Buried Pipe & Soil Restraint
Buried elements are restrained continuously by the soil rather than by discrete supports. PipeFEA generates non-linear soil springs along buried elements automatically, following the ALA buried-steel-pipe guidelines.
1. Set the Ground Elevation — Settings tab
Burial depth is computed from your model geometry: at every point along a buried element,
depth H = Ground Elevation (Settings ⚙️) − pipe centreline elevation.
Set Ground Elevation once (default 0) and model buried pipe below it — e.g. with ground at 0, a pipe at z = −1.2 m is buried 1.2 m to centreline. Depth therefore varies automatically along risers and sloping runs, and pipe at or above ground level gets no soil restraint.
A buried element that sits entirely at or above the Ground Elevation gets no soil springs — Check Model and the analysis will warn you: "Buried element(s) … lie at or above the Ground Elevation". Lower the pipe or raise the Ground Elevation.
2. Define a soil profile — Soil tab
| Field | Meaning |
|---|---|
| Name | e.g. Medium Sand |
| Soil Type | SAND / CLAY — selects strength terms and yield displacements |
| Density γ | kg/m³ (e.g. 1800 for a medium sand ≈ 17.7 kN/m³) |
| Friction Angle φ (degrees) | Sand/granular strength (0 for undrained clay) |
| Cohesion c (kPa) | Clay strength (0 for clean sand) |
| Coating Factor f | Interface friction angle δ = f·φ (≈ 0.6 rough steel/coated, up to 1.0) |
Soil profiles support the personal Library Bar (save/load across projects).
3. Assign soil to elements
Tick Buried Pipe (Soil Restrained) on an element in the Assign tab (or set the soil column in the Grid Editor, or the checkbox in the Inspector), and pick which profile. You can also set a soil profile while routing (the component panel has a buried toggle) so new segments inherit it.
Use Bulk Assign → Soil (Buried) to bury a whole line at once. Elbows between buried elements are buried automatically — the discretised bend arc inherits the soil from its adjacent legs.
4. What the solver does
At solve time each buried element is subdivided (≤ 2 m segments) and every sub-node receives soil springs per the ALA guidelines:
- Axial — adhesion + interface friction along the pipe,
- Lateral — horizontal bearing into the trench wall,
- Vertical — one-way: stiff/strong bearing downward, much softer and weaker uplift upward.
Every spring is elastic-plastic: it yields at its ultimate soil resistance (axial yield is only a few millimetres of movement), so large thermal movements near risers and buried bends shed load realistically instead of attracting unbounded spring force. The generated sub-nodes are internal — you'll see them in the Analysis Mesh preview and the post-analysis view, but your model keeps its original elements.
Tips
- The transition point between above-ground and buried pipe is usually the highest-stress location — put a node there and check it.
- Thermal expansion of long buried runs is resisted by axial soil friction, building compressive stress towards a virtual anchor point; make buried runs long enough to develop it or terminate them with a real anchor.
- Buried elements still need the model to be globally stable — keep at least one real restraint.
- Watch for uplift at overbends on hot lines: uplift resistance is by far the weakest soil direction, and the solver models it as such.