Designing D2: What Lies Beneath, Part 1

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.

About DART Daily

Dallas Area Rapid Transit (DART) gets you around 13 cities with rail, bus, paratransit, and rideshare services. We serve DFW International Airport and Fort Worth via the Trinity Railway Express (TRE). The service area consists of 13 cities: Addison, Carrollton, Cockrell Hill, Dallas, Farmers Branch, Garland, Glenn Heights, Highland Park, Irving, Plano, Richardson, Rowlett and University Park.
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4 Responses to Designing D2: What Lies Beneath, Part 1

  1. Randall Frost says:

    Unless a subway is way far more cost effective than the current above ground, if you are going to build a new line do it above the ground. Texans are used to have a view when driving, a subway is not for Texans. If you want Texans to give up driving and hop on a train, it has to be above ground. Just look at the current subway portion north of Dallas, it is boring.

    • Caden Adam says:

      The point of building underground is so that the rail lines don’t destroy the city in the way that the Interstate Highway Act destroyed many urban cores.

    • DART has it tough. They have to try and please lots of people….those who want a modern, durable solution that doesn’t “break up” and block parts of the city…and those that want the cheapest solution “with a view”. I don’t envy their task. As someone who rides every day, I know how important D2 is (along with platform expansion), and the advantages of the subway are clear to me.

      Thanks for the updates, DART!

  2. Dinah Waranch says:

    I am delighted with plans for a subway. It is the only far-sighted way to go in creating a truly urban environment for all as Dallas grows

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