alt Hacker NewsSo many questions ... which probably have been asked on prior HN threads ...
I wonder why 800 feet underground: Is that necessary to pass beneath all other infrastructure (to prevent flooding it?)? Remain beneath waterline to create negative pressure and reduce leaking? ?
Also, what is the general mathematical relationship between depth, rock pressure / weight, and energy required to drill? That is, what is the proportion of energy required to drill beneath 800 feet of material compared to drilling beneath 400 feet?
...
Replies
The depth allows it to be drilled through bedrock, which avoids a bunch of complications on an already complicated project.
This thing will probably be operating hundreds of years from now. What a project.
I don't know about New York in particular, but Chicago water engineering seems a related topic.
Here you do deep tunnels to avoid the surface, in ways another poster said; everything is easier when nothing is in the way.
For the mathematical difference, 400 feet below sea level and 800 feet below are almost exactly the same: difficulties are water getting in to your pit, but the machines that work on rock, work on rock at the same speed regardless of depth, so the difference between 400 feet and 800 feet is best described as 400 feet difference. A big issue here is that they do not drill; they hammer. Pounding base pylons into bedrock causes dramatic rhythms in the surrounding 500m, but that's to deal with the bedrock, not depth.
It's a 60 mile long tunnel and in order for water to flow through it, you need either pumps or a downhill gradient.
I'd guess the reason for the 800 ft is because the reservoir it'll draw from is near sea level.
Those are… actually some very good questions.
> Also, what is the general mathematical relationship between depth, rock pressure / weight, and energy required to drill?
There isn't any. It completely depends on the local geology.
Liquids are easy because there are no lateral load transfers, and the structures have to bear the weight of the entire water column above them. But with soil you get lateral load transfer, so the pressure on the tunnel is not easily relatable to its depth.
That's also why you can have mines that are kilometers deep, yet with tunnels held by wooden beams.
> energy required to drill
That depends on the rock type. In london, most things are clay, so not actually that solid (ie it needs shoring up immediately, and will collapse without supports, hence the travelling shield)
manhattan schist appears to be reasonably hard (not granite, but also no clay)