Dave's ESL Cafe's Student Discussion Forums

[Chan-Seung Lee]

[asterix]

This is a pretty old construction that goes back at least to the Latin:
non solum....sed etiam = not only... but also.
Nowadays it is OK, apparently, to write it as in your example, with the semi-colon.
However, I think it is better English to include the but.
As well means the same thing as also, so that is OK.
I also think you need to include to be
Not only was the Erie Canal an engineering triumph, but it quickly proved to be a financial success as well.

Not only was the Erie Canal an engineering triumph, but also it quickly proved to be a financial success.

Not only was the Erie Canal an engineering triumph, but it quickly proved to be a financial success also.