I'm working on plans to convert three bays of a 2x4 stud wall with drywall into a mass wall by adding material to the cavity. The material will be something like sand/clay/gravel (to be determined depending on the answer to this question and other factors). The goal is to catch heat from a wood stove around 8' from this wall and help moderate the resulting heat as well as transmit more heat through the wall to the adjoining room.
There are three 12/2 Romex cables running together horizontally through this wall at around 2' above the bottom plate.
What are the electrical concerns and considerations relevant to this idea? Is it safe and allowed by code to pour a bunch of heavy stuff around these wires? If not, what protection is needed and how can it be put in place given that the wires have already been run?
[Edit: I left the wires alone but I boxed them off so they have an airspace a couple feet tall and the mass only begins above them. This isn't really an answer to the question so I'm not leaving it as an answer. It's just the work-around I used since I didn't have the answer and it didn't seem like it was going to turn out to be okay to surround the wiring with mass.]