Actually, the "name" may be a bit misleading .... it might be more accurate to call them "branch piles" ...LOL!!
Many people swear by pine & cedar trees, others swear at them. I've caught Crappie from a lot of different types of wood. But, the best (non-natural occuring cover) I've encountered, so far, has been a large portion of a Sycamore tree ... that hung off a rock cliff over deep water. This scenario produced many 2lb fish, from a lake that normally produced hand sized fish from other wood cover. There are a couple of pluses to Oak (and other barkless hardwoods) ... one being there are few small branches and no bark to hang into ... another being that the branches are larger, thus creating a larger shade factor.
One problem with "brush", is that after a while underwater - they start breaking down, and your 4-6ft high pile of wood eventually becomes a 1-2ft pile of trunk carcasses and muck.
How deep you place your wood depends on several factors - how deep is the water in this lake, how much fluctuation does the water level go thru in a years time, what season do you want the fish to occupy it, how deep is the water in the area where you intend to place it, and what other features are in the general vicinity (channels, points, docks, etc). A lot of the answers to these questions depends on the region, state, lake, or type of body of water that you're planting your "piles" in. Typically, however, Summer & Winter piles should be in relatively deep water ... Spring & Fall piles should produce better in more shallow waters. That's not to say that a shallow (or deep) wood cover pile won't produce in all seasons. It's just a guideline based on the "normal" habits of Crappie, as I interpret them to be. I'm sure there will be others that have their own ideas & interpretations - and I'm sure they work for them. The great thing about Crappie, is that they will adapt to the conditions in which they exist ... so don't rule out any possible form of cover. I've caught them from a single stickup, barely as big as my finger ... to cedar tops in 1-2ft of water ... to floating mats of tree branches ... to all manor of standing and fallen trees of hardwood and softwood varieties. And if I ever find them ... I hope to add PVC structures, stake beds, brush buckets, rubber hose "bushes", and any other "hand-made" forms of cover ...LOL!! ...............luck2ya ...........cp