Soil permeability plays a critical role in construction, agriculture, hydrology, and foundation design. It refers to how easily water can flow through soil — a key factor in drainage, stability, and environmental safety.
Soil permeability is measured by the coefficient of permeability (k), which tells us how fast water passes through soil. It varies by soil type, texture, porosity, and saturation.
A highly permeable soil (like clean gravel) may be perfect for a wastewater system, but disastrous for a building foundation. That’s why soil testing professionals regularly assess permeability to evaluate project suitability, from embankments to irrigation.
In this guide, we’ll cover:
- What soil permeability is (and how it works)
- The difference between constant head and falling head test methods
- How to read and apply soil permeability charts and k-values
- What affects permeability (texture, porosity, Darcy’s law)
- Where to find ASTM-certified equipment for reliable test results
Let’s start with the basics and build up your understanding from the ground up — literally.
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 to Measure Soil Permeability (Constant Head 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 Permeability k-Value 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.
Read More: Essential Soil Testing Equipment for Accurate Land Analysis.
Factors That Affect Soil Permeability (Texture, Porosity & More)
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: What It Means for Soil Permeability
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.
Darcy’s Law in Action (Soil A vs B Example)
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.
How Soil Texture Affects Permeability
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.
How Soil Structure Influences Permeability
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.
How Soil Porosity Affects Permeability
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.
Understanding Soil Permeability Charts (With Use Cases)
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 to Test Soil Permeability (Constant & Falling Head Methods)
Soil permeability is commonly tested using two main lab methods: the constant head test and the falling head test. Each method is used depending on the soil type and its permeability characteristics.
Constant Head Test (For Coarse-Grained Soils)
Use when testing sands or gravels.
Steps:
- Prepare a soil sample to the required dimensions.
- Place it inside a permeameter, a cylindrical device connected to water-filled reservoirs.
- Use a constant head tank to maintain a steady water level.
Before the test, fully saturate the soil. Allow water to pass through the sample. Measure the volume of water collected over time, and apply Darcy’s Law to calculate the coefficient of permeability (k).
Falling Head Test (For Fine-Grained Soils)
Use when testing silts, clays, or fine sands.
Steps:
- Saturate the soil sample completely.
- Allow water to drain and track the fall in water level at set time intervals.
In contrast to the constant head method, this test lets the pressure drop gradually. It’s ideal for low-permeability soils where water flow is slow. Applying Darcy’s Law allows accurate determination of the soil’s hydraulic conductivity.
Best Soil Permeability Testing Equipment (ASTM Certified)
Choosing the right tools is the first step toward accurate soil permeability testing. For dependable results, your equipment must be ASTM-compliant, lab-ready, and suitable for your specific soil type and test method.
At Certified MTP, we offer a wide selection of soil permeability testing equipment, including:
- Flexible-wall permeability cells for variable-pressure applications
- Constant head permeameters for coarse soils (ASTM D2434)
- Falling head setups for low-permeability soils
- Latex membranes, manometers, and constant head tanks
- Shelby tube permeameters for undisturbed soil samples
- Double-ring infiltrometers for field-based infiltration testing
All our tools meet or exceed ASTM standards for lab and field testing. Whether you’re testing gravel, sand, silt, or clay, you’ll find reliable equipment built for precision and repeatability.
🔗 Browse our soil permeability equipment collection here »
Frequently Asked Questions About Soil Permeability
What is the meaning of soil permeability?
Soil permeability refers to the ability of soil to allow water to pass through its pores. It’s measured as the coefficient of permeability (k) and varies by soil type, texture, and porosity.
What is a good k value for soil?
A good k value depends on the application. For foundations, a low k value like 10⁻⁶ m/s (clay) is ideal. For drainage or filtration, a higher value like 10⁻³ m/s (sand or gravel) is preferred.
Which soil has the highest permeability?
Gravel and coarse sand have the highest permeability due to their large particles and wide pore spaces, allowing water to flow quickly.
What is the difference between constant head and falling head permeability tests?
Constant head tests are used for coarse-grained soils like sand and gravel, where water flow is fast. Falling head tests are used for fine-grained soils like clay, where flow is slower.
How is soil permeability calculated using Darcy’s Law?
Darcy’s Law:
k = Q × L / (A × h)
Where Q = flow rate, L = soil length, A = cross-sectional area, and h = hydraulic head.
How does soil texture affect permeability?
Soils with larger particles like sand allow faster water flow, resulting in higher permeability. Fine-textured soils like clay slow down water movement.
What equipment is used for testing soil permeability?
Common tools include constant head permeameters, falling head test setups, flexible-wall permeability cells, manometers, and double-ring infiltrometers.
Why is permeability important in construction?
Permeability helps engineers assess drainage, foundation suitability, and soil stability. It ensures proper water flow control around structures.
What is the unit of permeability?
Soil permeability is typically expressed in meters per second (m/s), centimeters per second (cm/s), or feet per second (ft/s), depending on the region.
What causes low permeability in soil?
Low permeability is caused by fine particle size (e.g., clay), high compaction, low porosity, or lack of connected pore spaces in the soil structure.
What is a Soil Permeability Chart?
A soil permeability chart is a reference table that shows typical k values (coefficient of permeability) for various soil types such as gravel, sand, silt, and clay. These charts help engineers and testers quickly estimate the water flow rate through a soil based on its type and texture.
They are commonly used in civil engineering, construction, and environmental testing to classify soils as high, medium, or low permeability based on numerical k values (e.g., 10⁻³ to 10⁻⁶ m/s).
