Also depends on what code your area has adopted, what kind of pipe you have installed and if its under the building or not.
2006 UPC (what we have adopted here in southern nevada) - 1211.1.2 states Underground piping shall be installed with a minimum of 18" of cover, unless external damage is not likely to result then minimum cover shall be 12".
Section 315.4 states that trenches sahll be completely backfilled in thin layers to twelve inches above the top of the piping with clean earth which shall not contain stones, boulders, cinderfill, frozen earth, construction debris or other materials that would damage the pipe.
I installed 3/4" Yellow PE pipe in a direct burial application that was 14" deep, giving me 12" of dirt cover. The inspector turned me down saying that 18" was required and that sand was also required for backfill material at which point I pointed him to section 1211.1.2 on the minimum cover requirements and stated that there is nothing in the code saying sand is required and he retorted with the installation standards for PE gas yard piping where it states 18" minimum requirements and gave in about the sand. Instead of redigging the trench, I spent 2 hours pushing an abs sleeve over the installed PE piping.
Have fun!