Slope stabilization for residential slope failure at Yerba Buena Island with PROPEX

Overview

Yerba Buena Island (YBI) is a natural island located in the San Francisco Bay, adjacent to the man-made Treasure Island, which was constructed in the 1930s. Together, these islands are the focus of a significant $1.5 billion redevelopment project spanning 405 acres (164 hectares). This redevelopment, planned by the City of San Francisco and private developers, aims to encapsulate the quintessential California lifestyle by incorporating residential homes, public transportation, parks and open spaces, public art, hotels, and restaurants. This ambitious project is a landmark endeavor, highlighting the integration of urban development with natural and man-made environments, aiming to create a sustainable and vibrant community.

Challenge

The first phase of the redevelopment project involved constructing a residential condominium on the northeast-central side of YBI, situated between Interstate 80 and Macalla Road. This construction necessitated significant infrastructure improvements and upgrades to Macalla Road to support the new multimillion-dollar complex. However, during a heavy precipitation event in the Fall of 2021, stormwater runoff accumulated within the gravel encasement surrounding the stormwater piping beneath Macalla Road. This accumulation of stormwater increased pore pressure below the subsurface, leading to a slope failure on the north side of the road. The slope, heavily eroded, measured 1.5H:1V and was 120 ft (36.6 m) long, consisting primarily of silty sandy soil. Adding to the complexity of the situation, a sanitary sewer line and a primary natural gas line were located on the slope face, necessitating a solution that would ensure slope stability while protecting these critical underground utilities.

Solution

To address the slope failure and provide a robust solution, ENGEO, a geotechnical engineering firm, partnered with Solmax to engineer a comprehensive approach. The objective was to ensure slope stability and provide erosion protection while safeguarding the underground utilities. Considering the design factor of safety (FoS) for both static and seismic conditions, ENGEO and Solmax determined that the PROPEX^® Armormax^® system was the optimal solution.

PROPEX Armormax is a cutting-edge system designed for surficial slope stabilization, offering vegetated reinforcement to enhance the factor of safety and significantly reduce the probability of slope failure. The system comprises Engineered Earth Anchors that are specifically designed and tested for compatibility and performance with PROPEX Pyramat®, a high-performance turf reinforcement mat (HPTRM) that significantly increases slope stability.

For this project, 6 ft (1.83 m) B3 anchors were used, spaced at 5 ft by 4 ft (1.52 m by 1.22 m), with each anchor providing a pullout capacity of 1,200 lb (544 kg). This configuration met the required design FoS. Following approval from all relevant environmental agencies and the City of San Francisco, the slope was rebuilt, and the PROPEX Armormax system was installed in February 2022 by Selby’s Seed & Erosion Control. Due to the slope's steepness, the installation team employed safety harnesses and ropes tethered at the slope's crest. After the installation, the HPTRM was sprayed with a bonded fiber matrix containing native seeds to establish vegetation.

One notable advantage of the construction techniques used was that no additional construction equipment was required, and there were no road closures or traffic control measures needed for Macalla Road. This minimized the disruption to local traffic and the community.

Performance

The implementation of the PROPEX Armormax system proved to be a resounding success. The slope stabilization and erosion control measures effectively addressed the challenges posed by the heavy precipitation event, ensuring the stability of the slope and the protection of the underground utilities. The use of the PROPEX Armormax system not only enhanced the factor of safety for both static and seismic conditions but also provided long-term vegetated reinforcement, contributing to the sustainability and aesthetic appeal of the area.

ENGEO, the geotechnical engineering firm overseeing the project, expressed satisfaction with the outcome and the performance of the PROPEX Armormax system. The success of this project has led to the identification of additional PROPEX Armormax installations within the broader Treasure Island/Yerba Buena Island Development Project. This highlights the versatility and reliability of the PROPEX Armormax system in addressing complex geotechnical challenges in urban redevelopment projects.

In conclusion, the Yerba Buena Island slope stabilization project serves as a testament to the effectiveness of innovative geotechnical solutions in urban redevelopment. The collaboration between ENGEO and Solmax, coupled with the advanced technology of the PROPEX Armormax system, ensured the successful stabilization of the slope, protection of critical infrastructure, and enhancement of the area's environmental and aesthetic qualities. This project underscores the importance of integrating advanced engineering solutions with sustainable development practices to create resilient and vibrant urban communities.