Page 251 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 215

grade. Thus, for the simplification purposes, the bore entry and exit
angles are defined relative to the average grade. For large surface
depressions or mounds (e.g., of height greater than the depth of inter-
est and extending over a long expanse, on the order of the minimum
drill rod bend radius or greater), the bore should attempt to follow a
path at the nominal specified depth below the average surface pro-
file. Available industry tools may be used to provide assistance dur-
ing the planning and construction phases.
The path should avoid unnecessary bends and consecutive left and
right, or upward and downward curves. Such trajectories are difficult
to follow and may lead to over-steering and excessive bends, resulting
in increased stresses in the drill rods and greater pulling forces during
the installation of the pipe. A heavy rope or line placed on the surface
may serve as a convenient means of defining a gradual curve consis-
tent with the desired path radius of curvature. The radius of curvature
of the path may be estimated by:

R = ΔS/Δφ (5.11)
avg
where R avg = average radius of curvature along path segment, ft
ΔS = distance along path, ft
Δφ = angular change in direction, radians

Thus, a change of 0.1 radian (approximately 6°) per 10 ft corre-
sponds to a radius of curvature of approximately 100 ft.

Accuracy and Tolerance
The deviation of the actual bore route from the proposed path should
be within a specified tolerance and such as to meet the intent of
required separations from existing facilities. Both vertical and hori-
zontal limits should be specified by the owner, consistent with local
regulations. Some regulations require that new construction activities
be a minimum of 18 in. from either side of the outer edge of existing
facilities (Telcordia, 2007).
Unintentional bore path deviations may result from attempting
to follow a bend with sharp curvature, drill head misalignments
caused by cobbles or other obstacles, or soil conditions in general.
Deviations may be reduced by decreasing the interval between suc-
cessive drill head location determinations. Unless otherwise speci-
fied, the drill head should exit at the surface or access pit within a
12-in. radius of the target point. It is generally less difficult to meet
such a condition for a target point in a pit than at the surface in a
gradually rising bore profile (see Fig. 5.16). The allowable horizontal
and vertical deviations along the bore route should also be specified
by the owner. Typically, horizontal and vertical deviations along the
bore route should be within ± 18 in. of the proposed path.