By default, RS2 resets the stresses in an element when it changes material. This is to model the removal and redeposition of the material through backfilling.
If you want to model ground improvement, and you want the stresses to remain in a particular finite element, when you change the element's material you have to disable the Reset Element Stress When Material Changes To This Material option in the Stage Properties dialog. You need to do this for the initial material, not the material you are changing the element to.
Staged Material Properties dialog: de-select Reset Element Stress to model ground improvement