What is the difference between permeability and hydraulic conductivity?

In geotechnical practice the terms are used interchangeably for k, the coefficient in Darcy's law, in units of velocity such as cm/s. Strictly, intrinsic permeability is a property of the soil alone, while hydraulic conductivity also depends on the fluid.

Which soil has the highest permeability?

Clean gravel, followed by clean coarse sand. Clays have the lowest permeability, typically 10⁻¹⁰ to 10⁻⁶ cm/s.

What is the permeability of sand?

It depends on the sand's grading and fines. FHWA's typical values are 10⁻² to 1 cm/s for clean coarse sand, 10⁻³ to 10⁻¹ cm/s for fine sand, 10⁻³ to 10⁻² cm/s for silty sand and 10⁻⁴ to 10⁻² cm/s for clayey sand.

When do you use a falling-head test instead of constant head?

For fine-grained, low-permeability soils, where the flow through a constant-head setup would be too small to measure. Constant head suits granular, free-draining soils.

Is a percolation test the same as a permeability test?

No. A percolation test measures how fast water level drops in a test hole, usually in minutes per inch, for sizing septic systems. It relates to permeability but isn't the same quantity as k.

Geotechnical

What Is Soil Permeability?

How easily water moves through soil, and why it shapes drainage design

BuildNumerics EditorialAugust 25, 2026Updated October 18, 20265 min read

What permeability describes

Permeability (or hydraulic conductivity) quantifies how easily water flows through a soil under a given hydraulic gradient. Coarse, granular soils like clean sand and gravel typically have high permeability — water moves through them relatively quickly — while dense clays typically have very low permeability, acting almost as a barrier to water flow.

The range is enormous. FHWA's typical values run from 1–100 cm/s for clean gravel down to 10⁻¹⁰–10⁻⁶ cm/s for clay — a difference of at least a millionfold — which is why permeability is usually discussed in orders of magnitude rather than precise values.

Darcy's law

Flow through soil follows Darcy's law: Q = k × i × A, where Q is the flow rate, k the coefficient of permeability, i the hydraulic gradient (head loss divided by flow length) and A the cross-sectional area. The law assumes slow, laminar flow through saturated soil, which holds for nearly all soils except very coarse gravels.

Typical values

Sands span a wide band depending on how much fine material they contain, which is why a clean sand and a silty sand can behave so differently in a drain.

Typical saturated hydraulic conductivity from FHWA NHI-05-037, Table 5-56 (after Coduto, 1999). Structure, fissures and layering can change field values greatly.
Soil	Typical k (cm/s)
Clean gravel	1 to 100
Sand-gravel mixtures	10⁻² to 10
Clean coarse sand	10⁻² to 1
Fine sand	10⁻³ to 10⁻¹
Silty sand	10⁻³ to 10⁻²
Clayey sand	10⁻⁴ to 10⁻²
Silt	10⁻⁸ to 10⁻³
Clay	10⁻¹⁰ to 10⁻⁶

How it's tested

For more permeable, granular soils, a constant-head permeability test (such as ASTM D2434) is common, maintaining a steady water flow through a sample and measuring the flow rate. For less permeable, fine-grained soils, a falling-head or flexible-wall method (such as ASTM D5084) is typically used instead, since flow rates are too slow for a practical constant-head setup.

Constant-head example

A sand sample 15 cm long and 10 cm in diameter (A = 78.54 cm²) runs under a constant head of 50 cm, and 400 cm³ of water is collected in 120 s. k = Q × L ÷ (A × h × t) = 400 × 15 ÷ (78.54 × 50 × 120) = 1.3 × 10⁻² cm/s — consistent with a clean coarse-to-fine sand.

Falling-head example

A fine-grained sample 10 cm long in the same mold is connected to a standpipe of area a = 0.5 cm². The water level falls from 100 cm to 50 cm in 3,600 s. k = (a × L) ÷ (A × t) × ln(h₁ ÷ h₂) = (0.5 × 10) ÷ (78.54 × 3,600) × ln 2 = 1.2 × 10⁻⁵ cm/s — within FHWA's typical range for silt.

The Soil Permeability Calculator handles both methods. Permeability depends on water temperature through viscosity, and the ASTM methods allow a correction to 20 °C, which the calculator does not apply.

What controls permeability

Grain size and gradation — permeability is closely tied to the finest part of the soil. Adding a few percent of fines to a clean sand can reduce it dramatically. For loose, clean sands, Hazen's empirical relation estimates k from the square of the effective size D10, read from a sieve analysis. See also What Is Aggregate and Why Does Gradation Matter?.
Density — compaction reduces void space and lowers permeability.
Structure — fissures, root holes and layering can make field permeability far higher than a lab sample suggests.
Saturation — trapped air blocks flow paths, so lab samples must be fully saturated.

Why it matters on a project

Permeability influences drainage system design (how quickly a French drain or drainage layer can move water away), seepage analysis for retaining structures and dams, and how quickly excess water drains from a compacted fill or subgrade — a low-permeability subgrade that can't shed water is a common contributor to pavement and foundation moisture problems.

Drainage layers and French drains need clean, open-graded stone far more permeable than the surrounding soil — the French Drain Calculator sizes the trench.
Compacted soil liners must be very tight: US federal rules for municipal solid-waste landfills (40 CFR 258.40) require the compacted-soil layer of a composite liner to be at least 2 ft thick with k no more than 1 × 10⁻⁷ cm/s. Requirements for ponds and other uses vary by agency.
Excavation dewatering and consolidation (settlement) rates both depend on permeability.
Infiltration and septic systems use field tests such as infiltrometers or percolation tests, which report results differently from lab k values.

Common mistakes

Using a constant-head test on a clay, where flow is too slow to measure accurately.
Testing a sample that isn't fully saturated.
Treating a single lab value as representative of a layered or fissured site.
Mixing units — k is often reported in cm/s, m/s or ft/day.

FAQ

What is the difference between permeability and hydraulic conductivity?: In geotechnical practice the terms are used interchangeably for k, the coefficient in Darcy's law, in units of velocity such as cm/s. Strictly, intrinsic permeability is a property of the soil alone, while hydraulic conductivity also depends on the fluid.
Which soil has the highest permeability?: Clean gravel, followed by clean coarse sand. Clays have the lowest permeability, typically 10⁻¹⁰ to 10⁻⁶ cm/s.
What is the permeability of sand?: It depends on the sand's grading and fines. FHWA's typical values are 10⁻² to 1 cm/s for clean coarse sand, 10⁻³ to 10⁻¹ cm/s for fine sand, 10⁻³ to 10⁻² cm/s for silty sand and 10⁻⁴ to 10⁻² cm/s for clayey sand.
When do you use a falling-head test instead of constant head?: For fine-grained, low-permeability soils, where the flow through a constant-head setup would be too small to measure. Constant head suits granular, free-draining soils.
Is a percolation test the same as a permeability test?: No. A percolation test measures how fast water level drops in a test hole, usually in minutes per inch, for sizing septic systems. It relates to permeability but isn't the same quantity as k.

Soil Permeability Calculator — do this calculation nowOpen calculator

Related Calculators

Soil Permeability Calculator

Related Guides

Aggregate Testing

How to Read a Sieve Analysis

10 min

Sources

ASTM D2434 / D5084
Standard Test Methods for Permeability of Granular Soils (Constant Head) / Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter.
FHWA NHI-05-037, Tables 5-55 and 5-56
Saturated hydraulic conductivity: test methods, Hazen's relation for clean sands and typical values by soil type.
FHWA NHI-05-037, Tables 5-55 and 5-56 — reference
40 CFR 258.40
Design criteria for municipal solid-waste landfill composite liners (compacted soil at no more than 1 × 10⁻⁷ cm/s).
40 CFR 258.40 — reference