Practice Question: Classify the given soil according to the Unified Soil Classification System

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Solution

The cumulative % retained on the No. 200 sieve (R200) = 79 (14+12+8+15+21+9).

% Finer then No. 200 sieve size (F200) = 100 − R200 = 100 − 79 = 21

Since F200 < 50, soil is predominantly coarse.

Therefore, first letter is S (sand) or G (Gravel).

Half the coarse fraction = 79/2 = 39.5

% retained on No. 4 sieve (R4) = 14 < 39.5 (half of the coarse fraction). Therefore, first letter is S (sand).

Since F200 =21 > 12, second letter of classification should be based on plasticity characteristics rather than gradation.

Use the Casagrande Plasticity chart with

LL = 25, PI = (25 – 12) = 13

This lies above the A-line (more like clay than silt).

Soil is classified as SC.

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