There's no way you're not going to have some condensation in an underground pipe. If you pump hot fog into underground pipe, it will cool the fog and leave some residue in the pipe. Those of us with fog chillers see this residue as an oily glycol film on the inside of the chiller. If you pipe fog out of a chiller into the underground pipe, the pipe being warmer will cause the fog to warm and some will drip out in the form of liquid.

Most people use 3" diameter pipe to move fog around their haunts. If you're piping with anything less, you'll need to start with a larger diameter pipe (e.g, 3") and then branch to smaller diameter pipes that at least equal the volume that could come through the original pipe unless you want to greatly change the density and/or flow rate. e.g., if you start with a 3" pipe, you will need at least 10 1" diameter pipes, or 2 2" diameter pipes. (see below for the math). If you don't have enough output pipe, your density and flow will increase. Increasing density will cause the fog to heat up and start to change back to a liquid.

Warning: Math follows....

The formula for the surface of a circle is pi*radius squared, so the 3" pipe yields 7 sq inches, a 2" pipe yields 3 sq inches, and a 1" pipe yields 3/4 of a square inch. If you begin piping in a 3" pipe, you can put a Y fitting in and split it into 2 2" pipes with an increase in flow rate of 16%, which is likely not deadly. If you split it with a Y into a 2 2" pipes and then split one of the 2" pipes into 1" pipes, your split from 2" to 1" will go from 3 square inches of input to 1.5 square inches of output. That is a 100% increase in flow rate, which would likely cause significant condensation in the pipe. To prevent that, you'd want to split the 2" pipe into 4 1" pipes.