Designing D2 What Lies Beneath, Part 1

Posted on Feb 1, 2017 by dartdallas
Journeying through D2’s project development process a few months ago, DART heard from the public that the mostly aboveground light rail route laid out in the original Locally Preferred Alternative (LPA) was not the best fit for Dallas or the region.  DART listened, and now we will be building the new route as a subway.  But what does it take to design one? Building a subway is no easy feat. Subways can offer benefits including: longevity (it’s a system that will last for 100 years), non-interference with traffic patterns above-ground, and less of a visual disturbance. However, there are significant challenges. Here are some of the elements that are out-of-sight but not out of mind to DART engineers as they think about the feasibility and design of a subway.
  • Soil and Natural Elements 
Texas is big, with beautiful plains, mountains, and coastal areas, and four physical geographic areas. We are home to the North Central Plains. Our geology is different from other regions in Texas, composed of clay soils and bedrock. The top layer of clay is made up of silt, sand and organic material. This layer tends to be weak and creates construction challenges when building a tunnel. The next layer is a bedrock layer, Austin Chalk (limestone). This limestone formation is generally about 25-40 feet thick and is ideal for tunnel construction. It’s easier to mine. The next layer of bedrock is Eagle Ford shale. This layer of bedrock is very weak and is prone to swelling upon exposure to wetting or water conditions, so it has its own challenges for tunneling. The feasibility of tunneling in these layers of clay and bedrock is a primary concern. The Austin Chalk layer provides for ideal tunneling construction conditions while the other layers of clays and Eagle Ford shale offer poor conditions of tunneling. The Trinity River basin has alluvial soils and fragile sediment which is very weak material and causes serious challenges to tunneling. In downtown Dallas, it is generally good tunneling material east of Lamar Street and poor tunneling material to the west where the original Trinity River meanders used to be. Unlike other underground systems in New York or Washington, where the bedrock is granite or other strong bedrock, Texas geology is very different and will play a factor in determining how DART will build the D2 subway. Making sure we consider every option and gain input from others before settling on a route is critical. Information from stakeholders, agency staff, public officials, and the public combined with DART’s technical expertise, we know that we will find the best route for D2 that is the most feasible, affordable, and reasonable. We’ll examine other surface and subterranean challenges we’re looking at in an upcoming post. Thanks for reading.
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