Originally Posted by jrumbaug
...The most pipes I had to lay was 4 of 1"poly, so 4" may of worked...
8" for multiple pipes makes sense... and no, you can't get 4 - 1" pipes in a 4" wide trench... not without stacking the pipes.
I found that working with 3/4" or 1" pipe, you had to account for 2" of space when placing multiple pipes in one trench. So 8" wide for 4 pipes side-by-side make sense (and the easiest to repair if anything had to be dug up). If you wanted to stack the pipes, you would have had to dig down at LEAST 14" to keep the upper pipe at that 12" level.
I was suprise at how quickly I filled a trench using multiple pipes. When I installed my system, I had 8 pipes in one trench running away from the manifold. I had to dig down an entire 24" to get 4 - 1" and 4 - 3/4" Sch 40 PVC in the one trench. (Yea, I know, it's a disaster waiting to happen... but that's why I used Sch 40 on laterals, screened the backfill, and the top of the top pipe is at least 12" below grade.)