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Pr oject Considerations for Horizontal Dir ectional Drilling 205
guidance tracking system incorporated into the drill string. The walk-
over system (Fig. 5.12) is more common, for which the receiver is
placed above the general vicinity of the drill head to allow a determi-
nation of its location and depth, and to indicate drill head orientation
for determining steering information to be implemented from the
drill rig. The information may be relayed to the drill rig operator by
direct communication from the tracker, or transmitted remotely.
Steering is achieved by controlling the orientation of the drill head,
which has a directional bias, and pushing the drill string forward
with the drill head oriented in the direction desired. Continuous rota-
tion of the drill string allows the drill head to bore a straight path.
The most common type of pipe utilized for Mini-HDD operations
traditionally has been high density polyethylene (HDPE), due to its
relatively high tensile strength characteristics and the absence of
potentially vulnerable joints. Other products, however, have been
successfully installed, including various types of polyvinyl chloride
(PVC) pipes and ductile iron pipes. Field fusible and restraint joint
PVC represent recently introduced products designed to be installed
by HDD or other pulling type applications. Either a single pipe or a
bundle of small pipes may be placed (e.g., HDPE “innerducts” for
communications applications). For the latter case, the desired com-
munication cable—fiber, coaxial, or copper—may subsequently be
installed into the innerduct path, as convenient. Although it is possi-
ble to directly pull back a bare cable into the borehole, such a proce-
dure is not recommended for most cables, unless special protection
(armor, etc.) is provided (Telcordia, 2007).
5.4.1 Mini-HDD Planning
General Considerations
The owner or project engineer will provide the general requirements
for the path of the pipe, including location within the right-of-way,
path of laterals to residence or building, etc. The actual layout, how-
ever, will vary from case to case, depending upon the specific obstacles
or specified utility line architecture. For Mini-HDD projects, the detailed
bore path for each segment will be determined on-site, in advance of
the operations, by the selected contractor, utility and/or regulatory
engineer.
Equipment and Product Restraints
Drill-Rod Constraints The planned path must be consistent with the
steering capability of the drill string and the allowable radius of cur-
vature of the steel drill rods based upon the corresponding bending
stresses in the steel rods and joints. Although some soil conditions
will inhibit sharp steering maneuvers, path limitations will often be
based upon fatigue strength considerations of the rods. A given rod