BRE digest 365 calculates the infiltration coefficient q which is in m/s or m/h. Typical values are between 1.0 x 10-3 and 1.0 x 10-6 m/s. If the result is lower than 1.0 x 10-6 m/s then the ground would be deemed unsuitable for infiltration drainage. You will need to look at an off-site solution to manage the surface water. Further information can be found on P.1.
The complete procedure is described in BRE digest 365 and CIRIA C697 but the main points are:
- The test pit should be in the same location as the proposed soak away.
- If the soakaway is longer than 25 m or the final position unknown then multiple tests within 25 m are required
- If the ground is fissured or suspect then the distance between test pits should be reduced to 10 m.
Test Procedure
- The trial pit should be the same depth as the proposed soakaway; for run-off from 100 m2 this will be 1 to 1.5 m below the invert level of the drain discharging to the soakaway, so overall depths of excavation will be typically 1.5 to 2.5 m. Depending on depth the pit should be 0.3 to 1 m wide and 1 to 3 m long with vertical sides trimmed square. The volume should be 1 m³
- Measure the depth and perimeter of the trail pit before the test. Check no groundwater is present.
- Fill the test pit quickly from a water bowser to the top or effective depth, and allow to fully drain. Partial drains should not be used as the test needs to ensure the ground has not become saturated. The pit doesn't have to be fully filled if the pit is deep or the proposed soakaway functions in the lower sections.
- Record the water level at enough intervals to display the level versus time.
- Repeat the test 3 times on the same or consecutive days.
Calculation
The soil infiltration coefficient, q is given by:
q = vp 25-75 / ap50 x tp75-25
Where:
tp75-25 = The time taken for the pit to empty from 75% to 25% of the depth of the pit,
Vp75-25 = The storage volume of the pit between 75% and 25% of the depth
ap50 = The area of the base and sides of the pit at 50% of the depth.
The smallest value of q obtained from all the tests carried out in the pit should be used.
