Underground Construction - September 2021 - 29

installation. For pipe constructed from HDPE, the bend radius
limitation of the drill rods is suffi ciently large to be compatible
with that of the product pipe.
FIGURE 2 shows the required setback distance of the drill
rig corresponding to the distances shown in Figure 1 and also
indicates the minimum possible depth for a level trajectory.
Pipe design, selection considerations
Th e guidelines indicate that essentially all the commonly used
wall thicknesses (e.g., DR 7 to DR 17) for HDPE (PE4710) pipe
would be suffi ciently strong to resist collapse for depths to
approximately 15 feet, with the possible exception of DR 17, the
typical limit for mini-HDD installations. A limit of 10 feet may
be more appropriate for DR 17.
Th ese depths assume an empty pipe during the installation
and pre-operational phase, in the absence of internal fl uids
or pressure, which would off set the eff ects of the external
pressure due to drilling fl uid/slurry. For depths greater than
15 feet, thinner wall pipe, or special situations, the adequacy
of the product for the application should be verifi ed using the
information provided in the appropriate appendix.
Th e following equation was developed as a convenient means
of estimating the peak force applied to the pipe as it is pulled
throughout the bore hole:
Tension (lbs) = [Bore Length (ft) × Buoyant Weight (lbs/ft) ×
(⅓ )] × (1.6)n
Th e buoyant weight may be calculated as:
Buoyant Weight (lbs/ft) = ½ [Pipe Outer Diameter (in.)]2
Pipe Weight (lbs/ft)
The parameter n is equal to the number (or fraction) of
90° route bends due to cumulative route curvature, where
n = n1
+ n2. The quantity n1
is the effective number of
deliberate/planned 90° route bends, and n2
is suggested:
is the cumulative
curvature (90° route bends) due to the unplanned undulations.
The following value of n2
n2 = [Bore Length (ft) / 500 ft] × [2-in / Rod Diameter (in.)]
Criteria for selecting an appropriate pipe strength then
corresponds to selecting a DR value with a safe pull tension at least
as large as the estimated tension, as determined above.
Th e safe pull tension for the PE4710 pipe is given below, for
both IPS and DIPS sizes. Th e quantitative values shown are based
on the minimal required wall thickness, as opposed to that of the
actual manufactured product and, therefore, underestimate the
average safe pull tension by approximately 6 percent.
-
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OptiTracHalfPageAd.indd 5
UCONonline.com | SEPTEMBER 2021 29
1/29/21 1:00 PM
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Underground Construction - September 2021

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