Page 307 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Pipe Replacement Methods 271

placement distances—can vary over a wide range, implying that it
will be difficult to confidently predict practical placement distances
for a given pipe system (specified material, diameter, wall thickness,
and the like).
While recognizing the inherent difficulty in providing a generally
accepted, conservative design protocol for the broad range of possi-
ble pipe-bursting operations, the availability of a “rule-of-thumb”
would be useful to help during the planning stages. Such a rough
guide is presented in Sec. 6.10.3, as well as in Sec. 6.10.4.

6.10.3 Pulling Loads—Planning Guide
The extremes inherent in the above theoretical models are associated
with the degree and extent to which the expanded borehole (cavity)
may collapse, and the method by which the resulting soil loads is
estimated. A reasonable approach in developing a more useful rule-
of-thumb or guide would be to assume a moderate degree of cavity
collapse and/or reduced soil loads that reflect a degree of soil arch-
ing, and compare the resulting predictions to the relatively recent
field experiences included in Table 6.1.
Previous studies (Atalah, 1998) have indicated that the potential
for collapse of the borehole encompasses a wide range of possibili-
ties, including collapse along the entire length as well as collapse of a
lesser extent, for example, approximately 25 percent of the length.
The same studies included estimates of estimated soil loads some-
what less than the simple prism load, reflecting soil arching. An over-
simplified, but convenient, means of attempting to account for these
effects is to use a modified version of Eq. (6.3), based on an effective
length equal to 25 percent that of the actual section , and an effective
∗ ,†
depth given by

d = d for d/D < 5
eff
= 2 / 3 d for 5 ≤ d/D < 10
= 1 / 2 d for 10 ≤ d/D < 20
= 1 / 3 d for 20 ≤ d/D (6.4)

where D is the nominal (trade size) pipe diameter of the HDPE
replacement pipe (in.). (The approximate nature of this simplification
obviates the need to carefully distinguish between the nominal and
the actual outer diameters of the product pipe.)

∗ The weight of the pipe itself, along the entire length L, is insignificant
compared to the soil loads, and may be ignored for these calculations.
† The 25 percent factor also reflects the possible (likely) reduced radial soil pressure
due to variation around the pipe circumference.