If you have spent time in San Diego, Calif., chances are you have seen the 2019 non-highway bridge award winner. The new Mid-Coast Corridor Retaining Walls extend the light-rail infrastructure from the Old Town neighborhood of San Diego north to La Jolla, Calif., and provides a second heavy-rail track through the entire corridor. The walls were built along the railroad right-of-way to increase the width of the corridor and allow for three additional sets of railroad tracks to be constructed.
The original design used soldier pile walls and mechanically stabilized earth to meet the project goals; however, the designers shifted to a precast concrete counterfort retention system when they recognized the many benefits that this solution would bring to the project, says Jeremy Kirkpatrick, precast concrete wall contractor with SRK Engineering. “Precast concrete elements significantly reduced the construction time by eliminating shoring, eliminating concrete-curing time, and reducing the delays associated with working within the active railroad corridor.”
Monolithic counterforts were developed for the project, and one tier of counterforts could be used for walls up to 14 ft high. Standard prestressed/precast hollow-core concrete panels, which are commonly used for parking structures and other buildings, were used for the wall panels. “The panels are lighter and stronger, and they can therefore span 15 ft or more between counterforts,” says Erick Aldrich, principal engineer with SRK Engineering. Controlled concrete mixes were used to create high-strength precast concrete with a high corrosion resistance. Some walls were built with post-tensioned concrete to increase their strength and allow for a two-piece counterfort with horizontal and vertical components.
Floods, earthquakes and moving trains
This project faced many challenges from the outset. The walls must be extremely durable to accommodate the significant railroad loads, extreme flooding events, and seismic demands (the Rose Canyon Fault runs directly beneath the project site). One of the walls was built adjacent to a 72-in.-wide sewer main, which had to be protected from the surcharge loading from the wall, Aldrich says. Also, throughout construction, workers had to stop work every time a train passed by the site.
When the project team began its work, the project was already behind schedule. That was when the benefits of precast concrete construction became very apparent, Aldrich says. Even with the many constraints, the precast concrete wall system went up rapidly, with one wall erected in just 48 hours during a track shutdown period. “Precast concrete elements were stored on site and erecting them very quickly within the railroad area reduced construction duration and limited disturbance,” Kirkpatrick says. “That mitigated the time constraints and the limited access to the site within an active railroad environment.”