These TBMs have the excavation front face physically separated from the tunnel as their main characteristic so the excavated material is stocked in a chamber located between the Cutting wheel and the machine.
Excavation: Close shields perform the excavation by means of a Cutting Wheel which fully covers the tunnel section, meaning that the excavation is performed as a full-face excavation, in contrast to the Open Shields and the conventional tunneling methods.
Advance: The close shields advance depends on the tunnel lining type. If the lining is a reinforced concrete pipe, the process is based on pushing the entire tunnel form the jacking frame. Otherwise, if the tunnel is covered by segmental lining, the machine will push itself against the last installed ring for the advance, leaving each segmental ring located at their final location.
Material extraction: In this case, the close shields may have different extraction methods of the excavated material, including wet extraction, dry extraction or semi-wet extraction.
Wet extraction defines the Slurry TBMs or Hydroshields which their basic performance principle comprises the extraction and the face support by means of a thixotropic fluid, mainly made of water, bentonite and additives, which also allows to transport the excavated material by suspension into the fluid and the mixing into the excavation chamber.
Dry extraction or semi-wet extraction, depending on the material and the additives included in the process, consist on that once the material is retired form the tunnel face, falls into a chamber with an conveyor belt or screw conveyor which pours the material into a wagon, allowing the transport of the material out of the tunnel.
Application: The applications for these kind of shields are very variable and diverse, from hard and abrassive rock, to very soft and non cohesive soils. The use of one or another close shield type will principally rely on the ground classification, which will define mainly the material extraction type as shown in the following figures.