1.) Chiofaro's new underground garage would not hold utilities; not allowed because resiliency requires these be above ground.
2.) The cost per space for HT's underground garage was $100,000 plus. The cost per space is on the very high end, an outlier, because of: the poor soil; within the historic tidal flow zone, and hydrostatic pressure; number of levels and excavation depth; proximity to the CA/T tunnel; and columns supporting a tower would run smack through the garage.
3.) An underground Dock Square garage has some of the same issues, though not all to the same extent.
4.) Storm surge. See interactive map for 2050.
http://seachange.sasaki.com/
As inundation becomes more frequent, activating protective barriers for an underground garage means it will be increasingly inaccessible. (They'll have to do something about the CA/T ramps too.)
What happens when floodwaters reach an underground parking garage, in this instance, a building with about 500 spaces on two levels.
https://youtu.be/pHYfnzb8LoM
The Dock Square developers have cut the number of spaces from 700 to 450. Now only a third of Chiofaro's 1400. The gsf of new building is about 450,000?, a little more than a third of the gsf that Chiofaro said he needed to pay for the cost of burying his garage.
The only deep garage constructed along the Greenway is the one at the Intercontinental, and that was dug by the Big Dig, because it wraps around a vent shaft.