Page 257 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 221

It is interesting to note that Eq. (5.18) does not include the depth
of the route, H, which is explicitly included in the original Eq. (5.12).
The mathematical simplifications have essentially eliminated this
dependency as a relatively minor effect, as appropriate for the simpli-
fication purposes.

Additional Path Curvature
The value of n in Eqs. (5.17) and (5.18) may be expressed as

n = n + n (5.19)
1 2
where n = effective number of deliberate/planned 90° route bends
1
n = cumulative curvature due to the unplanned undulations.
2
For example, if a deliberate horizontal (planar) bend of 45° to the
right, in order to avoid an obstacle or follow a utility right-of-way, is
followed by another 45° horizontal bend to the left, each 45° bend is
equal to half of a 90° bend, corresponding to a total of ½ + ½ = 1 full
90° bend (i.e., n = 1).
1
It is considerably more difficult to predict or determine the value
of the cumulative unplanned curvature, n , since this will obviously
2
vary among installations due to soil conditions, and expertise of the
crew. However, the following rule may be used to provide a reason-
able estimate for a Mini-HDD operation:

n ≈ L (ft)/500 ft (5.20)
2 bore
That is, there may be assumed to be effectively one 90° bend, due
to path corrections, for each 500 ft of path length. This rule is based
upon limited experiences, including analyses of sample as-built data
provided in Mini-HDD equipment user manuals. The above-
suggested value is consistent with the general magnitude of the
corresponding curvature of the actual installed paths. It is noted that
this value is not necessarily intended to be a conservative estimate,
and that significant variability may be anticipated.
The magnitude of unplanned path curvature provided by Eq. (5.20)
is intended to be applicable to a Mini-HDD operation, which typi-
cally uses steel drill rods of approximately 2-in. diameter. Larger
diameter drill rods are stiffer and, therefore, result in more gradual
path deviations and corrections, resulting in a reduced level of path
undulations. Thus, when applying the above procedures to a Midi-
HDD operation, a reduced value of n should be used. In particular,
2
since the rod stiffness is directly proportional to rod diameter, the fol-
lowing general value is implied

n ≈ [L (ft)/500 ft] · [2-in./d (in.)] (5.21)
2 bore