New frontier in post-grouting of tunnels in hard rock

Paper in proceeding, 2012

Post-grouting is a difficult task. It is often coupled with high costs and a result that is difficult to quantify. An infrastructure tunnel in an urban city in southern Sweden was used to develop and implement design methodology for post-grouting. The water inflow measured in weirs prior to post-grouting was about 3.8 l/min/100 m. The task of sealing low water inflows to achieve even lower ones takes a lot of effort and determination. The tunnel was built in the 1970s using the traditional drill-and blast method. During the pre-investigations of the project the analysis showed very good pre-grouting, giving a sealed zone of almost 10 meters. The tunnel had a large, sealed zone with a smaller hydraulic gradient and only narrow fractures were left unsealed. The grout chosen was the newly developed silica sol for the presumed narrow fractures and a robust cement grout if large inflows were hit by the grout holes. The tunnel is small, roughly 12 m2 and heavily congested with cables and heating pipes, making it difficult to use traditional machinery. The tunnel section, 90 m in length, was divided into three different sub-sections, each with specific goals. The result showed that the strategy developed worked well and production was more rapid than presumed. The targeted water inflow rate of 1 l/min was met. © 2012 American Society of Civil Engineers.