Project Location:

The BNSF Railroad Bridge Flood Risk Management project is a component of the U.S. Army Corps of Engineers’ (USACE’s) Santa Ana River Mainstem (SARM) Project to provide flood risk management to more than 3.35 million people in the growing urban communities across Orange, Riverside, and San Bernardino counties. The nominated project protects three existing Burlington Northern & Santa Fe (BNSF) Railroad bridges, including their supporting piers and abutments, from scour erosion due to flood flows. The three bridges are in parallel spanning across the Santa Ana River, approximately 2.25 miles downstream of the Prado Dam outlet works at the edge of the city of Corona in Riverside County, CA.

Project Description:

The BNSF Railroad Bridge Flood Risk Management Project (“BNSF Project”) is one of the components of the overarching Santa Ana River Mainstem (SARM) project that has been in planning, design, and construction phases for more than 45 years. Designed and managed by USACE Los Angeles District (LAD), the BNSF Project includes geotechnical investigation, numerical and physical modeling, conceptual and detail design, utility relocation, structural design, and construction of scour protection features for the bridges to maintain their stability for flood flows (up to 30,000 cfs) released from Prado Dam. The oldest of the three bridges and piers was designed and built by USACE as part of the railroad relocation for the original 1941 Prado Dam construction. The current BNSF Project provides protection to this critical transportation infrastructure and mitigates flood risks to BNSF’s delivery of goods and passenger services between the Pacific Coast and Southern California’s Inland Empire. The project involved multiple stakeholders.

Preconstruction Features
The BNSF Project consists of three separate railway bridge structures crossing the river: the upstream (north) track bridge, constructed in 1938, and two southerly bridges, both constructed in 1995. Each bridge is supported by abutments on the east and west ends and six intermediate bents (piers). The abutments and piers are supported by a group of driven H-piles. The abutments of the three bridges are protected with sheet piles and tiebacks constructed during the 1938 relocation. The sheet pile walls extend approximately 40 feet below the existing grade.
Since the construction of the bridges, Prado Dam was modified to handle larger flood flows in 2008. Larger releases from the flood control reservoir could potentially cause scour erosion deeper than what the bridges were originally designed to accommodate. Therefore, protection against additional scour was required to maintain the static and seismic stability of the bridges. Although much of the SARM construction work has been completed, a few elements remain. The BNSF Project is essential to ensuring that Prado Dam will be able to operate as intended under the SARM project and provide the level of flood risk reduction authorized by Congress.
The rapid growth and development of Southern California have decreased the effectiveness of the flood control system in place prior to the SARM project. Areas that would absorb rainfall-runoff have been reduced, along with the water-holding capacities of reservoirs. Today, the most severe flood likely to occur along the river would cover more than 110,000 acres to a depth of 3 feet and could amount to more than $40 billion in economic losses.
The overarching SARM project is designed to provide flood risk management for residences as well as business and public infrastructure in the Southern California communities of Orange, Riverside, and San Bernardino counties. Under contract with USACE-LAD, AECOM provided geotechnical design services for the BNSF Project and engineering support during construction.

Design Features

Due to the importance of the BNSF Project and the complexity of the restrictions and requirements (refer to the next section for details), an extensive study that included numerical (a three-dimensional computational fluid dynamics (CFD) model) and physical modeling (1:30 Froude-Scaled model) was completed to conservatively estimate the level of scour from Prado Dam releases with the application of safety factors. Once the final scour levels were determined, the soil structure interaction of the design alternatives was evaluated by performing nonlinear, time-domain dynamic numerical analyses using the computer program (FLAC3D) finite element models.

The major design features of the BNSF Project are the concrete slurry diaphragm wall enclosures around each pier group. A reinforced concrete cap on the enclosure ties the diaphragm wall panels together. The pier enclosure confines the soils surrounding the existing bridge foundation piles and prevents scour erosion. Pier nose extensions were constructed upstream of the enclosure to divert the river flows around the enclosure and mitigate local scour at the piers. The pier nose extension consists of reinforced concrete training walls supported by slurry diaphragm walls that taper upstream into the flow to a reinforced concrete nose wall. The nose wall is supported on 30 driven H-piles, with the bottom of the pile cap at 35 feet below the existing grade (below the estimated scour depth). The nose extensions were also designed to accommodate space for potential future bridge expansion by BNSF. The BNSF Project also includes concrete slurry diaphragm walls to protect the bridge abutment foundation piles. The abutment slurry diaphragm walls include T-section cantilever walls adjacent to the bridge foundations with anchored walls upstream and downstream of the bridge at the east and the west sides of the bridge (refer to the project’s overall plan). 

The abutment diaphragm walls below the bridge next to the foundation piles were constructed using a combination of a slurry excavation technique for the bottom 60 feet and a cast-in-place concrete bulkhead beam at the top 20 feet of the walls. The existing railroad bridge decks restricted the equipment that could be used to construct the walls below the bridges. Therefore, the diaphragm walls under the bridge were constructed using low overhead equipment.


Project Justification:

The BNSF Project is worthy of an award because of its unique design for mitigating bridge pier and abutment scour. The project stands out for successfully resolving constructability and site condition challenges despite the extreme environmental, geotechnical, hydraulic, and structural constraints as well as logistical issues. Listed below are some of these constraints along with brief descriptions of how they were addressed and resolved:

- The requirement for Reach 9 to remain a soft (earthen) bottom channel for wildlife usage and
native habitat preservation created unique environmental constraints in the design and construction of the bridge protection measures. For example, the measures had to avoid any vertical or horizontal barrier that restricted the natural meandering of the river and had to prohibit any significant increase in the velocity of low flows between piers. To meet these specific environmental requirements, the design team developed the pier nose extension and enclosure protection concept by streamlining the flow around the piers to minimize the local and abutment scour components. The hydraulic analyses used to confirm the effectiveness of the protection concept included: simplified two-dimensional analysis, three-dimensional CFD numerical analysis, and use of a 1:30 Froude-scaled physical model of the full channel width of the BNSF Bridge and piers to accurately estimate the local and abutment scour depths at the bridge. The project team performed the hydraulic analysis with expertise from the U.S Department of Federal Highways, Argonne Laboratory, and USACE’s Engineering Research and Development Center in coordination with USACE-LAD, peer reviewers, and project stakeholders.

- The combination of low overhead clearance under the bridge, active utility lines, and subsurface conditions restrictions, such as the high groundwater elevation and presence of cobbles and boulders in alluvial materials, also contributed to the complexity of the design and construction of this project. To overcome these unfavorable site conditions, the project team evaluated different design alternatives as part of the development of bridge protection measures. The team eventually determined that the concrete slurry diaphragm wall with its unique construction techniques would be the most appropriate design alternative. The reinforced concrete slurry diaphragm walls were constructed under low vertical clearance to protect the bridge abutments and piers and to maintain or improve their structural stability.

- The bridge crossing is a critical infrastructure component and major commercial transportation corridor for BNSF Railway in delivering freight and passengers to Orange County and the Inland Empire. The three bridges were actively used by passenger and freight trains consistently during construction. According to BNSF, approximately 130 trains (30 passenger trains and 100 freight trains) cross the bridges each day. Among the constraints it faced, the design team had to restrict any physical contact with the bridges; avoid any impact to bridge stability; protect the existing bridge foundation during construction of the concrete slurry wall; restrict maintenance access and construction inside the bridge envelope or BNSF’s right-of-way during the fourth quarter (from November through January), and plan for future monitoring of scouring. The effective technical collaboration between the project management team, BNSF Railway, other stakeholders, designers, and the Contractor during the project contributed to the successful delivery of this challenging project. The project’s design and construction ensured that the protection measures did not touch the existing bridge structure. An automated 24/7 instrumentation monitoring program was used to establish the bridge baseline signature and to monitor the bridge stability for an appropriate response during construction. In coordination with BNSF Railway, the project team sequenced construction activities to ensure that work would be completed with no or minimal effects on train traffic during the fourth quarter and that these activities would do no harm to the bridge.

- To build the pier protection measures, a river diversion had to be constructed to avoid adverse impacts to the existing fishery. The project team had to divert the river from the original channel to a temporary channel and then had to redivert the river back to its original channel at the project’s conclusion. Based on the original design, the river diversion duration was limited to the dry season (from mid-April through mid-October). Other unexpected environmental events occurred, such as an owl nesting under the bridge, which paused construction activities. Consequently, the diversion duration was extended through the rainy season. Despite a few severe flood events, the project team, including the designers and the Contractor, were able to adjust the construction plan and move the project forward successfully to meet the project completion deadline.

- Given the high groundwater elevation and granular nature of the subsurface materials, an exceptional excavation method and pile-driving process were needed. To construct the pier nose extension piles and pile caps below the design scour elevations (35 feet below the existing grade) and approximately 20 feet below the groundwater elevation, a temporary cofferdam was installed offset from the footprint of the pile cap. Consisting of a series of sheet piles and steel beam bracing, the temporary cofferdam provided protection for the excavation of the materials inside to the depth of the pile cap elevation. By using this unique excavation/pile-driving process, the project team was able to reduce the amount of excavation and associated dewatering of the project to less than one-third of the ordinary slope-cut and still deliver a quality job at a faster pace in a safer environment.

Special Circumstances:

The project encountered considerable challenges during design and construction, many of which are described in detail in the previous section.

The great SARM project is designed and managed by USACE-LAD to provide flood risk management for residences, businesses, public and private infrastructures in the Southern California communities of Orange, Riverside, and San Bernardino counties. The BNSF Project was identified and judged to be a necessary component of the original SARM project. All project stakeholders—including the Orange County Flood Control District, Riverside County Flood Control and Water Conservation District, and BNSF Railway—worked closely with USACE-LAD during its design and construction. The conceptual design was initiated with USACE-LAD in December 2011 and finalized for construction bidding in July 2017. The construction activities were started in December 2017 by MMB AJV (the awarded general contractor) and completed in November 2021. Since the beginning of the project, coordination and collaboration between the stakeholders were considered to be the key factor in the success of the project, given its many complexities and challenges. Despite unforeseen environmental events, adverse weather, differing subsurface conditions, delays, shortages of supplies and resources, and special safety protocols due to the global pandemic, the project was successfully completed. Simply put, the BNSF Project is a testament to the resolve and dedication of the design and construction team and all the stakeholders. Together they accomplished the project purpose, which is to protect the community and its critical transportation corridor from potentially damaging floodwaters.

Project Attachments:

OC Public Works Website

Award Citation::

The BNSF Railroad Bridge Flood Risk Management Project is one of the components of the Santa Ana River Mainstem project, which has been in progress for over 45 years. The BNSF Project is recognized for its innovative design and construction to mitigate local pier and abutment scour erosion from flood flows.

Suggested Award Summary:

The BNSF Railroad Bridge Flood Protection Project is one of the components of the Santa Ana River Mainstem project, which has been in progress for over 45 years. Designed and managed by the U.S. Army Corps of Engineers (USACE) Los Angeles District (LAD), the BNSF Project included geotechnical investigation, hydraulic analyses, numerical and physical modeling, environmental assessments, conceptual and detail design, utility relocation, civil and structural improvements, and construction of flood mitigation and scour protection measures for the BNSF Railway bridges to accommodate the design flood release from Prado Dam (up to 30,000 cfs). The oldest bridge piers were designed and built by USACE as part of the railroad relocation for the original 1941 Prado Dam construction. The BNSF Project protects this critical transportation infrastructure for BNSF Railways and provides flood risk management benefits to the public. The success of the BNSF Project is attributed to the critical coordination and collaboration between multiple stakeholders and the dedication of the project team from early concept development through final construction.

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