Abstract:
BESAB was contracted in April 2019 to seal a rock blasted underground sludge tank from leaking groundwater at a sewage treatment plant in the Stockholm area. Before excavation of the 24 m deep tank began during the construction time, the rock mass was pre grouted using cementitious suspensions, forming a ring curtain. The rock surrounding the tank consists mainly of quartzite, both competent or fractured by large and small cuttings. Fine and frequent water-bearing fissures largely dominated the surrounding rock causing groundwater leakages estimated to 46 cubic meters per day into the tank, lowering the effectivity of the plant. Previous grouting using cementitious suspensions had removed most leaks, however not within finer fissures. Based on the dominant direction of the fissures it was decided to repeat sealing as a post grouting in the form of an additional curtain around the tank, but instead now using polyurethane resins. Curtain holes were drilled according to the split spacing procedure with primary, secondary and, tertiary holes and consequently grouted. Core drillings detected a dominancy of open fissures smaller than 0, 2 mm. Water tests performed in the new grout holes, executed at three different levels showed values of less than LU = 3, 0. Therefore a two-component ultra-low viscosity polyurethane resin was chosen to avoid further rock fracturing caused by grouting. The polyurethane resin was specially developed with minimal foaming factor and extra prolonged start of thickening. The target was to ensure the expected and desired radius of penetration of the polyurethane while applying low pumping rate and pressure. The final reduction of groundwater leakage was over 90%. The rock water conductivity was reduced from 10-7 m/s to approx. 10-9 m/s. The client’ s plans to carry out an internal steel or concrete lining after the grouting was therefore never needed. Keywords: curtain grouting, hard rock, polyurethane