Page 473 - Trenchless Technology Piping Installation and Inspection

Glossary of Terms 423

HDPE: High density polyethylene, see polyethylene.

Head (static): The height of water above any plane or point of references.
(The energy possessed by each unit of weight of a liquid, expressed as the
vertical height through which a unit of weight would have to fall to
release the average energy posed.) Standard unit of measure shall be the
foot. Head in feet for water is 1 psi = 2.310 ft.
Heat cure: The application of either steam or hot water to cure a resin
saturated tube.
Heaving: A process in which the ground in front of a tunneling or pipe
jacking operation may be displaced forward and upward, causing an
uplifting of the ground surface.
Height of cover (HC): Distance from crown of a pipe or conduit to the
finished road surface, or ground surface, or the base of the railway.

High density polyethylene: A plastic resin made by the copolymerization of
ethylene and a small amount of another hydrocarbon. The resulting base
resin density, before additives or pigments, is greater than 0.941 g/cc.

Hoop stress: The circumferential force per unit areas, psi, in the pipe
wall owing to internal pressure.
Horizontal directional drilling (HDD): See directional drilling.
Horizontal earth boring machine: A machine used to bore horizontally
through the earth by means of a MTBM, cutterhead, rotating tool, or
ramming tool.
Horizontal auger boring: The use of auger boring machines to prepare
holes by the installation of a casing whereby the spoil is removed by the
use of augers.
Host pipe: In trenchless renewal and replacement methods. Existing,
old, or deteriorated pipe.
Hydraulic gradient line (HGL): An imaginary line through the points to
which water would rise in a series of vertical tubes connected to the pipe.
In an open channel, the water surface itself is the hydraulic grade line.
Hydrogen sulfide: An odorous gas found in sewer systems with chemical
formula of H S.
2
I/I: Infiltration/inflow; infiltration stands for ground water seepage
and inflow stands for surface water flow into the gravity collection
system.
Impact moling: Method of creating a bore using a pneumatic or hydraulic
hammer within a casing, generally of torpedo shape. The term is usually
associated with nonsteered or limited steering devices without rigid
attachment to the launch pit, relying upon the resistance of the ground
for forward movement. During the operation the soil is displaced, not