Page 243 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 207

For a typical Mini-HDD drill rod of approximately 2-in. diameter,
the corresponding radius of curvature given by Eq. (5.1) is 200 ft. In
practice, manufacturers will typically allow a 100-ft radius of curva-
ture for a 2-in. rod, confirming the conservative nature of Eq. (5.1).
Such a bending capability corresponds to a 90° bend radius of approx-
imately 155 to 160 ft, or an angular change of 5.5° to 6.0° per 10 ft drill
rod, consistent with Eq. (5.2). The corresponding allowable degree of
curvature or bending applies to bends in all horizontal (plan) or verti-
cal (profile)—or inclined—planes.

Product Pipe or Conduit Constraints
In general, the allowable radius of curvature for the product pipe
will be provided by the pipe manufacturer. For pipes or conduits
constructed from plastic or other relatively flexible material, and the
relatively small diameter pipe installed by a typical Mini-HDD
operations, the bending limitation of the drill rods as given in Eq.
(5.1) is typically sufficiently large to be compatible with that of the
product pipe. For these cases, related bending stresses need not be
explicitly considered in addition to the tensile stresses, as discussed
and estimated in Sec. 5.5 for polyethylene pipe. However, for steel
product pipe, guideline such as that given by Eq. (5.3) would be
applicable:

R = 100 D (5.3)
min
where R = minimum radius of curvature of product pipe, or bore
min
path, ft
D = nominal diameter of product pipe, in.
For other materials, including pipe or conduit assemblies con-
taining joints or couplings of lower strength than that of the basic
pipe element, the manufacturer’s recommendations should be fol-
lowed regarding minimum radius of curvature. In the absence of
such low strength joints or couplings, and other related product infor-
mation, the following formula may be used (Hair, 1988):

R = E· D/(SMYS · F · 24) (5.4)
min 1
where E = elastic modulus, for the material and temperature of
interest, psi
SMYS = material specified minimum yield stress, psi
F = design factor (< 1.0)
1
In this case, the corresponding bending stresses should be consid-
ered in addition to the estimated tensile loads, as determined for the
product pipe used (ASCE, 2005). It is noted that the peak pulling load
tensile stress for the pipe does not necessarily occur at the points of
peak bending stresses.