A machine excavated trial pit will be dug to a depth between 3-4.5m below ground level and filled with water to the proposed soakaway invert level. 1000L of clean water stored in an Intermediate Bulk Container (IBC), towable bowser or lorry mounted tank will discharged into the pit ensuring the sides of the soakaway pit are not damaged. Should the pit appear unstable or unsafe the test should be abandoned and the pit backfilled.
Once the soakaway pit has been filled and the water supply moved out of the way, a datum pole will be placed across the pit at the end closest to the safe zone. Following guidance outlined in BRE365 Soakaway Design Manual, depth to water from datum measurements will be taken at regular intervals. During excavation of the soakaway pit, details of the strata encountered, pit stability and other observations will be recorded by the logger. If any groundwater is present with the pit prior to the test, the water inflow should be recorded either as a seepage or as an entry. Seepage is where the quantity of water is insufficient to sample. Entry is a larger inflow of water where it will be possible to measure the water level after a certain time and sample the liquid. The water level will change over a period of time and should be recorded.
The soakage/ permeability tests are required to determine the soil infiltration rate and the drainage capacity of the soils. The tests are carried out in accordance with BRE Design 365 ‘Soakaway Design’. These tests are mostly used in soils with medium or low permeability, granular material.
On completion, any water still within the pit MUST be bailed out using the excavator and placed on the ground surrounding the pit (if safe to do so). The pit shall be properly reinstated so that no depression is left; a certain amount of mounding may be required to achieve this in the long term. Under no circumstance is the soakaway to be left overnight or left unattended.