What Is A Percolation Test?

A percolation test is a method of determining how quickly water drains away from soil.

Basically, the test involves digging a hole to a specified depth, filling it with water and measuring how long it takes to soak into the earth.

The results depend on various factors, the most significant being the composition of the soil.

Why would I need a percolation test?

Percolation tests are carried out at properties to see what size drainage field will be required for installation.

For a soakaway system to be considered, the site must be at least five metres away from your building and any roads.

But if the rate of drainage is too fast – or too slow – then a soakaway system may not be appropriate.

If the soil isn’t permeable enough and the drainage field empties slower than it is filled, it can cause a septic tank to back up.

All of these questions are ones that a percolation test will answer in order to ensure any work conducted is compliant.

As mentioned, a percolation test begins with digging a hole – but typically more than one is required, known as test pits.

Dig at least two holes of 300mm², at a depth of at least 300mm below the proposed invert level (measured from the bottom of the drainage pipe).

Fill the test pits with water (not while it’s raining) up to a depth of 300mm and note the time.

If the water hasn’t drained away completely after six hours, then your site likely isn’t suitable for a soakaway system.

Additionally, if it has gone completely in less than ten minutes, this may indicate the soil is too porous.

In either case, seek professional advice on the best way to proceed and available alternatives.

If the water has drained in an acceptable time frame, refill the holes to 300mm and start timing once the level drops to 75 percent (225mm).

Stop timing when the water seeps away to a depth of 75mm (25 percent full).

Take that time in seconds and divide it by 150, to give an average time it takes the level to drop by one millimetre.

Do this at least twice more in each pit – at different times of day – and record the findings.

Take the average of all your recorded numbers (at least six) to give you a percolation value (Vp).

Your Vp can then be used to calculate what size drainage field you need and whether a soakaway can be installed.

That’s it really, a relatively straightforward explanation of something that’s critically important to get right.

If you’re not 100 percent confident of conducting your own percolation test to the required standards, why not seek some support?

To arrange a visit from one of our specialists, or to discuss your needs over the phone, please get in touch.