Page 156 - Trenchless Technology Piping Installation and Inspection

122 Cha pte r F o u r

Term Description
The trench walls are comprised of undisturbed in situ
soils through which a trench is excavated. Depending
Trench walls
upon the type of soil they may be vertical or require
sloping.
Bedding is the soil placed on top of foundation
and provides uniform support and grade for the
Bedding
pipe, except at pipe bells or large couplings where
overexcavation is usually specified.
The foundation may comprise of undisturbed in situ
Foundation soil or have imported soils to replace any unsuitable
material at the bottom of the trench.
Embedment is comprised of material placed around
Embedment the pipe providing a supporting structure. It consists of
the bedding, haunching, and the initial backfill.
Haunch zone is the area between the bottom of
the pipe and the spring line. Backfill material in the
Haunch zone
haunch zone is critical in transferring forces in the
lateral direction.
Springline Springline is the horizontal centerline of the pipe.
The initial backfill protects the pipe from final backfill
Initial backfill placement. It typically begins at the springline of the
pipe and continues about 6 to 12 in. on top of the pipe.
Material placed over the embedment up to the ground
Final backfill
level.
Depth of trench wall occupied by the pipe equal to the
Pipe zone
outside diameter (OD) of the pipe.
Often specified in pipe design, this dimension must
allow realistic side clearances to the outside diameter
of the pipe, including belled ends and trench support
Trench width systems. Excessive widths caused by careless
excavation increase paving, loads on the pipe, quantities
of earthwork and possibly the top width of the trench,
affecting right-of-way, surface finishing, and so on.
TABLE 4.1 Explanation of Pipeline Installation Terminology (Howard, 1996)

4.3.1 Rigid Pipes and Flexible Pipes
Broadly speaking, pipe materials fall into two categories: rigid and
flexible. Rigid pipes sustain applied loads by means of resistance
against longitudinal and circumferential (ring) bending. Under
maximum loading conditions, rigid pipes do not deform suffi-
ciently enough to produce horizontal passive resistance from the soil