I reconstruct a model from an old report?¶
Yes. Load the report PDF (or a description in words) into the AI assistant: it reconstructs profile, layers, water table and structures in a preview to review before applying.
Is my project safe if I close the browser?¶
Slope NX saves an automatic draft in the browser. To avoid losing work, save the .slope file (or to the cloud with your account). When you open an example or a new file, the app warns if there is a project in progress.
Found an error on this page? Let us know or open a Pull Request.