concrete apron

[phut]

This past May 2020 I contracted with my local pool company to replace the corner fiberglass steps with steel swim ledges and steps (2) at the front end of my 18 x 35 in ground pool. To do so they had to take out the front end, which involved cutting the concrete apron and removing a section. The concrete they needed to remove was a section about 1 1/2feet wide but 20 in length. It had a pea gravel base. They dug out underneath the cut section, removed most of the pea gravel, leaving no support the length of the 20 long concrete. As a result, the entire 3 1/2 x 20 ft apron collapsed as it had no support. They had to then drive in sheet pilings to keep the pea gravel beneath the adjacent paver patio from spilling into the pool. What should they have done to prevent the collapse of the concrete?