ISO 14688-2:2017

Overview - ISO 14688-2:2017 (Soil Classification Principles)

ISO 14688-2:2017 defines the principles for classifying soils for geotechnical engineering. It sets out how soils can be grouped by composition and geotechnical properties using results from field and laboratory testing. The standard is intended to be used together with ISO 14688-1 (Identification and description of soils) and is applicable to natural in‑situ soils, reworked natural materials and synthetic materials. Classification is based on composition (not on water content or compactness) and supports consistent, project‑specific or national classification schemes.

Key topics and technical requirements

• Basis of classification
  • Soils are classified into groups of similar composition and geotechnical behaviour using laboratory and field test results.
  • Classification must follow tests referenced in ISO 17892 series (laboratory testing of soil).
• Primary characteristics used
  • Particle size distribution (grading) - sieving and sedimentation per ISO 17892‑4; coarse soils can be classified solely by particle size.
  • Plasticity - Atterberg limits (liquid/plastic limits) and derived plasticity indices for fine soils.
  • Organic content and carbonate content - influence classification and require recognition.
  • Other parameters - density index (relative density), undrained shear strength, sensitivity, consistency index and other geotechnical parameters where appropriate.
• Classification rules
  • Coarse/very coarse soils: classification primarily by particle size distribution.
  • Composite soils: consider both grading and plasticity.
  • Fine soils: classification based on grading and/or plasticity - the chosen basis must be reported.
• Soil groups (examples)
  • Boulders, Cobbles, Gravel, Sand, Silt, Clay, Peat, Gyttja, Humus, Made Ground/Fill, Synthetic materials. (Project or national schemes can define more detailed subdivisions.)

Applications and who uses this standard

• Geotechnical engineers - site investigation interpretation, foundation design, slope stability studies.
• Site investigators and drilling contractors - consistent classification during sampling and reporting.
• Testing laboratories - standardized testing and reporting aligned with ISO 17892 methods.
• Civil engineers and earthworks contractors - material selection, specification, and earthworks quality control.
• Regulators and project managers - harmonized soil data for contract documents and compliance.

Practical uses include preparing geotechnical reports, selecting appropriate laboratory tests, defining material acceptance criteria, and communicating soil properties across multidisciplinary teams.

Related standards

• ISO 14688-1 - Identification and description of soils (required companion document)
• ISO 17892 series - Laboratory testing of soils (referenced test methods)
• ISO 14689 - Identification, description and classification of rock
• EN 16907-2 - More detailed earthworks classification (regional/European)
• ISO 25177 - Soil classification for pedology and contaminated‑site remediation

Keywords: ISO 14688-2:2017, soil classification, geotechnical investigation, particle size distribution, plasticity, soil groups, ISO 14688-1, ISO 17892.