BIKE: In Trench Construction for Austin

[alan_drake]

Let me make clear that one of the appeals of the Copenhagen system is it’s flexability, depending upon what was best for each segment of the route.  In subway, in trench, on old RR ROW, taking up 2 lanes of freeway and two means of elevation (embankment – cheap, viaduct - more expensive).  I would like to enhance this flexibility by using vehicles capable of either OCS or 3rd rail operation.

For much of the old RR ROW Leander-Austin, I think non-grade separated, at grade rail on ballast construction is quite appropriate.  Rail on ballast is the cheapest to build (~$200/ft/track) and cheapest to maintain.  Copenhagen’s numerator grew much less than the denominator when they built 2 km on old RR ROW (and reused a 1930 bridge over a small canal).  Likewise the planned taking of two freeway lanes to the airport/rail terminal.  Just lay ties on concrete and affix rails with a concrete barrier to keep the cars out.

The low volume suburban crossings can be appropriately handled at grade.  Lamar @Airport is another issue (and there may be a few more as well).  I think the investment in a bridge (probably less than $15 million, that # came from a proposed streetcar bridge in New Orleans, with more difficult soil conditions) is worth it.

In the case of Austin, caliche limestone is, IMO, ideal for “in trench” construction.  It is soft enough to dig easily but strong enough to maintain an unreinforced side wall (per my understanding).  One can contact an engineer at the Water & Sewage Dept. for good cost estimates, since they have already done similar work throughout the city.

I will note some more on costs below.
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In Trench, How it could be done

Sacrifice a parking lane (or even a traffic lane) on, say, Trinity or San Jacinto.  Dig a trench (easy in caliche) for most of the way (tunnel if needed for a block or two with the hills).  Put a concrete barrier on the auto side of the trench and a fence (decorative or simple) on the other side.

The rail vehicle will be on 3rd rail so overhead clearances are minimal.  When passing under a cross street, go deep enough to clear a short (3.x m long) bridge.  About 4 m deep with the thickness of the bridge included.  If possible, block some cross streets and avoid the cost of a bridge and allow a shallow trench, saving money.  

Driveways will also, unfortunately, require a bridge as well.   However, from dim memory, there are few such on San Jacinto and Trinity from 5th Street to MLK.

Have a 7% or so rise between cross streets to allow a shallower trench.  Stops could be mid-block with loading platforms a meter or two below grade.  It is energy efficient to brake going uphill, and accelerate going downhill which this profile would do.  With a blocked cross street, an at grade station could be put on the former street.  Likewise for east side Trinity @ Waterloo Park.

Phase II would be single track with at least one passing section at a station.

Later, a second CBD-UT trench could be dug for dual track use.  Either a San Jacinto/Trinity pair or a San Jacinto/Lavaca type pair.

Integrating stations into existing development can be problematic in some cases.  However, artful selection of station locations can avoid stops next to buildings right on the sidewalk without an offset.

Digging a ~1.5 mile long trench with ~18 short bridges in Austin’s caliche limestone would not be difficult or cost too much. Water & Sewage has done comparable work already.  This could continue (per my Phase II) an electrified Red Line by the State Complex and into UT.  Precise routing (especially within UT) could be “interesting”, but there would be a full toolbox of options with tight turning radius’s to plan with for a next phase.

>From distant memory, “In trench” might work on Red River, but not closer in on campus.  A combination of Viaduct & Embankment elevation (if acceptable) would be probably be cheaper than in subway for parts of UT.

Extensions further north could go at grade but grade separated on the edge of the School for the Blind, etc.  

Just some rough ideas, subject to refinement.
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There are SO many costs associated with any project that it is hard to judge what that project would cost elsewhere.  But one major cost is consulting costs.  Often times, the hard costs (Civil, vehicles, electrification, land acquisition) are less than half the total cost of a US project.  Not so in the EU or Japan (who, IMHO, typically engineer their systems better).  But this is the price for FTA funding :-( and “The American Way”.

The Danes did, IMO, a superb and practical job.  Graffiti is controlled by planting ivy on elevated structures.  They standardized their stations (21 m x 60 m) and just dug holes straight down for the subway stations, with mirrors to direct skylight down to the subway platforms.  Instead of prefab (my initial choice) they cast viaducts in place (cheaper in their situation).  They adapted to what they had and searched for the best, most cost effective choice.  Sometimes this meant “stealing” from automobiles i.e. taking freeway lanes and converting sidestreets in Historic Copenhagen into subway stations.

Alan

It may be that a 1.5 mile TBM drive with a 1/2 mile at grade (grade seperated) may be the "best" solution north of UT.  Copenhagen needs just 4.9 m ID.  Grade seperation goes faster which reduces costs and increases ridership.

*OR* it may be that the slower, cheaper at grade plan proposed in 2000 is a better cost/benefit solution.  Will the community accept elevated sections north of UT ?  I do not know but somewhat doubt that.

However, averaging ~1.5 miles in trench, ~1.5 miles in subway and 1/2 mile at grade should be affordable, especially with mainly single track with passing sidings.   Just keep the consulting costs under control !