Page 67 - Trenchless Technology Piping Installation and Inspection

New Pipeline Installations 37

casing tubes with augers inside to remove spoils. If excessive
thrust force is applied at the excavation face and not enough soil
materials excavated and removed, there is a risk of heaving. Unless
a special push-on jointing system is used, the welding of two pipe
sections may take several hours. After completion of casing instal-
lation, the product pipe is installed using spacers or other means
(a two-phase process).

1.8.3 Horizontal Directional Drilling

Advantages
The major advantage of HDD is its steering capability. In case of hit-
ting obstacles, the drill head can be pulled back and guided around
the obstacle. As the HDD system can launch from the ground surface,
only small entry and reception pits (to collect drilling fluids) are
required. Therefore, the setup time is relatively shorter than other
trenchless installation methods. The single HDD drive length that
can be achieved (up to 6000 ft) is longer than any other non-worker-
entry trenchless method.

Limitations
The disposal of the slurry mixed with soil cuttings needs to be con-
sidered in advance, especially when no fluid recycling method is to
be used. Although the U.S. Environmental Protection Agency (EPA)
does not consider bentonite and certain polymers toxic materials, the
acceptability of such spoil material varies among local agencies as
well as landfill owners.
A thorough site and subsurface investigation is very important, as
corrective measures applied midway in the drilling or back-reaming
operation can be very time consuming and costly. When boring
under roadways or other environmentally sensitive areas, the use of
pressured fluid may cause serious concerns regarding the possible
deleterious effect of slurry migration laterally and vertically (also
called frac-outs). Care should also be taken to prevent possible ground
movement and loss of slurry to the pavement for shallow soil cove
installations.
While Mini-HDD equipment is portable, self-contained, and
designed to work in small, congested areas, Maxi-HDD operations
may require a relatively large area for the drilling rig and associated
equipment at the rig side. A comparable large area is generally
required at the product pipe side. Other limitations include the pos-
sibility that the bore may collapse in some granular (gravelly and
sandy) soils. In very soft soils, the steering ability may become diffi-
cult. Not maintaining enough fluid flow to remove cuttings may
cause a “hydrolock” situation, potentially causing heaving of soil or
pavement above.