Our clients often ask us how to figure out the size of a soakaway crate system, and the first thing you should do is conduct a percolation test. In this guide, we’ll walk you through the basics of performing a percolation test, which is essential when installing a septic tank or a soakaway crate system.
Key Takeaways:
- A percolation test needs to be done before installation to see if a soakaway system will efficiently disperse rainwater.
- VP values between 15 – 100 are ideal for a soakaway system.
- Soakaways are not designed for foul water disposal.
What is a Percolation Test?
According to Wikipedia, a percolation test (commonly referred to as a "perc test") determines the water absorption rate of soil, which is crucial for building a septic drainage field or infiltration basin. Properly designing a septic system requires the results of a percolation test. Simply put, a percolation test observes how quickly a known volume of water drains into the subsoil of a drilled hole of a known surface area. Although each jurisdiction has its own rules regarding the calculations for pit length and depth, the testing procedures remain consistent.
Different soil types affect water drainage rates. Sandy soils drain water much quicker than dense clay soils, which are common in the UK and have been used historically for waterproofing structures. Watching a documentary about the mystery of Oak Island, I learned that builders in the 1700s used blue clay to prevent seawater from entering their construction sites.
Therefore, the suitability of your soakaway system largely depends on the type of soil you're working with, and a percolation test helps determine this.
When Should I Perform a Percolation Test?
You should conduct percolation tests before installing any soakaway crate system, including our smallest 1m3 soakaway crate kits or a septic tank/treatment plant. However, only after performing a Groundwater Source Protection Zone search and a Trial Site Assessment Hole (we'll cover these in future guides) to fully evaluate the water table in the area.
Please note that the test differs depending on whether the soakaway will handle foul water or surface water. In this guide, we'll outline how to perform a percolation test for foul water. For a soakaway crate system handling large volumes of water in a short period, larger pits are generally required, typically 1x1 meter wide and 2 meters deep.
Soil Types to Consider for Your Percolation Test:
Identifying the type of soil in the area is critical for choosing the right drainage method. If the soil is too dense, water won't drain away quickly enough, potentially causing surface water buildup. On the flip side, if the soil drains too quickly, it becomes difficult to manage effectively, leading to long-term issues such as preventing the installation of a proper drainage solution.
Superior soil types suitable for soakaway installations include:
- Sandy-based soil
- Chalk-based soil
- Loamy soil
The worst soil types to use include:
- Clay soil
- Silt soil
After conducting the test, you should understand the soil type you're dealing with.
Steps for Performing a Percolation Test:
Now that we know what and why, here's the how.
A percolation test involves digging a hole (or multiple holes) in the ground around the proposed area of your soakaway system to a specific depth. Pre-soaking the hole ensures the soil is saturated before testing. Then, you fill the hole(s) to a certain level and measure how quickly the water level drops as it percolates into the surrounding ground. Based on the results, you can calculate the size of the soakaway or drainage field required. Remember, avoid conducting a percolation test in severe or abnormal weather conditions, as heavy rain, frozen ground, or drought will distort the results due to changes in the water table.
Here are the steps:
1. Dig the hole to 300mm wide by 300mm deep below the required invert level of the underground pipe. This could be up to one meter below the surface, so you might need to dig a large enough hole to stand in before excavating the percolation test hole.
2. Pre-soak the hole by filling it to at least 300mm and leave it for at least 12 hours to let the water drain away.
3. Refill the hole to 300mm with fresh water and time how long it takes to drain from 3/4 full (225mm) to 1/4 full (75mm). Divide this time by 150mm to get the average time in seconds for the water to drop 1mm. This is called the "VP."
4. Repeat this process several times, with a minimum of three holes, calculating the average time for each hole.
If the test results show an average VP value between 15 and 100, the ground is suitable for a soakaway. This ensures the water neither percolates too quickly into the groundwater nor remains stagnant in the system.
How to Perform the Percolation Test Calculations:
V = Time in seconds for the water in the test hole to drop by 1mm.
P = Maximum number of people the tank or plant is designed to serve.
Area (A) = V X P X 0.20 for sewage treatment units.
Area (A) = V X P X 0.25 for septic tanks.
The resulting calculation gives the area in square meters required for the soakaway system. The installer will then calculate the length of pipes needed, depending on the trench width.
This guide should give you a general idea of what a percolation test is and how to perform one to calculate the area in square meters required for a soakaway system (drainage field). However, always consult your local authorities, as regulations vary by region. Other factors, such as avoiding installing soakaways under driveways, parking areas, buildings, or near trees, dwellings, boundaries, rivers, and streams, should also be considered. Soakaway systems cannot be installed unless the Site Assessment and Soil Profile deep test hole shows a minimum of 1.2 meters between the bottom of the drain and the water table or bedrock. Less than this distance, and the water won't have enough soil to drain effectively.
Most importantly, soakaway crate systems MUST NOT be installed for foul water use under any circumstances. They do not comply with building regulations for this purpose and are unsuitable. Soakaway crates are only for rain or surface water, and using them for foul water drainage is unsafe and ineffective. If a company tries to sell you this, report them immediately to trading standards.
As always, we thank you for reading our guides here at EasyMerchant. If you need further assistance, feel free to reach out to us, and we’ll do our best to help.
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