Percolation testing: what it involves and why it cannot be skipped
A soakaway is a void in the ground — traditionally a rubble-filled pit, now more often a proprietary cellular crate system wrapped in geotextile — that receives rainwater and lets it infiltrate into the surrounding soil. Whether it works depends entirely on how quickly that soil accepts water, and that is measured rather than estimated. The standard test digs a trial pit at the intended location and to roughly the intended depth, fills it with water, and records the time taken for the water level to fall between defined proportions of the effective depth. The test is repeated, usually three times, because the first filling wets the surrounding soil and the later results are the representative ones. From the timings an infiltration rate is calculated, and that rate, combined with the area of roof and hard surface being drained and a design rainfall event, gives the storage volume the soakaway must provide. Several things can come out of that exercise and each changes the design. Where the ground is free-draining, the soakaway is modest and the design is straightforward. Where the rate is poor — as it can be on clay — the required volume becomes impractically large and infiltration is not a viable solution, which is the evidence needed to justify moving down the hierarchy to a watercourse or a sewer. Where groundwater is encountered at shallow depth, infiltration is compromised regardless of the soil, because there is nowhere for the water to go. And where the only viable position is too close to the house or a boundary, the risk of affecting foundations rules it out. The test costs relatively little and it is the difference between a designed drainage solution and an assumption that fails at building control. It is also worth doing at the same time as any trial hole for foundations, since both involve the same exercise of opening the ground.
