Before a soakaway can be designed, the ground has to be tested. Here's how a percolation test works, the BRE Digest 365 method, and how the result sizes a geocellular soakaway.
A percolation test (sometimes called an infiltration test) measures how quickly water drains out of the soil at the proposed soakaway location. The result — the soil infiltration rate — is the single most important ground input to a soakaway design. Without it, a soakaway cannot be sized reliably, which is why it is normally required before a drainage submission is approved.
BRE Digest 365 is the recognised method for soakaway percolation testing. In summary, a trial pit is excavated to the proposed soakaway depth, filled with water, and the time for the water level to fall from 75% to 25% of the effective storage depth is recorded. The test is repeated three times, and the slowest (most conservative) result is used to derive the soil infiltration rate for design. Testing the real soil at the real depth — rather than assuming a value — is what makes the resulting design defensible.
A pit is dug at the proposed soakaway location, to the intended depth, in the soil that will actually receive the water.
The pit is filled and allowed to drain once to wet the soil, so the recorded test reflects realistic, not dry, conditions.
The pit is refilled and the time for the water level to drop from 75% to 25% of the effective depth is timed.
The test is repeated three times; the slowest result is taken forward as the conservative design value.
The recorded times, pit dimensions and effective area give the soil infiltration rate used to size the soakaway.
The infiltration rate is combined with two other inputs — the design rainfall (storm intensity and return period) and the area draining to the soakaway — to calculate the storage volume needed. The system must store the design storm and then empty within an acceptable drain-down time so it is ready for the next event. A high-void geocellular system stores far more water per unit volume than gravel, so it meets the required storage in a smaller excavation. The geocellular soakaway design guide covers this, and the NGS Design Studio performs the sizing to recognised standards.
| Input | Where it comes from |
|---|---|
| Soil infiltration rate | Percolation test (BRE 365) |
| Design rainfall / return period | Hydrological data & the design standard for the project |
| Contributing area | The catchment draining to the soakaway |
| Drain-down time check | Confirms the system recovers before the next storm |
If the test shows the ground drains too slowly, or the groundwater table is high, infiltration is not viable. This is common in sabkha and similar conditions found in parts of Arabia. In those cases the design switches from infiltration to attenuation — storing the storm and releasing it at a controlled rate to the network — which the infiltration vs attenuation overview explains.
What is a soakaway percolation test?
A test that measures how quickly water drains into the ground at a site. A pit is filled with water and the rate the level falls gives the soil infiltration rate, which decides whether a soakaway is feasible and how large it must be.
What is the BRE 365 method?
The recognised soakaway test method: a trial pit is filled and the time for the level to fall from 75% to 25% of the effective depth is recorded, repeated three times, to derive the infiltration rate.
When is a soakaway not suitable?
Where the soil drains too slowly, the groundwater table is high, or in sabkha-type ground. An attenuation system with controlled discharge is used instead.
How does the test size a soakaway?
The infiltration rate is combined with the design rainfall and the contributing area to calculate the storage volume, checked against an acceptable drain-down time. The NGS Design Studio does this to recognised standards.