I am about to build a cavity wall that will have a 100mm drain passing through both leaves just above foundation level. I will build the hole for the drain into the wall with lintels over and the hole will be 150-200mm wide as the final position of the drain is subject to slight alteration. The cavity to the sides and above the drain will be filled with lean concrete as it is below ground level.
I am wondering what is the best method to use to close the holes on either side of the wall to keep the concrete infill in and the trench backfill out? As an architectural designer, I have always specified rigid cover sheets to be used for this purpose, but having visited my builders merchants today, they have never heard of such a thing. Does anybody have any suggestions of products or materials to use to cover these holes bearing in mind that they will eventually be below ground and subject to moisture?
Thanks in advance.