Sinclair Black has been talking about this for years...Decision Makers are not listening.
The City of Austin needs to get off the pot and DECIDE what needs to be done and STRONGLY repeat this message...over and...over...again.
The following link is from 2008...seven years ago and lists this idea as "Sinking I-35 (not executed)". The idea is much older.
http://www.austinchronicle.com/news/2008-03-28/606169/