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Percolation Testing – What is it about?

If a property requires a sustainable drainage system, especially for wastewater, which cannot be connected to main sewage lines, then a soakaway system or a septic drainage field is necessary. However, these systems cannot simply made anywhere because the ground on which you want to construct them might not be suitable.

One of the best ways to check this is through percolation testing. It is one of the many tests you’ll need to undergo, before you do finally build the septic drainage field. There are also many factors you need to consider, before carrying out the test, such as what you’re using the drainage field for; if it is for foul water or for surface water, and what the weather will be when you carry out the test itself.

There are many ways you can carry out the test, and most of the time, it is used for foul or wastewater. So, we’ll outline how to carry out a percolation test for foul water.

What Is A Percolation Test?
A percolation test is one of many tests that will help you determine how long it takes to absorb water into the ground. It’s as simple as digging a hole in the ground and saturating it with the foul water you’d expect the ground to absorb. As a result, you’ll be able to determine the permeability of the soil, and whether it’s suitable to be used for your septic drainage field.

Why Do You Need A Percolation Test?
The simplest answer to that question is the fact that not all land and soil is suitable for a septic drainage field. Some can be too dense to allow the system to drain water quickly enough, and some will be too porous to effectively treat the water that is being drained. That’s why it is important that a percolation test is done, to ensure the efficacy of the soil, for the drainage system to properly work, and to ensure less pollution in the surrounding environment.

When To Perform The Test
If you’re installing a new septic drainage field, you will need to perform the test before you even construct it. It is so you can be sure that the ground you’re using is suitable enough, and you can determine how large the field needs to be.

You also need to note that if you wish to replace the existing system, you will need to perform the test every time. This is to ensure the viability of the soil, and whether it is still suitable or not, especially if you’re planning on expanding the system or the operation of the property needs a larger septic drainage field.

It is also important to remember not to perform the test, after a hurricane or a major storm, as it will contaminate the results of your test, since the groundwater levels will be unusually high. Always perform the test on a dry day, to ensure that the results of the test won’t be skewed by any excess groundwater.

What To Consider Before Performing The Test
Make sure that the groundwater table, of the site where you wish to create the septic drainage field, is protected and far away enough, so you can construct the system. It would not be a good idea to construct the field if it can contaminate any local groundwater sources. It will ensure that these sources will still be useable and should there be any changes in the water table, both your septic drainage field, and local groundwater sources will be protected.

What Are The Best Soils For A Drainage System
As mentioned before, if the soil for the drainage system is too dense, then water, no matter if it is foul or fresh, will not be absorbed quickly enough, for the system to really work. If the soil is too porous, then it will easily soak up water, which could mean that it won’t be treated properly, for the system to work effectively. That is why it is important to consider what the soil is made from, before you consider constructing a septic tank or treatment plant, in the area you want.

The best soils for drainage systems tend to be those with low clay content, as clay can make the soil too heavy to treat the water. Clay soils also tend to absorb the water, and so will block the system and take up room. In general, the best type of soils to look for are:
• Sandy soils
• Loamy soils
• Non-retentive or non-absorbing soils, such as chalky soils

How To Perform The Test
The test is simple enough, you simply dig a hole in the ground, fill it with water and see how long it takes for the water to disappear. There is a lot more to performing the test, especially if you want to have an effective septic drainage field. There is a lot of preparation and calculations involved, so make sure you know what you’re doing.

First off, you’ll need to dig two pits, on the site where you’re planning to construct your drainage system. For each pit, carry out the following:
• Dig an area 300mm(h) x 300mm(w) x 300mm(d), below your proposed level of the outlet pipes.
• Clear the hole of any loose debris
• Fill the 300mm3 area with water and let it soak the soil overnight.
• Refill the area, the next day, and observe how quickly the water seeps away.
Carry out these steps at least three times, at different times of the day, to ensure a steady result. You should also note that if you are pressed for time, you can carry out a faster test, by monitoring the rate at which the first fill soaks away. If it continuously drains for 10 – 20 minutes, it’s too porous, and if it hasn’t completely drained away within 6 – 12 hours, then it is too dense.

How To Calculate The Results
All you need to do is to calculate the time it takes the water to drain from ¾ full to ¼ full, then divide it by 150. This gives an average time in seconds, the Vp needed for the water to drop 1mm. With this Vp, you’ll be able to calculate how large the soakaway system needs to be. You should also remember that a Vp with a result between 15 and 100 is the “sweet spot” for any soakaway system.