Hello, thanks for the reply. I have posted three pictures (left, bottom and right) showing the voids. the blue tile-lets are 2.5cm by 2.5cm (as a reference of the size of the voids). It appears that a repair job had been done before (especially the picture with the metallic screw? indicates a repair job) not sure what they had used before as it all seems to be crumbling.
What sort of solid material should i see when i remove the crumbling material?
i have posted a fourth picture (showing the depth of the mouth of the skimmer). roughly calculated to 17-18cm deep.
i inherited the pool when i bought the property 2 years ago, so i am learning pool maintenance. The pool was losing a lot of water, and i had suspected there was a leak in this area as the walls close to this skimmer were usually damp and peeling away.