Mineral King Road bridge team unveils state-of-the-art design

Mineral King Road bridge
Innovative design of new Oak Grove Bridge, which is actually two bridges, a new one for vehicle traffic and the historic one for pedestrians.

Mineral King Road will have two bridges 

On Thursday, November 19, a Tulare County-led project team unveiled an innovative new desgn for the preservation and future use of the historic Oak Grove Bridge. The meeting was held as a webinar via Zoom video-conferencing and featured remarks by Jason Vivian, Tulare County engineer and lead project manager; Shawn Cullers, a bridge engineer with Cornerstone Structural Engineering Inc.; and a consultant with GPA Consulting who reviewed the environmental work that was completed in 2018.

The Oak Grove Bridge is located 6.5 miles up the Mineral King Road from where the steep, narrow mountain road intersects with Highway 198 in Three Rivers. After being determined eligible for inclusion on the National Register of Historic Places, a National Environmental Policy Act (NEPA) analysis was completed by GPA Consulting in 2018.  

Mineral King Road bridge
One of two original Oak Grove Bridge brass plaques that were mounted on the concrete structural supports of the historic Mineral King Road bridge. The 1923 bridge will be preserved and a new bypass bridge will be buillt just upriver from the existing bridge to accommodate vehicular traffic.

Vivian recounted the progress of the last five years in the planning of the project. In 2015, a safety evaluation of the bridge found the Oak Grove Bridge to be structurally deficient. As a part of the evaluation, three alternatives were developed to mitigate safety hazards posed by the deteriorating bridge.

The first public meeting of the project was held in Three Rivers on February 23, 2016. The three alternatives that were proposed to fix the deteriorating bridge were (1) Retrofit/rehabilitate the existing bridge, leaving the historic appearance and structural integrity of the bridge essentially unchanged (estimated cost $4.5 million); (2B) Preserve the original bridge and replace its vehicular use with a new low maintenance bridge upstream (estimated cost $4 million); and (3) Preserve the original bridge within a hybrid design (estimated cost $5 million).

After detailed analyses of the three alternatives, Alternative 1, which in the next phase of the project’s documentation actually became Alternative 2 (a retrofit/rehabilitation of the existing bridge), was chosen because it preserved the original bridge and was the most cost-effective way to address the deterioration of the bridge. The estimated cost for completion of the project was $4 to $5 million and would cause only minimal impact to the bridge and its historic setting.

In 2017, new cracks developed in the concrete decking and the structural supports. Cornerstone Structural Engineering Inc., which had completed the initial evaluation, were hired to do a corrosion assessment of the concrete used in the bridge to determine if it was feasible to rehabilitate the existing bridge for its continued use for vehicular traffic. 

What they found was that the concrete throughout the bridge was rapidly deteriorating and in the advanced stages of “concrete cancer.” According to Shawn Cullers, project manager for Cornerstone, moisture had permeated the concrete throughout and caused swelling in the aggregate that was destroying the concrete from the inside out. The findings: The existing bridge, even after extensive rehab, was in danger of collapse if it continued to be used for vehicular traffic.

Mineral King Road bridge
New cracks were noted in both the the concrete arches and the structural supports of the 1923 Oak Grove Bridge on the Mineral King Road. 

Vivian said that’s when a new alternative was chosen that preserved the historic 1923 bridge for pedestrian use and also proposed building a new state-of-the-art structure up-canyon from and parallel to the existing bridge to accommodate vehicles on the Mineral King Road.

The new steel bridge superstructure will reportedly be barely visible from behind the historic bridge. It will utilize two large retaining walls at either end to stabilize the steep walls of the canyon.

Estimated cost: $8.5 million and, according to Vivian, will be 100 percent funded by the federal government. The design phase and bid process for the project is expected to be completed by the end of 2021.

As to when the funding might become available and construction actually begins, Vivian said the answers to those questions remain problematic.  





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