2.1.5. Geotech: NAD packing subdivision¶
The geotechnical model is the third and last level. It takes each unit of the lithological model and subdivides it by packing density or consistency, assigning a Belgian NAD class (1 to 31): a packing from los to zeer dicht for the coarse soils, and a consistency from weinig-vast to vast for the fine soils. The subdivision is driven by the CPT cone resistance, but the soil type itself is never touched: lithology is hard truth, and the cone resistance never promotes or demotes a unit from one soil to another.
2.1.5.1. How a unit’s class is chosen¶
Press “Subdivide”, and StrixGeo, for each litho body, reads the CPT cone resistances that fall inside the body, reduces them to a single representative qc, and looks that value up against the NAD bands for the unit’s soil and admixture variant.
The representative qc is a single, selectable statistic over the body’s readings. The default is the median, which is robust to spikes; the arithmetic, harmonic and geometric means are also available. The harmonic mean is the conservative choice, weighted toward the low readings.
The lookup is on raw qc, in megapascals, with no normalisation. This is the NAD table’s own method.
The class always belongs to the unit’s own soil column. A sand unit can only carry a sand class; a clay at a sand-like qc stays a clay and takes its densest clay class.
The geotech stage has its own minimum depth interval (default 0.35 m, tunable per project), coarser than the litho segmenter’s, so coarsening the packing bands never disturbs the lithology layering. A sliver thinner than that is treated conservatively: StrixGeo absorbs it into a neighbour only if the neighbour’s class survives the merge going looser. Where every absorption would come out denser, the sliver stands as its own thin band and you decide. StrixGeo will not silently upgrade a matig sand to a dense one.
2.1.5.2. The admixture choice¶
The NAD table has a clean column and a with-admixture column for each soil. What counts as an admixture here is specific: a clayey or silty admixture in a sand or gravel (“silt- or kleihoudend”), a glauconite admixture in a sand (common in the Belgian Neogene), or a sand admixture in a fine soil (“zandhoudend”). A gravel admixture in a sand does not count; a gravelly sand is treated as plain sand and classified in the clean column. The NAD table has no separate glauconitic-sand line, so a glauconitic sand you mark “with admixture” is placed in the same with-admixture (silt-/kleihoudend) column. For a sand whose only qualifying admixture is glauconite that answer is pre-selected in the dialog, since it is the closest line in the table; you can still switch it to clean sand before you divide. A sand with a silty or clayey admixture keeps an open question: nothing is pre-selected.
To keep the choice an explicit engineering decision rather than a silent guess, StrixGeo asks you, but only for a unit that actually carries a qualifying admixture (clayey, silty, or glauconitic in a sand; sand in a fine soil), whether it is pure or with admixture. A unit with no such admixture is not asked about. The question appears the first time you subdivide. Whether you press Subdivide on the Sections toolbar or Build on the geotech stage, both ask the same question before they run. Your answer is remembered and forces the column: “pure” classifies into the clean column, “with admixture” into the with-admixture column.
2.1.5.3. How boreholes are subdivided¶
A borehole records no cone resistance, so its packing cannot be read from its own data. StrixGeo classifies a borehole layer from the CPTs around it: it takes the packing of the same soil from the CPTs on either side of the borehole on the section, and carries it onto the borehole layer. Where no CPT of that soil is near enough to speak, it falls back to the loosest class of that soil’s own column, a conservative floor, and marks the result low-confidence. It never borrows a class from a different soil.
A borehole between two CPTs
When a borehole sits between two CPTs on a section, its packing bands come from those two flanks. If you have drawn a packing correlation across the borehole (tying the two CPTs’ bands together), StrixGeo blends the two flanks’ packing across the borehole in proportion to the distance, so the borehole reads as a smooth transition between them. Where the two flanks disagree on a band’s class, the nearer CPT names it. A drawn tie is preferred over a bare left-and-right blend, because the tie is your explicit statement that the two packings belong together.
2.1.5.4. How a CPTM is subdivided¶
A mechanical Begemann cone (a CPTM) still records a cone resistance, and the NAD lookup is a raw-qc lookup, so a CPTM is subdivided from its qc exactly like any other CPT. It needs no friction ratio for the packing class, which is why a CPTM that recorded no sleeve friction still classifies for packing. The soil-behaviour index and the relative density, which do need the friction, are simply not part of the packing lookup.
2.1.5.5. How the packing colours read¶
Each packing class has its own colour: a lightness ramp within each soil family, so a denser class of a soil is a darker shade of that soil’s hue. A band that has no packing class yet is drawn in a neutral grey and carries the “No CPT” badge, telling you it still needs a cone resistance to classify.
Gravel is the exception
Gravel is drawn in its own amber even without a packing class, rather than the neutral grey. A CPT cannot penetrate gravel, so a gravel band is expected to stay unclassified; painting it the same grey as an unclassified clay band read as a to-do it can never resolve, and the two greys were too close to tell apart. Amber says “this is gravel, and gravel does not classify from a CPT” — not “a CPT is missing here”. Every other soil keeps the grey, where an unclassified band genuinely means a CPT has not yet reached it.
2.1.5.6. The fill follows the same 2 / 1 / 0 rule¶
The geotech stage draws its packing bands with the same 2 / 1 / 0 contact-correlation rule as the litho stage: you correlate the packing bands between tests, and the fill follows the count of correlated contacts, two ties a band, one an onlap, none a lens. Where a CPT does not read a stretch of ground (an unread head or tail longer than the minimum thickness), StrixGeo leaves it as its own needs-review band rather than inventing a class across columns; you correlate the packing and decide.
2.1.5.7. Grouping: Join means group, never merge¶
Every geometrically-disconnected mass starts as its own body; StrixGeo does not auto-lump. When you want several bodies treated together, silt lenses in a sand pack with a similar packing, say, you Join them into a group, so you pool their data and end up with fewer export classes. Join groups, it does not merge:
Every body keeps its own separate body identity. A group can span two different litho parents, and merging would lose that provenance, so grouping adds a label instead.
A grouped body shows a “(group N)” tag against each member.
An export maps the whole group to one class.
The verbs are Join with… and Un-join from {label} (in Dutch, Voeg samen met… and Ontkoppelen van {label}). The body menu is available on both the geotech axis and the litho axis.
2.1.5.8. What the geotech stage produces¶
For each geotech unit StrixGeo shows the CPT-derived indicators in scope: the soil-behaviour zone, the relative density for sand units, the NAD packing class, and a unit-weight estimate. It also reads the NAD characteristic parameters from the table, one general value per body: the dry and wet unit weight, the effective friction angle, the effective cohesion, and, for fine soils, the undrained shear strength. These characteristic values are shown in the Parameters stage, kept separate from the measured statistics. All of them are estimates the engineer must verify.
The more advanced design-ready values (a stiffness E50, an over-consolidation ratio OCR, a shear-wave velocity Vs, and the like) are not produced in this version; they are future work.
2.1.5.9. The 3-D geotech model¶
The 3-D build works on the geotech axis exactly as it does on the litho axis: the same voxel labelling (explained in the Litho chapter) fills the space between the sections, colouring each voxel by its packing class instead of its lithology. The fence diagram and a split viewport, which shows two axes side by side (the litho and the geotech model, say), let you compare the lithological and geotechnical pictures of the same ground.