Soil Classification Sources: Soil Mechanics – Laboratory Manual,B.M. DAS (Chapters 9)

Class Outlines Soil Classification Systems USCS Classification System AASHTO USCS Classification System

USCS Classification System Originally developed for the United States Army The method is standardized in ASTM D 2487 as “Unified Soil Classification System (USCS)” USCS is the most common soil classification system among geotechnical engineers A typical USCS classification would be: SM Silty sand with gravel - Group Symbol Group Name

Naming Convention Fine-grained Soil Coarse-grained Soil First Letter Second Letter M – Silt L – Low plastic C – Clay H – High plastic O – Organic Coarse-grained Soil S – Sand P – Poorly graded G – Gravel W – Well graded M – Silty C - Clayey

Classification of Soils From sieve analysis and the grain-size distribution curve determine the percent passing as the following: > 3 inch – Cobble or Boulders 3 inch - # 4 (76.2 – 4.75 mm) : Gravel # 4 - # 200 (4.75 - 0.075 mm) : Sand < # 200: Fines First, Find % passing # 200 If (5%) or more of soil passes # 200 sieve, then conduct Atterberg Limits test (LL & PL)

Grain Size Distribution Curves GRAVEL SAND FINES Cobbles or Boulders

Classification of Soils If the soil is fine-grained (≥ 50% passes #200) follow the guidelines for fine-grained soils If the soil is coarse-grained (<50% passes #200) follow the guidelines for coarse-grained soils Find % Gravel & Sand Calculate Cu & Cc Calculate LL, PL and PI

Fine-grained Soils

Coarse-grained Soils

Example – Soil A Gravel Sand Fines = 8% Cu = 46.67 Cc = 0.95 98-62 = 36% Sand 62-8 = 54% Fines = 8% Cu = 46.67 Cc = 0.95 Soil A: D60 = 4.2 mm , D30 = 0.6 mm, D10 = 0.09 mm

Example – Soil A (Cont.) Gravel = 36% Sand = 54% Fines = 8% Cu = 46.7 Cc = 0.95 LL = 42 PL = 31 PI = 42-31 = 11 GO TO Plasticity Chart

Example – Soil A (Cont.) LL = 42 PL = 31 PI = 42-31 = 11 ML

Example – Soil A (Cont.) Soil A is then classified as SP-SM – Poorly-grades sand with silt and gravel