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Usefulness of plasticity chart

Short answer

Plots Ip against wL with A-line and 50% line. It classifies fine soils into clay, silt, or organic and low/high plasticity, gives CL-ML for hatched zone, classifies fines in coarse soils, and indicates compressibility and swelling.

How is the plasticity chart useful for classifying fine-grained soils?

The plasticity chart plots plasticity index (IpI_p) against liquid limit (wLw_L) and has the A-line (Ip=0.73(wL−20)I_p = 0.73(w_L - 20)) and a vertical line at wL=50%w_L = 50\%.

It is useful because:

  1. It classifies fine-grained soils (more than 50% passing 75 micron) into clay (C), silt (M) or organic (O) by position relative to the A-line (above = clay; below = silt/organic).
  2. It divides them into low (wL<50w_L<50) and high (wL>50w_L>50) plasticity (L or H), giving CL, CH, ML, MH, OL, OH.
  3. The hatched zone above A-line with Ip=4I_p = 4 to 7 gives the borderline class CL-ML.
  4. It is also used to classify the fines of coarse-grained soils with more than 12% fines (GM, GC, SM, SC).
  5. It indicates relative engineering behaviour: compressibility, swelling and strength rise with position upward and right.

✓ Answer checked by Claude Opus 5.5 (high effort)

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