Influence of synchronous grouting on surrounding sandy cobbles during shield tunnelling process: Insights from a scaled model test

Substantial soil deformation around tail-grouting activity is frequently observed during the shield tunnelling process in the sandy cobble strata, which is attributed to the convergence and potential collapse of the soils around the tail-void left behind the shield machine. The stability of the underlying strata is significantly impacted by synchronous grouting inside the tail-void. This study examines four-opening synchronous grouting, which is frequently used in shield tunnelling of metro-tunnels, to analyze the grout-spreading mechanism and soil displacement generated by the synchronous grouting in the sandy cobble layer. Laboratory tests are used to determine the characteristics of grouting mortar and soils in accordance with the similarity theory. The model experiment outcomes show that a rise in the GVR (Grouting-Volume Ratio) can cause the settlement at the ground surface. However, as the GVR improves, the pace of settlement growth diminishes. Additionally, the expanding cover depth of the tunnel contributes to the stability of the soil after synchronous grouting. As observed in the model test, when GVR achieves its maximum amplitude (i.e., GVR = 200 %), the grout mortar cannot spread to the crown of the tail-void, necessitating secondary grouting after the synchronous grouting during tunnelling practice. After grout hardening process, it was discovered that grout material above the tunnel waist was concentrated near the upper grout openings, implying a seepage spreading mode of the grout-mortar at these locations. In contrast, grout material below the tunnel waist was evenly distributed along the periphery of the tail void. The model test outcomes may be utilized as a guide for tail-grouting while shield tunneling in sandy cobble strata.