With March upon us, now is the perfect time to start thinking about percolation tests. Percolation tests are mandatory in gaining permission for the installation of a soakaway or other drainage systems as they determine the permeability and drainage capacity of soil. This is important as it ensures surface discharge won’t pool or flood, and that it will drain at a sufficient rate to remove contaminants before they reach the water table.
Preparation
Before a percolation test can be performed, we first must determine the height of the water table. This is needed so that we can guarantee that the installation of the soakaway won’t be within the range of peak seasonal groundwater changes thus limiting the effectiveness of a soakaway. To get this information we carry winter groundwater monitoring. Groundwater levels are usually at their peak between October to March and involves the installation of a borehole with a monitoring standpipe.
Once on site, we excavate a trial pit to the proposed level of the soakaway. The dimensions of the pit vary depending on the proposed drainage system but a depth of approximately 2m is typical. The water table should not rise to within 1m of any proposed outlet pipes. Where the natural soils are granular, coarse gravel is used to fill the trial pit to a safe level reducing the risks associated to working adjacent to an open excavation as well as maintaining stability of the pit walls.
Method
Once the pre-checks have been made and results satisfied, the percolation test can begin.
- Firstly, a 1m3 pit, usually rectangular in shape is excavated to the required depth of the proposed plans and filled with gravel if necessary.
- This pit is then saturated by filling it to the proposed dimensions of the soakaway with clean water and allowing it to seep away, monitoring the time period for the water to reduce between 75% and 25% capacity of the pit.
- The test should be repeated a minimum of three cycles per trial pit, with the longest time period for the water to seep taken as the given infiltration rate.
The infiltration rate dictates the suitability of the natural soils to permeate excess water for your proposed design.
Act Now
With the improved weather conditions spring usually brings, the end of March is the best time to perform a percolation test. Contact us now to get your percolation test booked in. To find out more about our other groundwater and soakaway testing services, click here.
