Yes, digging dry frozen dirt is possible. Freestall barn stantions???? /forums/images/graemlins/smile.gif
In my part of Minnesota I hear pipes need to be 5' deep but 6' deep under driveways & other heavy compation areas (this assumes heavy compaction & clearing the insulating snow allows the frost to go deeper) so the standard is 6' deep minimum. Someone above said 8', that would be news to me in south central MN, but other areas might need it or codes may have changed.
On the other hand, septic laterals can be a max of 1' deep here. Jee I wonder why folks have problems?????
--->Paul