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Unified Soil Classification System
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<p>The Unified Soil Classification System (USCS) is a <a href="/facts/Soil_classification/gru7kDvL">soil classification</a> system used in <a href="/facts/Engineering/RFVg8H3I">engineering</a> and <a href="/facts/Geology/WGvVRIYF">geology</a> to describe the <a href="/facts/Soil_texture/1PQkqW0u">texture</a> and <a href="/facts/Particle_size/KWwwGkn4">grain size</a> of a <a href="/facts/Soil/5Ddpr3QM">soil</a>. The classification system can be applied to most <a href="/facts/Unconsolidated/p4vcyUq1">unconsolidated</a> materials, and is represented by a two-letter symbol. Each letter is described below: </p> <table><tbody><tr><th>First and/or second letters</th><th>Second letter</th></tr><tr><td><table><tbody><tr><th>Letter</th><th>Definition</th></tr><tr><th>G</th><td><a href="/facts/Gravel/IwHG9JfI">gravel</a></td></tr><tr><th>S</th><td><a href="/facts/Sand/MdTIt0M8">sand</a></td></tr><tr><th>M</th><td><a href="/facts/Silt/HAxJkhDP">silt</a></td></tr><tr><th>C</th><td><a href="/facts/Clay/xnwouEkB">clay</a></td></tr><tr><th>O</th><td><a href="/facts/Organic_material/t17tVCvk">organic</a></td></tr><tr><th>PT</th><td><a href="/facts/Peat/F0kJcsLt">peat</a></td></tr></tbody></table></td><td><table><tbody><tr><th>Letter</th><th>Definition</th></tr><tr><th>P</th><td>poorly graded (many particle of about the same size)</td></tr><tr><th>W</th><td>well-graded (many different particle sizes)</td></tr><tr><th>H</th><td>high liquid limit</td></tr><tr><th>L</th><td>low liquid limit</td></tr></tbody></table></td></tr></tbody></table> <p>If the soil has 5–12% by weight of fines passing a #200 sieve (5% < P#200 < 12%), both grain size distribution and plasticity have a significant effect on the engineering properties of the soil, and dual notation may be used for the group symbol. For example, GW-GM corresponds to "well-graded gravel <i>with silt</i>." </p><p>If the soil has more than 15% by weight retained on a #4 sieve (R#4 > 15%), there is a significant amount of gravel, and the suffix "with gravel" may be added to the group name, but the group symbol does not change. For example, SP-SM could refer to "poorly graded SAND with silt" or "poorly graded SAND with silt and gravel." </p>