Mountain Lake Embankment Slope Stabilization by Vertical Vibrated Stone Columns, Highway 1, San Francisco Bay Area

The objective of this project was to enhance the lake sustainability by dredging the lake bottom sediments transported by surface runoff and improve the drainage facilities of the adjacent areas to collect the runoff. Compacted stone columns (SC) were installed within the shoulder of State Highway 1 adjacent to Mountain Lake in the City of San Francisco. The purpose of the ground mitigation was to densify the subsurface soils below the roadway shoulder and stabilize the western slope of the lake during the dredging, as well as for long-term seismic ground motion. This paper presents the experiences gained from the design and construction of this project. This method was selected over the conventional structural and other ground mitigations involving cements or other chemically processed construction material to preserve the lake natural environments. The SCs were placed in holes formed by a self-penetrating vertically vibrating probe of a heavy steel casing into the ground through bottom feed. The impact of the installation was controlled through monitoring the ground movements by telltales installed along the edge of the adjacent road and ground vibration by seismographs. The subsurface soils densification around the SCs was evaluated through cone penetration testing (CPT) performed before and after the SC installations. A total of 393 SCs were installed to depths varying between 12 ft to in excess of 40 ft successfully. Ground settlements in range of 0.1 to 7.2 in. and vibration with sum peak particle velocity (PPV) from 0.1 to in excess of 1.35 in./s were recorded during the SC installation, causing hairline cracks and minor faulting on the adjacent roadway. This project demonstrates that geotechnical engineering practice has knowledge and tools to help design and construct sustainable civil projects.