Technical interpretation workflow of the application.

0A.1 Coverage

The documentation covers the current implemented interpretation workflow: CPT file parsing for GEF, Excel, and CSV sources, per-point CPT classification, layer detection, layer parameter assignment, stiffness derivation, and experimental m-fitting. It is intended as the full technical account of Stages 1 to 5 rather than a short product summary.

Where the application contains an engineering simplification, the simplification is stated explicitly. The purpose is not to replace engineering judgement, but to make the implemented mathematics auditable and readable.

• Classification and boundary logic are documented separately from parameter assignment.
• Stage 6 uses the interpreted CPT state produced by the earlier stages rather than reclassifying the profile independently.
• The notation follows geotechnical convention with compression positive and depth measured downward.

0A.2 Layer quantities carried into engineering use

Per interpreted layer, the application carries the quantities needed by the engineering modules: top and bottom depth, γ and γ_sat, φ′, c′, c_u, E_oed,ref, E_oed,i, E_50,ref, E_ur,ref, m, K_0,nc, ν_ur, k_h, and k_v.

Geometry, load combination, hydraulic scenario, and optional Stage 5 tuning are then applied on top of those layer values.

1.1 Supported file structures and column mapping

GEF files declare their physical quantities through #COLUMNINFO lines. The application reads the quantity identifier and maps it to the relevant column index; column order is therefore never assumed.

Excel workbooks are read from a Data sheet and, when present, a Header sheet. The Data sheet may contain depth, q_c, f_s, and R_f; the Header sheet can provide project, test, location, date, water level, surface level, coordinates, and the net area ratio.

CSV files are the reduced-input route. They must contain headers named depth and qc. Optional headers are fs and rf. Comma, semicolon, and tab delimiters are detected automatically.

A recommended minimal CSV header is depth [m], qc [MPa], fs [MPa]. If decimal commas are used in the values, a semicolon-delimited CSV is recommended.

• GEF quantity 1: penetration length.
• GEF quantity 2: q_c.
• GEF quantity 3: f_s.
• GEF quantity 4: R_f.
• GEF quantity 6: u₂.
• GEF quantity 11: corrected depth, used before quantity 1 when both are present.
• Excel/CSV column headers may include units, for example qc [MPa], fs kPa, or Friction ratio (Rf) in %.

1.2 Unit conversion and row filtering

The parser converts q_c and f_s to MPa based on the declared GEF unit string or the Excel/CSV column header. Headers containing MPa are used directly; kPa is divided by 1000; Pa is divided by 1 000 000.

If the unit declaration is absent or ambiguous, a heuristic fallback is used so the file remains workable: q_c values above 100 are treated as kPa, otherwise MPa; f_s values above 1000 are treated as Pa, values above 10 as kPa, otherwise MPa. Explicit unit labels are therefore strongly recommended.

Rows are removed when they clearly do not represent meaningful CPT measurements, namely negative depths, values before cone engagement, or all-zero terminal rows.

• Depth is expected in metres below surface.
• R_f is optional; when absent or invalid, the app computes it from f_s and q_c.
• Rows with z < 0 are discarded.
• Rows with q_c < 0.02 MPa are treated as cone-not-engaged and discarded.
• All-zero rows appended by logging software are discarded.

1.3 Water table, elevation, and header metadata

The phreatic level is first taken from the GEF measurement variables or the Excel Header field Waterniveau when available; otherwise a default depth below surface is assigned. CSV files do not carry this metadata, so the engineer should review or override the default after import.

Surface elevation is read from the GEF ZID header, the Excel Header field Grondniveau, or entered manually. When present, all depth values can be expressed relative to TAW as well as depth below surface.

2.1 Robertson (1990) — normalized SBT / I_c

The Robertson route uses a normalized q_t–F_r framework. Because final layer unit weights are not yet available at Stage 2, the app uses a preliminary stress estimate with fixed unsaturated and saturated unit weights.

If pore pressure u₂ and net area ratio a are available, the cone resistance is corrected to q_t; otherwise the measured q_c is used directly.

• The current app groups the normalized result into Gravel, Sand, Silty sand, Sandy clay, Clay, and Peat / organic.
• Sensitive fine-grained soil is not inferred separately from I_c alone.

2.2 Robertson (2016) — iterative normalized SBT / Q_tn

The Robertson 2016 route keeps the same broad SBT / I_c family mapping as the earlier normalized Robertson workflow, but replaces the fixed-stress-exponent cone normalization with the iterative Q_tn formulation. The stress exponent n varies with the inferred soil behaviour and is solved iteratively.

This route may be preferred when the input is a CPTu, but the implementation does not require measured u₂. If u₂ is absent, the app uses q_t = q_c in the same way as the Robertson 1990 fallback.

• The implementation reuses the existing broad app families: Gravel, Sand, Silty sand, Sandy clay, Clay, and Peat / organic.
• The displayed method metric is Q_tn; the I_c boundaries remain the same as the Robertson 1990 route.

2.3 CUR 3 layers — broad q_c–R_f zoning

The CUR 3 layers route is implemented as a direct q_c–R_f zoning rule based on the published broad chart with four material fields: Sand, Silt, Clay, and Peat. It is a boundary-generation method, not a detailed parameter catalogue.

The implementation follows the chart as nested zones checked from the most specific region to the broadest envelope: sand first, then silt, then clay, with peat as the remaining outside region.

To keep the downstream parameter workflow stable, the chart field “Silt” is carried internally as the app’s intermediate family and tagged with the subtype marker CUR3 silt. The chart logic itself remains four-zoned.

Show published CUR 3 chart

• This route is direct and non-normalized: no stress correction is applied to q_c before classification.
• The current app uses this route for broad layer boundary logic; detailed geotechnical parameter assignment is still handled later by the Stage 3 parameter method.
• Internal mapping note: the chart field “Silt” is stored as the app’s intermediate type family so that later parameter and compatibility tables remain usable.

2.4 NEN 6740 — stress-dependent material classification

NEN 6740 uses a stress-corrected cone resistance together with friction ratio. The source method is graphical: a semilog chart with fourteen material areas rather than a closed algebraic decision tree.

The app therefore separates the method into two parts. First it evaluates the published stress correction documented in the Deltares D-SHEET Piling manual. Then it applies a transparent representative-area rule that selects the nearest material area from a digitized fourteen-material set.

Show published NEN 6740 chart

• The stress-correction equation with exponent 0.67 is part of the documented Deltares implementation of the published NEN route; the one-dimensional score projection is the app’s deterministic way of reproducing the graphical chart.
• The score coefficient magnitude 0.34 is not a normative NEN coefficient. It is the regression-fit semilog projection magnitude through the fourteen stored representative centres used by the app.
• The representative materials are: gravel, slightly silty, moderate; sand, clean, stiff; sand, slightly silty, moderate; sand, very silty, loose; loam, very sandy, stiff; loam, slightly sandy, weak; clay, very sandy, stiff; clay, slightly sandy, moderate; clay, clean, stiff; clay, clean, weak; clay, organic, moderate; clay, organic, weak; peat, moderately preloaded, moderate; peat, not preloaded, weak.
• The weakest transition remains the area-5/area-6 pair (loam, slightly sandy, weak versus clay, very sandy, stiff). Near ties are resolved by the fixed chart order of the stored representative set.
• A repo verification script re-projects all fourteen centres at several effective stresses to ensure the stored centres and the implemented score formula stay in sync.
• The selected NEN material is then mapped into the app’s internal type families so that later layer grouping and parameter workflows remain consistent.

2.5 NEN Tabel 3 — direct subtype classification

The Table 3 route uses raw q_c and R_f and returns a detailed subtype together with the characteristic parameter row used later in the workflow. The direct source documented here is the implemented NEN Tabel 3 catalogue itself, not a separate Eurocode classification chart.

Because the table contains overlapping q_c–R_f envelopes, the app evaluates the rows deterministically in table order rather than assuming the source is mutually exclusive. The implemented family order is grind, zand, leem, klei, then veen. Within each family, subrows are checked top-to-bottom, with q_c lower bounds inclusive and upper bounds exclusive. For R_f, the open bands R_f < 1 and R_f > 6 remain strict, while the bounded intervals are treated as closed.

For this method, the raw boundary logic follows the detailed subtype result rather than only the broad family. This means subtype changes such as density or consistency transitions inside one family can create provisional layer boundaries before smart merge and minimum-thickness correction simplify them.

• If no table row matches, the app keeps a deterministic fallback so the workflow can continue, but that fallback is not itself part of Table 3.
• The app UI may still show a legacy “EC7” label because the later parameter workflow is Eurocode-aligned. The direct q_c–R_f table documented in this section is the implemented NEN Tabel 3 source.

2.6 Classification versus parameter assignment

Stage 2 is boundary logic only. The classification method decides which soil type each CPT reading belongs to and therefore where soil-type changes occur with depth.

Stage 3 parameter assignment is independent of the chosen classification route. A layer identified by Robertson, CUR 3 layers, or NEN 6740 may still be assigned a Eurocode Table 3 subtype and its associated γ, γ_sat, φ′, c′, and c_u values.

3.1 Raw segmentation and boundary placement

The first pass creates provisional raw segments directly from the pointwise Stage 2 classification sequence. For Robertson, CUR 3 layers, and NEN 6740, the raw segment key is the broad internal soil family. For NEN Tabel 3, the raw segment key is the pair {type, subtype}, so detailed subtype changes can create provisional layer boundaries before later simplification.

Boundary depths are then constructed from the retained CPT rows. The first raw segment starts at ground level. Other raw segments start at the midpoint between the last CPT row of the previous segment and the first CPT row of the new segment, unless a downward merge has already handed them an inherited top boundary. If that straddling sample pair is unavailable or degenerate, the app falls back to the legacy 20 mm offset from the first row of the new segment. Segment thickness is always evaluated on these active layer boundaries rather than on the bare difference between first and last CPT reading.

• This means the NEN Tabel 3 route is intrinsically finer than the other Stage 2 routes, because subtype transitions inside one soil family are preserved in the raw segmentation.
• NEN 6740 currently uses the internal family only for raw layer generation; its 14 detailed material labels remain available for interpretation but do not by themselves create raw boundaries.
• The midpoint rule is self-calibrating to the actual CPT logging step and remains bounded by construction between the two samples that straddle the raw boundary.
• For uniform 40 mm sampling, the midpoint rule collapses exactly to the earlier 20 mm offset.

3.1A Smart merge correction

The smart-merge correction is an in-house MADEP post-processing heuristic. It is not a published external classification standard or code method.

The app also provides a smart-merge mode. This mode is applied only after the original raw layering has already been created from the unmodified point-by-point classification sequence.

In smart mode, the algorithm first removes highly compatible existing boundaries when the continuity score exceeds a sensitivity-controlled threshold. Only after that reduction step does it enforce the minimum thickness by merging every remaining sub-minimum layer with its most similar neighbor.

The resulting smart-merged configuration becomes the active layer model. Layer boundaries, per-layer averages, subtype suggestion, and downstream parameter assignment are recalculated from the final merged geometry.

• The original boundary detection step is never redefined by smart merge; smart merge operates only after the base layering algorithm has already run.
• In smart mode, similarity-based reduction is applied before the final minimum-thickness enforcement. The minimum thickness therefore acts as the last hard constraint, not as the first distortion of the layering.
• Very thin sections are intentionally down-weighted in the resistance to merging: sliver layers receive a small merge bonus, and sharp-transition penalties are attenuated when the layer thickness is small relative to a fixed sliver reference. This early similarity reduction is therefore independent of the chosen minimum thickness.
• Low sensitivity keeps smart merge closer to the conservative baseline behavior; high sensitivity allows smaller continuity advantages to influence the merge direction and makes it easier to remove highly compatible boundaries. At the extreme right-hand end of the range, the smart pass becomes willing to absorb almost all remaining compatible boundaries before the minimum-thickness rule is applied.
• The final layer count is not strictly monotonic with the sensitivity value. Because minimum-thickness enforcement runs after the smart-merge pass, a higher sensitivity can change the topology of the intermediate layering in a way that later produces either fewer or more final layers than a lower sensitivity case.
• If the two candidate directions are nearly equal, the app resolves the tie by merging into the thicker neighboring layer; if the thicknesses are equal, upward merge is chosen.
• The penalty term reduces the score for large q_c jumps, large R_f jumps, peat/non-peat transitions, gravel/non-gravel transitions, and thin layers that act as critical markers such as very weak peat, very coarse gravel, very high R_f, or extreme q_c.

3.1B Minimum-thickness enforcement

The minimum layer thickness remains the final hard constraint on the smart-merge chain. In baseline mode, it is enforced directly by repeated upward merging. In smart mode, it is enforced only after the similarity-based boundary reduction described above.

In smart mode, each remaining sub-minimum layer is merged with the more similar neighboring layer according to the directional score.

• This means the minimum-thickness setting does not control the early smart-similarity reduction itself; it only controls the last mandatory cleanup step.
• As a result, low t_min preserves more of the similarity-reduced layering, while high t_min forces stronger final consolidation of the remaining thin layers.

3.1C Intended engineer workflow

The intended use of the layering controls is sequential. First select the Stage 2 classification method that best represents the CPT behaviour for the project. That method defines the provisional pointwise geology and therefore the raw segment sequence.

Then use smart merge to remove boundaries that are judged highly compatible in the context of the surrounding profile. Only after that should the minimum thickness be used as the final hard cleanup rule for layers that remain too thin to retain.

The minimum-thickness setting is therefore not intended to replace the geological interpretation performed by the selected classification method or by the smart-merge pass. It is the final practical consolidation rule applied after those interpretation steps.

• Choose the classification method first; it defines the raw boundaries that everything else works from.
• Use smart merge to smooth obviously over-segmented or highly compatible boundaries, not to force an arbitrary target layer count.
• Use minimum thickness last to eliminate remaining thin layers that are still impractical after the similarity-based cleanup.
• After every change, review the resulting boundaries against the q_c and R_f trends rather than judging only by the final number of layers.

3.2 Final layer statistics and boundary continuity

After the final merged geometry is fixed, the app recomputes each layer summary from the merged CPT rows. Mean q_c, mean f_s, and mean R_f are taken over the valid CPT rows inside the final segment. The representative subtype string is the modal subtype within that final row set.

The final layer table is then rebuilt with continuous boundaries: each layer top is taken from the previous final layer bottom, and each layer bottom is forced not to lie above its own top. These recomputed averages are the Stage 4 input for stiffness and conductivity derivation.

• Rows with q_c ≤ 0.02 MPa are excluded from the averages when possible; if that would leave no rows, the full segment row set is used as fallback.
• So every time the merging changes, the boundaries and the engineering averages are recalculated consistently from the new merged geometry.

3.3 Subtype suggestion and parameter assignment

The final merged layer first inherits provisional parameters from the merged rows. For the NEN Tabel 3 classification route, the app can average the pointwise row parameters available from the table-matched CPT rows. For the other classification routes, it falls back initially to the generic family defaults.

After that, the app auto-suggests the best Eurocode Table 3 subtype from the catalogue using the final layer average q_c, average R_f, and the compatibility matrix of the CPT family. If the active Stage 3 parameter method is the Eurocode Table 3 route, the suggested subtype also supplies the layer parameters. All assigned values remain editable, and manual overrides always take precedence over automatic assignment.

• The suggested Eurocode subtype is selected from the full catalogue by compatibility first and q_c/R_f fit second.
• The final layer warnings below the table are then derived from the chosen subtype versus the CPT-family compatibility matrix, so the engineer can still see when a selected subtype lies only in an adjacent transition family.

3.4 PLAXIS simulated CPT export

Stage 3 can export the active interpreted layer model as a measurement-style CPT text file for PLAXIS. The goal is to reproduce the final interpreted layering exactly, not to retain the original pointwise cone variability.

The export reuses the original CPT depth rows and coordinates. For each original depth, the app finds the active final layer and writes that layer's representative values back as a synthetic CPT row. The result is therefore piecewise constant per interpreted layer, but sampled at the original CPT spacing.

• The header contains X[m], Y[m], Z[m], followed by a D[m] Q[MPa] F[MPa] x table.
• One exported row is written for every original CPT depth row, so the synthetic file remains aligned with the measured depth sampling.
• The final x column is currently written as 0, so the exported file behaves as a PLAXIS-compatible CPT with depth, cone resistance, and sleeve friction.

4.1 Effective stress at layer midpoint

Both model-parameter routes use the effective vertical stress at layer midpoint. The phreatic level therefore directly affects the reference stiffness correction.

Above the water table, γ is used; below it, γ_sat is used. Pore pressure is hydrostatic.

4.2 q_c-to-E_oed,i correlation and α methods

The first stiffness step converts the representative cone resistance into an oedometric modulus through one of two α correlations selected in the Stage 4 UI: Method A — Sanglerat literature (fixed α per behavioural soil type) or Method B — SB260-21-6.4.10 (q_c-graded family rules). Method A and Method B are alternative routes that produce different α — and hence different E_oed,i — for the same layer.

Method A applies fixed Sanglerat-style α defaults keyed to the broad layer type (Peat, Soft clay, Clay, Sandy clay, Silty sand, Sand, Gravel). All rows in the layer share one α regardless of q_c.

Method B applies the SB260-21-6.4.10 Tabel 21-6-5 rules, structured around three families — cohesive (klei, leem, veen), transition / overgangsgronden (the (zh) and (lh) subtype variants), and granular / zandgronden (zand, grind, grind (kh)). The family is selected from the EC7 subtype string first, falling back to the broad type when no subtype is set, and α (or E_s) varies with q_c within the family.

The full implemented α mapping is shown in the expandable reference table below so the reader can audit exactly which family, q_c band, or modulus formula is applied.

• Method A produces the higher α values for granular soils (Sand = 13, Gravel = 15) characteristic of Sanglerat literature. Method B uses the SB260 zandgronden rule E_s = 4q_c / 2q_c+20 / 120, which gives much smaller effective α (e.g. α = 4 for q_c ≤ 10 MPa). The two methods are not interchangeable — Method A and Method B will give different E_oed,i for the same layer.
• Method A keys off the broad layer type (l.type) only. Method B keys off the EC7 subtype string first and falls back to type when no subtype is available.
• For peat, water content w is not available in the app, so Method B uses the SB260 default α = 1.5 (the conservative branch of Tabel 21-6-5 for veen).
• The (zh) / (lh) suffix on klei / leem / zand routes to the transition family. The (kh) suffix on grind routes to the granular family.

4.3 Reference-stress correction (shared by Methods A and B)

Both Method A and Method B apply the same Hardening Soil reference-stress correction to convert the in-situ E_oed,i from §4.2 into the reference-stress quantity E_oed,ref at p_ref = 100 kPa. The cohesion-corrected form is used so the correction remains well-behaved in cohesive layers where σ′_v0 can be small relative to c′cotφ′.

Methods A and B differ only in how E_50,ref and E_ur,ref are derived from this shared E_oed,ref; the depth correction itself is identical in both routes.

The CUR 2003-7 stress-exponent default is binary: m = 0.5 for granular soils (Sand, Silty sand, Gravel) and m = 1.0 for cohesive soils (Sandy clay / leem, Clay, Soft clay, Peat / organic). The Stage 5 m-fitting routine can override the m default per layer when site-specific evidence supports a different value.

• The stress-exponent default mirrors the CUR 2003-7 binary 0.5 / 1.0 split rather than the more nuanced PLAXIS-style 0.5 / 0.7 / 0.85 / 1.0 grading. The intent is to align the documented method (CUR 2003-7) with the implemented default; per-layer engineer override and Stage 5 m-fitting remain available.
• The cohesion-corrected denominator uses c′cotφ′ as a Mohr-Coulomb consistency term so the depth correction stays bounded in cohesive layers; this matches Schanz, Vermeer & Bonnier (1999) and the SB260-21-6.4.10 form.

4.4 Method A — CUR 2003-7 stiffness ratios

Method A takes the shared E_oed,ref from §4.3 and applies the CUR 2003-7 family rule to derive E_50,ref. E_ur,ref is taken as three times E_50,ref. The family rule treats klei and leem together, so Sandy clay (leem) is in the cohesive set with the 1.25 factor; for granular soils E_50,ref is set equal to E_oed,ref. ν_ur = 0.20 (CUR 2003-7 / PLAXIS 2D Manual).

• The cohesive set for the E₅₀/E_oed = 1.25 ratio includes Sandy clay (leem) — the CUR 2003-7 rule is "klei en leem", not "klei alone".
• E_ur,ref = 3E_50,ref follows aGEO practice and is a Hardening-Soil convention shared with the PLAXIS 2D Manual.

4.5 Method B — E_50,ref = E_oed,ref

Method B takes the shared E_oed,ref from §4.3 and sets E_50,ref equal to E_oed,ref for all soils. E_ur,ref remains three times the selected E_50,ref.

This gives a single consistent reference stiffness and is sometimes preferred in practice when the engineer wants to avoid the cohesive-soil E₅₀/E_oed split.

4.6 Hydraulic conductivity basis

The app uses indicative hydraulic conductivity values tied to Belgian and USDA-style texture classes, with OVAM (Tabel 2-44) and De Smedt / VUB (Tabel 2-45) as the principal reference sources for the I/RA/11461.15.066/JSW guideline. The representative value is treated as a geometric-mean estimate within the adopted class range rather than a deterministic measurement.

Anisotropy is then introduced through a k_h/k_v ratio. Sand and gravel are taken as isotropic. Cohesive soils (clay, sandy clay / leem, peat) are taken as k_h/k_v = 3. Silty sand ("fijn zand") sits between the two regimes; the engineer selects between a Stage 4 anisotropy method that controls how that intermediate case is treated.

For the PLAXIS material-command export, the internally stored conductivities are converted from m/s to m/day before they are written to the command file.

• Method A — OVAM / I/RA/11461 (default): conservative engineering practice. Silty sand is grouped with the fine soils, k_h/k_v = 3.
• Method B — Bear (1979) academic: literature-typical intermediate value for fine / silty sand, k_h/k_v = 2. Reflects the partly-cohesive nature of silty sand without lumping it fully with cohesive soils.
• Sand and gravel remain isotropic (k_h/k_v = 1) under both methods. Cohesive soils (clay, sandy clay, peat) keep k_h/k_v = 3 under both methods.
• ψ_unsat (height of partially saturated zone above the water table) follows the PLAXIS 2D Manual: 0.1 m for sand / silty sand / gravel, 1 m for sandy clay (leem), 3 m for clay and peat.
• In-situ measurement takes priority over indicative table values.

4.7 PLAXIS material-command export

Stage 4 includes a dedicated PLAXIS material export in addition to the CSV layer table. The implemented export writes a text file containing supported soilmat commands, not a directly generated native .matXdb database.

That choice is deliberate: in the modern PLAXIS workflow the app reliably creates project materials through command-line material creation, after which the engineer can copy those project materials into a reusable global material database from inside PLAXIS if desired.

For every interpreted layer, the export writes two material definitions: one Mohr-Coulomb material and one Hardening Soil material.

• Mohr-Coulomb export writes Identification, SoilModel = 2, DrainageType, gammaUnsat, gammaSat, ERef, nu, cRef, phi, psi, PermHorizontalPrimary, and PermVertical.
• Hardening Soil export writes Identification, SoilModel = 3, DrainageType, gammaUnsat, gammaSat, E50Ref, EOedRef, EURRef, PowerM, pRef = 100, cRef, phi, psi, PermHorizontalPrimary, and PermVertical.
• For the MC material, ERef follows the current-stress loading stiffness E50,i from the selected Stage 4 stiffness route rather than the Hardening Soil reference-stress quantity E50Ref.
• Method A gives E50,i = 1.25 EOed,i for the cohesive set — Sandy clay (leem), Clay, Soft clay, and Peat / organic — and E50,i = EOed,i for the granular set (Sand, Silty sand, Gravel). Method B gives E50,i = EOed,i for all soils.
• Only clean sand and clean gravel are exported as Drained by default. Fines-bearing granular subtypes such as zand (lh) and grind (kh) are exported as Undrained A.
• The export intentionally omits RF, nuUR, and K0NC in the current tested PLAXIS workflow so PLAXIS can keep those automatic or read-only values under its own control.
• cu and psi_unsat are not written by the material-command export. Undrained A uses the effective stress strength parameters, and psi_unsat remains available in the app without yet being exported.

4.8 Drained Poisson ratio, β, and the deformation modulus E_def (SCIA Engineer)

SCIA Engineer’s subsoil and Soil-In input does not take the oedometric modulus directly. Each layer of the geologic profile is characterised by the deformation modulus E_def, defined as the ratio of a normal-stress increment to the corresponding linear-strain increment, and SCIA converts E_def internally to the constrained (oedometric) modulus of the ČSN 73 1001 settlement formulation through the transfer coefficient β (SCIA Engineer 25.0 Online Help, “Defining a new geologic profile”). The identical relation is implemented by GEO5 for the same settlement method, and the convention originates in ČSN 73 1001 (effective 1988–2010, since replaced by ČSN EN 1997-1), whose layered-settlement model Soil-In computes.

β is not an empirical correlation factor. It is the exact isotropic-elasticity ratio between the Young-type modulus and the constrained modulus: inverting Hooke’s constrained-modulus identity M = E(1 − ν)/[(1 + ν)(1 − 2ν)] gives E = βM with β = (1 + ν)(1 − 2ν)/(1 − ν), which is algebraically identical to the form β = 1 − 2ν²/(1 − ν) printed in the SCIA documentation. Stage 4 therefore reports, per layer, the drained Poisson ratio ν′, the derived coefficient β, and the deformation modulus E_def = β·E_oed,i.

The conversion is applied to the in-situ modulus E_oed,i at the layer mid-depth stress state — not to the reference quantity E_oed,ref, which is a Hardening-Soil construct normalised to p_ref = 100 kPa and would misstate the layer stiffness at its actual stress level. E_def consequently inherits the selected α method (§4.2): Method A and Method B produce different E_oed,i and hence different E_def for the same layer. The app reports E_def in kPa; the SCIA geologic-profile dialog expects MN/m² (MPa), so the reported value is divided by 1000 on entry.

The selection of ν′ follows a two-stage proposal cascade, each stage overridable by the engineer. The soil type selected in Stage 3 (NEN/EC7 Tabel 3 subtype) proposes a drained Poisson ratio from the default table below; the engineer may override ν′ per layer in the Stage 4 Mohr-Coulomb panel; and β and E_def are always derived from the resolved ν′, never edited directly. Selecting a different soil type in the Stage 3 dropdown — including a consistency-only refinement within the same family, since the defaults are graded per subtype — re-proposes ν′ for that layer even when ν′ had previously been overridden: the manual value was chosen for the previous selection and is deliberately invalidated, after which the engineer may override again.

The default table is anchored to the ČSN 73 1001 directive normative characteristics — the native parameter set of the β/E_def convention, reproduced in the SCIA help — wherever a Belgian Tabel 3 class has a ČSN analogue (gravels G1–G5: 0.20–0.30; sands S1–S5: 0.28–0.35; fine soils F1–F4: 0.35, F5–F7: 0.40), and is capped at ν′ = 0.40. Two families depart deliberately from ČSN: veen, for which ČSN has no class and for which measured drained values of peat are far lower than mineral fine soils, and leem, which is kept in the international silt band 0.30–0.35 rather than the ČSN fine-soil value 0.40 because Belgian (Hesbaye) leem is a low-plasticity loess silt.

• E_def is a drained, long-term settlement parameter, so ν must be the drained ratio ν′. The undrained value ν_u ≈ 0.5 describes the short-term incompressible state — EN 1997-2:2007 Annex K.2(3) (informative, in the plate-load module context) gives 0.5 for fine soil undrained and 0.3 for coarse soil, and the second-generation draft prEN 1997-2 clause 9.1.2.2 generalises ν′ = 0.3 drained / ν_u = 0.5 undrained — and at ν = 0.5 the coefficient β vanishes and E_def loses meaning. The Stage 4 input is therefore clamped to 0.05–0.49, and all table defaults stay at or below 0.40.
• The previous implementation assigned ν = 0.40–0.45 to veen and soft klei. These are near-undrained magnitudes: at ν = 0.45, β = 0.26, which would underestimate E_def by a factor of roughly two to three against the drained literature. The veen ladder is rebuilt at 0.15/0.20/0.25 from measured drained data (O’Kelly 2017: ν′ = 0.10–0.15 for fibrous peat, with a K₀-implied ceiling of about 0.21–0.26), and klei is capped at the ČSN soft-fine-soil value 0.40.
• ČSN 73 1001 itself tabulates 0.42 for class F8 (high-plasticity fat clays) — a class the Belgian Tabel 3 does not isolate; in SCIA’s reproduction of that table the printed β of 0.37 also differs from the formula value 0.392. High-plasticity clays should therefore be handled by an explicit per-layer override rather than by a higher table default.
• Grading directions are physically distinct and both sourced: in granular soils ν′ rises with density (Kulhawy & Mayne 1990: ν_d = 0.1 + 0.3(φ′ − 25°)/20°), while in clays the proposal falls with consistency (soft NC clay has higher K₀ = 1 − sin φ′, hence a higher K₀-matched ν; stiff OC clay trends toward the PLAXIS unloading range 0.15–0.25, Material Models Manual §3.3.2). The family ordering Gravel (0.28) < Sand (0.30) is correct because ν′ tracks fines and plasticity content rather than grain size: ČSN gives clean gravels 0.20–0.25 below clean sands 0.28.
• Per the SCIA Topic Training, smaller E_def values “are on the safe side” — for the computed settlement magnitude. This is not universally conservative: the Soil-In C-parameters also redistribute internal forces in the supported plate, so deliberately understating stiffness is not a substitute for a representative value where bending moments or differential settlements govern.
• The same drained ν′ feeds the PLAXIS Mohr-Coulomb material export (§4.7). Note the interplay with drainage typing: PLAXIS requires ν′ < 0.35 for Undrained (A)/(B) materials (Material Models Manual §3.3.2), while cohesive layers are exported as Undrained A — a soft klei layer defaulting to ν′ = 0.40 will therefore be flagged on import, and the engineer should review ν′ or the drainage type in PLAXIS in that case.
• E_def and β are exported in the CSV layer table (columns nu, beta, Edef_kPa) and carried in the Stage 7 report payload; the resolved ν′ continues to feed the in-app Stage 6 deformation materials as before.

5.1 Log-linear basis of the fit

The oedometer law of the Hardening Soil model becomes linear in logarithmic form. This allows the app to fit a straight line to the pointwise CPT-derived E_oed,i values inside one layer.

The fitted slope is interpreted directly as m, while the fitted intercept gives E_oed,ref.

5.2 OLS solution, fit quality, and acceptance

The regression is performed on all valid CPT points inside the layer, using the current α method and the current water table. The resulting m is clamped to the engineering range used by the app, and the fit quality is reported in log space.

The current interface shows the default line, the fitted line, and the point cloud; the engineer may still adjust the previewed m before acceptance.

• The fit is experimental and remains an engineer preview.
• m is expected to remain between 0 and 1 in normal interpretation, even though the raw regression may suggest otherwise.
• Accepting the fit overrides m only; the reference stiffness is then recomputed from the accepted m.

6.1 Sign convention and in-situ stresses

Compression is taken as positive. Depth z is measured downward from ground level. Elevation values are interpreted in Belgian TAW convention where relevant.

The in-situ effective-stress profile is reconstructed from the active groundwater level and the interpreted unit weights.

• Above the phreatic level, the dry unit weight γ is used; below it, γ_sat is used. Pore pressure u(z) = γ_w · max(0, z − z_w) is subtracted from σ_v(z) to form σ′_v(z).

6.2 Hardening Soil reference-stress convention

Stage 6 uses the Stage 4/5 Hardening Soil stiffness convention. E_oed,ref is referenced at p_ref = 100 kPa and stress-dependent stiffness is recovered from the interpreted m-value and effective stress level.

For the current settlement and dewatering routes, the governing stress is the vertical effective stress on the evaluated centreline.

6.3 Integration and sublayering

The engineering calculations use stratification summation. The profile is divided into sublayers, stresses are evaluated at sublayer mid-depth, and settlement or stress changes are integrated over depth.

The application keeps interpreted layer boundaries and phreatic boundaries explicit in the discretisation so stiffness and stress changes are not smeared across those interfaces.

Eurocodes and Belgian National Annexes

• EN 1997-1:2004+A1:2013: Eurocode 7 — Geotechnical design — General rules.
• NBN EN 1997-1 ANB:2022: Belgian National Annex to EN 1997-1, Design Approach 1.
• EN 1990:2002+A1:2005: Basis of structural design.
• NBN EN 1990 ANB:2005: Belgian National Annex to EN 1990.
• EN 1992-1-1: Eurocode 2 — Design of concrete structures.
• NBN EN 1992-1-1 ANB: Belgian National Annex to EN 1992-1-1.

Books and Journal Papers

• Terzaghi & Peck (1967): Soil Mechanics in Engineering Practice, 2nd ed.
• Vesić (1975): Bearing Capacity of Shallow Foundations, in Foundation Engineering Handbook (Winterkorn & Fang, eds.).
• Robertson (1990): Soil classification using the CPT. Canadian Geotechnical Journal, 27(1), 151–158.
• Robertson (2016): Cone penetration test (CPT)-based soil behaviour type (SBT) classification system — an update. Canadian Geotechnical Journal, 53(12), 1910–1927.
• Robertson & Wride (1998): Evaluating cyclic liquefaction potential using the CPT. Canadian Geotechnical Journal, 35, 442–459.

Belgian and Dutch practice documents

• SB260: Standaardbestek 260, artikel 21-6.4.10: karakteristieke grondparameters op basis van elektrische sondering.
• CUR 2003-7: CPT-correlated geotechnical parameter guidance used in Belgian and Dutch practice.
• NEN 6740: Dutch geotechnical design standard with stress-dependent CPT material classification.
• Deltares D-SHEET Piling User Manual: Version 24.1; §34.2.2 documents the NEN stress-correction formula q_c,NEN = q_c(100 / σ′_v0)^0.67.
• PLAXIS 2D 2018 Reference Manual: Reference manual showing the broad CUR 3 layers classification chart with Sand, Silt, Clay, and Peat fields.

PLAXIS workflows

• PLAXIS 2D Material Models Manual (2025.1): Official material-model manual giving the Mohr-Coulomb Young’s modulus guidance and Hooke-law stiffness relations.
• Bentley KB0109063: How to define and edit a material via the command line; documents the supported soilmat workflow.
• Bentley KB0109071: PLAXIS soil model numbers for command-line material creation.
• Bentley KB0043470: Re-using materials from other projects in PLAXIS, including project-to-database workflows.
• Bentley KB0108936: Material parameter datasets article whose sample package shows modern .matXdb content on disk.

Hydraulic conductivity and constitutive parameter sources

• Schanz, Vermeer & Bonnier (1999): The Hardening Soil model: formulation and stress-dependent stiffness basis.
• OVAM / I-RA-11461 (2002): Indicative hydraulic conductivity ranges by Belgian texture class.
• De Smedt / VUB (2005): Indicative hydraulic conductivity ranges by USDA texture class.

Classic theoretical references

• Boussinesq (1885): Application des potentiels à l’étude de l’équilibre et du mouvement des solides élastiques.
• Newmark (1935): Simplified computation of vertical pressures in elastic foundations.
• Fadum (1948): Influence values for estimating stresses in elastic foundations.
• Dupuit (1863): Études théoriques et pratiques sur le mouvement des eaux.
• Thiem (1906): Hydrologische Methoden.
• Bear (1979): Hydraulics of Groundwater.
• Freeze & Cherry (1979): Groundwater.
• Kyrieleis & Sichardt (1930): Grundwasserabsenkung bei Fundierungsarbeiten.
• Louwyck et al. (2022): The Radius of Influence Myth. Water, 14(2), 149.
• Powers et al. (2007): Construction Dewatering and Groundwater Control, 3rd ed.

Foundation models

• Hetényi (1946): Beams on Elastic Foundation.
• Vesić (1961a): Bending of beams resting on isotropic elastic solid.
• Vesić (1961b): Beams on elastic subgrade and Winkler’s hypothesis.
• Pasternak (1954): On a New Method of Analysis of an Elastic Foundation by Means of Two Foundation Constants.
• Kerr (1964): Elastic and viscoelastic foundation models.

SCIA Engineer, E_def, and drained Poisson-ratio sources

• ČSN 73 1001:1987: Zakládání staveb. Základová půda pod plošnými základy (Foundation of structures — Subsoil under shallow foundations), Praha; effective 1.10.1988, withdrawn 1.4.2010, replaced by ČSN EN 1997-1. Origin of the E_def/β convention and of the directive normative ν–β–E_def characteristics per soil class (bibliographic data via secondary records).
• SCIA Engineer 25.0 Online Help (Soilin): “Defining a new geologic profile” and “Subsoil in the 3D model”: E_oed = E_def/β with β = 1 − 2ν²/(1−ν); reproduces the ČSN 73 1001 ν/β tables. help.scia.net, accessed 12 June 2026.
• SCIA Topic Training — Foundations (2024): SCIA Engineer 25.0 Topic Training, Ch. 5 Soilin: per-layer E_def entry in MN/m² and the guidance that smaller E_def values are on the safe side for the computed settlement.
• GEO5 Online Help — Oedometric Modulus: Fine spol. s r.o., Theory — Settlement Analysis: independent implementation of the identical E_def/β relation for the ČSN 73 1001 settlement method.
• EN 1997-2:2007: Eurocode 7 — Geotechnical design — Part 2: Ground investigation and testing. Annex K.2(3) (informative, plate-load settlement module context): ν = 0.5 for fine soil undrained, 0.3 for coarse soil — the standard’s only Poisson-ratio guidance.
• prEN 1997-2 (2021 draft): Second-generation Eurocode 7 Part 2, CEN/TC 250/SC 7 document N1506, clause 9.1.2.2: ν′ = 0.3 drained and ν_u = 0.5 undrained in the absence of direct measurement.
• Buildwise (2022): Richtlijnen voor de toepassing van de Eurocode 7 in België volgens NBN EN 1997-1 ANB — beschoeiingen, §4.2.5: gedraineerd ν = 0.2 à 0.3; ongedraineerd ν = 0.5 (zand, leem en klei).
• Kulhawy & Mayne (1990): Manual on Estimating Soil Properties for Foundation Design, EPRI EL-6800, Electric Power Research Institute: drained ν_d = 0.1 + 0.3(φ′ − 25°)/20° for granular soils (after Trautmann & Kulhawy 1987).
• O’Kelly (2017): Measurement, interpretation and recommended use of laboratory strength properties of fibrous peat. Geotechnical Research (ICE) 4(3), 136–171: measured drained ν′ of peat 0.10–0.15; K_0,nc of seven peats 0.26–0.36.
• AASHTO LRFD (2020): Bridge Design Specifications, Table 10.6.2.2.3b-1 “Elastic Constants of Various Soils” (after U.S. Dept. of the Navy 1982 and Bowles 1988): silt 0.3–0.35; sand loose 0.20–0.36, medium 0.25–0.40, dense 0.30–0.40; gravel 0.20–0.40.
• Bowles (1996): Foundation Analysis and Design, 5th ed., McGraw-Hill, Table 2-7 typical Poisson values: sandy clay 0.2–0.3; silt 0.3–0.35; sand and gravelly sand 0.3–0.4 (values via secondary reproduction).
• Ameratunga, Sivakugan & Das (2016): Correlations of Soil and Rock Properties in Geotechnical Engineering, Springer, Table 3.17: saturated clays drained 0.2–0.4 and undrained 0.5; dense sand 0.3–0.4; loose sand 0.1–0.3.