Large-Scale Post-Grouting Works in the Stockholm Bypass-Paper

Paper
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The Stockholm Bypass is a major underground highway project whose northern tunnels encountered more fractured, water-conductive rock than pre-construction investigations predicted. Elevated water ingress threatened groundwater levels and compliance with environmental limits, prompting systematic post-grouting with cement-based grout.<br /><br />The paper describes three drilling geometries designed to intersect fractures that conventional pre-grouting may have missed, particularly structures parallel to the tunnels. Grout mixes of differing water-cement ratios are selected according to fracture size, with controlled pressure and fixed grouting-time criteria. Areas for treatment are identified through geological mapping and water-ingress observations.<br /><br />Post-grouting performance is evaluated using grout take per hole, visualized in spatial grids and histograms, alongside geological mapping and changes in visible water seepage. These data help identify areas that may need further treatment. Water ingress is monitored using flow-measuring weirs and, occasionally, temporary pumps. However, measurements are difficult to interpret: monitoring sections are often much longer than the treated areas, and readings are affected by tunnel construction, groundwater recharge, weather, and equipment variability. Consequently, decisions about additional grouting often rely more on initial water ingress and grout-take results than on short-term flow measurements.<br /><br />The effectiveness of post-grouting varies from substantial reductions in ingress to no measurable change. Challenges include limited rock confinement after excavation, grout leakage back into the tunnel, difficulty reaching effective pressure, and ensuring micro-cement is properly dispersed to penetrate small fractures. Geological conditions—especially connected weakness zones, shallow weathered rock, and the tunnels’ large, flat profile—also constrain success.<br /><br />Overall, grout-take data provide useful and relatively reliable support for evaluating treatment, while water-ingress trends are slower and more difficult to distinguish from other influences. The authors emphasize continuous monitoring and adapting the design to local conditions.
Keywords
Stockholm Bypass
underground highway
post-grouting
cement-based grout
fractured rock
water ingress
groundwater levels
drilling geometries
grout take
geological mapping