Our ground is 1/3 rock. The pit was dug, lined on the bottom with bricks, sides lined with concrete blocks. The hole was quite a bit larger than what we ended up needing, so we back filled the sides with the dug up rock, and then filled with decomposed granite. It
should hold in the heat quite well. We will soak the area with
water the day before starting the fire. I'm following the method suggested by a
local. Wrapping the primal cuts in foil, then wet
newspaper, then more foil. It will cook overnight for 12-14 hours.
Half of it will be just shredded for tacos, the other half will be for birria, a stew. My future in-laws are from Jalisco where the dish hails from.