Infiltration testing (also called permeability testing) measures the rate at which water moves through soil. This is a geotechnical assessment for designing drainage systems, stormwater management, on-site wastewater disposal, and assessing groundwater recharge.
What Is Infiltration / Permeability Testing?
Infiltration testing determines the hydraulic conductivity (K) of soil — a measure of how easily water can flow through the soil's pore spaces. The result is expressed in metres per second (m/s), millimetres per hour (mm/h), or metres per day (m/day).
Understanding the infiltration rate of a site is essential for:
- Designing sub-surface drainage systems
- Sizing stormwater detention and infiltration basins
- Assessing on-site effluent disposal suitability
- Evaluating groundwater recharge
- Verifying drainage assumptions in earthworks design
Factors Affecting Infiltration Rate
| Factor | Effect |
|---|---|
| Soil texture | Sands have high permeability; clays have very low permeability |
| Soil structure | Well-structured soils (with cracks, root holes) have higher permeability |
| Moisture content | Saturated soils have slower infiltration than dry soils |
| Compaction | Compacted soil has significantly reduced infiltration |
| Vegetation | Root channels increase infiltration; leaf litter reduces surface sealing |
| Temperature | Warmer water has lower viscosity and infiltrates faster |
Typical Infiltration Rates
| Soil Type | Hydraulic Conductivity (m/s) | Classification |
|---|---|---|
| Clean gravel | 10⁻² – 10⁻¹ | Very high |
| Sand | 10⁻⁵ – 10⁻² | High to moderate |
| Silty sand | 10⁻⁶ – 10⁻⁵ | Low to moderate |
| Silt | 10⁻⁸ – 10⁻⁶ | Low |
| Clay | 10⁻¹⁰ – 10⁻⁸ | Very low |
| Compacted clay liner | < 10⁻⁹ | Practically impermeable |
Testing Methods
1. Field Infiltration Test (Constant Head)
Used for sandy and granular soils where water flows readily.
Method:
- A test hole is excavated or a ring infiltrometer installed
- Water is maintained at a constant level in the hole
- The rate of water addition required to maintain the constant head is measured
- Steady-state flow rate is used to calculate hydraulic conductivity
2. Field Infiltration Test (Falling Head)
Used for less permeable soils where flow rates are slower.
Method:
- A test hole or piezometer is filled with water
- The rate of water level drop over time is recorded
- The falling head data is analysed to determine K
3. Double Ring Infiltrometer (DRI)
The most accurate field method for near-surface infiltration testing.
Method:
- Two concentric rings are driven into the ground and filled with water
- The outer ring acts as a buffer to ensure vertical flow in the inner ring
- Only the inner ring's water level drop is measured
- Eliminates lateral flow errors common with single-ring tests
4. Permeameter Test (Laboratory)
A soil sample is tested in a laboratory permeameter under controlled conditions.
- Constant head permeameter — for granular soils
- Falling head permeameter — for fine-grained soils
5. Borehole Permeability Test
Used at depth within a borehole to determine in-situ permeability.
- Packer test — isolates a section of the borehole with inflatable packers
- Slug test — instantaneous water level change monitored for recovery
- Pumping test — constant rate pumping with observation wells
Australian Standards
| Standard | Title |
|---|---|
| AS 1289.6.7.1 | Determination of the permeability of a soil — Constant head method |
| AS 1289.6.7.2 | Determination of the permeability of a soil — Falling head method |
| AS 1726-2017 | Geotechnical site investigations — field testing |
| NCC | National Construction Code — drainage and stormwater requirements |
Applications
Drainage Design
Infiltration rates determine the spacing and depth of sub-surface drainage systems. Low-permeability soils require closer drain spacing.
Stormwater Management
Local councils require on-site stormwater detention (OSD) for new developments. Infiltration testing determines whether soakage pits, infiltration basins, or rainwater gardens are feasible.
On-Site Wastewater
AS/NZS 1547 requires infiltration testing to design effluent disposal systems. The tested infiltration rate determines the required area of absorption trenches or beds.
Earthworks
Infiltration testing verifies whether compacted fill meets permeability specifications (e.g., for clay liners in landfills or water retention structures).
Frequently Asked Questions
How many infiltration tests are needed?
It depends on the site area and variability. A minimum of 1 test per 1,000 m² for uniform sites, or more for variable soils. Council requirements typically specify minimum test numbers.
Can infiltration testing be done in wet weather?
Testing during or immediately after heavy rain can give misleadingly low results. Allow the site to drain for at least 48 hours after significant rainfall.
What is a good infiltration rate?
For stormwater infiltration basins, a rate of > 10 mm/h is desirable. For on-site wastewater, rates of 15–80 mm/h are ideal. Below 5 mm/h, alternative drainage solutions are usually required.
How does compaction affect infiltration?
Compaction can reduce infiltration by 90% or more. This is critical for earthworks — a compacted clay liner may achieve K < 1 × 10⁻⁹ m/s, while the same soil uncompacted may have K > 1 × 10⁻⁶ m/s.