If you're talking about the crate type of soakaway, such as the Polystorm or similar, mine had a 150mm hole, with knockouts, in several sides of each crate, so a reducer and a small piece of 150mm pipe was easy to fit into an appropriate hole.
As Studips, says just cut the fabric around the pipe.
Incidentally, mine came with some 50mm or 100mm, (I can't remember the height correctly now.) spacers to retain the height of the shingle bed underneath, and around the sides if required.