Warsaw Metro Project C21 Station Soil Bottom Plug Using Big Diameter Jet Grouting Technique

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Warsaw Metro Line II’s C21 station required a deep soil plug to control groundwater inflow and reduce uplift and settlement risks during excavation. The station’s 19,964 m² excavation was enclosed by a diaphragm wall, and the plug was installed near the wall toe, approximately 34–36 m below ground.<br /><br />The solution comprised 4,515 large-diameter jet-grout columns, totaling about 46,100 m³. Columns were designed to be 3.5 m in diameter, arranged on a 2.6 m triangular grid, with a 2 m plug thickness. The ground consisted mainly of silty fine sand, with SPT-N values of 20–25. A specialist double-fluid jet-grouting monitor on single-pass rigs was used to deliver the required energy.<br /><br />Five trial columns were constructed to optimize production parameters. Cyljet electrical testing and painted-steel checks were used to assess column diameter; these informed a production energy target of about 270 MJ/m. Production began in September 2019, with two rigs achieving approximately 25 m of jetting per rig per day and less than 1% verticality deviation.<br /><br />Quality assurance included diameter checks, unconfined compressive strength testing of spoil and cores, and permeability testing at column intersections. The project specified a minimum UCS of 0.5 MPa and maximum permeability of 1 × 10⁻⁷ m/s. Reported permeability results met the specified limit, and the authors concluded that the plug passed the project’s QA/QC requirements. The station excavation was subsequently completed within the specified seepage tolerance.<br /><br />The case study demonstrates how trialing, high-energy double-fluid jet grouting, and multiple verification methods can be combined to construct a large, deep seepage-control plug in challenging sandy ground.

Keywords

Warsaw Metro

C21 station

deep soil plug

jet grouting

groundwater control

diaphragm wall

silty fine sand

quality assurance

permeability testing

uplift settlement control