Permeability and its Testing method - TS

 

Permeability in soil is the property that describes the ease with which water (or air) flows through the interconnected voids or pores between soil particles. It is a critical engineering property used to predict seepage, calculate the settlement rate of structures, and design drainage systems. 

·  Coefficient of Permeability

  This is the standard quantitative measure, often called hydraulic conductivity. It represents the flow velocity under a unit hydraulic gradient and is typically measured in meters per second (m/s) or centimeters per second (cm/s).

·  Darcy's Law: The fundamental principle governing water flow in soil, stating that discharge velocity is directly proportional to the hydraulic gradient.

·  Soil Type Variation: Permeability varies vastly by soil texture:

o High Permeability: Coarse-grained soils like gravel and sand

o Low Permeability: Fine-grained soils like silt and clay

Factors Affecting Permeability 

                Particle Size and Shape: Larger particles create larger voids, leading to higher permeability. Angular particles typically result in lower permeability than rounded ones due to increased surface area.

·  Void Ratio: Generally, a higher volume of voids relative to solids increases permeability.

·  Degree of Saturation: Saturated soils (all voids filled with water) are more permeable than partially saturated soils, as entrapped air blocks flow paths.

·  Soil Structure and Stratification: Flocculated clay structures are more permeable than dispersed ones. Additionally, natural soil layers often have higher permeability horizontally than vertically. 

·  Testing Methods 

·  Laboratory Tests:

o  Constant Head Test: Used for coarse-grained, highly permeable soils like sand.

o  Falling Head Test: Used for fine-grained soils with low permeability, such as clay.

·      Field Tests:

o Pumping-out Tests: Involve pumping water from a well and measuring the drop in surrounding observation wells; more accurate for large-scale projects.

o Percolation Tests: Simple tests measuring the rate at which water level drops in a bored hole.