Page 261 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 225

Pipe Collapse
ASTM F1962 specifies that the effective applied pressure to be less
than the collapse load as given by Eq. (5.14), but with a safety factor,
such as 2-to-1. As applied to Eq. (5.23) for Mini-HDD applications,
the requirement, therefore, corresponds to:
H (ft) ≤ 1000 hour water head (Table 5.1)/6.0 (5.25)

Such additional margin is intended to account for loads or degrad-
ing effects not previously directly considered, such as a possibly
extended period for the drilling fluid/slurry to thicken and provide
the anticipated lateral support.
This procedure is considered reasonably conservative for Mini-
HDD installations for drilling under roads or most other obstacles.
For river or creek crossings, in which the soil loads may be influenced
by the overlying water, additional considerations may be warranted,
such as discussed by Petroff (2006). For such cases, it may be desired
to use a greater safety factor or a longer term collapse strength.
5.5.4 Design Example
The appropriate wall thickness of an HDPE pipe, for a given diame-
ter, may be conveniently determined by the application of Eqs. (5.24)
and (5.25). As an example, consider the feasibility of installing a 4-in.
HDPE pipe of DR 11 rating, for a relatively long 600 ft Mini-HDD
route, including one deliberate 90° planar bend, and placed at a rela-
tively large depth of 30 ft.
The following physical properties apply to the HDPE pipe and
specified route:
L = 600 ft
bore
D = 4.50 in.
w = 2.3 lb/ft
a
H = 30 ft
Based upon these values, the following values may be directly
calculated:
n = 1.0 (one deliberate 90° bend)
1
n = L (ft)/500 ft
2 bore
= 600 ft/500 ft
= 1.2 (additional equivalent 90° bend)
n = n + n
1 2
= 1.0 + 1.2
= 2.2
2
w = 0.5 · D – w (lb/ft)
b a
2
= 0.5· (4.50) – 2.3 (lb/ft)
= 7.8 lb/ft