Concrete deck piers

During the rebuild of my deck around my pool, I had to excavate soil from around some of my deck piers to accommodate a retaining wall.

In some instances, I had to lower the grade 18" and now I am concerned that the frost line will now be lower and may promote heaving in the winter.

There is only one side of the pier that is essentially lower as grade slopes towards that other sides.

My question, is there any use in trying to build up the grade on the lower side to try and replace the lost height? My thoughts were to use sand bags filled with the soil from around the area and stack them up the side of the pier. Am I waisting my time?

Thanks in advance!

Geoff