Jet Grouting in Stratified Soils

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The study examines single- and double-fluid jet grouting for a deep excavation in stratified Manhattan soils: fill, organics, silty sand, and varved glacial silt. Jet-grouted columns formed a support slab and gravity wall, and sealed gaps between secant piles. Trial and production performance was assessed using grout and spoil specific gravity, spoil strength, and verification cores.<br /><br />The Specific Gravity Ratio (SGR)—spoil specific gravity divided by the specific gravity of the grout actually injected—was proposed as a qualitative indicator of soil removal. SGR values above 1 generally indicated that soil had been cut and mixed into the grout. However, compressed air, grout variability, installation sequence, and nearby hardened columns or obstructions affected spoil measurements, so SGR was not a definitive measure of column quality.<br /><br />The findings emphasize extending starter casings through the fill and organics into the silty sand to reduce borehole instability. Double-fluid grouting generally produced greater spoil-specific-gravity variability, likely because of air-induced turbulence. Single-fluid grouting produced stronger spoils and greater strength gain, although double-fluid grouting’s compressed-air shroud improved soil cutting in varved silt. Spoil strength at 28 days provided an indication of the lower-bound strength of hardened grout cores.<br /><br />Verification cores met the project’s specified strength and stiffness requirements, but results varied widely and some cores contained untreated or incompletely mixed soil, highlighting the need to account for defects in design. Varved-silt cores were more vulnerable to drilling disturbance than silty-sand cores; their failure behavior ranged from brittle to ductile, while silty-sand cores failed brittly. Core recovery in varved silt was generally poorer when coring began within about 91 days of grouting.

Keywords

jet grouting

deep excavation

Manhattan soils

Specific Gravity Ratio

spoil strength

single-fluid grouting

double-fluid grouting

varved glacial silt

verification cores

secant piles