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Caisson
A foundation constructed at surface and sunk to its final position. Caissons can have solid or open bases depending on the ground they are being sunk into (generally open caissons are sunk into softer ground e.g. clay). Caissons can also be filled with compressed air, which provides dry working conditions when using an open-base caisson.

Canopy tube
canopy tubeA metal tube drilled into the tunnel face above the ground to be excavated, the tubes are pumped full of grout once in place. A series of tubes are drilled forming a ‘pipe umbrella’. This umbrella helps poor ground to arch over the tunnel, reducing the risk of crown failure.

Cantilever diaphragm walls
These are walls supporting the sides of an excavation which have no support at the top. The wall is prevented from tipping over by burying the base of the wall below the bottom of the excavation. Sometimes the cantilever diaphragm walls and cut and cover technique are combined with the use of a roof hat which is conciliated over the diaphragm walls and the spoil removed after.

Caulking
This is sealing a seam to make it watertight. This is often used in tunnels to seal around the segments.

Caulking groove
A detail formed in a construction joint to allow caulking to be placed if required.

Chainage
A measure of length along the alignment of a project.

Circumferential joint
A joint (typically between segments) which runs circumferentially around the tunnel bore. Except in rock tunnels where the TBM uses grapples.

Cladding
This is lining placed on the internal surface of the tunnel. Cladding provides a smart finish, can be fireproof and is easily retro-fitted. It is also expensive to replace and can be damaged easily be errant vehicles.

Closed-form-solutions
Perfect analytical solutions to (often theoretical) problems. Closed-form solutions allow quick calculation of problems by making assumptions and likening reality to an idealised situation.

Cofferdam
A temporary wall built to keep water out of a construction area. These will often be constructed of diaphragm walls or piles sunk into the ground around the construction area. Triad is removed by grate in dry or wet condilim.
Example presentation on building a cofferdam tunnel accross a river

Compressed air
It is used during excavation to apply pressure to the face and prevent the face coming in. Also to prevent water influx and keep the excavation dry. This method carries health and safety implications as it involves miners working in pressurised air conditions.
Used to gain access to face when using enclosed face TBM in certain eradiation.

Compensation grouting
A method of reversing ground settlements by injecting grout into the ground. The volume of grout must be carefully controlled in order to prevent heave whilst eliminating settlement.

Convergence
A measurement of the movement of the walls of a tunnel inwards. The walls will tend to deflect in with the load of the ground on them, this is often monitored to provide information on the performance of the lining during construction.

Contiguous pile wall
contiguous pile wallA piled wall where the piles are placed adjacent to each other with small gaps between the piles. This type of wall is used only where ground conditions are suitable.


Conveyor
Used in tunnelling to remove excavated material from a tunnel face or shaft. The conveyor takes the material from the face to the tunnel spoil handling point, where it is dealt with.

Crossover
crossoverUsed in rail tunnels to allow trains to access different lines / platforms. Often designed as a large cavern with multiple tunnels joining it.


Cross passage
A small tunnel used to connect between adjacent bores in a multiple-bore tunnel. Cross passages provide a means of escape from an incident bore and allow equipment to be placed out of the main bores. It also provides access for O/M.

Crown
The highest point of the internal curved surface of a tunnel cross section.

Culvert
It is a small channel or drain used to carry water beneath an obstacle (e.g. road).

Cutterface
This is the front side of a tunnel boring machine.

Cutterhead
cutterheadThe head at the front of a tunnel boring machine used for cutting into the ground. Cutterheads have different designs depending on the type of ground they are built to operate in. Hard rock conditions are tackled by installing cutting discs which shear the rock off the face. Softer conditions require picks to be fitted to the face, these scoop away the ground.

Cut and cover tunnel
A method of tunnel construction involving excavating a trench, installing the structure and covering it over. This method is typically used for shallow tunnels. Great care is required to ensure the walls of the excavation are well supported while the structure is installed.

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