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Chapter 3 — Soil Classification

3.1 Why classify

Classification groups soils with similar engineering behaviour into a few named categories. It lets an engineer communicate, correlate with local experience, and make first estimates of strength, compressibility, and drainage — before any testing. It is a language, not a design method.

3.2 The Unified Soil Classification System (USCS)

The USCS is the standard in geotechnical practice. It uses:

  • the grain-size distribution (percent passing the No. 200 sieve) to separate coarse-grained from fine-grained soils;
  • the \(C_u\) and \(C_c\) to distinguish well-graded (W) from poorly-graded (P) coarse soils; and
  • the Atterberg limits (via the plasticity chart) to classify fines as silt (M), clay (C), or organic (O) of low (L) or high (H) plasticity.

Major groups: GW, GP, GM, GC (gravels); SW, SP, SM, SC (sands); ML, CL, OL, MH, CH, OH (silts, clays, organics); and Pt (peat).

3.3 The AASHTO system

The AASHTO system (used in highway engineering) classifies soils into groups A-1 through A-7 for use in pavement and embankment design, with a group index reflecting quality. A-1 and A-2 are granular and good; A-6 and A-7 are clayey and poor.

3.4 What classification does not tell you

Classification is based on index properties, so it cannot give:

  • the actual strength or stiffness (two CLs can differ widely);
  • the density state (loose vs. dense sand both classify as SP);
  • the in-situ structure (cementation, fabric, fissures);
  • the stratigraphy — one sample does not describe a site.

Classification must always be confirmed by testing and observation.

3.5 Key takeaways

  • Classification is a language for grouping soils by behaviour.
  • USCS uses grain size + Atterberg limits; AASHTO uses groups A-1 to A-7.
  • It gives first estimates but not actual strength, density, or structure.
  • Always confirm with testing and field observation.