Before you can instal a septic tank you need to establish firstly if the natural land drainage is suitable to support the drainage and if so you then need to calculate the amount of soakaway required to enable the septic tank to work effectively. To do this you need to conduct what are called Porosity tests.

To carry out the porosity tests you firstly need to dig a number of pits accross the land, all precisely the same diameter and depth, each 1 metre square and deep with a further smaller square pit in the centre at the bottom. This all has to be dug by hand and once you get down to a meter deep you find that as you leaning into the pit to dig further down falling headlong into your freshly dug pit becomes a regular hazard. 

Once all of the pits are dug, you can start the tests. Over a set number of days a specific amount of water is poured into each pit and recordings are taken to measure the time the water takes to fall between various levels in each pit. In addition other factors such as weather conditions and temperature are also recorded and factored in and the measurements which are sent to the Surveyor who uses them to calculate the size and layout of soak away drainage trenched required to support a septic tank.