Page 469 - Trenchless Technology Piping Installation and Inspection

Glossary of Terms 419

Deformed and reshaped: See modified sliplining.

Diameter of reamer: Largest diameter of reamer (in horizontal directional
drilling).
Dimension ratio (DR): See standard dimension ratio (SDR).
Dimple: A term used in tight fitting pipeline renewal, where the new
plastic pipe forms an external departure or a point of expansion slightly
beyond the underlying pipe wall where unsupported at side connections.
The dimples are used for location and reinstatement of laterals.
Directional drilling: A steerable system for the installation of pipes,
conduits, and cables in a shallow arc using a surface launched drilling
rig. Traditionally the term applies to large-scale crossings in which a
fluid-filled pilot bore is drilled using a fluid-driven motor at the end of a
bend-sub, and is then enlarged by a washover pipe and back reamer to
the size required for the product pipe. The positioning of a bent sub
provides the required deviation during pilot boring. Tracking of the drill
string is achieved by the use of a downhole survey tool.

Dog plate: See backstop.
Drill bit: A tool that cuts the ground at the head of a drill string, usually
by mechanical means, but may include fluid jet cutting.
Drill string: (1) The total length of drill rods or pipe, bit, swivel joint and
so on in a drill borehole. (2) System of rods used with cutting bit or
compaction bit attached to the drive chuck.
Drilling fluid or mud: A mixture of water and usually bentonite and/or
polymer continuously pumped to the cutting head to facilitate cutting,
reduce required torque, facilitate the removal of cuttings, stabilize the
borehole, cool the head, and lubricate the installation of the product pipe.
In suitable soil conditions water alone may be used.
Drive or entry or launch or jacking shaft or pit: Excavation from which
trenchless technology equipment is launched for the installation of a
pipeline. In pipe jacking, it incorporates a thrust wall to spread reaction
loads to the soil.
Dry bore: Any drilling or rod pushing system not employing drilling
fluid in the process. Usually associated with guided impact moling, but
also some rotary methods.
Earth piercing: (1) Term commonly used in North America as an
alternative to impact moling. (2) The use of a tool, which comprises a
percussive hammer within a suitable casing, generally of torpedo shape.
The hammer may be pneumatic or hydraulic. The term is usually
associated with nonsteered devices without rigid attachment to the
launch pit, relying upon the resistance (friction) of the ground for forward
movement. During operation, the soil is displaced not removed. An
unsupported bore may be formed in suitable ground, or a pipe drawn in,
or pushed in, behind the tool. Cables may also be drawn in.