Methods of permeability – what’s the difference?
Permeability in its most basic soil-related form is determining the amount of water that passes through a soil in a set amount of time. But how is it measured?
Permeability goes by many names: hydraulic conductivity; drainage; infiltration; or Ksat, and can be expressed in all variances of units: mm/hr; cm/min; and m/day to name a few. There are more than a few ways to measure it: in-situ or disturbed (in the laboratory); constant-head or falling-head; 95 % maximum compaction or varying rates of compaction.
To help understand the various methods SESL Australia offers each will be described here, along with units and appropriate applications but before we do, a few important terms:
Constant-head: where a head of water is maintained over the test portion, thus the pressure remains constant throughout the test.
Falling-head: where the head decreased as water infiltrates through the test portion, thus the pressure diminishes throughout the test.
The double-ring infiltrometer is a device that is used to determine the in-situ permeability of sports fields. It uses a falling-head apparatus that has been inserted carefully into the soil profile and reads the permeability over numerous appropriate time intervals. Each reading is calculated and then averaged to report in mm/hr. The double ring saturates the surrounding soil profile to remove interference caused by lateral flow.
Single Ring Constant Head
This method is outlined in the Adoption Guidelines for Stormwater Biofiltration Systems as the method for determining the in-situ hydraulic conductivity of a bioretention basin. A single cylinder is inserted into the soil profile and maintaining a constant head above the profile by pouring water in. When the readings stabilise, the volume poured is recorded and the permeability calculated. Some materials take time to stabilise and some drain very quickly making it a difficult test to undertake. The guidelines also require numerous monitoring points, so it can make for a long day in the field!
Steel cores are hammered into the soil profile in the field and extracted to be carefully delivered to the laboratory for analysis. A constant-head apparatus is used after the core has been soaked overnight to exclude all the air with results reported in mm/hr but can be converted into any applicable units. Cores can vary in size, so soils with high gravel contents can create channels with smaller cores when being driving into the profile. This is the most accurate method of measuring the in-situ permeability when being tested in the laboratory.
This method is prescribed in ASTM F1815-11 specifically for athletic field root zones. It is most commonly referred to in the United States Golf Association (USGA) specification for putting green construction. It uses a constant-head apparatus to determine hydraulic conductivity after the soil has been compacted using a falling hammer to achieve the equivalent of 95 % maximum compaction. The method is designed for sands that are used in professional grade sports field constructions, however SESL has used the method principles to develop a modified method for determining the hydraulic conductivity of capping and dam construction clays to determine their sealing capacity after compaction at their plastic limit.
HC3 and HC6
This method was developed by Soil Physicists Keith McIntyre and Bent Jakobsen as a robust manner of determining a soils hydraulic conductivity over various levels of compaction. Using a falling-head approach, in tubes of 30 mm diameter, the method really is restricted to use on mostly a-structural sandier soils for landscaping and sports fields. Up to six levels of compaction can be applied, with 8, 16 and 32 drops the most common to reflect the compaction commonly experienced in passive amenity turf, active amenity turf (sports fields) and horse racing tracks/professional performance sports fields respectively. Results are in mm/hr and are accompanied by density measurements for each level of compaction applied.
The method outlined in AS 4419 “Soils for landscaping and garden use” uses a constant head apparatus and reports the permeability in cm/hr. The method uses a cylinder of 100 mm diameter, meaning it can be used for soils with some structure and variability in particle distribution. Hard particles (stones, organic matter) greater than 10 mm are screened out of the test portion and a light compaction is applied. The sample is soaked overnight in the cylinder before running. Where heavy soils are used, if the aggregates are stable, the permeability may be high as they could act like a gravel. If they slake readily, the soil will effectively seal and prevent infiltration. Useful for landscaping purposes where bulk soil volumes will be used in garden beds.
A cost effective means for indicative permeability estimation. Uses the relationship and principles of the soil texture (approximate sand:silt:clay content by working and manipulation of an adequately prepared bolus) and the structure (description of the organisation of the soil aggregates or peds) to determine the permeability class. Classes are: Very Rapid (> 120 mm/hr); Rapid (60 – 120 mm/hr); Moderately Rapid (20 – 60 mm/hr); Moderate (5 – 20 mm/hr); Slow (2.5 – 5 mm/hr); and Very Slow (< 2.5 mm/hr). Useful for heavier soils where other laboratory methods have constraints to use or for when a simple indication for interpretive purposes holds the most value.
Numerous other methods exist for various applications including geotechnical and in-situ and range from easy to undertake to expensive requiring complex equipment. The fundamanetals of permeability can be applied to various apparatus and applications.
Whatever your permeability needs, contact an SESL Soil Scientist to discuss today!
(Image featured: Double-ring infiltrometer analysis performed in-situ to determine permeability performance)