Grout Mix Bottom Problems / Liner Replacement

[wtrlvr]

Hello to you all

I am about to replace a liner in an in-ground pool with a grout mix bottom according to the original bill of material. The bottom is obviously deteriorating as there are little "pebbles" that must be a breakdown of the grout from water damage I assume. These pebbles are starting to create pinholes in the liner in a few spots.

My plan is to remove the liner and resurface and patch the grout as needed. Then I was thinking of gluing some thin foam down to entirely cover the grout bottom to protect the liner.

Any suggestions? Type of grout etc?

BTW the pool is 16x32x8 hopper bottom with Polymer walls and an 8 foot plastic step on the shallow end. The steps are cut in and not wrapped in vinyl. Pool was built in 1985 by Schertyle Pools of Virginia Beach. This is will be the third liner according to the detailed paperwork the original owner turned over to me including photos of the original construction! I have owned the pool for five years