This is probably a stupid question whose answer is as simple as “they build a ramp” but I’ll ask anyway.
Large construction projects often begin with digging a large hole, and this process involves a lot of large equipment (loaders, excavators, cranes, trenchers, pile drillers etc.), all of which ends up at the bottom of the hole when they’re done. How do they get all of that stuff out of the hole?
From what I’ve seen they do build a ramp to get the equipment in there; only the ramp wraps around the edge of the hole (think pathway down a canyon) and is part of the hole. When they are done and need the equipement out they slowly remove the ramp behind them on thier way up.
When I was working on the WTC gig, we built two ramps (on either side of “the hole”). Some equipment got pulled out by cranes, though. Gotta’ use the cranes for something… other than paying $30k/shift for them to sit idle…
According to an authoritative reference I ran across many years ago, they just leave the equipment there and it becomes part of the new building’s HVAC system.
In most of the projects I’ve worked on, the ramp is an additional excavation outside the footprint of the actual hole. You need some way for the dump trucks to get in and out to haul the material being dug out of the hole, so it’s continuously dug as the hole is. When the hole is complete, the ramp is backfilled. The ramp might curve to fit a limited-space building site.
That was going to be my answer as well but I couldn’t manage to remember the name of the book. You saved me a cafe society question.
All I can find right now are Japanese sites, but often the equipment used is brought up/down to the location in a dismantled state, put together for the work, then dismantled to be removed piecemeal.
The next question is: how do they build the cranes? it always looks to me like you’d need a crane to build a crane, and that’s a “turtles all the way down” argument.
First thing I thought of as well. One of the reasons I had children was so I could read that book to them, like my parents did with me.
I came up with my own theory on how they extend cranes, but I don’t think it’s how they actually do it. My idea was:
You have the main support shaft. The crane cab is on the top, with a sheath at its base, slid over the shaft.
Cables are looped from the ground, over pulleys on the top of the shaft, to the base of the sheath. By pulling on the cables from ground level, it pulls the sheath and cab higher, enough for another piece of the shaft to be slipped inside the gap and assembled, so that when the cable is released it rests on what is now a taller shaft.
Repeat as necessary.
But I haven’t seen any physical evidence that this is how it’s done, so they may have some other more practical method.
It surprises me that I have never witnessed a crane get built, despite seeing hundreds throughout my life.
They actually do use the crane to build the crane. The self-erecting tower cranes have a module that “climbs” the tower as it’s being built. They first erect the base, then set this module on it. The module is a kind of “two-story” thing, with the control room and a cut-out space underneath. They keep adding sections in the cut-out space with the crane, then jacking the module up to the top of the new section, where they put a new, new section, jack the module up to the top of that, etc, etc.
Years back they were building a communications tower near where I was working. They brought in a crane and added a boom extension to increase the working height.
They found that the crane couldn’t raise the boom from the horizontal position, so they brought in another crane to give it a boost to about 30 degrees or so.
Same but in reverse for the disassembly.
I had a kid so I could tell him that they drive bigger and bigger trucks over the bridge until it collapses.
We all have our role models to aspire to.
Stole my point! 
I’ve seen cranes being used to build another crane, and the first one was either self-erecting or a hydraulic crane on a truck.
For a related engineering problem, the way you get certain kinds of heavy machinery inside a factory building is by laying down the floor, then building the machine in place, then putting up the rest of the building (I’ve also seen floor, then framework and cover, then machine, then walls).
I’ve seen this too, and I was amazed at how quickly they erected the crane. One of the workers explained that it’s a health issue: If the erection lasts for more than four hours, they have to call a doctor.
Don’t try to float the equipment out.
Depends on the project and type of equipment. If the item is self propelled, they will usually drive it out on an incline created for the purpose. Some equipment can really climb a steep grade. If the quarters are confined or the incline not possible because of further construction such as walls, a crane can lift it out. The day-rate on a crane of this nature is quite high. Therefore, you may rarely see it in action because it comes and goes back quickly like that truck you rent from Penske.