Page 252 - Trenchless Technology Piping Installation and Inspection

216 Cha pte r F i v e

Separation from Existing Utilities
To help maintain an 18 in. separation from an existing facility along
the route, the proposed bore path, including the radius of the widest
cutter/reamer, should be at least 36 in. laterally offset from, or below
or above the verified depth of the closest edge of the facility (Telcordia,
2007). An exception includes the case of the bore path crossing an
exposed utility, at which point the desired physical separation, or
noninterference, can be positively verified (IEEE, 2007).

5.5 Pipe Load Calculations
As said earlier, the product pipe must be capable of withstanding the
corresponding various loads experienced during the operational (ser-
vice) phase, and during the installation and preoperational phases. It
is therefore recommended that the product pipe be independently
verified to be able to withstand the service loads corresponding to
internal pressurization, if appropriate, as well as soil and surface loads,
such as experienced in a conventional trench installation; see The Plas-
tics Pipe Institute Handbook of Polyethylene Pipe, Plastics Pipe Institute
(PPI, 2008). Regarding the installation phase, a method intended for
Maxi-HDD projects installing polyethylene pipe is provided in ASTM
F1962, Standard Guide for Use of Maxi Horizontal Directional Drilling for
Placement of Polyethylene Pipe or Conduit under Obstacles, Including River
Crossings (ASTM, 2005). In comparison, a simpler technique would be
more appropriate for typical lower cost, less sophisticated Mini-HDD
operations. Thus, a simplified methodology is presented in this sec-
tion to estimate the required pull loads on polyethylene pipe installed
by Mini-HDD systems. This simplified procedure also evaluates the
potential collapse tendency of the polyethylene pipe during the instal-
lation or post-installation (preoperational) phases.
The methodology and associated formulae are based upon
approximations to the more complex set of equations and procedures
provided in ASTM F1962. The objective is to provide a convenient
means of identifying potentially problematic Mini-HDD installations
and/or to aid in the pipe selection process, in contrast to the exten-
sive planning or analytical investigations characteristic of typical
Maxi-HDD projects. The proposed mathematical model reflects the
major route parameters (bore length, planned bends) and buoyant
force for an empty polyethylene pipe, and also accounts for unplanned
curvatures (undulations) resulting from path corrections in a typical
Mini-HDD installation.
Although the methodology is primarily conveniently described
with respect to Mini-HDD installations, the results are also applicable
to Midi-HDD operations. The procedure is applicable to commonly
used high-density polyethylene (HDPE) pipe, based on its character-
istically low-bending stiffness.