Describe one classification system, limits

Unified Soil Classification System (USCS)

Proposed by A. Casagrande (1948) and adopted by the US Army Corps of Engineers and Bureau of Reclamation; it is the most widely used engineering system. Soil is classified by grain size distribution and plasticity.

Procedure

1. Coarse-grained if more than 50% is retained on the 75 micron (No. 200) sieve; fine-grained if 50% or more passes.
2. Coarse-grained: G (gravel) if more than half of the coarse fraction is retained on 4.75 mm sieve, else S (sand).
   • Fines < 5%: GW, GP, SW, SP. Well graded if $C_u \ge 4$ (gravel) or $\ge 6$ (sand) and $1 \le C_c \le 3$.
   • Fines > 12%: GM, GC, SM, SC using Atterberg limits (A-line).
   • Fines 5 - 12%: dual symbols (e.g. GW-GC).
3. Fine-grained: use the plasticity chart. Below A-line: M (silt); above: C (clay); organic: O; $w_L < 50\%$ gives L, $w_L \ge 50\%$ gives H: ML, CL, OL, MH, CH, OH. Pt is peat.

Limitations

• Only the fraction smaller than 75 mm is considered; cobbles and boulders are not classified.
• Gives no direct measure of engineering properties such as strength or compressibility.
• Dual symbols and borderline cases cause ambiguity.
• Particle shape, density and structure (in-situ condition) are not considered.
• Single boundary at $w_L = 50\%$ gives no medium plasticity class (the IS system corrects this).
• Based on the remoulded soil, so natural structure effects are ignored.