man digging a hole
Calendar 17 November, 2023

Trial Site Assessment Holes (TSAH): A Guide

For septic tanks and soakaways to function efficiently, they need to be surrounded by sufficient unsaturated soil to absorb the effluent. While a percolation test will establish the porosity of the soil, a trial site assessment hole (TSAH) is required to determine the level of the winter water table.

Why is a TSAH a mandatory requirement?

A trial site assessment hole is a mandatory part of the septic tank and soakaway installation process, stipulated by BS 6297:2007 and Part H of the Building Regulations.

Whatever time of the year the installation is to take place, it is important to determine the height of the groundwater table during the winter months, when it is likely to be at its highest. If the water table is too high, soil saturation will occur too soon and the septic tank or soakaway will quickly fill up. If this happens, effluent can flow back up the system, causing flooding and contamination.

When should a TSAH test be carried out?

A TSAH should be the first priority once it has been established that the site isn’t in a Source Protection Zone . It should also be carried out before a percolation test as a septic tank or soakaway cannot be installed if a TSAH test fails, regardless of the results of a percolation test.

How is a TSAH made?

To create a test site assessment hole, you must excavate a hole that covers at least 1 square metre of ground and 1.5 metres deeper than the level of the pipework for the system. With pipework typically being installed at a depth of 600-700mm, the TSAH is likely to be at least 2.1 metres deep.

How does a TSAH show the height of the groundwater table?

As the groundwater table rises in wetter seasons, it carries minerals up through the soil. When the water recedes, these minerals remain in the soil and are visible as a grey or grey-brown mottled layer between  100mm and 2000mm deep. This is a more accurate indicator of the groundwater table than excavating until standing water is found, as the water table will vary considerably with the seasons.

How are the TSAH results interpreted?

There must be at least a 1-metre height difference between the winter groundwater table and the pipework of the septic tank or soakaway, so if the tidemark left behind by the winter groundwater table is less than 1 metre lower than the pipework, you will not be permitted to install a septic tank or soakaway.

If your soil passes the TSAH test, congratulations! Now it’s time to carry out a soil percolation test and if that passes, you’re all set to install your septic tank or soakaway.


We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call the friendly team of drainage experts at Drainfast on 01420 555600 or email [email protected]

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!

Vicki James Drainfast Team Portrait

Written by
Vicki James

Sales & Marketing Coordinator

Vicki is a vital part of the marketing team; from reporting to copywriting, she ensures we complete projects on time.

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