shaking over the excavations, as it faces Colonel Shaw and his bell-cheeked N**** infantry
The shaking refers to the statehouse; it's presiding over this whole scene below. Maybe this is the tingling—an actual physical tremble from the construction going on below. We're certainly bracing ourselves. Aren't you?
Finally we meet members of the Union that we've been anticipating since we read the title! Colonel Shaw was in charge of the 54th all-black infantry in the Civil War.
The "N****" 54th infantry was the first of its kind in the Civil War; they entered the war in 1863.
Today the word "n****" sounds dated and even offensive, but at the time this poem was written (late 1950s), it was the word used to describe Africans and African Americans and was not considered derogatory.
What to make of "bell-cheeked"? Well, it's likely a decent description of anyone's cheek. You've got the rounded cheekbone as the top, or dome of the bell, and the cheek skirts out from there, much like a bell does. Does that description ring true enough for you? Get is? Bell? Ring? Ah, nevermind.
on St. Gaudens' shaking Civil War relief, propped by a plank splint against the garage's earthquake.
Note: Augustus Saint-Gaudens was an Irish-American sculptor who made the Civil War statue that Lowell is writing about.
It seems this memorial is shaking because it's butted up against this crazy construction that's going on, and that there's only a propped up plank to separate it from the dizzying construction.
It's worth pointing out that there isn't an actual earthquake in the garage, but that the construction is causing the area to shake like one.
Think about all these "shaking" "tingling" "earthquake" words the speaker has used so far to describe the scene. It's as if it might all fall apart at any second.