The International Society for Trenchless Technology

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Term Definition
Entry Ring

See Launch Seal

Exit Seal

Same as launch seal except for the retrieval of the machine at the reception shaft. Used in high ground water to prevent the loss of ground.

Exit Shaft

See Reception/Exit Shaft/Pit

Eye (Pipe Jack Eye)

A sealing device placed in the shaft walls of the thrust and reception pits to allow the shield and pipes to enter and exit without soil, water and slurry flowing into the shaft.


The undisturbed soil immediately ahead of the shield.


Material comprising cementitious and steel elements to form a structural lining, which is either placed in situ in a man-entry size pipeline or tunnel, or is preformed into segments for later installation.

Filter Cake

A thin layer of bentonite drilling fluid that seals the borehole preventing the flow of liquids from the borehole into the native soil.

Fluid Assisted

Method of guided drilling using a combination of mechanical drilling and Boring/Drilling pressurised fluid jets to provide the soil cutting action.

Fluid Density

See Mud Weight

Fold & Form Lining

Method of pipeline rehabilitation in which a liner is folded to reduce its size before insertion and reversion to its original shape by the application of pressure and/or heat.


See Hydrofracture

Free Boring

Method of auger boring without a casing.

Gel Strength

The measure of electrical attractive forces of the drilling fluid that allows the drilling fluid to suspend drilled solids as they are transported by the slurry out of the bore hole.

Ground Penetrating Radar

Nondestructive method for detecting subsurface structures, voids, discontinuities, and changes in material. GPR uses radar pulses transmitted into the ground to image the subsurface by recording electromagnetic signals reflected back to the GPR system.


The process of filling voids or modifying/improving ground conditions. Grouting materials may be cementitious, chemical or other mixtures. In microtunneling grouting may be used to fill voids around the pipe or shaft or to improve ground conditions.

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