Just started a new pool build 20x40 inground vinyl liner pool. I am doing the install myself. During excavation we hit some rock and had to over dig the deep end of the pool. The walls are not within the 1-2 inches desired for the finishing of the pool with sand/mortar. How do I "build up" the walls again to get back to where it should be to move on to the next step? Any help is appreciated!!