Hybrid Injections – Laboratory Testing and Field Experience During Tunnel Excavation-Paper

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The paper describes hybrid grouting for controlling severe water inflows during tunnel construction. Cement suspension and two-component polyurethane (PU) resin are injected simultaneously, with the PU content adjusted from 0% to 40% according to ground conditions and injection pressure. Cement is economical and penetrates small fissures, while PU expands on contact with moisture, reacts rapidly, and resists washout in large, water-bearing fractures. Their combination can seal larger pathways first and allow cement to fill smaller fissures.<br /><br />Laboratory tests found that increasing PU content substantially changes fresh-grout behaviour and shortens reaction time: mixes with 30–40% PU typically react within 2–4 minutes, compared with up to 15–30 minutes for 10–20% PU. PU addition reduces compressive strength, but higher PU contents produced no major further reduction; tensile strength was not significantly affected, and elastic modulus remained comparable to cement grout. Shrinkage decreased as PU content increased.<br /><br />The method was applied at Switzerland’s Ritom pumped-storage project after excavation encountered an unexpected inflow of about 135 litres per second in fractured rock. Hybrid injection screens reduced residual flow to less than 5 litres per second, enabling excavation to proceed mostly under dry conditions. The limited spread of grout also avoided unnecessary treatment of more distant fractures. Project comparisons estimated savings of more than 50% in time and cost versus the alternatives assessed.<br /><br />The authors conclude that hybrid injection offers a flexible, potentially economical way to treat variable, highly permeable ground. Success depends on suitable equipment, continuous monitoring and quality control, and trained operators; environmental requirements must also be addressed.

Keywords

hybrid grouting

tunnel construction

water inflow control

cement suspension

polyurethane resin

fractured rock

grout injection

Ritom pumped-storage project

groundwater sealing

water-bearing fractures