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Old Posted Nov 16, 2020, 4:18 PM
Novacek Novacek is online now
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Originally Posted by H2O View Post
Me 3. I didn't want to be a naysayer before the election, but I am still a little skeptical about having two river crossings within the budget. Once the engineering details get worked out, I think the crossings will be the most obvious hit in Value Engineering. For instance, I have some trouble imagining bridge crossings if the goal is to enter a tunnel before 4th Street.

Bridges would have to be high enough to be out of the flood plain and allow the hike and bike trail to pass underneath. The trail near Trinity is at a higher elevation to go around the boat house and Waller Creek Tunnel. The tracks would have to cross Cesar Chavez at the surface and descend into a tunnel before 4th Street. Light rail trains cannot exceed about 6% grade, so the tunnel portal has to be more than 1 block long just to get into a shallow cut and cover tunnel. Cap Metro has been selling the idea that the downtown underground stations will be in deep tunnels, with an extensive amenity rich mezzanine level. I think that means that at least 2nd and 3rd would have to be cut off by the tunnel portals. I think that will be unacceptable to the City of Austin and the public. Cutting Cesar Chavez off with a tunnel portal would be a fatal flaw, in my opinion.

For these reasons, I am predicting deep tunnels under Lady Bird Lake, which also means that the tunnels will need to extend a significant distance north and south before they can come up to grade because they will be fighting against the surface topography which rises fairly steeply either side of the river.

I remember back in 2000, when some of us were advocating for tunnels, the consultant from Parsons Brinkerhoff did a quick estimate that tunnel portals would need to be between 1 and 2 miles long.

For this reason, I think we might end up with a single river crossing shared by both the Blue and Orange lines in the first phase, with provisions to add a second crossing in the future when there is higher ridership and the gold line is upgraded to light rail.

There is also the issue of turning 90 degrees in the tunnels. Light rail vehicles can barely make a 90 degree turn within our 80 foot ROW widths. They have to be travelling at the lowest speed possible, and will create a lot of vibration and noise. Fortunately, you want the trains to be traveling slowly close to a station. However, stations cannot be located on a curve and have to be about half a train length down a straight tangent from any curve. That means there will be an interest in clipping the corners of properties. This would be a bigger issue at the surface, but even in a tunnel, they will need to thread between the foundations of existing and future buildings at corners.

Knowing all of this, it makes me chuckle when I see people question why it takes so long in the design and engineering phases before construction can even start. I think the next couple years are going to be fascinating for transportation geeks between Project Connect and the rebuilding of I-35.
I think you're seriously discounting the topography difference of downtown from the lake. I'd expect (and what seems to be the proposal) is a bridge crossing of the lake, and then entering the tunnel south of Cesar Chavez.

The nominal elevation of Town Lake is 428 ft. The elevation of CC/Trinity seems to be ~488 ft.

It's a little hard to find a specific flood elevation for town lake. The document linked here

Seems to suggest a rise to 439 feet at Longhorn dam during a 100 year event.

That's quite a bit of elevation to work with.

Honestly, if improving or replacing Longhorn dam is needed to avoid a tunnel crossing of the lake, I'd expect that to be on the table for Prop A funds (and an appropriate use).

At your 6% grade, then the 1000 feet from CC to 4th is enough to be 60 ft deeper, enough for a mezzanine level (under the park?) above the tracks.
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