San Francisco Public Works Relies on Local Expertise to Replace Retaining Wall

Within the confines of every American city lie patches of nature set aside and protected from the surrounding development. These parks defy the encroaching concrete and asphalt to offer respite for urbanites seeking natural beauty. Across the country, from New York’s Central Park to Seattle’s Gas Works Park, many of these spaces have become iconic, representative of the collective local cultures. But not all city parks are vast, grassy expanses dotted with trees or ponds where residents visit for leisure and tourists for photo ops. Some are smaller habitats, nestled into the nooks and crannies of urban districts, serving a greater purpose.

San Francisco’s Hawk Hill Park, perched two miles inland from the Pacific coast between a schoolyard and a residential cul-de-sac in the Forrest Hill neighborhood, spans little more than a city block. A towering remnant of ancient sand dunes, the rise and fall of the park’s hills provide an idyllic space for short hikes and bird watching year-round. More importantly, though, Hawk Hill’s sandy soil and tufts of foliage supply shelter for sensitive species, including three California state treasures: the rare California croton, the San Francisco wallflower and the green hairstreak butterfly.

But Hawk Hill is a delicate ecosystem, a fact reflected in the city’s efforts to keep part of the dunes free from pedestrian traffic. That’s why, when a wooden retaining wall between the dunes and the Funston Avenue cul-de-sac began to fail, city officials knew timely action was needed. If the retaining wall at the base of Hawk Hill collapsed, the hillside could tumble, and its unique ecosystem would likely collapse. The wall needed to be replaced to ensure the line between residential space and protected habitat remained unbroken. Doing so, though, would require careful planning and flawless execution – for multiple reasons.

If the retaining wall at the base of Hawk Hill collapsed, the hillside could tumble, and its unique ecosystem would likely collapse.

Challenge: Toeing the Line between Nature and Neighborhood

Like many places in San Francisco, Funston Avenue is short on parking. The southern bend of the cul-de-sac, which is devoid of houses thanks to its border with Hawk Hill, operates as a pseudo parking lot for neighborhood residents – a luxury they don’t take for granted. Each night, about a half dozen vehicles are carefully positioned around the curve. So when news broke about the retaining wall’s degradation, residents quickly contacted local officials, knowing that a failure in the wall was certain to bury the cul-de-sac – and any vehicles that happened to be parked there – under tons of sand.

As it turns out, the city had been monitoring the wall for years and was keen to replace it before a catastrophic failure. But the risks were substantial. In addition to the potential for a collapsed ecosystem, the endangerment of several species and the loss of vital parking spaces, the replacement project could damage a nearby home sitting broadside to the dune.

A failure in the wall was certain to bury the cul-de-sac – and any vehicles that happened to be parked there – under tons of sand.

Working in such a tight space would require careful attention to detail and creative problem solving, two things that are difficult to balance with the accelerated timelines of emergency repair projects. The city needed an experienced partner who knew how to navigate the complexities of San Franciscan urban construction – one that could avoid any missteps that might make the sliding hillside worse. Luckily, the city knew exactly who to call.

Solution: Agile Procurement meets Local Expertise to Complete Public Works Project

When you look up large construction firms in San Francisco, one company you won’t find among the results is Yerba Buena Engineering and Construction. A small, local firm, Yerba Buena has quietly built an extensive resume of successful close-quarters construction projects over the past 20 years. Whatever Yerba Buena may lack in advertising budget or name recognition, they’ve more than made up for in their relationship with the city and its residents.

“They’re one of our oldest JOC contractors,” said Teenchee Le, the JOC Manager for San Francisco Public Works. JOC – shorthand for Job Order Contracting – is a unique construction project delivery method that allows project owners, including state and local government entities, to procure contractor services for multiple projects through a single, competitively bid contract.

“This is their fourth or fifth contract with San Francisco Public Works,” Le continued. “Since Yerba Buena has performed this type of work in the past with a lot of success, we felt confident assigning them this project.”

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The positive impacts of Yerba Buena’s expertise were demonstrated even before the replacement work began. Their team knew the challenges of working amid the iconic rowhomes and steep hills of San Francisco. “Their experience working on city streets was key for us,” Le stated. “They understand the soil conditions. They understand how to work with the city politics. In a crowded city, it’s always a balance between development and keeping the environment protected.”

This project made such a positive difference for the community that it won San Francisco Public Works a 2021 Harry Mellon Award of Merit for outstanding use of Job Order Contracting. Click here to find out more about the Harry Mellon Awards, including how to submit your exceptional JOC project for this distinguished honor.

Yerba Buena went door-to-door along the cul-de-sac to open lines of communication with residents, discuss the project plans with them and listen to their concerns. After gathering resident input and surveying the site, they recommended four items be added to the scope of work:

• Hiring a third-party engineering firm to document the condition of nearby homes before and after construction
• Setting survey control on critical points of houses to confirm no movement occurred
• Monitoring real-time vibrations to be notified of any unanticipated ground movements
• Providing homeowners with a record of vibration readings after the project was finished

And while these extra steps may have taken up valuable time with traditional construction procurement, the quick access to contractor services provided by JOC enabled Yerba Buena to demonstrate additional care for the neighborhood. Le explained it this way:

“What made this project successful was the timeframe. This would’ve taken six-to-eight months with a formal bid, but we didn’t have that kind of time with something as urgent as this. With JOC, we were able to select a contractor that we trust and who knows the work and complete the Joint Scope Meeting in significantly less time that bidding the contract.”

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Result: Preserving Habitats, Relationships and Businesses

Because of the compactness of the turnabout and the slope of the road, Yerba Buena had to back their trucks and equipment 1,500 feet down Funston Avenue. Once they reached the cul-de-sac, they used an innovative equipment bridging system with crane mats to displace the equipment weight away from the hillside in order to minimize its impact on the wildlife preserve.

With all the equipment in place, Yerba Buena could begin the replacement project. The plan involved installing a new steel and concrete wall behind the wooden one, then removing the timber once the work was finished. The first step was to drill holes for the 18 steel posts that would serve as the main structure for the new wall. The soil from those postholes was then used to buttress the old wooden wall during construction.

The postholes, however, proved to be unstable due to the sandy nature of the soil. To make up for the instability, Yerba Buena utilized on-site volumetric cement trucks, so they could promptly mix and pour the concrete needed to set the steel beams after the holes were dug. Once the beams were in place, another 1,100 linear feet of steel beams were run through 24-inch-diameter holes and encased in concrete to complete the new structure.

With the new wall in place and the old one removed, Yerba Buena used the soil removed during drilling to regrade the hill, installed erosion-prevention features and planted 3,000 native plants on the hillside to help stabilize the soil.

As no damage was done to Hawk Hill Park or any of the adjacent homes, the project was a resounding success. But the park and neighborhood weren’t the only two benefactors.

“We use our JOC contracts as a way to invest in local, small businesses in San Francisco,” Le told us. “We strive to provide opportunities for them to grow. We need to provide a space for smaller companies [like Yerba Buena] to thrive, since the construction industry is so competitive.”

This investment opportunity is a well-documented benefit of JOC, and it’s been shown to improve contractor-owner relationships over the life of the contract. Le attested to this, saying,

“There’s a sense of respect and trust between us. If they see something that doesn’t work, they’ll just let us know. With other contracting methods, if you have a disagreement or argument, the other party shuts down – they don’t know who you are or trust you. But with JOC, we actually listen to the contractor on how to do things better. They’re the expert.”

The residents of Funston Avenue benefited from that level of trust between Yerba Buena and the city, and there will undoubtedly be many more San Franciscans who will in the future.