The International Society for Trenchless Technology

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Term Definition

One of several drilling fluid additives that reduces clay balling and keeps tooling clean.


The variation from line and level to that specified.

Directional Drilling

See Horizontal Directional Drilling (HDD)

Discrete Sliplining

Method of lining with pipes shorter than the section to be renovated which are not jointed prior to insertion to form a continuous pipe, and which have not been shaped to give them a cross sectional diameter smaller than their final diameter after installation.

Downhole Drilling Assemblies

Mud motors and mechanical downhole motors used primarily to excavate hard soils and rock.

Drill Bits

Tools that excavate soil or rock and faciliate steering at the face of the bore. Common types of drill bits used in HDD include slant-face bits, slanted-face rock bits, rotary rock bits, and percussive bits.

Drill Rig

A trenchless machine that installs pipes and cables by drilling a pilot bore to establish the location of the underground utility before enlarging the hole if needed and pulling back the product.

Drill Rods

High strength hollow steel pipes joined to form a string used to transmit rotational torque and thrust, and to transport drilling fluid from the drill rig to the downhole tools.

Drill String/Stem

The total length of drill rods/pipe, bit, swivel joint etc. in a bore.

Drilling Fluid

Largely comprised of water, drilling fluids usually include bentonite and if warranted other additives such as soda ash, polymers, and detergents to address water and soils properties that could compromise the successful completion of the HDD installation.

Drive Shaft

See Jacking Shaft

Dry Bore

Method of creating a bore without the use of a drilling fluid. Usually associated with guided impact moling, but may also apply to some rotary methods.

Earth Piercing

See Impact Moling

Earth Pressure Balance Machine (EPBM)

Type of microtunnelling machine in which mechanical pressure is applied to the material at the face and controlled to provide the correct counter-balance to earth pressure to prevent heave or subsidence. The term is usually employed where the pressure originates from the main jacking station in the drive shaft or to systems in which the primary counter-balance to the earth pressure is supplied by pressurised drilling fluid or slurry.

Entrance Seal

See Launch Seal

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