Curtain Grouting in Hard Crystalline Rock Using Ultra-Low Viscosity Polyurethane Resin-Paper

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The paper describes post-grouting work to control groundwater leakage into a 24 m-deep underground digester at the Käppala wastewater treatment plant near Stockholm. The surrounding quartzite contained numerous fine, water-bearing fissures that conventional cement grouting had not sealed. Leakage was initially about 42 m³ per day, and measurements indicated that the required reduction in rock permeability was unlikely to be achieved with cement alone.<br /><br />The team installed an additional curtain around the digester using split-spacing primary, secondary and tertiary boreholes. Water-pressure tests generally showed low Lugeon values, while core drilling identified open fissures smaller than 0.2 mm. A specially developed, two-component polyurethane resin was selected for its low viscosity, extended reaction time and limited foaming. These properties enabled penetration into fine cracks while allowing low pumping rates and pressures, reducing the risk of widening fissures or damaging the concrete slab.<br /><br />The grouting campaign reduced leakage by more than 90%. Leakage fell from approximately 42 m³ per day to 16 m³ per day after grouting, and to below 2 m³ per day after supplementary sealing near an access tunnel. Rock-water conductivity was reduced from roughly 10⁻⁷ to 10⁻⁹ m/s. As a result, the planned internal steel or concrete lining was no longer necessary.<br /><br />The authors conclude that low-viscosity polyurethane can effectively seal fine fissures that cement grouts may not reach, but success depends on selecting suitable materials and equipment and having an experienced design and site team.

Keywords

post-grouting

groundwater leakage

underground digester

polyurethane resin

cement grouting

rock fissures

quartzite

Lugeon tests

water permeability

grouting curtain