Jet Grouting for Strength in Marine Sediments with Hydrocarbons-Paper

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The Charlestown Bus Garage shoreline stabilization project used overlapping jet-grouted columns to support an earthen embankment along the Mystic River. Because the site contained soft marine sediments and organic soils, pre-production field tests were required to verify column strength. Testing unexpectedly identified fuel oil and coal-tar hydrocarbons in sediments, along with highly variable strengths and localized uncured material.<br /><br />A geo-probe investigation mapped contamination from approximately Sta. 1100 to 1800. Most measured hydrocarbon concentrations were below 10,000 ppm (about 1% by wet weight). A second field test program compared grout mixes, air pressures, and installation methods in contaminated and uncontaminated areas. Results showed that lower air pressure generally produced higher unconfined compressive strength (UCS): columns installed with 0–2 bar air pressure achieved minimum UCS values of about 450 psi, while medium- and high-pressure tests sometimes fell below the 250 psi design requirement. Double-pass columns showed improved core uniformity.<br /><br />Production proceeded using low air pressure, with double-pass installation reserved for areas with unusually high contamination. Quality-control cores showed that the 250 psi design strength was generally achieved across the site, regardless of hydrocarbon presence. Isolated low results were investigated with additional cores and were found to reflect localized variability; average column strengths met requirements. All production columns were accepted, and double-pass columns were not needed.<br /><br />The project demonstrates the value of comprehensive field testing, environmental investigation, and production quality control when ground conditions are uncertain. Literature review helped inform decisions, but site-specific testing was essential. The authors also emphasize that contaminant type and concentration, soil variability, and contractor-selected installation parameters all affect performance; collaborative problem-solving among the owner, designer, and contractor was key to success.

Keywords

Charlestown Bus Garage

shoreline stabilization

jet-grouted columns

Mystic River

marine sediments

hydrocarbon contamination

field testing

unconfined compressive strength

quality control

ground improvement