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Chapter 11 HDD
i
Table of Contents
Foreword
iii
Introduction
1
Background
1
Horizontal Directional Drilling Process
2
Pilot Hole
2
Pilot Hole Reaming
2
Drilling Mud
2
Pull-Back
3
Mini-Horizontal Directional Drilling
3
Geotechnical Investigation
4
Geotechnical Data For River Crossings
5
Summary
5
Product Design: DR Selection
5
Design Considerations for Net External Loads
7
Earth and Groundwater Pressure
8
Stable Borehole - Groundwater Pressure Only
9
Borehole Conforms/Collapse With Arching Mobilized
9
Borehole Collapse with Prism Load
11
Combination of Earth and Groundwater Pressure
11
Live Loads
12
Performance Limits of HDD Installed Pipe
13
Time-Dependent Behavior
13
Ring Deflection (Ovalization)
14
Ring Deflection Due to Buoyancy
15
Ring Deflection Due to Earth Load
15
Ring Deflection Limits (Ovality Limits)
16
Unconstrained Buckling
17
Wall Compressive Stress
18
Example Calculations
19
Installation Design Considerations
19
Pull-Back Force
20
Friction Drag Resistance
20
Capstan Force
21
Hydrokinetic Force
22
Tensile Stress During Pull-Back
22 ii
External Pressure During Installation
24
Resistance to External Collapse Pressure During Pull-Back
25
Installation
Bending Stress
25
Thermal Stresses and Strains
26
Torsion Stress
26
Example Calculations
27
References
29
Appendix A
31
Appendix B
37
FIGURES
Figure 1 - Borehole Deformation
6
Figure 2 - Performance Limits of HDD Pipe Subjected to Service Loads
13
Figure 3 - Ovality Compensation Factor
17
Figure 4 - Estimated Pull-Back Force Calculation
22
TABLES
Table 1 - H20 Loading Under Rigid Pavement (AISI)
12
Table 2 - Apparent Modulus of Elasticity and Safe Pull Tensile Stress
14
@ 73
0
F
Table 3 - Design Deflection Limits of Buried Polyethylene Pipe
16
Long Term, %
Table 4 - Critical Buckling (Collapse) Pressure for Unconstrained HDPE
18
Pipe @ 73
0
F
Table 5 - Safe Pull Loads for HDPE Pipes
24 iii
Foreword
Polyethylene Pipe for Horizontal Directional Drilling is one of the chapters being pre-
pared for inclusion in the Plastics Pipe Institute’s PPI Handbook of Polyethylene Pip-
ing, which will be issued as a complete volume in the future. This handbook will cover
other uses of polyethylene piping systems including municipal, mining, and industrial
applications. Other topics to be addressed in the handbook will include engineering
principles, design and installation of polyethylene piping systems, and relevant codes
and standards.
PPI is a division of The Society of the Plastics Industry, Inc. (SPI), the major U.S. trade
association representing all segments of the plastics industry.
The Municipal and Industrial (M&I) subgroup of PPI are producing the PPI Handbook
of Polyethylene Piping. M&I membership consist s of major North American manufac-
turers of polyethylene (PE) pipe and fittings, PE piping materials, machinery, and
equipment used for joining and installing PE piping, related test laboratories, and
professional organizations.
PPI maintains additional subgroups, which address other applications, such as gas
distribution. PPI and its subgroups provide technical and promotional support for the
effective use and continued application of thermoplastics pipe and related products,
consistent with the best public interest. PPI membership also includes producers of
polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polybutylene (PB),
and crosslinked PE (PEX) piping products and materials.
For a list of other publications available from PPI and/or further information, please
contact:
The Plastics Pipe Institute, a Division of
The Society of the Plastics Industry, Inc.
1801 K St. N. W., Suite 600K
Washington, D.C. 20006
Toll Free: (888) 314-6774
Phone: (202) 974-5318
Fax: (202) 293-0048
http://www.plasticpipe.org
The information in the publication was prepared by SPI as a service to its members and
the industry. While prepared in good faith and believed to be accurate, SPI, its mem-
bers, and contributors disclaim all liability for any loss or damage arising from reliance
on this information by any person. SPI does not endorse the proprietary products or
processes of any manufacturer. Consult the product manufacturer for specific informa-
tion.
August, 1998 Horizontal Directional Drilling 11 - 1
INTRODUCTION
The Horizontal Directional Drilling (HDD) Industry has experienced so much growth
in the past decade, that HDD has become commonplace as a method of installation.
One source reported that the number of units in use increased by more than a hun-
dred-fold in the decade following 1984. This growth has been driven by the benefits
offered to utility owners (such as the elimination of traffic disruption) and by the inge-
nuity of contractors in developing this technology. To date, HDD pipe engineering
has focused on installation techniques and rightfully so. In many cases, the pipe
experiences its maximum lifetime loads during the back-pulling operation.
The purpose of this document is to acquaint the reader with some of the important
considerations in selecting the proper polyethylene pipe. Proper selection of pipe
involves consideration not only of installation design factors such as pull-back force
limits and collapse resistance, but also of the long-term performance of the pipe
once installed in the bore-hole. The information herein is not all inclusive; there may
be parameters not discussed that will have significant bearing on the proper engi-
neering of an application and the pipe selection. For specific projects, the reader is
advised to consult with a qualified engineer to evaluate the project and prepare a
specification including design recommendations and pipe selection.
Background
Some of the earliest uses of large diameter polyethylene pipe in directional drilling
were for river crossings. These are major engineering projects requiring thoughtful
design, installation, and construction while offering the owner the security of deep
river bed cover with minimum environmental damage or exposure, and no disruption
of river traffic. Polyethylene pipe is suited for these installations because of its scratch
tolerance and the fused joining system which gives a zero-leak-rate joint with design
tensile capacity equal to that of the pipe.
To date, directional drillers have installed polyethylene pipe for gas, water, and sewer
mains; electrical conduits; and a variety of chemical lines. These projects involved
not only river crossings but also highway crossings and right-of-ways through devel-
oped areas so as not to disturb streets, driveways, and business entrances.
Chapter 11 gives information on the pipe selection and design process. It is not
intended to be a primer on directional drilling. The reader seeking such information
can refer to the bibliography of this document. A suggested document is the “Mini-
Horizontal Directional Drilling Manual” published by the North American Society for
Trenchless Technology (NASTT).
Chapter 11
POLYETHYLENE PIPE FOR HORIZONTAL DIRECTIONAL DRILLING Horizontal Directional Drilling 11 - 2
HORIZONTAL DIRECTIONAL DRILLING PROCESS
Knowledge of the directional drilling process by the reader is assumed but some
review may be of value in establishing common terminology.
Briefly, the HDD process begins with boring a small, horizontal hole (pilot hole) under
the crossing obstacle (i.e. a highway) with a continuous string of steel drill rod. When
the bore head and rod emerge on the opposite side of the crossing, a special cutter,
called a back reamer, is attached and pulled back through the pilot hole. The reamer
bores out the pilot hole so that the pipe can be pulled through. The pipe is usually
pulled through from the side of the crossing opposite the drill rig.
Pilot Hole
Drilling the pilot hole establishes the path of the drill rod (“drill-path”) and subse-
quently the location of the PE pipe. Typically, the bore-head is tracked electronically
so as to guide the hole to a pre-designed configuration. One of the key consider-
ations in the design of the drill-path is creating as large a radius of curvature as
possible within the limits of the right-of-way, thus minimizing curvature. Curvature
induces bending stresses and increases the pull-back load due to the capstan effect.
The capstan effect is the increase in frictional drag when pulling the pipe around a
curve due to a component of the pulling force acting normal to the curvature. Higher
tensile stresses reduce the pipe’s collapse resistance. The drill-path normally has
curvature along its vertical profile. Curvature requirements are dependent on site
geometry (crossing length, required depth to provide safe cover, staging site loca-
tion, etc.) But, the degree of curvature is limited by the bending radius of the drill rod
and the pipe. For small size pipes the stiff drill rod usually controls the curvature and
thus significant bending stresses do not occur in the pipe. The designer should
minimize the number of curves and maximize their radii of curvature in the right-of-
way by carefully choosing the entry and exit points.
Pilot Hole Reaming
The REAMING operation consists of using an appropriate tool to open the pilot hole
to a slightly larger diameter than the carrier pipeline. The percentage overs