Rail revival challenge

January 2014 » Features » PROJECT CASE STUDY
Bridge design and construction methods complete double-tracking project under budget and ahead of schedule to improve rail passenger service.
John Eschenbach
The Carlsbad Double Track and Bridge Project eliminated bottlenecks and provided additional passenger travel options on Amtrak and the North County Transit District's Coaster commuter trains.

Across the United States, class one railroads are experiencing a revival. Energy-efficient and sleek trains now replace fuel-guzzling and blocky ones. Trains are again contenders in the mass transit game, competing for both passengers and freight. In Fiscal Year 2013, Amtrak carried a record 31.6 million passengers. Ridership has increased more than 50 percent since 2000 and Amtrak expects the growth trend to continue.

Project
Carlsbad Double Track and Bridge Project, Carlsbad, Calif.

Civil engineer

J.L. Patterson & Associates

Project focus

Extending an existing second main track 1.9 miles eliminated a significant bottleneck to passenger and freight service in the Los Angeles-to-San Diego rail corridor.

One of the major challenges is rail infrastructure. The class one railroads of yore cannot handle the soaring ridership and increase in freight demands by themselves. Demand exceeds capacity, forcing bottlenecks and delays in rail traffic. The solution for many railroads is easy to understand, but hard to execute: Add a second track. In some cases, add a third or fourth track.

The state of California is a representative case study. The Los Angeles-to-San Diego (LOSSAN) rail corridor is Amtrak's second busiest rail corridor in the United States. As such, the LOSSAN rail corridor is the target of a multimillion-dollar upgrade project designed to improve the capacity and reliability of on-time passenger and freight train services. The first phase is in Carlsbad, Calif., called the Carlsbad Double Track and Bridge Project.

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 on each end of 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.

Challenge 1: Spanning the Agua Hedionda Lagoon
Projects of this scale and magnitude face many complex engineering challenges. At Carlsbad, the major challenge is spanning water, specifically the Agua Hedionda Lagoon. The Carlsbad Double Track and Bridge Project is not the first to span the Agua Hedionda Lagoon.

  • 1881 – The first railroad bridge across the lagoon was the California Southern Railway, a subsidiary of Atchison, Topeka & Santa Fe (AT&SF) Railway. The bridge was a classic trestle – a series of short spans across a number of timber bents – and reflected the early trend of railroading. The trestle had 14 spans with an open bridge deck, where the rail track fastened directly to ties on the spans. The trestle support posts were pile-driven into the ground. In short, the bridge was a 14-span ballasteddeck pile trestle bridge.
  • 1948 – AT&SF field records indicate a third timber trestle in a new alignment.
  • 1958 – The City of Carlsbad added 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) built a three-span concrete bridge on a new alignment west of the 1948 timber trestle, meant to replace the trestle. NCTD demolished the timber trestle to accommodate a future second main track or "double track."
  • 2009 – Amtrak issued a design contract for the Carlsbad Double Track and Bridge Project.
  • 2010 – Amtrak issued a construction contract for the Carlsbad Double Track and Bridge Project.
  • 2012 – The Carlsbad Double Track and Bridge Project finished and both main tracks were opened 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 (2006) has spans made from standard concrete bulb-tee girders. The girders, four per span, support a ballast bridge deck, where the train track 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 ballast retainers.

The structural bridge designer on the Carlsbad Double Track and Bridge Project made a unique design improvement. He noticed that two components of the 2006 concrete bridge perform the same function. He decided to eliminate the separate bridge deck altogether from the new concrete bridge. He redesigned the standard bulb-tee girders, widening and thickening the top flanges and web. The redesigned bulb-tee 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 highlighted the Carlsbad Double Track and Bridge Project, saving about $750,000 of 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 2006 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 2006 bridge team made ground modifications prior to drilling.

However, the modified ground work still caused a number of stability changes, 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.

The Carlsbad Double Track and Bridge Project designer had to determine a safe way to install the substructure piles adjacent to the first new concrete structure. The designer had to consider various pile installation methods. From history, he knew that the drilling method, even with ground modifications, does not work. From the geotechnical evaluation, he knew 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, decided to use the oscillating method.

For added stability and strength, the designer used 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. 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, 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 places the tremie concrete in the CISS steel casings.

Prior to the pile installation, temporary sheet shoring protected the 2006 concrete bridge and track from collapse, rail settlement, and other disruptions. The protective measure allowed train service to continue amid construction of the new intermediate piers and abutments.

Two cranes, at either end of the bridge, set the redesigned girders onto the new intermediate piers and abutments. The bridge deck (the top flanges of the redesigned girders) were waterproofed and the bridge deck was flooded with ballast. The concrete ties were installed and rail fastened in place.

Challenge 2: Maintaining existing train service
The design of the train track itself was an ordinary fare. The challenge was developing a new track alignment while maintaining existing train service. The alignment had 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.

The universal crossover does not make use of the existing turnout or switch, relying instead on four newly installed turnouts. Train service could continue on the existing turnout.

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 track work that passing trains would otherwise disrupt as long as certain conditions are met. The Carlsbad Double Track and Bridge Project secured Form B protection, which allowed construction to occur between trains with the use of flags for protection. Form B protection proved to be the most cost effective for the majority of work.

Challenge 3: Considering the total system
The new track passes over three existing grade crossings: Tamarack Avenue, Cannon Road, and a private power plant grade crossing. They all required new crossing warning devices. The new universal crossover required new control point signals, intermediate signals, and switch machines.

The footprint of the new track required 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.

Project outcome and future trends
The purpose of the Carlsbad Double Track and Bridge Project was to improve the capacity and reliability of on-time passenger train service along the Carlsbad portion of the LOSSAN corridor. A recent Amtrak decision is a testament to the success of the project. Before the project, only NCTD's Coaster commuter trains stopped along the Carlsbad portion of the LOSSAN corridor. After the project and effective on Oct. 7, 2013, Amtrak added its service, with four new station stops between Oceanside and San Diego. Amtrak's Pacific Surfliner trains are servicing these stops, with three northbound trains and three southbound trains every day. The addition of Amtrak gives passengers another travel option in the Carlsbad area. NCTD still runs its Coaster commuter train.

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.

Use of redesigned bulb-tee girders eliminated the need for an intermediate concrete bridge deck and saved about $750,000 in construction costs and time.

Projects such as the one at Carlsbad are happening all over the United States. Adding tracks has proven to be a winning solution in most bottlenecking problems. Most bottlenecking occurs because trains have to stop and wait for other trains passing on the same track. An additional track allows trains to continue moving past each other and increases train route options. Some markets are already past double tracking and into triple and quadruple tracking, such as Philadelphia to Baltimore and Washington, D.C. to Baltimore area.

The interrelationship between passenger and freight train services motivates both industries to invest in railroad infrastructure upgrades. Passenger and freight trains compete for precious track time with many passenger trains running along existing freight lines. Passenger train delays contribute to freight train delays and vice versa.

The opposite is also true. Passenger train efficiencies contribute to freight train efficiencies and vice versa. Thus, both the passenger and freight train industries have been investing in railroad infrastructure upgrades, specifically to train tracks, bridges, and tunnels. In fact, both industries combined have invested more than $75 billion since 2009. The upgraded infrastructure, in turn, improves the capacity and reliability of on-time service to both passengers and freight.

The Carlsbad Double Track and Bridge Project has a specific performance purpose, which has been achieved. The project was also under budget and ahead of schedule. However, a less touted fact that has particular relevance is that the project had no injuries or accidents. The push for performance metrics becomes moot if it comes at a human cost. The Carlsbad team was dedicated and committed to the project, and safety was a concept they recognized and emphasized. Safety is an aspect of effective project and construction management. Going forward, other projects would benefit from continuing this emphasis on safety.

John Eschenbach is a senior project manager at J.L. Patterson & Associates, a Southern California-based trackwork engineering firm that spearheaded the inventive engineering of the Carlsbad Double Track. He has more than 37 years of experience in the design, maintenance, and construction management of class I railroads. He can be reached by email at jeschenbach@jlpatterson.com.


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