To establish the overall length of a drain run needed when installing a septic tank, a soil porosity (percolation) test is required. The following procedure should be adopted:

• Excavate a hole 300mm square by 300mm deep below the proposed invert level of the land drain.
• Fill hole with water to a depth of 300mm and allow to drain away overnight.
• Refill to a depth of 300mm and observe the time taken to drain from 75% full to 25% full level (i.e a depth of 150mm). Divide this time by 150mm. The answer gives the average time in seconds (Vp) required for the water to drop 1mm.
• Repeat this exercise two more times with at least two trial holes. Calculate the average time.

This time will be used by whoever installs your septic tank and soakaway system, to determine the size of drainage field needed.