# Percolation Test - AKA the Soakaway Size Calculator

The purpose of a percolation test is to see how long a length of field drain (soakaway) you need for your Sewage Treatment Plant. One might also call it a soakaway size calculator

See our full percolation test method document for more information. You’ll need to record your findings in this results table.

### Step 1: Dig

Excavate at least three holes 300mm square to a depth at least 300mm below the proposed invert level (300mm) of the infiltration pipe, spacing them along the proposed line of drainage.

If the water table is reached, please note at what depth.

### Step 2: Fill

Fill with water to 300mm and allow to seep away completely. If the water drains rapidly (within 10 mins), then the hole should be filled again up to a maximum of 10 times.

### Is It Suitable?

If the water continues to drain away rapidly, then the ground is not suitable.

Also, if the water has not soaked away within 6 hours, the area is not suitable in this case either.

### Calculate

• Determine the percolation rate by refilling each hole with water to a depth of at least 300mm and observing the time in seconds for the water to seep away from 75% full to 25% full (225mm to 75mm).
• Divide this time in seconds by 150. This gives the average time in seconds required for the water to drop 1mm.
• Repeat the test at least 3 times in each hole.
• Take the average figure from the tests to produce the percolation value Vp (in seconds).
• Obtain the average figure for the percolation value (Vp) by summing up all the values and dividing by the number of values used.

### Results

The time difference is the vp rate or percolation rate

Email the results to sales@allertonuk.com.

We will calculate the length of drains needed. We will need the number of dwellings involved with the sewage treatment system, and the number of bedrooms.

Record the results on this chart:

Here is an example of a completed form:

Once you have done the percolation test and recorded your findings on the results sheet provided, you can send it over to us to calculate, or you can work out the length of the drainage field yourself. All of this is fine, but what about the result? With drains needing to be 2 metres apart (between trenches) the area calculated from the test results can be bigger than the garden. It may not fit in the space available, taking into account trees, paths, greenhouses and other features. What then?

### Building a Septic Tank Drainage Field

The answer is the re-do the test, but instead of a hole as deep as the outlet pipe plus the small test hole. Simply dig a hole that is 500mm deep, and 300 square. This will be much easier and quicker than one 1100mm or even 1800mm deep. By using a pump station (which is a bit more expensive I’m afraid) we often find the porosity of the topsoil is so much better, especially on clay or other heavy soils. We can install the drains at a very shallow depth. That would be about 100mm of cover over the pipe, and 300mm of gravel below just as before.

We know from experience that the test results from this would allow us to build a drainage field a fraction of the size. Not only will it be smaller, but we can move it’s position, such as placing it higher up the garden if a slope is against you. The pump makes this easier too, it could even pump to a less used part of the garden. The additional cost of the pump station is often saved by the reduced size of the drainage field, and it can be placed in an area of your choosing, not necessarily adjacent to the Sewage Treatment Plant

You might think that by using the pump station next to the original septic tank you would save money. It might do so for a while, but bare in mind that the effluent from a septic tank is 300ppm, compared with the 20ppm you’ll get once wastewater is filtered through a sewage treatment plant or ConSept and the smaller soakaway. It wouldn’t be long before you need a new soakaway in a fresh area of the garden, which may take even more space than you originally calculated.

Returning to the test, Allerton would advise opting for a pumped outlet and a smaller soakaway system. There is no harm testing thoroughly though, digging a deep hole and a shallow one near it, so you know the exact size needed, and avoid spending any more than you have to. In clay soils you often realise the futility of digging deep holes, as the hole fills with water as you dig, or it fills up overnight anyway.

Now you know how to do the test and you have everything you need to get digging and calculating the size of a soakaway. Remember, we can even do the calculation for you, providing the results are satisfactory and there is a map showing the holes and land to work with – But don’t worry, we can provide advice to get you there. There can be a long period of time for the water to drain away, if you’re lucky. The test should not be done during wet periods because of this, or very dry periods.

With this in mind, get out there and start digging!

### Summary

A percolation test is essential to find out how long your septic tank soakaway should be. it’s important you get this part right, to ensure your system works the best it can. Having a big enough soakaway, and appropriate soakaway maintenance, will ensure the soakaway and septic tank last years to come.

Give us a call at 01529 305 757 to ask about our options!

## FAQ

Q: What size should a soakaway be?

A: That depends on the nature of the water percolation test. Rainwater at 5 ppm? Treated effluent from a sewage unit at 20 ppm? Or effluent from a septic tank at 100 ppm?

Q: Do I need building regs for a soakaway?

A: Yes

Q: What is a rainwater soakaway?

A: A soakaway to take water from roofs and hard surfaces, yards and patios for example.

Q: Can a soakaway get blocked?

A: Soakaways from Septic tanks are bound to get blocked. Raw sewage has 300 parts per million (ppm). Water from a septic tank is not much better at 200 ppm. So those fine particles start to block the soakaway and the ground below and around the soakaway pipes from day one!

Q: Do soakaways need maintenance?

A: It is very hard to maintain a soakaway. Like all filters, they block!

Q: How long do soakaways last?

A: The more permeable the soil, the longer the soakaway lasts. It is important to not let the floating solids escape into the soakaway by having the septic tank emptied annually.

Q: How do I know if the soakaway is blocked?

A: The water in the outlet pipe is covering the bottom of the pipe. It should be free of water!

Q: Why do soakaways fail?

A: Soakaways have a limited life, even if the septic tank is emptied annually. It is a filter in effect. All filters get blocked eventually.

Q: How deep is a soakaway?

A: The soakaway is as deep as the outlet pipe as it leaves the septic tank. If it is in the ground too deep it may not work at all! Clay at depth is much more like Plasticine and clay is used to line reservoirs and canals! If sandy, the soakaway will work much better, but not if the table rises in a wet winter to a level near the soakaway pipe. A wet flannel is useless!