What is the purpose of pressure control in grout injection behind tunnel linings?

Pressure control during grout injection behind tunnel linings is crucial for several reasons, all aimed at achieving effective ground support and preventing damage to the lining itself. The primary purpose is to fill voids between the tunnel lining and the surrounding ground completely and uniformly. This ensures that the lining is fully supported and that loads are evenly distributed. Without proper pressure control, the grout may not reach all areas, leaving voids that can lead to instability and settlement. Controlling the pressure also prevents over-pressurization, which can damage the tunnel lining segments or cause them to move out of alignment. Excessive pressure can also cause the grout to flow into unwanted areas, such as drainage systems or adjacent tunnels. The injection pressure is carefully controlled to match the ground conditions and the lining's structural capacity. In weak or fractured ground, a lower injection pressure is used to avoid fracturing the surrounding rock or soil. In stronger ground, a higher pressure may be used to ensure that the grout penetrates into smaller voids. Monitoring the grout flow rate and pressure provides valuable information about the effectiveness of the grouting process. A sudden drop in pressure or an increase in flow rate may indicate that the grout is flowing into a large void or a fracture. Pressure control also ensures that the grout cures properly and achieves its desired strength. Excessive pressure can disrupt the curing process and weaken the grout. 'Grout injection' refers to the process of injecting a pumpable material into the ground to fill voids, stabilize the soil, or reduce permeability. 'Tunnel lining' refers to the structural support system that is installed inside the tunnel. 'Void' refers to an empty space or cavity.