Impact of Overcut on Interaction Between Shield and Ground in the Tunneling with a Double-shield TBM

Double-shield TBMs (DS-TBM) are among the most technically sophisticated excavation machines in use by tunneling industry. The use of shields around the TBM allows the machine to pass through weak grounds and adverse geological conditions. However, there are limitations in applicability for DS-TBM in some ground conditions where large deformations are anticipated. The presence of the shield limits access to the tunnel walls for observation of ground conditions. This means limited possibilities of observing and analyzing ground conditions to avoid certain problems. Similarly, the presence of the shield does not allow the intrusion of the ground into the tunnel envelope, which is the main objective of using a shielded machine in the first place, yet it also creates the possibility of ground pressing against the shield. In such conditions, TBM may get stuck (including shield jamming and cutterhead blocking) in complicated geological structures, especially under high ground cover or in weak rocks, where large convergences are expected. This could cause major delays and impose a heavy and expensive burden on the tunneling operation. Some of the issues related to application of DS-TBMs in squeezing ground have been discussed in Hasanpour (2014) and Hasanpour et al. (2014a, b) and some possible scenarios and concepts for mitigating the related problems are offered. There are several performance parameters that should be considered with high accuracy at the design stage of a TBM for preventing machine entrapments. Size of the annular space or gap between ground and shields (created by overcut), length and diameter of shields, thrust force and torque, and machine advance rate are the most important performance parameters in tunneling by a shielded TBM. However, selecting the correct overcut, compared to other performance parameters, has a significant impact on preventing shield jamming. Selecting an appropriate or optimum value for overcut at the design stage of DS-TBM tunnel and implementing the predetermined overcut is the easiest way to address machine jamming, with the possibility of adjustment along the tunnel by using movable gage cutters. The adjustments can be directly related to ground properties and optimized to reduce the risk of machine jamming, while minimizing both the amount of material that is excavated and hauled out of the tunnel and the amount of grout that is placed behind the segments. For preventing the shield seizure, increasing the annular gap between the rock and shield is often utilized at the machine design stage. This feature can be included in the design of the cutterhead to accommodate a given overcut as a base design, and as needed, the excavated diameter of the tunnel, and hence the gap above the shield can be increased to react to bad ground where large convergences are & Rohola Hasanpour roha93@gmail.com