1) Soils and their classification

1) Soils and their classification

1. Geotechnical engineering

   • Engineering properties of earth materials
   • Application of engineering geology, hydrology, soil mechanics, rock mechanics & mining seismology
   • Soil mechanics:
     • Application of the laws and principles of mechanics and hydrolics to engineering problems dealing with soil as an engineering material

2. What are soils?

   • Unconsolidated material sediments and deposits of solid particles from disintegrated rocks
   • Formed by:
     • Residual soil (broken from parent rock)
     • Transported soil
   • Particulate materials
   • Un-cemented/ weak cemented accumulation of mineral and organic particles &sediments found above bedrock

3. Soil structure: three phase system

   • Solid particles, pore fluid, pore gas

4. Soil grain shapes

   • Angular, subangular, subrounded, rounded

5. Unique nature of soil materials

   • Little control over their properties
   • Highly variable
   • Non-linear stress strain relation
   • Inelastic
   • Anisotropic
   • Multiphase system

6. Classification of Soils

   • Grain size distribution
     • Gravel: 60-2mm
       • Sand: 2-0.06mm
         • Silt: 0.06-0.02mm
           • Clay: 0.02-0.002mm
     • Shape parameters
       • Coefficient of uniformity
         • Cu=D60/D10
       • Coefficient of curvature
         • Cc=D3^2/D60*D10
       • Well graded soil
         • 1<Cc<3, Cu>4 for gravels
           • 1<Cc<3, Cu>6 for sands
   • Consistency limits
     • Liquid limit
       • Moisture content at which soils behave as a liquid
       • Determined by Cassagrande method
     • Plastic limit
       • Moisture content at which soil behaves as a plastic material
       • Plasticity Index
         • PI=LL-PL
     • Shrinkage limit
       • Moisture content at which no further volume change occurs

7. Unified Soil Classification Systems

   • Based on grain size and plasticity characteristics
   • Coarse grained soils
     • 50% or more >0.075mm
       • Divided into gravel and sands
         • Cohesionless soils
   • Fine grained soils
     • 50% or more <0.075mm
       • Divided into silt and clays
         • Cohesive soils
   • Gravel (G)
     • W if Cu>4,1<Cc<3 otherwise P
   • Sand (S)
     • W id Cu>6,1<Cc<3 otherwise P
   • Clay (C)
   • Silt (M)
   • Organic (O)
   • Well graded (W)
   • Poorly graded (P)
   • Low plasticity (L)
   • High plasticity (H)