Why is clayey soil so important while studying geotechnical engineering?

Clayey soils are very important in geotechnical engineering because of their complex behavior:
1- High plastic clays (generally plastic Index >30%, or LL>50%) pose high swelling and shrinkage potential with change in moisture content and may cause excessive total and differential deformations to structures 
2- Clays generally have low hydraulic conductivity. The higher the plasticity, the lower is hydraulic conductivity. Hence, plastic soils may act as seepage barriers in many geo-structures (such as dam cores, cut-off walls, sealant, seepage barrier for waste dumps, etc)
3- High plastic clays are prone to cracking if desiccated: cracking may substantially increase hydraulic conductivity of the soil in cracked state (a soil which otherwise would have had a very low hydraulic conductivity)
4- In natural state, clay soils generally have moisture content and their deformation under loads is generally consists of two parts: immediate deformation and time-dependent deformation (consolidation and creep)- consolidation is due to dissipation of excess pore-water pressures and creep is due to plastic deformation of soil structure
5- The strength of the clayey soil depends on the loading and boundary conditions and generally drained and undrained conditions are considered for short term and long term behavior of the clayey soils. 
6. Pore-water pressure play a major role in strength and deformation of clayey soils under various loading conditions
7. Chemistry of pore water can significantly affect the behavior of the clayey soil
8. In unsaturated state, clayey soils have “apparent cohesion” due to matric suctions. The suction in clay soils can significantly increase their shear strength and has a major role in slope stability analysis of unsaturated soils
9. strength and deformation properties of the clayey soils depends on their loading history (pre-consolidation pressure) (i.e. over consolidated and normally consolidated soils) 
10. Generally, if the clay mass percentage in a soil (such as clayey sand) is more than 15%, clay could effectively control hydraulic conductivity properties of the soil. If the clay content is more than 30%, clay could effectively control mechanical behavior properties of the soil. Soil classification systems generally put huge imphasis on clay or fine contents of the soil. 
11. Clay contents of the soil significantly affects its frost -heave and thaw weakening and thaw-settlement properties

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