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

PPT Presentation

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The C21 station forms part of Warsaw Metro Line II. Its approximately 18 m deep, top-down excavation was supported by perimeter diaphragm walls. Before excavation, a jet-grouted bottom plug was installed to limit water ingress and prevent wall movement that could affect nearby buildings.<br /><br />The ground included silty fine sand and clay, with groundwater 3–4 m below ground level. The plug was designed at about 35 m below ground level, near the diaphragm-wall toe, with a 2 m thickness. Its targets were a unconfined compressive strength of at least 0.5 MPa and permeability of 1 × 10⁻⁷ m/s. The design used nominal 3.5 m diameter columns on a 2.6 m triangular grid, with four cutoff walls dividing the station.<br /><br />Five trial columns and two verification methods—a resistivity-based Cyljet system and painted steel rods—were used to establish and confirm the jetting parameters. Production used double-fluid jet grouting, with verticality controlled to within 1%. In total, 4,515 columns were installed, representing approximately 46,100 m³ of grout.<br /><br />Quality assurance included calibration and painted-steel tests, strength testing of spoil and borehole samples, and borehole permeability tests. Testing met project requirements. Visual checks found column diameters of up to 4.75 m. The completed plug was successfully delivered, and excavation proceeded without problems while remaining within the specified seepage tolerance. The presentation emphasizes the importance of suitable jet-grouting parameters and equipment, verticality control, and rigorous quality assurance.

Keywords

Warsaw Metro

C21 station

Line II

jet grouting

bottom plug

diaphragm walls

groundwater control

double-fluid jet grouting

quality assurance

permeability