Why is a TSAH a mandatory requirement?
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.
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!
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