The Gulling Street Bridge over the Middle Fork Feather River has become a focal point of concern following the release of two critical reports from the California Department of Transportation and MGE Engineering. The concern is centered around scouring, the erosion of sediment around bridge foundations by fast-moving water that can compromise structural stability. Both reports highlight significant structural issues related to scour at Pier 3, but they present differing opinions on the bridge’s current safety and necessary steps forward.
The Caltrans and MGE Engineering reports agree on the fundamental issue: the bridge’s spread footing at Pier 3 is critically undermined. This scour has compromised the structural integrity of the bridge, making it unsafe for unrestricted traffic. Both reports emphasize the urgency of addressing this issue to prevent potential structural failure.
Caltrans identified the undermining of the spread footing at Pier 3 during an underwater inspection Sept. 26, 2023. MGE Engineering confirmed the severe scour at Pier 3 during their December 2023 inspection, further evaluating the structural implications.
Both reports recommend immediate load restrictions to mitigate the risk of collapse. Caltrans specifically advises posting the bridge for a maximum of 6 tons gross traffic load in each direction.
The reports agree that immediate actions, such as updating the plan of action and implementing monitoring systems, are essential to ensure the safety of the bridge while longer-term solutions are developed.
While there is consensus on the critical nature of the scour issue, the reports differ in their approach to remediation and the details of their recommendations.
The Caltrans report includes the following recommendations:
- Immediate load restrictions: Posting the bridge for a maximum 6 tons gross traffic load in each direction.
- Daily monitoring: Implementing a daily monitoring plan to track any further deterioration. “We have to visually monitor daily and measure when water levels are high,” said Todd Roberts, Portola’s director of public works.
- Bridge closure plan: Developing a bridge closure plan during high flows to prevent further damage.
- Update plan of action: Revising the 2006 action plan to reflect current conditions and incorporate the new restrictions and monitoring requirements.
The MGE Engineering report includes the following recommendations:
- Data gathering and analysis: Collecting necessary data; performing hydraulic, structural and geotechnical assessments; and conducting a hydraulic engineering analysis for countermeasure design.
- Settlement sensitivity analyses: Evaluating structural stability to develop criteria for monitoring and closures during flooding.
- Geotechnical borings: Drilling to determine the depth and quality of rock at pier locations.
- Countermeasure evaluations: Assessing alternatives for addressing the scour issue.
- Revised plan of action: Developing a revised plan of action based on gathered data and evaluations.
In addition to their recommendations, the reports also provide detailed information about the soil and material conditions under the bridge, which are crucial to understanding its stability
The Caltrans report includes the following information:
- General geologic conditions: The bridge is underlain by undifferentiated deposits of sand, gravel and silt, with local lacustrine clay and coaly beds. These deposits are highly variable and include interstratified fluvial, glaciofluvial, colluvial and lacustrine sediments occurring along the Middle Fork Feather River.
- Alluvium and colluvium: These deposits underlie the floor of the valley and the lower slopes of Long Valley.
- Andesitic tuff and breccia flows: These deposits are present immediately north of the bridge.
The MGE Engineering report provides additional information:
- 1952 log of test borings: This log indicates gray-brown sand and gravel in the upper 10 feet, underlain by very compact gravel and coarse sand. The borings did not encounter rock below the alluvial layer.
- 2005 Taber Consultants investigation: This investigation encountered alluvium over intensely weathered igneous rock. The alluvium consisted of sand and gravel with silt and cobbles, with a very dense sand, gravel and silt layer below it. The rock below the alluvium was classified as intensely weathered igneous rock at an elevation of 4,799 feet.
These varied subsurface materials significantly impact the bridge’s stability and contribute to the scour problem, both reports state. They highlight the need for detailed geotechnical evaluations to fully understand the foundation conditions and inform necessary remedial actions. Despite these concerns, Roberts noted, “Even with minor scouring on the pillars, the cohesion of the earth under the ground support pillars is stable.”
According to Caltrans, the bridge cannot sustain unrestricted live loading in its current condition. That necessitates the 6-ton load limit. The MGE Engineering report, while agreeing on the immediate need for restrictions, emphasizes the importance of continuous monitoring and phased evaluations to ensure long-term safety.
Further compounding these issues, recent evaluations have shown that even with a 6-ton vehicle weight limit, the bridge’s foundation at Pier 3 is operating close to its reduced capacity, providing less room for error and a smaller margin of safety. A safety factor of 2.6 is still above a 1, when the foundation can still theoretically handle the expected loads. A zero rating means the structure has failed.
Despite the posted 15 mph speed limit and 6-ton weight limit signs, citizens have reported seeing vehicles over 6 tons driving unrestricted on the bridge, raising concerns about enforcement and safety compliance. “I’ve seen vehicles 80,000 pounds, which is 40 tons, tractor-trailer and other trucks that haul equipment over the bridge,” said Tony Buckelew, of Portola.
Safety issues known in 2006
The history of critical scour issues at the bridge goes back decades to a 2006 bridge report produced when Mayor Pro Tem Jim Murphy was city manager. In that report, the bridge was given a scour rating of 3 with warnings of potential additional scour. City Engineer Dan Bastion recommended countermeasures to include installing a gabion mattress and adopting a scour-monitoring plan totaling $194,000. The scour monitoring plan recommendation included monitoring for vertical or horizontal movement at any location in excess of one-half inch, which would initiate a bridge closure.
The latest flurry of concern started in September 2023, when Caltrans did a routine inspection of the bridge. The City of Portola received email correspondence from Caltrans on Dec. 22, 2023, documenting the critical findings from the underwater inspection. The email confirmed the undermining at Pier 3, reinforced the need for the load restriction to 6 tons and reduced the Pier 3 safety factor from 3.0 in 2006 to an estimated 2.6 today.
At the Portola city meeting Jan. 10, 2024, both Caltrans and MGE Engineering reports agreed that the critical scour issue was exacerbated by the previous winter’s storm disasters. MGE Engineering’s December 2023 inspection included former interim City Manager Jon Kennedy, Director of Public Works Todd Roberts and Councilmember Stan Peiler. The final Caltrans report, received in January 2024, reiterated the severity of the scour at Pier 3. City staff proposed a budget amendment to cover the cost of MGE Engineering’s investigation and recommended evaluations totaling $162,768. Kennedy advocated for approving the full amount to expedite necessary actions.
On May 22, the city council adopted Resolution No. 2578, outlining the receipt of $53,744 in road maintenance and rehabilitation funding for fiscal year 2024-25. These funds will support essential road maintenance, rehabilitation projects and safety improvements, including the Gulling Street Bridge rehabilitation. However, no estimates have been provided for the total cost to fix the bridge.
The project, originally slated to start this summer, aims to address the scour issues at piers 3 and 4. The estimated useful life of the completed project is between two and five years. In April, new interim City Manager Susan Scarlett highlighted the fact that the maintenance and rehabilitation funds are earmarked for projects in the next fiscal year, ensuring continued focus on the bridge’s safety and functionality.
“The next step is to get Caltrans and MGE Engineering together in the same room,” Scarlett said. “We don’t have decision-making at the moment.”
Portola has spent $8,777.88 on renting an electric sign to warn drivers to proceed slowly over the bridge, emphasizing the ongoing commitment to public safety.
Who has the ultimate authority?
If MGE Engineering and Caltrans have a dispute regarding the bridge’s safety, Caltrans ultimately has the authority, according to state regulations. As the state transportation agency, Caltrans’ assessments and recommendations typically carry the final say in matters of bridge safety and regulatory compliance.
A Portola native who has been following the controversy was critical of the city’s response to the bridge crisis.“They haven’t taken any measures to fix the bridge. They spent all that money on a report; it says immediate action required. Somebody dropped the ball. What are they going to do if it collapses?” said Lisa, who asked that her last name not be used.
There have been no new updates from City Hall about how the negotiations between Caltrans and MGE Engineering are progressing. Portola retiree Buckelew said, “The city and Caltrans are putting the public in danger. Time will tell.”
Several community members feel like they are kept in the dark about final decisions on the necessary actions to secure the Gulling Street Bridge. “The communications need improvement for sure. I feel we weren’t told everything and the bridge isn’t as safe as they say. I believe the city owes us answers on what they’re going to do about it,” Lisa said.
