Denver's Engle P3 "Flies" Reinforced Earth Walls

Denver's Engle P3 "Flies" Reinforced Earth Walls

Reinforced Earth MSE wall with 5X10 facing panels.
Reinforced Earth Product in Field
Denver, CO
Denver Transit Partners
Fluor Enterprises, Uberior Investments & Laing Investments
Pacheco Concrete Products

Eagle P3 (East ­and ­Gold Line Enterprise Public-Private Partnership) project is part of the Denver Regional Transportation District's (RTD) FasTracks program, a voter-approved, 12-year, multi-billion-dollar transit expansion program that is building new commuter and light rail services throughout the RTD service area. The Eagle P3 Project is being delivered and operated under a concession agreement that RTD has entered into with a "concessionaire" that has been selected through a competitive proposal process. The selected concessionaire is known as Denver Transit Partners (DTP), a design-build-operate-maintain (DBOM) joint venture owned by Fluor Enterprises, Uberior Investments and Laing Investments. Other leading firms involved in the team include Ames Construction, Balfour Beatty Rail, Fluor/HDR Global Design Consultants, PBS&J, Parsons Brinckerhoff, and others. 

The Eagle P3 project is not simply about designing and building a commuter rail line from Wheat Ridge, west of Denver, through Union Station in downtown, and on to Denver International Airport (DIA), plus a 2-mile segment of the Northwest Electrified Rail Corridor, a maintenance facility and more. Eagle P3 also requires an investment into the project of several hundred million dollars by the DBOM partners, followed by their responsibility – for nearly 30 years – to operate and maintaining the system they are building. That is a pretty powerful incentive to build it well and build it right and a good reason to select Reinforced Earth walls from RECo – The Reinforced Earth Company.

RECo's work on Eagle P3 included almost 510,000 sq. ft. of Reinforced Earth MSE walls using the company's 5 ft. x 10 ft. rectangular precast panels (Fig. 1), supplied by Pacheco Concrete Products, Commerce City, with four different architectural finishes defined by RTD and DIA. Many walls are back-to-back, meaning the rail line is on a raised retained fill contained between two Reinforced Earth walls. In this configuration, because the total structure width is narrow, typically defined by the right-of-way needed for the two tracks, the reinforcing strips of the two back-to-back walls may meet or overlap in the center of the backfill between the walls (often the case with back-to-back designs). Viewed in cross section, the top layers of strips angle downward to go beneath a duct bank running down the centerline between the tracks.

Not all the Reinforced Earth walls are back-to-back; some simply retain fill needed to provide level grade for the trains. But for all the walls, a thorough geotechnical report, carefully developed design details and well thought out specifications governed the work. For instance, reinforcing strips are generally longer than the normal 0.7H (70% of the wall height) in compliance with AREMA (American Railway Engineering and Maintenance-of-Way Association) requirements to design for the railroad surcharge on top of walls. The resulting extra length of the reinforcing strips increases their pullout resistance to accommodate this added load. Drainage flow was carefully designed, with inlets collecting runoff on top of the retained fills and box culverts allowing existing at-grade drainage to pass through the Reinforced Earth walls and continue on its natural course. At each box culvert, vertical slip joint panels created a flexible joint between the Reinforced Earth soil structure and the more rigid concrete culvert, allowing the structures to behave independently without damage to either one. And each Overhead Contact (power) System pole, spaced approximately every 158 ft., had a special RECo-designed foundation (Fig. 2) integrated into the tops of the walls.

One thing the specifications did not have to address, however, was stray current corrosion of the MSE reinforcements. Multiple in-service structures throughout the U.S. and around the world, along with several expert studies, have clearly documented that hot dipped galvanized steel strip MSE wall reinforcements do not collect stray currents the way pipelines and other long, electrically-continuous structures do. Stray current protective features are not needed for RECo MSE walls and the Denver RTD constructs all its Reinforced Earth walls to reflect that conclusion.

Creative contractors are always looking for ways to build a project faster and for less cost, including changing the design and/or the means and methods of construction.  So DTP was happy to receive RECo's suggestion of a way to eliminate some cast-in-place structures, especially those requiring form liners to achieve the project architectural requirements. Some structures retaining cuts were replaced by top-down-constructed soil nail walls, eliminating the need for temporary excavation support during construction. Reinforced Earth rectangular facing panels – like those on RECo's MSE walls – were cast with protruding anchors on their back side, erected a small distance in front of the soil nail wall with its protruding nail heads, and the space between the walls was filled with concrete. The result was a less costly permanent retaining wall with the desired architectural facing. Figure 3 shows some of the rectangular panels on walls forming the entrance to a box-culvert-type bridge near the DIA end of the East Line; note the custom-designed panel finish, called "Wing Tip", created by the airport for walls on their property.

A tangent pile wall – a different type of top-down retaining structure – was constructed at another location where permanent support of an excavated slope was required. Multiple reinforced concrete piles were cast in place in predrilled holes such that the piles were flush against each other along the line of the wall. RECo proposed casting a grade beam across the tops of the piles and embedding anchoring devices to which the plain-finish full height facing panels would later be connected. It was simple to cast the grade beam on grade before the soil in front of the piles was excavated. Following this excavation, the panels were erected and connected to the anchors in the grade beam. The top of the wall was finished by pouring a combination wall cap and drainage ditch, which also encased and protected the panel anchors. Altogether, RECo supplied nearly 42,000 sq. ft. of precast fascia panels for various soil nail and tangent pile walls.

Another RECo innovation that helped the contractor build faster and at less cost was thickened MSE panels to meet railroad "corridor protection" requirements. The Eagle P3 East Line traverses an existing BNSF railroad corridor where a future active rail line may be built in front of walls supporting the East Line tracks.  To prevent a train from penetrating the retained fill In case of a railroad derailment, BNSF required a 2.5 ft. thick cast-in-place crash wall in front of several of the Reinforced Earth walls. Such a structure is feasible but awkward to construct and requires a foundation that conflicts with the MSE wall's leveling pad. RECo's solution was to add 2.5 ft. of concrete on the back side of all facing panels in the corridor protection zone, up to the 1-2 panel high "top of crash wall" line defined by BNSF (Fig. 4). Adding the crash wall to RECo's 5 ft. x 10 ft. precast panels was simple and efficient and seamlessly combined crash wall construction into the MSE wall erection process, eliminating the separate step – and cost – of cast-in-place crash wall construction.

The Eagle P3 project is headed for a 2016 completion and start-of-service. The Reinforced Earth Company is proud to be contributing by supplying its cost-saving retaining wall systems as well as by creatively engineering several unique, contractor-friendly alternatives. RECo’s Regional Engineer Blake Nelson, P.E. states “One of the reasons I feel this project was successful for us was the level of communication and coordination throughout.  We started meeting with the precaster and design/build team early on, months before we submitted our first design.  The submittal process, approvals, and material deliveries went smoothly because the design details and schedules were worked out ahead of time.” So next year, when you need to go between the airport and Denver's downtown or western suburbs, you will be able to hop on commuter rail lines of the RTD's Eagle P3 project and speed along atop or in front of numerous Reinforced Earth MSE and fascia walls. Swift transportation and beautiful Reinforced Earth walls – a great way to travel for visitors and citizens of Denver!