These kind of TBMs show characteristics from grippers and from close shields, with a telescopic shield articulated into two shields, one acting as a simple close shield and the second including the struts of the gripper system for grounds with hard soils or rocks.
Excavation: The double shield performs the excavation in an equivalent way as the close shields, meaning that it is a full-face excavation by means of a Cutting Wheel which includes different set of tools like cutters, rippers and scrappers.
Advance: The double shield TBMs advance comprises mainly 2 phases, since the presence of the telescopic shield allows continuing the excavation during the installation of the segmental lining. In the first place, the advance is the same as in a close shield by pushing the machine against the tunnel lining and, in the second place, once the necessary length for the next ring installation is reached, the gripper struts are activated so that the advance can be continued by directing the forward push against the excavated ground walls.
Material extraction: This kind of tunneling machine performs the excavated material extraction by dry or semi-wet extraction, in a similar way as the Earth Pressure Balance (EPB) TBMs.
Application: The double shield TBMs are only used in ground with good support capacity, either rocks or cohesive highly resistant materials, due to the gripper advance system, and always under no phreatic level presence conditions.