Do I have to use a drainage blanket?
The tops of the wick drains need to have a backpressure of zero. Stated otherwise, the wick drains need to be vented to the atmosphere. This can be done naturally if the upper soils are clean sands or gravels. Otherwise a drainage blanket of clean sand or gravel is required. Alternatively the wick drains can be connected to horizontal strip drains, with the strip drains being vented to the atmosphere.