Can I bury a 1 1/2" shower/tub drain pipe in an earthen floor?
I'm not aware of any restrictions on the minimum size of a drain pipe encased in concrete in either the BC or National plumbing codes.

If the floor is truly classified as concrete, there may be issues on how literal the inspector's interpretation of the code is.

The 1998 BC Plumbing code included the following:

Section 7.3.5 Protection of Piping, Subsection 7.3.5.3 Isolation from Loads:


Where piping passes through or under a wall it shall be installed so
that the wall does not bear on the pipe.
All pipping shall be
installed without undue strains or stresses and provisions shall be
made for expansion, contraction and structural movements.
No piping shall be embedded in concrete or masonry.


The BC building inspectors issued interpretation 98-0056 regarding this issue. The bottom line was that pipes embedded in concrete needed to be sleeved. However, the current BC plumbing code (2006) has removed both sentience 2 and 3 from 7.3.5.3. The national code (2005) also reflects this change. (The BC Plumbing code is almost an exact copy of the national code).

The short answer is that it comes down the the inspector's opinion. It would probably be beneficial to contact your local inspector to discuss.