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secant pile wallSecant pile wall
A method of constructing a concrete wall in poor ground by means of a continuous row of concrete-filled boreholes, alternate holes are bored in two successive series such that adjacent piles overlap.

Secondary lining
Lining in addition to primary lining for decoration, improved fluid flow, protection, structural enhancement or other purposes.

segmentSegment
Arc shaped preformed component that forms part of the tunnel or shaft lining.


Settlement
Downward movement of the ground surface.

SGISGI
Spheroid Graphite Iron, a form of segmental cast-iron tunnel lining.





Shaftsshaft
A shaft is a vertical or steeply inclined excavation used as a passage from the surface to the workings, used for ventilation, travelling, hoisting, or all three. Shafts are usually of limited cross section in relation to their depth.



shieldShield
A protective tube used in soft ground, inside which a TBM works, the shield eliminates timbering.

Shield driven
Method of excavation in the front of a tunnel or pipe jack using a shield. (see shield)


shot creteShot crete
A commonly used term for mortar or concrete sprayed through a hose and pneumatically projected at high velocity onto a surface.



Side drift
Continuous ribbed support, usually steel for construction of tunnels.

Single pass
A tunnel which only has one layer of lining.

Slab
Usually is a concrete section used for supporting loads, providing cover, or in the case of a base slab, acting as a plug against water filling a shaft.

Slurry
A mixture of bentonite and water.

Slurry shield
Method using a mechanical tunnelling shield with closed face which conditions the ground and employs hydraulic means for removing the excavated material and balances the ground water pressure.

Soffit
The highest part of the underside or an arch shape.

Soft ground
Normally consisting of sands/gravels, extra consideration is required in tunnelling through this material, as soft soils are unstable over a certain period and must be considered as less predictable than hard rock.

Soil nail
Slender elements (usually steel reinforcing bars) inserted into the ground to act as ground improvement.

Spile
Bars inserted into a tunnel face to act as a form of ground improvement.

Spoil
Earth material from an excavation.

Sprayed concrete (lining)
SCL is an established method of tunnelling using sprayed concrete to support the excavation both temporarily and permanently. (see shotcrete for picture)

Spring line
This is the point where the curved portion of a tunnel roof meets the top of the wall. In a circular tunnel the spring lines are at the opposite ends of the horizontal centreline.

Squeezing rock
Difficult tunnelling ground conditions characterised with (usually) the rock being strongly jointed and fractured and having low strength.

Stank
A method of damming up a water course or channel/pipe to control water flow.

station concourseStation concourse
This is a large open area where people can gather.




Steel fibre reinforced concrete (fibre)
A concrete mix that contains short discrete steel fibres that are uniformly distributed and randomly oriented throughout the mix opposed to conventional steel rebars used in reinforced concrete.

Steel sets/arches
Steel support structure for tunnel construction.

Step (plate junction)
Where two tunnels lined with plates of different diameters meet, special vertical plates are required to close the vertical faces - so forming a step. Often in situ, concrete is used instead of plates to avoid the heavy cost and time delay in making special plates.

stubStub
Small tunnel structure, constructed for ease of connection to future tunnels.



Sump
A pit in which water collects before being baled or pumped out.

Surge tunnel
A tunnel constructed to take into account the surge pressures associated with the flow of water.

^{If you think we have omitted a term, email us at tunnels.webmaster@mottmac.com}