Page 262 - Trenchless Technology Piping Installation and Inspection

226 Cha pte r F i v e

Thus, Eq. (5.18) predicts a peak pull load of

T = [L · w · (1/3)] · (1.6) n
1
D bore b
= [600 ft · 7.8 lb/ft · (1/3)] · (1.6) 2.2
= 4387 lb

Equation (5.24) then requires that this predicted installation load,
4387 lb, be significantly less than the relevant safe pull strength
(nominal 4-in. pipe, DR 11) indicated in Table 5.2 for HDPE pipe. The
corresponding safe pull strength of 7524 lb, allows a safety factor of
1.72, representing a reasonable margin. Although the example con-
siders a 4-in. pipe, for a given DR value, the predicted pull load, T ,
1
D
and the safe pull strength are both proportional to the square of the
outer diameter. The conclusions are, therefore, independent of the
pipe diameter. It is noted that the use of the DR 11 pipe in a longer,
nominally straight route of 800 ft—beyond the generally accepted
limit (600 ft) for Mini-HDD applications—would also be predicted to
be have an equivalent margin of safety.
Regarding the potential vulnerability to collapse, either during or
after installation, Eq. (5.25) requires that the peak installation depth,
or 30 ft, be no greater than one-sixth the relevant head of water (1000
hours, DR 11) indicated in Table 5.1 for HDPE pipe. This corresponds
to a safe depth of 255 ft divided by 6.0, or 42.5 ft, independent of pipe
diameter. Thus, the relatively large 30 ft proposed installation depth
is within the capability of the DR 11 wall thickness.
This relatively difficult (long, deep) installation(s) demonstrates
that a DR 11 HDPE pipe represents a reliable selection for the large
majority of Mini-HDD applications, and is, in fact, consistent with
field experience. Thinner-walled pipe (higher DR rating) may be
successful in many cases, as may be verified by specific calculations
for the route of interest. It is also emphasized that the present
methodology for pipe DR selection does not prove that a thinner-
walled pipe, such as DR 17 commonly used, would not be successful
in practice in individual installations, but as in most design proce-
dures, it should be noted, is intended to serve as a caution that this
design may be marginal (nonconservative).

5.6 Summary
This chapter has presented general background as well as basic
design and project management considerations for HDD. Both Maxi-
HDD and Mini-HDD operations were discussed. Although typical
Mini-HDD installations are individually less critical than more com-
plex, extensive Maxi-HDD projects, it is nonetheless important to
follow proper planning and installation practices to help assure a