Boreholes and soils¶
Tabs 2 and 3. This is where the subsoil profile goes in — the data that governs everything else.
Borehole / NSPT¶
The borehole is entered metre by metre: for each depth, an \(N_{SPT}\) value and a soil classification.
| Column | Content |
|---|---|
| Depth | 1 m, 2 m, 3 m… |
| \(N_{SPT}\) | Blow count |
| Soil type | Classification by fractions — see below |
Soil types¶
The soil is classified by the fractions it consists of, in order of predominance — fifteen combinations in all:
| Sand | Silt | Clay |
| Silty sand | Sandy silt | Sandy clay |
| Silty-clayey sand | Sandy-clayey silt | Sandy-silty clay |
| Clayey sand | Clayey silt | Silty clay |
| Clayey-silty sand | Clayey-sandy silt | Silty-sandy clay |
The classification selects the design parameters
It is not a label. It selects \(K\) and \(\alpha\) in Aoki-Velloso, \(C\) in Décourt-Quaresma, \(\alpha_T\) in Teixeira and the factor \(m\) for the springs.
Swapping silty sand for sandy silt reduces \(K\) from 0.80 to 0.55 MPa, 31 % less toe resistance. See Parameter tables.
Multiple boreholes¶
SPX accepts more than one borehole profile in the same project, each one identified. Different piles can be associated with different boreholes — which is what allows a site where the subsoil varies from one side to the other to be handled.
Editable PDF reader¶
SPX reads borehole logs directly from the PDF, extracting depths, blow counts and soil descriptions into the table. The extraction is editable: whatever comes out wrong can be corrected before calculating.
On a site with fifteen boreholes twenty metres deep, there are three hundred rows to type, and each one is a chance to swap silty sand for sandy silt.
Check the soil classification
Automatic extraction depends on the layout of the log. Always check the resulting table against the original, paying attention to the soil classification — it is what selects the parameters of every method.
In SPX AI the reading is intelligent
SPX AI handles format variations, tables with merged cells and non-standard descriptions — the reports that the standard reader cannot interpret.
3D profile¶
With more than one borehole, SPX interpolates the stratigraphy and presents a three-dimensional profile of the ground. It is the feature SPO does not have, and it serves two purposes: spotting layers that dip, and visually justifying why two nearby piles were given different lengths.
Soil Properties¶
The third tab. It holds the parameters per layer — unit weights and whatever else feeds the geostatic stress used in settlement.
The program offers a table of typical unit weights by consistency and density, accessible from the tab itself, for those without test data.
Value adopted when data is missing
When there is no match in the table, the program adopts \(\gamma = 18\) kN/m³. It is a reasonable value for an average soil, but record the assumption in the calculation report when it governs the result.
Good practice¶
- Do not design with the toe at the last metre investigated. The methods need layers below the toe to build \(N_p\); at the last level, the program repeats the last value, which is optimistic if the profile was improving.
- Be wary of \(N_{SPT} > 50\). It is outside the calibration range of most correlations.
- A shallow borehole produces a shallow design. If the profile does not reach the required depth, the capacity table simply ends — and the correct reading is "investigation is missing", not "the pile doesn't work".