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EPB (Earth Pressure Balance machine)
A type of tunnel boring machine which retains a prescribed amount of excavated soil in the cutter head. Hydraulic jacks are used to force this soil against the face of the tunnel, ensuring the ground remains stable. Normally used in granular soils.

Elephant’s foot
The enlargement of a tunnel lining at the joint between the side walls and the invert.

Erector
A mechanical arm present in a tunnel boring machine which is used to put the tunnellining segments into place.

Escape sets
Emergency breathing apparatus which may be used in an underground space if the oxygen level reduces.

Extrados
The outside face of a structural element ie the tunnel extrados.

Extensometer (inclinometer)
A device for measuring the change in distance between two points. Often used for measuring the ground movement induced by tunnelling.

Expanded lining
Primary lining that consists of tunnel segments that are expanded circumferentially against the surrounding ground.

Eye (tunnel eye)
The start of a tunnel, normally at a junction between a shaft and a tunnel.

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