This post shows how to breach a 2-level aquifer with only the wagon wood using cave-ins in concentric circles. It was inspired by the wiki an threads on the cave-in method but these assume unlimited building materials, which we don't have.
The general plan is to cave in a circle (actually: rectangle outline) of soil into the first aquifer layer, dig out the center of the rectangle and pump it dry so we can channel to the second layer, and then cave in the center of the rectangle into the second aquifer level so we can dig a stair down to the rock.
With the starting wagon we can (barely) make a pump by deconstructing the wagon, building a carpenter with a log, making a block, deconstruct the carpenter, building the carpenter again with the block, building the pipe section and corkscrew, and deconstruct the carpenter again to get the block back. We don't have any additional building materials, so no constructing supporting arms, floors, etc. We can use the one block to build a temporary floor to rescue stranded miners, but that should not be needed if we do it correctly.
The schematic overview:We assume two soil layers above two aquifer layers. The first thing to do is channel the inner ring and then the outer ring around the first cave in. It is important to start with the first ring so you can get the miner out easily. The structure is assymetric since we need to place a pump later. The trick behind the cave in method is that the soil we drop into the water will form a new wall which shuts out the aquifer.
After the first cave-in we can channel out the inner part of the first aquifer layer. Since no water can come from outside, the only water generated is from the one pillar we need to leave standing to provide support. We can then pump away the water and let in the miner to mine away the aquifer part of the pillar so no more water is produced. The pillar is now supported by a single floor. Next, we channel out the second aquifer layer and cave in the pillar by channeling away the floor. After the dust settles, you can dig a staircase in the middle of the 3x3 pillar that was dropped down. And pray that there is no third aquifer layer :-)
The step-by-step guide with screenshots0. While the miner is mining, make the pump components as outlined above.
1. Channel out the inner ring from above to the last dry soil layer, leaving a 3x3 pillar standing.
2.
BuildDig a staircase to get the dwarf back up, and then channel the outer ring, with one side 2-wide to allow for the pump. Make sure to leave one tile unchanneled for support (only on the top level)
3. Channel out the space under the ring that will be caved in and the ring around that. Make sure to leave a little bridge in the outer ring so the inner ring will be accessible later. The channeled out ring around the cave in part is there to drain the water, so it doesn't matter that one tile is not channeled out. Edit: if you're lazy / pressed for time, only dig out the outer ring on the side where the pump will be, that is more than enough to capture the water.
4. Cause the first cave in by channeling out the last tile from the top. After the cave in, the first aquifer layer should look like this:
The soil we dropped turned into the same type of soil as the aquifer, but it will not be water producing. So, if we get everything inside the new 'wall' channeled out, there won't be any new water in this part and we can move down.
5. First, channel away the inner part of the aquifer layer. Make sure to give orders row by row so you won't have stuff leftover in the middle. Leave one wall standing. The last dry layer should now look like this:
EDIT: actually, place the tile that you leave around one to the right so that it is in a corner of the 3x3. This is needed because you want to use a constructed floor to cause the second cave-in to avoid losing your miner to a tragic accident.
And the first aquifer layer looks like this:
We first need to get the water out. It will still be producing some water due to the two tiles that we couldn't channel out, but a pump can beat two aquifer tiles and as soon as the water is mostly gone we can mine the last two tiles while leaving the floor/wall/ramp above standing.
6. Build a pump on the wide part of the soil layer. In fact, you could have done this while waiting for the miner to finish channeling. Set to pump from inside out and pump manually
Once the water is down a bit mine the two tiles that were left. Once these are gone, the level will be completely drained. You can now safely stop pumping and deconstruct the pump, if desired.
7. Channel out the inner part into the second aquifer layer. Be sure to leave one tile in the outer part so we can reach the center.
8.
We are now ready to cause the second cave-in. WAIT! Don't actually do that, it might kill your miner and then you have a big problem. Instead, build a floor next to the remaining pillar, then channel out the ramp, bring the miner to safety, and deconstruct the floor with a brave volunteer:
(the screenshot is from my second failed attempt, where I forgot to channel out the ring around the 3x3, which is needed to catch the water. I dug some space behind the volunteer to increase his chances. The death rate so far is 2 out of 3, which according to trusted sources, "ain't bad". Also note that I only dug the outer trench on one side, which is quite enough to capture the water after the first cave-in before you get the miner down.)
9. After the second cave-in, the remaining water will drain into the trench around the 3x3. Once the water is mostly gone, dig a down stair in the center and dig down to the rock. You are now through the aquifer!
You have struck native gold! I knew piercing the aquifer would be worth it :-) It is now 21st of Granite. I did waste some time waiting for the 1/1 water on the 3x3 to evaporate but it turns out 1/1 water doesn't really flow into a hole, so that was unnecessary.
If you follow the method here you should be able to get a 100% success rate quite easily and be able to breach the 2 level aquifer in around 20 days. The only risk is losing your volunteer, but that is difficult to guard against. It might be good to store the pump material somewhere else to avoid them being knocked around, if you want to preserve the parts.
In retrospect, you can probably skip the floor construction as long as there is no trench behind the miner. We don't actually care that much if he dies, as long as his pick remains accessible. That should be slightly quicker.
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As an epilogue, here are the stories of the brave dwarves that died to bring you this information, both starting after the second cave-in.
Failed attempt #1:
And my miner was knocked into the trench and drowned. Oops. This didn't happen on my test run, I guess it is random what the cave in dust does...
Okay I don't have time to redo the whole thing now, might edit it later. Proper procedure would have been to build the supporting pillar one closer to the edge, so you can build a floor as a temporary support. Then, you can safely channel the ramp and use a replaceable peasant to remove the floor. I guess mining out some area behind the and behind the trench will also improve his survivability as being knocked back is not what killed the miner.
Edit: Failed attempt #2:
Edit: did the run again. As you can see: (1) I built a nice little floor from the deconstructed pump block, which is currently being removed by a brave volunteer, (2) I only dug out the outer channel on the side of the pump, which was more than enough to drain the water after the first cave-in (3) Just to be sure, I dug out a "volunteer landing strip" on the side of the floor to decrease the odds of the volunteer to attempt a Hudson style landing.
Unfortunately, things don't end well for our brave volunteer:
As stated before, this is quite random, I have no idea how to improve his odds. A floor higher up would cost at least three logs, which we don't have, so I guess we have to take the odds with the volunteer. Good thing we start with seven dwarfs, eh?
The bad thing is that the pump components were knocked into the trench, and we need
the pump to drain out the water, or at least realize that we need to dig a drain into A2 to let the water out...
In this run, after building the (now drowned) staircase and some trenches, it is the 19th of Granite, so that seems like a feasible end time.