Well, I have a yard that slopes a bit, and my pool is installed around 8" at one end and 14" at the other end. When I installed it I brought home a truck and trailer and tractor from work. I also had a compactor and rotating laser level. I dug for the pool and a sidewalk at the same time, and what I did was removed all of the black dirt, then I leveled off the clay and loaded that on the truck, I kept all of the black dirt for the yard I spread it around and redid the grading.

So with the pool dug down to the clay, I compacted the clay, then I put in around three inches of sand, leveled with an asphalt lute, and then I did the happy bottom, with the foam cove and duct taped everything together, so far four years later everything is great still.

In my situation, I don't think that I needed to use an underlayment because my pool is actually in the clay. I did surround the pool with mulch though, which I may change to rock eventually, I do get grass growing into the mulch on the one side, but that is the same with all of my beds and is just a maintanance thing.

I have some plants by the pool, most of them are away from it, I have a hosta close to it, which I will move when the deck goes up, and some varigated grass by there that I just threw there when I moved it from digging in the gas line for the heater. It does not send up shoots and tends to just grow in a clump so it does not bother me by being there.