What is the purpose of a thrust ring during TBM launching?

During TBM launching, a thrust ring serves as a temporary structural element that distributes the high thrust forces generated by the TBM's jacking system evenly across the face of the launch tunnel or shaft. The primary purpose is to prevent damage to the initial tunnel lining or the surrounding ground during the TBM's initial push into the earth. The thrust ring is typically a large, heavy-duty steel ring that is installed at the tunnel portal or within the launch shaft. It provides a stable and rigid surface against which the TBM's jacking system can push. By distributing the thrust forces over a larger area, the thrust ring reduces the stress concentration on any single point, preventing localized failures or deformations of the surrounding ground or lining. The design of the thrust ring must consider the TBM's maximum thrust capacity, the geometry of the launch tunnel or shaft, and the strength of the surrounding ground. The ring must be strong enough to withstand the thrust forces without deforming or failing. The contact area between the thrust ring and the surrounding ground is often increased by using a layer of concrete or grout to provide a more uniform bearing surface. The term 'thrust ring' refers to a temporary structural element used during TBM launching. 'TBM launching' refers to the process of starting the TBM's excavation from the launch shaft or tunnel portal. 'Jacking system' refers to the hydraulic cylinders used to push the TBM forward.