ABSTRACT
When underground rock constructions are excavated, it is common practice to reduce the water ingress into the tunnel by grouting. In hard rock, the water ingress mainly occurs in the open fractures inside the rock mass.
When conducting a grouting design, it is important to have a proper understanding of the hydrogeological conditions in the rock mass to correctly assess the hydraulic gradient which is acting on the grout. One phenomenon that occurs and damages the grout is viscous fingering. Viscous fingering occurs when a more viscous fluid is displacing a less viscous fluid, creating the appearance of “fingers” in the more viscous fluid. The viscous fingering will occur in the region where the pressure gradient of the grout is lower than the hydraulic gradient, resulting in a poor grout spread. Today, when a grout design is conducted, the penetration length is of high importance. However, the designed penetration length includes the region where viscous fingering occurs, which in turn results in an incorrect design or poor grout quality.
To study this, a mathematical model is derived from the Navier Stokes equations to predict the region where viscous fingering occurs during grouting of cement. In conjunction with this mathematical model, a common type of cement grout is characterized by conducting rheological measurements which are then implemented into the mathematical model.
The results show that stable grouting can be achieved in the early stage of process and be mechanically stable over a long term by properly predicting the region where viscous fingering occurs.
