This thread isn't about how to do drainage, but why we do it like we do. This came about while building a large wall last fall. I was talking to the engineer who designed the wall and just making sure everything was right. I mentioned that we daylighted the draintile every 20 feet and questioned why so close together ? This is when it got interesting for me. The engineer asked if I had ever seen water come out of one? I replied that I had seen a trickle but nothing more. He replied that if I saw water flowing out that I should run and run fast. Of course I asked why and this is what he told me. Drain tile behind a wall does absolutely no good, but they still use it because it is common practice. If their is enough water behind the wall to flow out, not just trickle, the hydrostic pressure will blow out the wall. I should have asked for a more technical reason, but I was too busy to discuss it. I have been thinking about this for months now and it does make sense. We place the tile on packed ground and cover it with rock that will let the water flow. To get water into the tile and flowing it needs to be at least an inch deep. (remember the corrigated tile is not smooth inside) If the tile is sloped for drainage, the water will flow through the rock before it is deep enough to enter the tile. With this, the water will enter the tile at the lowest point. At the same time it will flow between the block and depending on the block your using, through the center of the block to your base. Any one else have any thoughts on this? To try to stop the pointless comments and repeating what books and manuals say, I am certified and i've stacked a block or two. If you want to see the wall that started this, search here for Manta Ray earth anchor.