GeoStudysGEOTECHNICS | ENVIRONMENT | GEOTHERMAL
Technical articles  /  Method sheets  /  M03
GeoStudysMethod sheets
SHEET 03 · SEPTEMBER 2026 · CONE PENETRATION TEST
METHOD M03CPT CLASSIFICATION

Robertson chart in glacial till

Why the CPT classifies overconsolidated glacial till as sand – and why every group of CPTs therefore has a borehole alongside.

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
Interpret the soil behaviour type from the CPT correctly where the ground moraine is glacially preloaded.
Input
qc and fs per centimetre; borehole log of a key borehole; water level for the effective stress.
Output
Robertson zone as behaviour type, not as soil type; a flag wherever CPT and borehole disagree.
Limitations
From qc and fs alone, glacial till cannot be reliably distinguished from dense sand. Without u2 measurement, qt ≈ qc.
Standards
DIN EN ISO 22476-1, DIN EN ISO 14688-1, DIN 18196
The CPT measures how the soil behaves – not what it consists of.Principle, after Robertson [1]
Robertson chart and share of data points in glacial till classified as sand — anonymised
← SWIPE IMAGE SIDEWAYS →
Figure 1. Wind farm in Saalian ground moraine, four sites each with one key borehole and three CPTs (anonymised). a) Data points from 2 m depth, coloured by borehole description; b) share of points in weathered till and till classified as sand by Ic or IB, zone 5 shown separately. One site has no cohesive section down to 11 m.

01Findings at approx. 5 300 data points in weathered till and till

25 %sand by Ic (zones 6–7), a further 59 % sand mixtures (zone 5) – not wrong for SU*/ST*
44 %sand-like by IB (> 32)
15 %clay-like by IB (< 22) – correctly identified
96 %dilative (CD > 70) – consistent with heavy preloading

02Why this happens

The CPT responds to the mechanical behaviour in the ground, not to the grain size distribution [1]. Very stiff, heavily overconsolidated fine-grained soils behave like coarse-grained soils and dilate on shearing [1]. For these, the chart provides zones 8 and 9, “overconsolidated or cemented” [1]; the Ic circles of zones 2 to 7 do not capture them. At low overburden, normalisation also raises Qtn. IB [2] separates only partly as well: the till plots where dense silty sand also plots.

03How we evaluate

  • Key borehole per site: it shows what is in the ground; the CPT provides depth, variability and strength [3].
  • Zone as behaviour type, never as soil type; always both Ic and IB.
  • House rule: zone 6–7 with qc above 30 MPa where the borehole shows ground moraine → flag “sand or preloaded till – compare with borehole”.
  • Parameters within their range of validity: no relative density in till, cu there as an upper estimate.
Ic = √[(3.47 − log Qtn)² + (log F + 1.22)²]IB = 100(Qtn + 10) / (Qtn·F + 70)CD = (Qtn − 11)(1 + 0.06F)17(1)

Ic after Robertson & Wride [4], IB and CD after Robertson [2]; Qtn normalised cone resistance, F normalised friction ratio [%]. Without u2 measurement, qt ≈ qc; σ′v0 = σv0, as no free groundwater was encountered down to final depth. Five CPTs refused at 34 to 46 MPa before target depth – possibly on cobbles, but unconfirmed.

Standards and references
  1. Robertson, P. K. (2010) Soil behaviour type from the CPT: an update. 2nd Int. Symp. on Cone Penetration Testing (CPT’10), Huntington Beach; cpt-robertson.com, accessed 24 Sept. 2026
  2. Robertson, P. K. (2016) Cone penetration test (CPT)-based soil behaviour type (SBT) classification system – an update. Canadian Geotechnical Journal 53, 1910–1927
  3. GeoStudys Method sheet 01 (2026) Layer boundaries with confidence level · DIN EN ISO 22476-1:2023-04 · DIN EN ISO 14688-1:2020-11 · DIN 18196:2023-02
  4. Robertson, P. K.; Wride, C. E. (1998) Evaluating cyclic liquefaction potential using the cone penetration test. Canadian Geotechnical Journal 35(3), 442–459
GEOSTUDYS METHOD SHEETS · M03 · 2026WWW.GEOSTUDYS.COM

SUGGESTED CITATION: GeoStudys Consulting (2026): Robertson chart in glacial till. GeoStudys Method sheets, Sheet M03. www.geostudys.com/en/technical-articles/robertson-chart-glacial-till.html