Jet Grouting for Deep Excavation Support at Re-entrant CornersÂ-Paper

Paper

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The paper examines jet grouting used to seal gaps in diaphragm and secant pile walls at re-entrant corners of a deep New York City subway excavation. Existing utilities prevented wall installation, so overlapping jet-grout columns were installed in clusters around or near electrical duct banks to provide temporary excavation support and groundwater control.<br /><br />Two approaches were compared. At Entrance 3, contractors used one set of jetting parameters across the soil layers to form initial columns, followed by a second pass targeting the organic layer. At Ancillary 2, parameters were adjusted for each soil type, with columns formed in a single pass. Both approaches generally produced columns with reasonable continuity and met the specified 400-psi strength requirement. The simpler, two-pass approach gave more consistent results, though it was potentially more conservative because its parameters had to suit the most challenging soil conditions.<br /><br />Spoil specific gravity varied with grout-mix consistency, compressed-air use, soil type, and the proximity, age, and installation sequence of neighboring columns or embedded walls. The authors propose the Specific Gravity Ratio (SGR)—spoil specific gravity divided by the actual grout specific gravity—as a qualitative indicator of soil removal. SGR values at or above 1 generally indicate effective erosion, although air entrainment can make the measure less reliable.<br /><br />Installation sequence mattered: later columns could remove less soil when adjacent columns had hardened, while closely spaced work could disturb columns that were still soft. Verification coring found generally good continuity, and laboratory strength tests at Entrance 3 exceeded the target. Excavation exposed blocks that provided effective support and watertightness. The paper concludes that SGR can help monitor jetting, but coring and testing remain essential to confirm column continuity and strength.

Keywords

jet grouting

diaphragm walls

secant pile walls

deep excavation

subway excavation

groundwater control

jet-grout columns

Specific Gravity Ratio

soil removal

verification coring