What is the primary difference in application between a single shield and a double shield Hard Rock TBM?

The primary difference lies in the timing and method of applying ground support. A single shield TBM progresses by extending grippers against the tunnel walls for stabilization, then advancing the cutterhead to excavate, and subsequently installing the tunnel lining sections within the protection afforded by the shield. It is suited for geological conditions where the excavated tunnel face and surrounding rock mass maintain stability, even briefly, before lining installation. Conversely, a double shield TBM utilizes both a front and a rear shield. Following the front shield's gripping action, the rear shield extends forward to facilitate the placement of tunnel lining, while still benefiting from the front shield's protective structure. This enables concurrent excavation and lining processes, making it ideal for less stable conditions or situations necessitating rapid advancement. The double shield's key benefit is providing continuous and immediate ground support through simultaneous excavation and lining, particularly crucial in fractured or squeezing rock formations susceptible to deformation. The 'shield' refers to a protective cylindrical structure that prevents collapse. 'Grippers' are hydraulic mechanisms using opposing force against the tunnel walls to propel the TBM. 'Tunnel lining segments' constitute the permanent structural framework of the tunnel.