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SHEET 01 · SEPTEMBER 2026 · GROUND MODEL
METHOD M01BOREHOLE + CPT

Layer boundaries with confidence level

How one borehole and several CPTs become a geotechnical cross-section – and how the sheet shows which boundary is confirmed and which is adopted.

GeoStudys Consulting, Geotechnical consultancy for ground engineering, environment and geothermal energy, Berlin

Download PDF A4 · 1 PAGEREADING TIME APPROX. 3 MINUTES · AS OF 25 SEPT 2026
At a glance
Purpose
Geotechnical cross-section per site, e.g. beneath a wind turbine foundation.
Input
Borehole log of a key borehole (soil type, soil group, origin, carbonate content); raw CPT data qc, fs; collar elevations with source and accuracy.
Output
One A4 landscape sheet per site: section at true spacing, confidence level per boundary, legend, site plan.
Calculation
Reproducible: same input, same boundaries. Thresholds (in-house, not standardised) are stated on the sheet.
Limitations
Without a difference in strength or material behaviour, a boundary stays invisible in the CPT. Below the borehole, only the position of a boundary is confirmed, not the soil type.
Standards
DIN EN ISO 22476-1, DIN EN ISO 14688-1, DIN 18196, DIN EN 1997-2, DIN 4020
The CPT tells you where a boundary lies. The borehole tells you what lies there.Principle of the method
Geotechnical cross-section with confidence level and contrast function — anonymised
← SWIPE IMAGE SIDEWAYS →
Figure 1. Wind turbine site in Saalian ground moraine (anonymised). a) RKS window sample and three CPTs, each 10 m apart, qc grey (0–40 MPa); filled dot: boundary detected in the CPT, open dot: adopted from the borehole. b) Contrast function of CPT 3 with search windows (beige) and acceptance threshold. Section approx. 2.8× vertically exaggerated.

01Procedure

  1. Reference boundaries from the borehole. The borehole log gives the depth and content of each boundary.
  2. Contrast per CPT. Step in log qc and in the modified SBT index IB across a 0.50 m window, Eq. (1).
  3. Assign. Search window ±0.25·z (0.30 to 1.50 m); accepted from 1.8 times the median, top down and monotonic.
  4. Set the confidence level per Table 1 and draw it as line style in the section.
  5. Below the borehole only maxima above 2.5 times the median, spacing ≥ 1.0 m, as a short mark without correlation.
  6. Final check. Every borehole boundary visible, elevations with source, exaggeration stated, one sheet per site.
C(z) = |⟨log qc⟩u − ⟨log qc⟩o| + |⟨IB⟩u − ⟨IB⟩o| / 40IB = 100(Qtn + 10) / (Qtn·F + 70)(1)

⟨ ⟩o, ⟨ ⟩u mean over 0.50 m above and below z · Qtn normalised cone resistance · F normalised friction ratio [%] · IB modified SBT index [4], clay-like below 22, sand-like above 32 · weighting 1/40 and thresholds 1.8 and 2.5 are in-house method parameters. IB complements log qc where material behaviour changes but strength hardly does. Beneath CPT 1, qc alone separates the layers (sand 5–10 MPa, glacial till 25.5 MPa); IB classes the till as sand-like (approx. 85) – only the borehole settles the soil type.

02Confidence level

LevelConditionLineExample
highdetected in all CPTs1.0 · 7.0 · 9.5 m
mediumpartly detected, otherwise from the borehole0.4 · 4.5 m
lowdetected in none–

Table 1. Levels and result at the site in Figure 1; depths of the borehole boundaries.

03What the check reports

  • Map vs borehole: map shows ground moraine, borehole meltwater sand to 7.0 m – flagged as a discrepancy.
  • Boundary without contrast: at 4.5 m, CPT 3 reaches only 0.9 times the median. It remains visible as a borehole boundary.
  • Foundation level: not drawn without a confirmed working platform elevation.
  • Elevations: from the terrain model, ±0.15 m, marked as not surveyed.
Standards and references
  1. DIN EN ISO 22476-1:2023-04 Felduntersuchungen – Teil 1: Drucksondierungen mit elektrischen Messwertaufnehmern
  2. DIN EN ISO 14688-1:2020-11 Benennung und Beschreibung von Boden · DIN 18196:2023-02 Bodenklassifikation für bautechnische Zwecke
  3. DIN EN 1997-2:2010-10 Eurocode 7 – Erkundung und Untersuchung des Baugrunds · DIN 4020:2010-12 Geotechnische Untersuchungen für bautechnische Zwecke
  4. Robertson, P. K. (2016) Cone penetration test (CPT)-based soil behaviour type (SBT) classification system – an update. Canadian Geotechnical Journal 53, 1910–1927. doi:10.1139/cgj-2016-0044, accessed 24 Sept. 2026
GEOSTUDYS METHOD SHEETS · M01 · 2026WWW.GEOSTUDYS.COM

SUGGESTED CITATION: GeoStudys Consulting (2026): Layer boundaries with confidence level. GeoStudys Method sheets, Sheet M01. www.geostudys.com/en/technical-articles/layer-boundaries-confidence-level.html