We did one a few years back, after digging the hole with a backhoe, we burned a major fire for 12 hours which left lots of coals, then added a pickup load of volcanic rock, and the pig wrapped in chicken wire and wet burlap bags MAJOR mistake, LOL!!
Add the rock BEFORE starting the fire.
The pig come out half raw, good thing we had a few gas grills near-by and lots of alcohol to keep the crowd under control.
Good luck and please share you pics!!!