Overcoming Site Challenges for Santa Margarita River Bridge
North San Diego County Transit Development Board (NCTD) owns and operates the San Diego Northern Railway in San Diego County, California, providing Coaster commuter rail service between Oceanside and San Diego. Amtrak's Pacific Surfliner and Metrolink commuter trains also provide intercity service along this busy stretch of railroad. Passenger trains routinely operate at speeds up to 90 mph along this line. Developing additional capacity to increase service and reliability along this important transportation corridor is a high priority, and design of a second main track and new railroad bridge across the Santa Margarita River are key components of the plan.
The Santa Margarita River Bridge is located along the Los Angeles to San Diego (LOSSAN) Rail Corridor just north of Oceanside. The project site is within the boundaries of Marine Corps Base Camp Pendleton, and will connect two passing tracks. The longer section of double track allows for moving meets when trains pass each other. The bridge also is nearing the end of its economical life, and portions of its south timber approach trestle recently required replacement.
The proposed project replaces the existing Santa Margarita River Bridge with a new 755-foot two-track bridge, composed of a 500-foot main river bridge and a 255-foot approach trestle at the south end. It adds 0.8 miles of new double track to connect the two existing passing tracks and upgrades the existing 1.7-mile Fallbrook Junction Passing track. The resulting double track is 4.8 miles in length with a design speed of 90 mph.
Design Details
A unique feature of this project is the structure type selected to replace the 212-foot steel through truss span and two 106-foot deck plate girder spans that cross the river. The proposed main river structure is a 32-foot wide cast-in-place concrete box girder with continuous spans of 150 feet, 200 feet and 150 feet.
This bridge type was selected over the more common rail structures because of some unique characteristics inherent to the project site. Among the chief obstacles designers had to overcome were poor soil quality and a high propensity for seismic activity. And in doing so, the design team needed to develop a solution in which the piers matched an adjacent three-span Interstate 5 structure for stream hydraulics. This eliminated all simple short span options. Long-span simply supported steel options included through trusses and deck plate girder structures with 200-ft main spans.
In looking at the challenge of building the structure on poor soils, it was determined that a continuous structure would perform better during events of seismic activity. Furthermore, the marine environment requires that steel bridges have expensive coating systems to prevent corrosion. This would mean more substantial future maintenance costs for the steel option.
Maintenance of steel structures is further complicated by the sensitive river/lagoon environment at this and many of NCTD’s other bridges. Obtaining permits for even minor maintenance can be a time-consuming and expensive process. Adjacent Caltrans cast-in-place bridges have been in service for 40 years with minimal structural maintenance. Because of the limited work area, any cost advantage that might have come from using a simple span steel bridge was eliminated by the need for extensive falsework in erecting the bridge.
All of these factors combined to make the cast-in-place concrete box girder the most economical option. While this structure type is not typical within the industry nationally, it has been successfully used on rail bridges throughout California.
The substructure also comprises some unique characteristics. Liquefiable soils and high seismic forces again contributed to design considerations, resulting in a substructure design for the main piers that consists of a pier cap that is supported by a 12-foot drilled shaft surrounded by a ring of 30-inch steel pipe piles. The 12-foot diameter drilled shafts are mainly to resist the lateral load, while the 30-inch driven steel pipe piles carry the vertical loads.
The approach trestle has nine 28-foot pre-cast concrete box girder spans, erected on piers that incorporate a concrete cap supported by driven steel piles. The cap will be cast-in-place concrete. The design is based on Metrolink’s standard trestle plans.
Focus on Constructability
Creating a design that could be implemented presented a major concern for the project team; the existing track profile had to be raised six feet to clear the 100-year flood plain. Either the new bridge would need to be constructed offset from the existing alignment or crews would have to build a temporary shoofly and trestle. The shoofly option was eliminated early in the design process because of higher costs and the impact on sensitive marine tidelands.
The offset alignment, on the other hand, offered several advantages. The new bridge can be constructed without interrupting train operations or conflicting with the existing bridge footings, and the new embankment can be constructed upland from the sensitive tidelands. The design for the embankment encapsulates the existing embankment without encroaching into the marine tidelands along the east side of the track, and will be constructed in two phases. Once the bridge and new track (including the partial embankment) are completed, the existing track can be removed and the remaining embankment constructed.
Building this large of a structure over water will require the use of a temporary work trestle to support the large equipment needed to construct the three main river piers. It is anticipated that the contractor would construct the work trestle across the river, immediately downstream from the new piers. Platforms for cranes, pile drivers and drill rigs can be built to jut out perpendicular to the work trestle and along the side of the pier sites.
Pier foundation construction would begin by driving a steel sheet pile cofferdam around the perimeter of the two in-water pier footings to retain the excavation. A mud seal will be installed in the bottom of the cofferdam to prevent groundwater infiltration. The work must be completed within the fish window for the endangered tidewater gobie. Once the cofferdams are in place, bridge work can continue without interruption.
The large diameter drilled shaft in the center of the footing would be constructed prior to driving the steel piles. A larger drill rig and one or two smaller service cranes can be positioned alongside the cofferdam. The steel piles will be driven once the drilled shaft is complete.
Summary
The design of complex projects such as the Santa Margarita River Bridge must consider constructability and maintenance of train operations as well as design and environmental factors. While a cast-in-place concrete box "structure is not a "typical” railroad bridge, it is the recommended solution given the specific set of challenges this site poses, and will provide years of economical service.