Here's a copy of a post that I made in our Paradox forum, to help a poster called
Coinneach breach an Aquifer by the collapse method:
@Coinneach: Here's your little tutorial on how to breach an Aquifer
and survive to brag about it to the other Dwarves.
Dwarven Engineering 101 : Breaching an Aquifer.To make it simple, I'm going to assume that you have at least three Soil levels above the Aquifer (although this technique will still work with only two levels) and that the Aquifer is only one level thick (although this same technique can punch through multiple Aquifer levels with a slight modification).
Assume that you want to run a 3x3 stairway down through a one-level-thick Aquifer.
Start by digging out a room on the level just above the Aquifer. Make it a large room... with at least four or five times the floor area of your intended stairway. Be generous in your measurements. Since I'm planning a 3x3 stairway (nine tiles in area), I've dug out a room with a floor area of 127 tiles... about 14 times larger.
Now channel out a big hole in the floor, large enough for the stairway itself plus one tile of overlap all around it. We're planning to drop a caisson into the Aquifer to hold the water away from our stairway, so we'll need to channel out enough room for the 3x3 stairs plus a one-tile-thick caisson wall all around it... so we need a 5x5 hole. Channel out one row at a time, so your Miners don't end up taking an unscheduled bath.
The work-room just above the Aquifer:
... and the channeled-out hole in the Aquifer itself:
Now go up a level, to the spot just above your work-room (ie: two levels above the Aquifer level). Dig out a donut-shaped room, leaving untouched a 5x5 square of solid Soil in the center, positioned directly above the 5x5 hole you've channeled down into the Aquifer. This is the caisson that we are going to drop into the hole.
Now go up one more level (ie: three levels above the Aquifer) and hollow out the entire room. Build a Support in the center, directly over our caisson block.
At a safe distance... which means a LONG, LONG way away... set up a lever and link it to the Support. That way, we'll be able to evacuate the area and lock all the doors before pulling the lever to drop the caisson block. Did I mention the doors? Put doors on all three rooms, so that you can lock everyone out just before pulling the switch.
The caisson block is now supported only by the floor and ceiling, and by the support that we built in the top room.
Now channel out the strips of floor that are directly orthogonal to the caisson block (ie: on the North, South, East and West... ignore the corners) and do the same in the room above.
After the channels are finished...
... the caisson block is now held up only by our lever-activated Support. Evacuate the area, lock the doors, and pull the lever.
KABOOM!
The caisson block drops directly into the channeled-out hole, and splashes the water all over the place. This is why we made the bottom room so big... the 7/7 water from the hole is now only one or two units deep, and will evaporate quickly.
Notice that our dropped caisson block fits snugly into the hole we had channeled out earlier... you can't even see where they join.
Once the water is mostly evaporated and is down to scattered patches only one unit deep...
... dig a 3x3 set of down-stairs in the exact center of the dropped caisson block. Make sure not to dig adjacent to the original Aquifer walls! You must leave a one-tile-wide border of caisson intact, all around your stairs.
... and now we are through the Aquifer, and down into the stone layer below.
This method can be adapted to breach Aquifers many levels thick, by successively dropping O-shaped caissons... starting at the outside with the largest ring, excavating and pumping dry everything inside the ring, then setting up one level lower and dropping a smaller ring inside that one.
Any questions?