Permeability — GeoTechnical Solutions

Permeability — also known as hydraulic conductivity or k-value — is a measure of how easily water flows through a porous medium such as soil, rock or a geosynthetic. Expressed in metres per second (m/s), permeability spans many orders of magnitude and is one of the most important parameters in geotechnical, drainage and contaminated land design.

Typical k-values for UK soils

Permeability varies dramatically between soil types — from highly permeable gravels down to effectively impermeable clays. Indicative ranges relevant to UK ground conditions are:

Soil type	k-value (m/s)	Behaviour
Gravel	1 × 10⁻² – 1 × 10⁻¹	Very permeable
Sand	1 × 10⁻⁵ – 1 × 10⁻²	Permeable
Silt	1 × 10⁻⁸ – 1 × 10⁻⁵	Low permeability
Clay	1 × 10⁻¹¹ – 1 × 10⁻⁸	Effectively impermeable
Glacial till (variable)	1 × 10⁻⁹ – 1 × 10⁻⁵	Highly site-specific
Chalk (UK upland)	1 × 10⁻⁷ – 1 × 10⁻⁴	Variable, fracture-controlled

Measurement methods

UK practice uses a mix of laboratory and field methods depending on soil type and required accuracy:

• Laboratory tests — constant head permeameter for granular soils (BS 1377-5); falling head permeameter for fine-grained soils.
• Field tests — variable head tests in boreholes (Hvorslev or US Bureau of Reclamation methods); soakaway percolation tests to BRE Digest 365 for SuDS design.
• Indirect estimation — from CPT or piezocone testing using empirical correlations (Robertson 1990).
• Pumping tests — for confined aquifers and major excavations below the water table.

Applications in design

Permeability governs many design decisions. For SuDS infiltration components, k ≥ 1 × 10⁻⁵ m/s is required for effective soakaway operation per BRE Digest 365. For dewatering of excavations below the water table, permeability determines the discharge rate from sumps or wellpoints. For contaminated land containment, k ≤ 1 × 10⁻⁹ m/s is the typical regulatory threshold for clay liners.

Slope stability analysis, dam design, and groundwater modelling all rely heavily on representative permeability values determined for the site.

Related ViaCon solutions

ViaCon supplies geosynthetics and drainage components specified by ground permeability. See our geotechnical solutions and stormwater solutions. Related glossary entries: soakaway, geotextile, French drain and permeable paving.

Frequently asked questions about permeability

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