Tuesday, April 15, 2014

Overcoming obstacles: The Carlsbad Double Track and Bridge Project

 

Long girders over the Agua Hedionda Lagoon.
An illustration of the modified bulb tee girders.
Passing trains meet at Agua Hedionda Lagoon.
Long girders over the Agua Hedionda Lagoon.
by John P. Eschenbach, senior project manager at J.L. Patterson & Associates

Creating capacity within an existing infrastructure footprint can be a tricky task, but one the Carlsbad Double Track and Bridge Project succeeds in doing.

When commercial airfare first emerged as an option for long distance travel, it quickly became the most popular choice of travel for Americans. Yet as fuel costs – and, in turn, ticket prices – continued to rise along with airport security, railroad operators seized the opportunity to return to the mass transit game. Travel by commuter rail is experiencing a unique resurgence across the United States, as trains are now more energy efficient than ever before. Furthermore, passenger rail has emerged as a convenient and cost-effective travel alternative, allowing passengers to sidestep many of the headaches involved with airport travel.

Train ridership has doubled since 2000 and, in 2013, Amtrak broke its own record for the number of passengers carried in a single calendar year with more than 31.6 million passengers. While mass transit is shifting in favor of the railroads, much of the country's existing rail infrastructure is inadequate to handle the influx in both ridership and freight demands. Fortunately, there is a solution, albeit one that is often more challenging to execute than it is to conceptualize: Laying down additional track on pre-existing rail lines to increase routing options.

Usually one additional track is sufficient, but in some cases a third or even a fourth track may be necessary. The passenger and freight train industries have invested more than $75 billion since 2009 in upgrading railroad infrastructure, largely through the addition of new track. This task may seem straightforward, but it can pose logistical nightmares without the proper planning or use of technology. One case study indicative of how such a project can be successfully undertaken can be found in the state of California's Los Angeles-San Diego-San Luis Obispo (LOSSAN) Rail Corridor. LOSSAN is Amtrak's second busiest rail corridor in the U.S., making it the target of a multimillion dollar upgrade project designed to improve the capacity and reliability of on-time performance for passenger and freight train services. The first phase, called the Carlsbad Double Track and Bridge Project, is located in Carlsbad, Calif.

Spanning the Agua Hedionda Lagoon

Projects of this scale and magnitude face a multitude of complex engineering challenges. At Carlsbad, the biggest challenge is spanning water, specifically the Agua Hedionda Lagoon. The Carlsbad Double Track and Bridge Project is not the first to span this body of water. Below is a brief history of other expansions involving the bridge:

  • 1881: The first railroad bridge across the lagoon is the California Southern Railway, a subsidiary of Atchison, Topeka & Santa Fe Railway (AT&SF). The bridge is a classic trestle, a series of short spans across a number of timber bents and reflects the early trend of railroading. The trestle has 14 spans with a ballasted deck, where the rail fastens directly to ties on the spans. The trestle timber bents are six pile bents driven into the ground. In short, the bridge is a 14-span timber pile trestle bridge.
  • 1948: AT&SF field records indicate a third timber trestle in a new alignment.
  • 1958: The city of Carlsbad adds a 12-inch sanitary sewer timber pile trestle bridge across the creek adjacent to the timber trestle on the inland side.
  • 2006: North County Transit District (NCTD) builds a three-span concrete bridge on a new alignment west of the 1948 timber trestle, meant to replace the trestle. They demolish the timber trestle to accommodate a future second main track or double track.
  • 2009: Amtrak issues a design contract for the Carlsbad Double Track and Bridge Project.
  • 2010: Amtrak issues a construction contract for the Carlsbad Double Track and Bridge Project.
  • 2012: The Carlsbad Double Track and Bridge Project finishes and both main tracks are open for revenue-generating train service.

The two main tracks stand side by side across the Agua Hedionda Lagoon, exhibiting the next evolution of bridge design.

Developing the girders

The first concrete bridge constructed in 2006 has spans made from standard concrete bulb tee girders, which are beams that look like elongated capital "I" letter. The girders support a ballast bridge deck, where the rail fastens to ties that are in a bed of stable ballast material on top of the bridge deck. The outermost girders on both sides support the ballast retainers.

The structural bridge designer on the Carlsbad Double Track and Bridge Project makes a unique design improvement. He notices that two different components of the 2006 concrete bridge perform the same function, allowing him the option of eliminating one. He decides to eliminate the separate bridge deck altogether from the new concrete bridge. He redesigns the standard bulb tee girders, widening and thickening the top flanges and web. The redesigned girders now support the ballast directly, eliminating the need for an intermediate concrete bridge deck.

As before, the outermost girders on both sides support the ballast retainers. The short girders have the ballast retainers precast and the long girders have the ballast retainers cast in place after setting.

The redesigned bulb tee girders are the highlight of the Carlsbad Double Track and Bridge Project, which saved approximately $750,000 in construction costs and two months of construction time. The savings are the estimated construction costs and time of building a separate bridge deck. As a result, the overall project was completed under budget and ahead of schedule.

Developing the foundation piles

The concrete bridge has conventional cast-in-drilled-hole (CIDH) pile supports, which are concrete piles cast in holes drilled to predetermined depths. Whenever drilling in any scenario, the surrounding ground moves and the ground level shifts to some degree. The ground level also shifts naturally from soil movement or volume changes. The bridge structure and train track may displace, if the ground level shifts too dramatically. To mitigate this risk, the bridge team made ground modifications prior to drilling.

However, the modified drilling still caused a number of stability changes to the environment, most notably ground movement and soil liquefaction. In turn, the track on the adjacent 1948 timber trestle displaced (which the Carlsbad Double Track and Bridge Project has now replaced). To further complicate matters, the geotechnical evaluation concluded that the loose soil in the area could potentially liquefy below the water table, which is the imaginary line separating ground saturated with water and unsaturated ground. The soil, sediment and rocks are full of water below the water table.
The Carlsbad Double Track and Bridge Project designer has to determine a safe way to install the substructure piles adjacent to the first new concrete structure. The designer has to consider various pile installation methods. From history, he knows that the drilling method, even with ground modifications, does not work. From the geotechnical evaluation, he knows that the driving, boring and vibrating methods would not work because they cause high water table and the risk of more soil liquefaction. The designer, through process of elimination, decides to use the oscillating method.

For added stability and strength, the designer uses cast-in-steel-shell (CISS) piles instead of CIDH piles. CISS piles are steel casings with epoxy coated steel rebar cages and filled with concrete inside of each. Each intermediate pier uses two larger diameter CISS piles, while each abutment uses two smaller diameter CISS piles. To stabilize the earth, the designer used auger cast concrete piles and installed seven days before the CISS steel casings. The casing installation process does not cause or encounter any significant ground movement or soil liquefaction. After installation of the rebar cages, a concrete pump truck fills all of the CISS steel tubes with concrete.

Prior to the pile installation, temporary sheet shoring protects the 2006 concrete bridge and track from collapse, rail settlement and other disruptions, allowing train service to continue during construction.

Maintaining existing train service

The design of the train track itself is an ordinary fare. The challenge is developing a new track alignment while maintaining existing train service. The alignment would need to accommodate a second main track, a second main bridge and a universal crossover control point that allows trains to switch tracks when traveling in either direction.

Train service continued on the existing No. 20 turnout at CP Farr until the new No. 24 universal crossovers were installed at New CP Farr.
The railroad at Carlsbad operates according to the General Code of Operating Rules (GCOR), a compilation of operating rules for some railroads in the United States. The GCOR allow crews to perform trackwork that passing trains would otherwise disrupt, as long as certain conditions are met. The Carlsbad Double Track and Bridge Project secures Form B protection, which allows construction to occur between trains with the use of flags and an Employee In Charge for protection.

Considering the total system

The new track passes over three existing grade crossings: Tamarack Avenue, Cannon Road and a private power plant grade crossing, which all required new crossing warning devices.

The new universal crossover requires new control point signals, intermediate signals and switch machines. The footprint of the new track requires a retaining wall. The first segment of the wall is a post and panel design and the second segment is an environmental concrete block design.

Evaluating project outcome, future trends

The purpose of the Carlsbad Double Track and Bridge Project is to improve the capacity and reliability of on-time passenger train service along the Carlsbad portion of the LOSSAN corridor. The problematic bottleneck was the single main train track at either end of the Carlsbad Poinsettia Station. The solution was to create two main tracks, each 5.1 miles long, by extending an existing second main track 1.9 miles.

Recently, Amtrak made an announcement that indicates the high success level of this project. Before the project, only NCTD's COASTER commuter trains stopped along the Carlsbad portion of the LOSSAN corridor. After the project, effective October 7, 2013, Amtrak has added the service of its Pacific Surfliner trainers, with four new station stops between Oceanside and San Diego.

The San Diego Association of Governments (SANDAG) has 19 rail projects on the southern tip of the LOSSAN corridor under development, with 15 of them already funded. The area extends from the Orange County/San Diego County line to the City of San Diego.

Projects such as the one at Carlsbad are happening all over the U.S., with some markets already moving into triple and quadruple tracking. Adding tracks has proven to be a winning solution in most bottlenecking problems, as it enables trains to continue moving past each other and increases train route options.

The Carlsbad Double Track and Bridge Project achieved its specific performance purpose, under budget and ahead of schedule. The enhanced LOSSAN corridor will also help facilitate high-speed trains, the next generation of track transportation. In addition to its notable feats of railroad engineering, this project is also praiseworthy due to the fact that it had no injuries or accidents. The Carlsbad team was committed to ensuring the safety of the project. This dedication to safety is one of the most important qualities of effective project and construction management and one that must continue to be valued highly as the current rail industry renaissance looks to continue unabated.

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