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Tutorial 5 — A Buried Pipeline Transition

The classic riser problem: an above-ground line drops below grade and runs buried. The air/soil transition is where the action is.

Setup: New project; DN100 SCH 40; carbon steel; pressure 2 MPa; T_max 80 °C; Ground Elevation 0 (Settings).

1. Soil profile​

Soil tab → create Medium Sand: type SAND, density 1800 kg/m³, friction angle 32°, coating factor 0.6 → add. (Save it to your library for reuse.) Burial depth isn't a soil property — it comes from where you route the pipe relative to the Ground Elevation in Settings.

2. Geometry​

  1. Anchor at 0,0,1 (1 m above grade — Manual Node).
  2. Route dX 6 (above-ground run), then dZ -2.2 (the riser drops to z = −1.2 m — centreline burial depth), then dX 20 (the buried run), and anchor the far end.
  3. Convert the two corner nodes to Elbow / Bend (radius 0.152 m).

3. Bury the underground run​

Assign tab → the 20 m element's card → tick Buried Pipe (Soil Restrained) → profile Medium Sand. (For many elements: Bulk Assign → Soil.)

4. Support the above-ground part​

Give the 6 m run a resting support (Insert Support Along Element at 3 m, Z:POS, μ=0.3).

5. Solve and interpret​

Run All Cases, then:

  • ⛓ Analysis Mesh (or the post-analysis view): the buried element has been subdivided ~every metre, each sub-node carrying axial/lateral/vertical soil springs.
  • Displacements: the buried run's axial growth decays with distance as soil friction absorbs it — far enough in, the pipe is effectively anchored (the virtual anchor).
  • Pipe Stresses: expect the governing location at or near the transition elbow — the above-ground thermal growth is fed into soil restraint there.
  • Reactions: the real anchors carry much less than an unburied equivalent would — the soil took it.

6. Experiment​

  • Deepen the burial (route the buried run at z = −2.5 instead of −1.2) → stiffer springs → shorter virtual anchor length, higher local stress at the transition.
  • Switch the profile to CLAY with a cohesion value (e.g. 50 kPa, φ = 0) and compare.
  • Delete the far anchor entirely and re-run: a long enough buried run is stable on soil alone (the pre-solve check still needs one restraint — keep the top anchor).

Reference: Buried Pipe & Soil; theory in the Technical Reference.