I have recently acquired a leak in an underground PVC pipe, which is under the concrete slab for my underground pool. While digging to find the leak I observed that there were serious erosion issues underneath the slab (4-6 inches average). At first I thought that this was due to the leaking pipe, as I'm losing a lot of water when the pool pump is running. After having dug around the edge of the slab I observed that whoever built the pool never connected the pool overflow drain into the drain pipe. There's about a 4 inch gap between the two, and no evidence of a broken pipe, glue or sealant on either ends of the pipes, or other missing connections to indicate that the two pipes ever were connected. This leads me to believe that the erosion has been ongoing since the first day the pool was filled. As I am at least the third owner since the pool was installed, I have no way of finding out who built it and the pool is out of warranty anyway.
Once I fix the leaking pipe and the missing drain section (both under the slab, but it's only 4-5 feet wide at that point, so I'm going to try to fix them without breaking up the slab) I need to shore up the concrete so it doesn't crack. I'm surprised that there have been no issues with it so far, considering how bad the erosion is. My original plan was to bring dirt in and fill in the washed out areas, but now that I've got 40 feet of slab dug out it's probably not going to be possible to fill in around the entire pool with dirt. I'm not even sure how far around the slab the erosion goes, but I'm going to presume a worst case scenario. I'm thinking about placing concrete cinder blocks under the slab to support it, possibly every 10 feet or so. I may need to place some as far under the slab to help support the slab in varying places, and also some on the edge of the slab, but at one point the slab is 30 or so feet wide, and I won't be able to get very far underneath it at that point.
If anyone has some suggestions on how to shore up the slab safely I am all ears. As I just had to buy a new pool filter and a new refrigerator hiring someone to do it is not an option. This will be a DIY and it's definitely not my strong suit. Thank you all for your time in advance.