Grouting Methods Involved in Stabilization of TBM Tunnel near CP-06, UAA-04, Chennai Metro-Paper

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CP-06, an NATM cross passage connecting the up-line and down-line Chennai Metro tunnels near Kilpauk Medical College, was excavated through loose, permeable silty sand with a high groundwater level. Jet grouting proved ineffective under the high water pressure, so Tube-a-Manchette (TAM) grouting was carried out from the surface. Although the heading was excavated successfully, heavy seepage began during bench excavation, carrying an estimated 12–15 m³ of sand and silt into the tunnel.<br /><br />The inflow worsened after heavy rains and created voids beneath the running tunnel, causing settlement, lateral movement, ovalisation and cracks in tunnel rings. Road-level sinkholes also formed. Temporary props, bracing, shotcrete and sandbags were installed, while utilities were investigated and repaired.<br /><br />Stabilization involved extensive grouting from both the surface and inside the tunnel using ordinary Portland cement, microfine cement, polyurethane and sodium silicate. Cement was used to fill and strengthen soil, while polyurethane helped stop water inflow. Electrical resistivity surveys and dynamic cone penetration tests were used to identify weak zones and check treatment effectiveness.<br /><br />Finite element hydraulic analysis using RS2 indicated that water was entering mainly through the cross-passage invert, where the hydraulic gradient exceeded the critical gradient for the soil. The resulting erosion and loss of ground support explained the tunnel deformation. The incident suggested that TAM grouting had not adequately treated the ground below the invert.<br /><br />After seepage and movement were controlled, damaged segments were repaired and strengthened with steel rings. The cross passage was later excavated using a revised sequence and completed in May 2017. The authors emphasize the need to verify grouting below the invert, monitor tunnel movements, and use investigation results to guide targeted treatment before removing temporary supports.

Keywords

NATM cross passage

Chennai Metro tunnels

TAM grouting

groundwater seepage

silty sand

tunnel settlement

sinkholes

electrical resistivity survey

finite element hydraulic analysis

tunnel deformation