DAUB recommendations for the selection of tunnelling machines - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 366

11.3 Definitions and abbreviations

11.3.1 Definitions
Abrasiveness The abrasiveness describes the influences resulting from the geology on
the wear of tools. The abrasiveness of hard rock is often characterised with the CAI test
(Cerchar Abrasivity Index) and that of soft ground often with the LCPC test (Test of the
“Laboratoire Central des Ponts et Chaussées”), in addition to the mineralogical composi-
tion and strength parameters.

Active face support Measured and monitored support of the face by a suitable medium
(for example slurry or remoulded earth) based on a support pressure calculation.
Air pressurisation The excavation chamber is pressurised with compressed air to hold
back groundwater. Support against ground pressure is only possible in almost imperme-
able soil or if the face is sealed, e.g. by a filter cake.

Annular gap Cavity between the sides of the excavated cavity and the outer face of the
segments.
Articulated shield Shield machine with more than one shield section, which are articu-
lated with active steering cylinders or passive hydraulic cylinders in order to improve
steering around curves.

Blowout Uncontrolled escape of compressed air to the ground surface or riverbed asso-
ciated with loss of support effect.
Breasting (plates) Additional mechanical support to the face with extendable plates.

CAI (Cerchar Abrasivity Index) Value from the test of the same name for the characteri-
sation of the abrasiveness of solid/hard rock.
Clamping units Side-mounted bracing apparatus in double shield machines intended to
transfer the thrust forces radially into the surrounding rock mass, resist rolling and stabi-
lise the tunnelling machinery.

Closed mode In closed mode, the excavation chamber of a tunnelling machine is held
under a measured and monitored positive pressure. The pressure is applied by slurry, re-
moulded earth or also compressed air.
Cuttability The facility of excavating rock with a tunnelling machine depending on the
rock properties. The most important process technology parameters for the quantitative
description of the ease of boring are the cutterhead penetration and the contact pressure.
Cutterhead A tool carrier in hard rock tunnel boring machines fitted with disc cutters for
full-face excavation. In soft ground, the term cutting wheel is normally used.
Cutting wheel Mechanism for the full-face excavation of a tunnel cross-section in soft
ground. The ground is excavated as the wheel rotates and the design and tool equipment
of the wheel are suited to the relevant ground properties. In hard rock, the term cutterhead
is normally used.

Disc cutter (disc) Hard rock tool with a rotating hardened cutter ring, which can destroy
the structure of the rock at the face.