I've got an unfinished basement (1 year old). A shower and toilet were roughed in, but I want to add a bathtub. Attached are photos showing where I've broken through the slab. I've come across some plastic and underneath the plastic is some 1/2 " broken rocks (gravel?). My question is, can I assume there is a drain pipe under the plastic/rocks(gravel)? The dug out portion to the right is where I'd like to add the bathtub drain and was hoping to run a drain to the main stack on left side. If there is a drain pipe below the plastic/rock, can I just tap into that drain. If there is not drain to the right, can I just run a drain (to the left) and directly into the main stack?