dwpc,

Did you leave most of the gunite shell in place? Did you just filled it in without punching holes in it? Aloha

Elsewhere on garden web, it was suggested that the material wasn't properly compacted.

If holes were not punched through the pool shell, then the water can accumulate in the soil's void spaces and replace the air. Without drainage the soil when soaked loses it's bearing capacity because the particle can float apart and not bind together to resist the downward load forces. This is similar but not quite like a quicksand condition. All the mechanical compaction will not create a stronger surface if the water can not drain. Aloha