The hole has been dug exactely the correct measurements for concrete 150mm (6") thick to be sprayed on.
If the pool is being built with block or poured concrete then the hole will be over size.
Note hole in the middle for the gravel pit for the hydrostatic valve tube.
Also note that a soft patch in the floor was dug out & filled with concrete.

Photo courtesy:
Clearview Pools, Adelaide Australia Ph Mobile 0418300400

Web Page written by:
Victor K de Fontenay © Sept 1998
Page Last updated 3.00pm 27th Feb 2000