Filler under slab for drainage

[WWilts]

re underslab drains,

Building regs approved doc H (drainage) says 
flexible filler and at least 75mm of granular material between the top of the pipe and the underside of the flexible filler below the slab

 

What can serve as the flexible filler?

 

 

[saveasteading]

If this refers to preventing the drain from being crushed by foundations/differential movement, then polystyrene or packing foam.

Can be wrapped around and tied, or laid as blocks to 4 sides.

 

This is also how to pass a drain through a footing.