Bypass Tunnel Options

3 Sep 2009 by admin, Comments Off on Bypass Tunnel Options

The City of Austin Watershed Protection Department has been busy evaluating the Bypass Tunnel (BT) repair/construction options. So far around $100k has been spent researching different options to prevent continued deterioration and extend the life of or replace the thirty-four year old structure. The original assessment included three options, namely repair, modification, and complete replacement of the BT.  The three main categories have been expanded with sub-options, namely 1., 1a., 1b (Stabilization), 2, 2a (Pipe-in-a-Pipe), and 3 (complete replacement), with the addition of two options, namely Piers and Other Options.  All of these options were presented to the Environmental Board on August 19 by Mr. David Johns of the COA Watershed Protection Department.  He was so kind as to furnish the slides depicting the various cross sections of the various options.  These slides can be found on our website.  As you review the slides, keep in mind that they generally are arranged from the least expensive to the most expensive.  At this point, everyone has their own opinion as to the best option, depending on which considerations are valued at that moment, considerations being pool closure time, cost, flow capacity, endangered species, longevity, site disturbance, and constructability.

Option 1, the least expensive, least intrusive, and probably the least durable, is Stabilization.  It adds a floor to the existing structure to stop the leaks in the bottom, add strength and weight, as well as side slope control.  Option 1a lines the inside of the BT and includes poolside and side slope stability components.  Option 1b adds even more ballast inside the BT and replaces the lost capacity of the BT with a sidewalk which doubles as an overflow ditch during floods, with a wall on the poolside.  The advantage of options 1 is the cost.  If funding is limited, better this than nothing.  The minus is that you continue to maintain an older structure with unpredictable problems in the future, failing joints, new holes, who knows.

Picture 1

Option 2 is a Pipe-in-a-Pipe, and 2a, with a liner, additional concrete on the walkway for ballast and durability, and limestone blocks on the pool side.  The plus of Pipe-in-a-Pipe is you keep and strengthen the old BT and wouldn’t have to rip out the old structure. A minus is the reduced capacity of the BT which means more pool closings in the long run.

Option 3, complete replacement is the most expensive, would enlarge the capacity of the BT which would reduce pool closings, could incorporate new entrance grates and dam modifications.

Option 5 is the placement of Piers to stabilize the structure.  As a geologist this option is not recommended.  Anytime you drill in an active spring area, any hole you drill runs the risk of becoming a new Barton Springs, with water bubbling up, eroding the drill hole quickly and changing the whole underground water pathway.  The same thing applies to Other Options, diverting water around the pool with a trench or tunnel.  As soon as you hit a hydrogeologically active part of the formation with your trench or tunnel, you will have a new spring, and the other springs could dry up.  Sticking with the original footprint of the BT minimizes that risk.

-Gary Beyer, Vice President, FBSP

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