Trial Site Assessment Hole for Septic Tank Soakaways

The Trial Ste Assessment Hole (TSAH) is dug to find out how deep the water table or bedrock is in the proposed septic tank soakaway area.

In order for a septic tank or sewage treatment plant soakaway drainage field to have any chance of working, you must have a minimum of 1 metre of unsaturated soil below the bottom of the soakaway drains at all times of the year and in all weather conditions. It is impossible to soak effluent away in already saturated soils or into rock. Over 60% of all sites fail this test and it is pointless going on to do percolation tests.

Trial Site Assessment Hole - Method

The TSAH must be a minimum of 1m² with a depth that is 1.5m deeper than the drain field pipework. As the drains are normally between 600mm and 700mm deep, this results in a hole that is at least 2.1m deep. The water table or bedrock is not allowed to ever get within 1 m of the pipework.

Obviously, bedrock is easy to determine but unless the TSAH is dug in the wettest period of the year, you are not going to get a true result  of how high the water table gets by testing the amount of actual water in the hole at the time of the TSAH. However, the water in the water table has a high percentage of minerals in it and these are left as a tide-mark when the water table goes down again. It is this 'tide-mark' that you are looking for on the sides of the hole.

The features that you are looking for either grey layers in the soil or a grey/brown 'mottling' that occurs in a line in the soil. The line may wave up and down, it is not always straight and it can be anything from 100mm to 2 metres deep.

Grey layer shows that the winter water table saturates this soil mottling due to high water table

The grey layer is the high water table level

If you see this then you cannot have a soakaway

This mottling shows that the soil is periodically saturated by the water table

If you see this then you cannot have a soakaway.

 If your site fails this test, you cannot have a septic tank. You may be able to have a sewage treatment plant and discharge to a ditch or stream, if you have one, or to an above ground 'mound' soakaway which we can design for you free of charge.