Permeability is the property of soil that permits the flow of water through its interconnected void spaces. The ease with which water flows through soil is measured by the coefficient of permeability.

The coefficient of permeability, denoted by $k$, represents the velocity of flow through a soil under a unit hydraulic gradient and has the dimensions of velocity.

Importance of Permeability

Permeability influences:

• Seepage through earth dams.
• Drainage of foundations.
• Groundwater flow.
• Consolidation of soils.
• Stability of slopes and retaining structures.

Knowledge of permeability is essential for many geotechnical engineering problems.

Principle of Permeability Tests

The determination of permeability is based on Darcy's law, which states that the rate of flow through a porous medium is proportional to the hydraulic gradient.

$$Q=kAi$$

where,

• $Q$ = discharge per unit time,
• $k$ = coefficient of permeability,
• $A$ = cross-sectional area,
• $i$ = hydraulic gradient.

Constant Head Test

The constant head test is suitable for coarse-grained soils such as sands and gravels.

$$k=\frac{QL}{Aht}$$

where,

• $Q$ = quantity of water collected,
• $L$ = length of soil specimen,
• $A$ = cross-sectional area,
• $h$ = hydraulic head difference,
• $t$ = time.

Variable Head Test

The variable head test is suitable for fine-grained soils such as silts and clays.

$$k=\frac{2.303aL}{At}\log_{10}\left(\frac{h_1}{h_2}\right)$$

where,

• $a$ = cross-sectional area of standpipe,
• $L$ = specimen length,
• $A$ = specimen area,
• $t$ = elapsed time,
• $h_1$ = initial head,
• $h_2$ = final head.

Other Methods

Permeability may also be determined by:

1. Pumping out tests.
2. Pumping in tests.
3. Indirect methods based on grain size distribution.
4. Consolidation test data.

Engineering Applications

Permeability determination is important in:

• Seepage analysis.
• Earth dam design.
• Groundwater investigations.
• Drainage systems.
• Foundation engineering.
• Slope stability studies.

Since the movement of water significantly influences the behaviour of soils, the accurate determination of permeability is an essential part of geotechnical engineering investigations.