CivilLab · Study Aid

Particle Size & Soil Classification

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What am I seeing?
  • The grading curve is read at 10, 30 and 60 percent passing. Those three sizes give Cu and Cc, which say how wide the range of sizes is and how smoothly it is filled.
  • A steep curve holds one size, so Cu is small and the soil is poorly graded. A flat curve covers many sizes, so the small grains fill the gaps between the large ones and the soil packs tighter.
  • Once more than 10 percent passes the 75 micron sieve, D10 lies below the finest sieve. Cu and Cc can no longer be read from sieving and a hydrometer test is needed.
  • The plasticity chart only matters when there are enough fines. Under 5 percent it is ignored, between 5 and 12 percent it adds a second symbol, and above 50 percent it decides the whole name.
  • The A line separates clay from silt at the same liquid limit. Above it the fines are clays, below it they are silts. Nothing should plot above the U line.