From foundation design and land development to construction projects and agriculture, soil permeability is vital in many industries. Assessing the rate at which water passes through soil gives us a clear understanding of a mixture’s suitability for different purposes. A highly permeable soil is essential when constructing a wastewater system, but the worst nightmare of a foundation designer. As such, material testers regularly assess the permeability of soil samples to understand the properties of a soil mixture.
But let’s get back to basics. In this article, we’ll explore what ‘soil permeability’ means, how we test for it, and which factors affect it. We’ll also reveal where you can find the best permeability equipment for your next soil testing application.
What is Soil Permeability?
Soil permeability is a measurement dictating how quickly water can pass through a soil sample. We use permeability testing to assess how effectively soil allows water to travel through it.
In general, water travels quickly through highly permeable soils and slowly through soils with low permeability.
Assessing permeability is essential in many industries. It is critical for designing effective drainage systems, building structures, and creating foundations. It also helps us with agriculture and irrigation, aids with hydrogeology, and allows us to measure the stability of slopes and embankments.
How Do We Measure Soil Permeability Using the Constant Head Permeability Test?
We measure soil permeability using various methods, including the constant head method, by assessing the rate at which water flows through a sample over a predetermined period. Material testers express this water flow rate as the ‘coefficient of permeability’. Specific methods for measuring permeability include the constant head permeability test for large-grained soils and the falling head permeability test for fine-grained soils. The constant head permeability method is particularly suitable for testing coarse granular soils such as sand, where maintaining a constant flow and head loss is crucial for accurate results.
The permeability coefficient is represented by k, where ‘k’ refers to the hydraulic conductivity of a soil or porous medium. In other words, the coefficient indicates the intrinsic ability of soil to allow water to pass through it. A higher hydraulic conductivity shows that soil can flow through the medium more easily. A lower measurement reveals that the soil has a greater resistance to water flow. We typically express the permeability coefficient in meters per second (m/s), centimeters per second (cm/s), or feet per second (ft/s).
Soil Coefficient of Permeability (k) Chart
Once we understand a soil’s properties, we can use a reference table to sort the sample into one of many soil permeability classes. A table for classifying soils in civil engineering may look something like this.
Soil coefficient of permeability (k)
In this table, the leftmost column refers to the soil’s permeability coefficient in meters per second (m/s). The central column gives us the equivalent measurement in millimeters per hour (mm/s). The final column shows us what this means in practicality. For example, the table tells us that soil with a permeability of 10-4 (360 mm/h) has a medium permeability degree.
Best Soil Testing Equipment for Accurate Land Analysis.
Which Factors Impact Soil Permeability Tests?
Let’s explore some of the different factors that impact soil permeability.
Granular soils, such as sands and gravels, play a significant role in determining permeability due to their texture and particle size distribution.
Darcy’s Law and Hydraulic Conductivity
Soil profiles consist of multiple layers. The soil quality between each layer of soil can vary wildly. Some layers are heavily saturated; other layers aren’t. Some may be permeable, while other layers may be completely impermeable, allowing no water to pass through them. Different soil properties in different layers can hugely impact the flow rate of water. Let’s explain why.
When soil is completely saturated with water, the water molecules in the pore spaces exert pressure on the soil particles. This increases the hydraulic head (water pressure) of the soil. The difference between the water pressure of two soil layers directly impacts the ability of water to flow from one layer to the next. We refer to this correlation as ‘Darcy’s law’.
Darcy’s law states that the rate of water passing through soil layers is directly proportional to the unit hydraulic gradient (difference in water pressure) between soil layers. This principle is fundamental to understanding permeability and is often tested using various test methods.
In short, water can only flow from one layer to the next if there is a difference in pressure between the two layers. The steeper the hydraulic gradient (difference in water pressure) between two layers, the more readily water can flow between them.
Example of Darcy’s Law and Hydraulic Conductivity
To better explain this concept, let’s imagine two equally permeable soil layers stacked on top of each other. Soil A is the top layer of soil. It is heavily saturated due to recent rainfall. Soil B is three feet below soil A. It is currently dry. As soil A is saturated, it has a high water pressure. Soil B currently has no water pressure because it is completely dry.
Soil A’s water molecules are in constant motion and will look to move from an area of higher concentration to an area of lower concentration until they reach an equilibrium. As such, soil A will rapidly transfer water to the dryer, equally permeable soil B, meaning the soil profile will exhibit a high permeability coefficient.
In contrast, if soil B was already as saturated as soil A, the water pressure from soil A would have nowhere to flow, meaning the permeability coefficient between soil A and soil B would likely indicate a low permeability coefficient.
Note that while Darcy’s law can significantly impact a soil’s permeability, typical values will fluctuate depending on other factors, such as soil texture, grain size, and porosity.
Soil Texture
A soil’s texture is dictated by its proportion of sand, silt, and clay particles. This can significantly affect permeability. Generally, the larger the particle sizes in the soil, the easier it is for water to flow. Soils that consist of coarse or medium sand typically have a high permeability. Silty sand or clay soils often have a much lower permeability. The combination of materials that make up your soil sample can be the difference between a rapid or slow permeability coefficient.
That said, soil texture isn’t always black and white. Poorly graded gravel containing a range of particle sizes may make a soil’s permeability inconsistent. Likewise, a mixture of fine sand and silty clay may cause problems when attempting to obtain a reliable, accurate reading.
Soil Structure
Some layers within a soil profile are more permeable than others due to their texture and porosity. We refer to a soil layer that is significantly more permeable than others as a ‘permeable horizon’.
If a soil profile contains a lot of permeable horizons, water has a much easier time traveling through the soil because it has more pathways to flow through. As such, the way soil is arranged can significantly impact permeability. Soil permeability tests are essential for assessing the permeability of different soil structures.
Soil Porosity
Porosity refers to the volume of pores or voids interconnecting soil particles. We refer to soil with a lot of pores as ‘high porosity’ and soil with a low volume of pores as ‘low porosity’.
It’s important to note that water doesn’t flow between soil particles. Instead, it flows between the voids that connect the soil. Therefore, a highly porous sample has more soil pores to which the water can flow, allowing water to move through the soil more readily.
In general, the higher the soil’s porosity, the greater its permeability. Of course, this depends on several other factors, such as the hydraulic gradient of the soils and the textures of different soil types. Soil permeability testing equipment is crucial for accurately measuring soil porosity and permeability.
What is a Soil Permeability Chart?
The soil permeability chart is an accessible, simple way to see the typical values of permeability for different soil types. The constant head permeability method determines permeable soils like sand. This test provides valuable data for permeability charts.
Permeability charts vary in complexity. Some charts give broad ranges for materials like gravel or sand. Others classify precise blends, showing minimum and maximum permeability for clays, sands, and gravels. These measurements consider coarseness, plasticity, and uniformity.
For this example, we’ll use a simple permeability chart:
Soil permeability by soil type
The left of this table details and describes different types of soil. The right side of the table details the expected permeability (k) of each soil in meters per second.
Permeability charts serve two primary purposes. First, they let construction experts quickly estimate the suitability of different soil types for construction applications. For example, a residential foundation typically requires low-permeability. In this case, it would be quick to ascertain that a gravel and sand mixture would be unsuitable.
Second, construction experts can use a permeability chart to compare their actual and expected results. If a clean sand mixture is expected to fall in the range of 10-4 to 10-6, but the actual permeability was 10-3, this would indicate that the mixture is more permeable than expected. A material tester can then take steps to understand why this is the case.
How Do You Test for Soil Permeability?
There are several ways to test a soil’s permeability. Let’s explore two primary methods in more detail:
The Constant Head Test
1. Carve a soil sample to the desired length and width.
2. Place the sample in a permeameter, which is a cylindrical chamber with water-filled reservoirs.
3. Maintain a constant water level in the reservoirs using a constant head tank.
Saturate the soil fully before the test begins. Allow water to flow through the sample, then measure the flow rate over a set period. Use Darcy’s law to calculate permeability.
The Falling Head Test
1. Fully saturate the soil sample at the start of the test.
2. Allow water to drain and measure the fall in water level over time.
Unlike the constant head test, the falling head test set allows the water level and pressure to decrease. This test is typically reserved for low soils needing accurate measurement and using like fine sands or clays because the water would drain too quickly in highly permeable samples.
Where Can I Find the Best Equipment for Soil Permeability Tests?
The most important first step for any successful soil permeability test is selecting the right tools for the job. Your chosen equipment must be ASTM standardized, high-quality, and reliable.
If you’re looking for the best tools for your next soil permeability test, CertifiedMTP can help. We stock a wide range of permeability testing equipment for every test application. From flexible-wall permeability cells and constant head permeameters to latex membranes, manometers, and constant head tanks, we have the first-class equipment to give you first-rate results.
Looking for something different? We also offer a range of Shelby tube permeameters and double-ring infiltrometers. Whatever your material testing needs, we have the equipment to ensure you get accurate, reliable results time and time again.
