Improving a scour-critical and seismically deficient bridge and housing local wildlife
By Mike Pugh, PE
In 1964, the Hickman Road Bridge was constructed to improve the connection of the City of Waterford to the southern part of Stanislaus County, in California, and maintain a major regional north-south roadway in the central part of the county. The bridge is located just south of the City of Waterford and crosses the nearly 150-mile-long Tuolumne River. The existing structure was approximately 58 years old and consisted of a seven-span concrete box girder with concrete abutment and pier walls supported on pile caps founded on driven piles. When a structural and seismic evaluation was conducted on the existing bridge, it was found to be scour critical and seismically deficient. Over the bridge’s almost six decades of service, the pile caps at the piers located adjacent to the main river channel had become undermined during high flow events, which led to its status of being scour critical. Because the Tuolumne River channel was continuing to degrade the structure, there was no practical solution to repairing the scour damage at the pile caps. Coupled with the fact that the bridge did not meet existing seismic design criteria, it was now eligible for replacement under the California Department of Transportation’s (Caltrans) Highway Bridge Program (HBP).
California Department of Transportation — Highway Bridge Program
While many states have their own HBPs, the California Department of Transportation (Caltrans) HBP was created to “replace or rehabilitate public highway bridges over waterways, other topographical barriers, other highways, or railroads when the State and the Federal Highway Administration determine that a bridge is significantly important and qualifies under the HBP program Guidelines.” The HBP allows for certain project elements to be reimbursable, including “replacement, rehabilitation, painting, scour countermeasure, and preventative maintenance activities.” With funding on the table and a bridge in need of replacing, the design, environmental clearance, and construction of a replacement bridge was authorized.
Initially, a preliminary environmental study (PES) was developed and designers and environmental scientists worked with Caltrans and Stanislaus County during the design process to complete and clear the environmental technical studies. These studies included an Initial Site Assessment (ISA) for hazardous waste, Water Quality Report, Natural Environment Study (NES), Biological Assessment (BA), Wetland Delineation, Section 4(f) for an adjacent public park, Visual Impact Assessment (VIA), and Historic Property Survey Report/Historic Resources Evaluation Report/Archaeological Survey Report(HPSR/HRER/ASR). The environmental clearance documents were an Initial Study/Mitigated Negative Declaration (IS/MND) for the California Environmental Quality Act (CEQA) and Categorical Exemption (CE) for the National Environmental Policy Act (NEPA).
A Wildlife Habitat
During the design effort it was discovered that local wildlife, including birds and roughly 6,000 bats, had taken residence on the bridge, specifically under the deck overhangs and between the bridge’s east exterior girder and the exterior bridge-mounted communication conduits. The team considered how these bats and birds could make the new bridge their home once construction was complete. Deck overhangs and bat houses mounted to both exterior girders were incorporated into the design of the new Hickman Road Bridge, which created ideal roosting conditions, a place where the wildlife could birth and raise their young. Within a few months after bridge completion, bats had already begun residing in the new bat houses.
Maintaining Traffic During Construction
Because the Hickman Road Bridge is such a critical piece of infrastructure in the area and there was not a reasonable detour available for rerouting traffic (the shortest detour length was 10 miles), it was determined that the existing bridge was structurally sound to remain in service until the new bridge opened. With 17 months of construction completed, the new bridge consists of a 750-foot long, 75-foot tall, five-span, cast-in-place, post-tension concrete box girder structure, supported by seat abutments at each end and intermediate two-column piers. Each pier is supported by two large-diameter cast-in-drilled-hole piles, ranging in diameter from 100 inches to 125 inches, and measuring up to 125 feet in depth. The bridge features two 12-foot traffic lanes, two 8-foot shoulders for bicyclists, and one 5-foot sidewalk for pedestrians. Additionally, 960 feet of roadway was reconstructed to conform the new bridge to the existing Hickman Road alignment.
Like with any infrastructure project, the design team faced numerous challenges. The first was justifying the need for a new bridge, given that the existing structure was still structurally sufficient. The design team developed conceptual designs and associated costs for a seismic retrofit of the existing bridge, a permanent repair on the existing bridge to protect the pier foundations from scour, and a bridge replacement. To compare the alternatives, the team prepared a life cycle cost analysis that was used as justification to replace the existing bridge.
The second challenge was addressing the geometric needs for the roadway approaches that would place the new bridge on a parallel alignment with the existing bridge; and so the existing bridge could remain in service during project construction. To minimize the necessary roadway approach work, the team prepared a design that placed the new bridge as close to the existing bridge as possible. Despite this effort, the necessary approach lengths still exceeded the HBP allowable limit of 200 feet at each end of the bridge. To address this, the designers prepared a long-approach justification memo citing road geometric design requirements, design speed justification, and safety concerns. Caltrans concurred with the justification and additional funding was secured for the long approaches.
The third challenge involved numerous elderberry bushes and migrating fish species found at the site. Environmental scientists considered two alternative road alignments for the new bridge to avoid environmentally sensitive areas. The right of way (ROW) costs were reviewed against the Valley Elderberry Longhorn Beatle (VELB) mitigation costs, and ultimately the alignment with increased ROW was chosen, saving Stanislaus County nearly half a million dollars in VELB mitigation costs.
Now that the construction of the new bridge is complete and the existing bridge has been removed, the new Hickman Road Bridge meets current seismic criteria, addresses the Tuolumne River channel erosion issues, and provides additional capacity and a safer crossing for vehicles, pedestrians, and bicyclists. The new bridge also carries a new waterline and other vital utilities across the Tuolumne River that are critical to the region. The $19-million project was funded through the HBP, Caltrans Local Seismic Safety Retrofit Program, and transportation funds created by local sales tax measures.
Mike Pugh, PE is an associate vice president and principal engineer with Dewberry in Rancho Cordova, California. He has nearly 40 years of structural design experience, including new bridges, bridge repairs and widenings, and seismic retrofits.