Intervention for Improving Characteristics in Fault Zone in Tunnel Alignment - Consolidation Using Chemical Grouting with Mono and Bi-Component Resins-Paper

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The paper describes ground treatment used to free a double-shield tunnel boring machine (TBM) trapped in a water-bearing fault zone during construction of a 2.85 km power-line tunnel in southern Italy. The tunnel crossed highly variable crystalline-metamorphic rock, including faults containing severely broken, clay- or sand-like material. Four fault zones caused TBM stoppages; the most difficult event occurred near kilometre 2,100.<br /><br />At this location, the TBM broke through a clay layer that had confined pressurized water. Water and material then flowed cyclically into the excavation chamber and shields, filling and blocking the machine. The fault zone was more than 20 m thick, with water pressure of about 13 bar and flows of 50–80 litres per second.<br /><br />Before grouting, crews used drilling through the TBM shield to investigate the fault and installed self-drilling drainage pipes. Their large water-soluble valves resisted clogging by fine material. Drainage reduced hydraulic pressure and stopped the recurring inflow, improving conditions for resin injection.<br /><br />The team injected mono-component resin to fill voids and protect the TBM, followed by bi-component resin to strengthen the collapsed ground. Because conventional injection pipes could not be installed in the unstable, water-bearing material, self-drilling bars were used. Laboratory-scale trials helped determine resin mixes and setting times that would provide strength while allowing the resin to spread despite residual water. Injections were arranged on a one-metre grid, and seismic and geoelectric tomography assessed the treated ground between stages.<br /><br />The combined drainage and grouting program consolidated the fault sufficiently for the TBM to pass through and resume excavation. The case demonstrates how tailored drainage, resin selection, staged injection, and verification testing can address severe water inflows and unstable fault material around a trapped TBM.

Keywords

ground treatment

tunnel boring machine

water-bearing fault zone

drainage pipes

resin injection

grouting

hydraulic pressure

fault zone stabilization

self-drilling bars

tunneling