Pre-grouting is an essential part in the low-cost tunnel concept. It is also work to be carried out on the critical line. It is therefore important to correctly describe the conditions for the works in the technical specification. Intense research has clarified the theoretical base for pre-grouting. However, describing the water bearing structures of the rock mass is still a challenge. It has in many projects been found that estimation of the grouting effort regarding number of grouting fans, hole spacing, grouting sequences, grouting pressure and number of re- grouting fans is to a large extent related to geology.
The tunnels in the Stockholm By-Pass are pre-grouted. Totally more than 2500 grouting fans have been carried out. The grouting efforts have been followed up and adapted to geology in order to fulfill the requirements of acceptable groundwater ingress to the tunnels. The objective of this paper is to describe the achieved experiences from the follow up.
The follow up, shows and confirms the intuitive conclusion that the achieved sealing efficiency and tightness depends on the grouting effort, more specifically to a significant extent on the number of
grouting rounds. It has also been found that the need for grouting also to a large extent depends on the geology. We have found that the hydrogeological conditions of the rock mass can be divided into 3 classes as simple, open and complex flow paths. Simple flow paths are characterized by open unfilled fractures in massive rock with relatively few discontinuities more or less perpendicular to the tunnel alignment. Complex flow paths are characterized by channels (1D flow) possible together with 2D flow in less connected networks. The fractures are often filled. Geologically, the rock can in many cases be described as fault or fracture zones. Open flow paths are often connected with large grout takes but with
2D flow and generally good grout results. In the paper it is suggested how to analyze the grout take to identify the hydrogeological conditions and to verify the grouting results. It is also suggested how to use the data during the execution of the work to decide whether to continue excavation or when an additional complementary grouting round is mandatory.