I suppose it's about time we had an opportunity to discuss the very basic task of running pipe in the ground. After all, the DOL calls us "inside wiremen," so it's fair to say many have probably never run wires for any distance underground.
I'm going to give the 'gold standard,' and point out a few issues along the way ....
First off, use big pipe, and bury it deep. I say this without regard for either the type of pipe or the location of the trench.
You want 'big pipe' because pipe is cheap, digging is expensive, pulling can be a PITA, and you will love the extra room.
I say 'deep' - meaning about 2 ft. to the top of the pipe - for two reasons. The first is protection; you want the pipe to be well past the reach of the casual shovel probe or sprinkler repair. You also want to use sweep 90's to come into your handholes from beneath; this makes wire pulls so much easier.
There's a misconception about steel and rust. In my experience, once the pipe is a foot or so under the soil, the soil is packed tightly enough that there's not enough air/oxygen for rust to form. Still, I will admit that I like PVC for the horizontal runs.
If serious, or repeated, wire pulls are expected, you will want your sweeps to be steel. This is so the pull string does not cut into them. Wrap the steel in heavy vinyl 'pipe wrap tape' to prevent rust.
Just before you place the pipe in the trench, level the bottom with a layer of sand. This sand will also help prevent rocks, roots, etc., from stressing the pipe at any single point. Then put a few inches of sand over the pipe to hold it in place. If you're running more than one pipe, you might want to strap the pipe to bits of wood or scrap pipe to maintain spacing during your filling operation. Also seal your pipe ends against dirt entering; duct tape works well. Now is when you call for your 'open trench' inspection.
A proper handhole will be about 16" deep. Make little earthen 'dams' at the ends of the trenches, then fill the handhole area to this level with 'pea' gravel. The gravel will support the handhole, allow you to level it, and allow for drainage. Once you've placed the boxes, you can fill everything in. Make sure to 'tamp,' or compact, the soil around the handholes.
If your handhole is smaller than a 5-gallon bucket, you're using the wrong product. Nor are metal "Bell" boxes suitable (soil alkalinity rapidly corrodes them away, and dirt enters past the gasket no matter what you do).
It is allowed for the gardener to cover the handholes with light layers of mulch, crushed bark, etc.
Wire splices need to be absolutely waterproof, yet you need not use 'direct burial' methods. Personally, I stay away from the expensive silicone-filled wire nuts. For lighter duty applications I will simply fill ordinary wire nuts with Scotch-kote and point them so as to drain. For severe applications, I will submerge the wire nuts completely in one of the Scotch epoxy-packs (this is a product also approved for direct burial). I won't do any sealing until I am certain that the circuit is good and the connections correct.