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.