Design, Construction Challenge and Quality Control of A Deep Soil Mixing System for Waterfront Slope Stabilization-Paper

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The paper describes the design and construction of a deep soil mixing (DSM) system to stabilize 3,500 feet of waterfront at a high-density development in Redwood City, California. The site contained loose fill over soft, compressible Bay Mud and potentially liquefiable sand. Seismic analyses indicated that the untreated shoreline could experience substantial lateral deformation, threatening planned structures.<br /><br />An interlocking DSM buttress was selected over steel sheet piles and auger-cast piles for cost and schedule reasons. Five-foot-diameter columns were installed with overlapping spacing and keyed into underlying stiff alluvial soil. The system was designed to resist static and seismic loads and reduce ground movement to tolerable levels. Vinyl sheet piles protected the DSM from water exposure and scour, while a flood wall retained engineered fill. DSM spoils were reused to raise site grades.<br /><br />Construction challenges included the confined, tidal site, unstable soft soils, tension cracking, buried debris, and old retaining structures. Timber mats reduced equipment pressure; pilot trenches helped control lateral pressures and cracking. Obstructions were removed, and temporary steel sheet piles restrained buried structures where needed.<br /><br />Quality control included real-time monitoring and daily reporting of mixing and installation parameters, along with full-depth coring of 3% of the DSM elements. Testing of 205 core samples showed 28-day strengths of 199–738 psi; none fell below the specified 90-day minimum threshold of 110 psi. Laboratory testing also indicated that DSM spoils had improved properties compared with untreated Bay Mud and could be reused as engineered fill.<br /><br />The project demonstrates that DSM can provide effective waterfront slope stabilization when supported by detailed seismic design, construction adaptations, and systematic quality control.

Keywords

deep soil mixing

DSM

waterfront stabilization

Bay Mud

seismic design

lateral deformation

interlocking buttress

soft soil improvement

quality control

Redwood City