Page 259 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 223

Pipe Diameter to Thickness Ratio (DR)
Duration 7.3 9 11 13.5 15.5 17 21
Short term 2896 1414 724 371 238 177 91
1 hour 1714 837 429 219 141 105 54
10 hours 1436 702 359 184 118 88 45
100 hours 1205 588 301 154 99 74 38
1000 hours 1019 498 255 131 84 62 32
1 year 880 430 220 113 72 54 28
50 years 649 317 162 83 53 40 20

TABLE 5.1 Ideal Critical Pressure (Water Head, ft) for Unconstrained HDPE Pipe
(73° F)

during which the drilling fluid applies hydrostatic pressure on the
pipe, subsequent to which it is conveniently assumed to thicken and
actually provide support against possible earth-imposed loads, or by
which time the soil has locally redistributed is relaxed around the
pipe to provide some lateral support against buckling. For this post-
installation phase, the tension reduction factor, f , is equal to 1.0.
R
Based upon the ovality reduction factor dependency trend pro-
vided in ASTM F1962, and consistent with the present simplified
approach, it is reasonable to assume a maximum overall value of f of
o
approximately 0.5 to account for ring deformation due to initial oval-
ity plus the value induced by installation or post-installation load-
ings. Such deflections may be induced by bending, aggravated by ten-
sion-induced wall bearing pressure, or possible postinstallation soil
loads.
The greater collapse strength at 1 to 10 hours relative to that at
1000 hours, would tend to be offset somewhat by the tension reduc-
tion factor, f , during installation as well as the degradation at possi-
R
ble elevated temperature as the polyethylene pipe rests on the surface
prior to installation. However, these latter degrading effects would
not be experienced simultaneously, or with the pipe being placed
at its maximum depth and distance. Therefore, the 1000-hour (postin-
stallation) collapse characteristics, as adjusted, are conveniently
employed to evaluate the vulnerability to collapse. Based upon the
above discussion, Eq. 5.23 is derived
H (ft) = 1000-hour water head · f · f /γ
o R b
= 1000-hour water head · (0.5) · (1.0)/(1.5)
or,
H (ft) = 1000-hour water head/3.0 (5.23)

where H is the allowable head of drilling fluid (ft) (i.e., maximum
pipe depth).