The first test that you should carry out when considering installing a septic tank or soakaway is the trial site assessment hole (TSAH). Whilst many believe they need to do a percolation test first, this is in fact pointless because if the TSAH test fails you will not be allowed to install a soakaway regardless.
What is a Trial Site Assessment Hole and why is it important?
It is a test to determine the highest level of the water table of the area in which you wish to install a septic tank or soakaway.
After you have determined the area isn’t a Groundwater Source Protection Zone, the first test done on site is the Trail Site Assessment Hole, this is part of the Building Regulations and BS 6297 2007 requirement).
The test is to determine the position of the groundwater table, and is needed, because if the groundwater is too high, there is not enough unsaturated soil under the soakaway pipe to soak the effluent away and the tank will fill very quickly. In the winter, groundwater can be so high that it flows back from the soakaway, filling the tank, which means the position of the highest groundwater level also needs to be determined.
The TSAH is a mandatory test that most people are unaware of, it has been mandatory since 2000, yet it is hard to find information on it and how to carry it out. It is mandatory for a good reason too, as finding out the winter water table is imperative for ensuring the performance of a soakaway can hold up to winter and wet conditions throughout the whole year.
How do you carry out a trial assessment hole?
According to the instructions in section H2 of the Building Regulations and the BS 6297 2007:
The trail hole should be a minimum of 1m² with a depth that is 1.5m deeper than the proposed drainage field pipework. As the drains are normally between 600mmm and 700mm deep, this usually results in a hole that is at least 2.1m deep.
The groundwater table or bedrock is not allowed to get within 1m of the proposed effluent drainage pipes.
This means that the water table must be at least 1m below the soakaway drain-pipes at all times, even during the wet winter season. This is determined by looking at the soil on the side of the hole – as said above, if you can see a mottled band you can’t have a soakaway.
How do you determine the height of the winter water table?
In winter, the water table rises. It goes down during spring, and is the lowest in the summer months. Unless the TSAH is dug in the wettest and coldest part of the year you are not going to get a true results of the maximum water table gets just by testing the hole at the time.
As the water rises in the winter it carries with it minerals from below ground level. These minerals then oxidises when the water recedes and leave a tide-mark. The tide-mark is a band in either grey layers in the soil or a grey/brown mottling. The band can be between 100mm-2m deep.
If you see this within the trial site assessment hole, the water table is too high in the winter and you won’t be allowed to install a soakaway or septic tank as it will not work.
You can also read our blogs on:
How to size a drainage field?
How to carry out a percolation test?
How to establish if you are in a groundwater protection zone?
New septic tanks regulation coming into force
Helpful guide to appointing a sewage treatment installer