Geotechnical Results¶
The fourth tab, and the decision point of the project: this is where the pile length is chosen.
The tab has four sub-tabs:
| Sub-tab | Content |
|---|---|
| Aoki–Velloso | Capacity by level |
| Décourt–Quaresma | Capacity by level |
| Teixeira | Capacity by level |
| Settlement (Cintra & Aoki) | Settlement, load × settlement curve, axial force diagram |
How to read the table¶
Each method presents one row per possible toe level:
| Column | Unit | Content |
|---|---|---|
| Levels | m | Toe depth, negative |
| Cumul. \(R_l\) | kN | Shaft friction accumulated from the head down to the level |
| \(R_p\) | kN | Toe resistance at that level |
| \(R_t\) | kN | Sum of the two |
| Final \(P_a\) | kN | Allowable load, with factors and limits already applied |
The procedure is straightforward: go down the Final \(P_a\) column until you find the first value that exceeds the pile load. That level is the required length.
That's why the table is by level
Programs that return a single number force you to try lengths until you get it right. With the whole \(P_a(z)\) curve in one run, the choice becomes a matter of reading a column — and it also shows how much you would gain by going one metre deeper, which is the information needed to decide between lengthening the pile or increasing the diameter.
Extra columns for bored piles¶
For a bored pile in compression, Décourt-Quaresma adds:
| Column | Content |
|---|---|
| Bored Pa | The \(1.25\,R_l\) limit |
| Dec-Qua Pa | The method's value, without the limit |
The Final \(P_a\) column shows the smaller of the two. Having all three side by side shows which criterion governed — information that often justifies changing the pile type.
The three methods diverge¶
And they should. The main source of divergence is which \(N_{SPT}\) each one uses at the toe:
| Method | Toe \(N\) |
|---|---|
| Aoki-Velloso | The layer immediately below |
| Décourt-Quaresma | Average of three layers |
| Teixeira | Average over the window \([-4D, +D]\) |
How to interpret it
- A large divergence usually indicates a profile with an abrupt change near the toe. Aoki separates from the other two because a one-metre stiff lens supports its toe on its own.
- Teixeira standing out from the others points to the influence of the diameter: only Teixeira sizes the \(N_p\) window as a function of \(D\).
- Excessive agreement is more suspicious than divergence — it usually means a homogeneous profile, where all three reduce to the same average.
The defensible practice is to adopt the smallest of the three, or the method with known regional calibration, recording the choice in the calculation report.
Settlement¶
The fourth sub-tab provides:
- Estimated settlement by the Cintra & Aoki method, split into the elastic shortening of the pile and the soil contribution.
- Group settlement, by the \(\rho_{max}\sqrt{n}\) estimate.
- The Van der Veen load × settlement curve, anchored at the working point.
- The axial force diagram along the shaft.
The result is given in millimetres.
Allowable settlement is not the program's decision
SPX estimates how much the foundation settles. How much the structure can tolerate is a property of the structure — and differential settlements between supports, which are what actually damage buildings, require comparing the foundations with each other.
See limits of validity.
Geotechnical check¶
The program issues a check for each pile, comparing the applied load with the allowable load at the adopted level. Changing geotechnical parameters after calculating triggers a warning — the result on screen no longer matches the input, and it must be reprocessed.