Page 235 - Trenchless Technology Piping Installation and Inspection

Pr oject Considerations for Horizontal Dir ectional Drilling 199

• Thermal: The difference between the temperature at initial
construction and the later operating temperature of the pipe-
line can result in tensile or compressive longitudinal stresses.
• External pressure: The pipeline is subjected to long-term exter-
nal pressure resulting from vertical loads that may be imposed
on the pipe from the overhead soil as well as possible surface
live or dead loads. These effects will typically be of greater
significance for a plastic (e.g., polyethylene) pipe than a steel
pipeline.

External Coating
External coatings used in HDD installations should be smooth and
resistant to abrasion. Historically, pipelines installed by HDD in allu-
vial soils have usually been coated with corrosion coating only.
Weight coating is generally not required. The deep, undisturbed
cover provided by HDD installation has proven adequate to restrain
buoyant pipelines.
The corrosion coating most often used on HDD crossings is thin
film fusion bonded epoxy (see Fig. 5.11). This coating is popular
because it is highly durable, and the field joints can be coated using a
compatible fusion bonded epoxy system. For crossings installed in
rock, highly abrasive soils, or soil conditions which might involve
point loads, a protective coating should be used in addition to the cor-
rosion coating. The protective coating need not have corrosion preven-
tion properties, but must protect the underlying corrosion coating.

5.3.6 Specifications and Drawings
The HDD technical specifications should clearly define all details rela-
tive to HDD performance. Job specific details such as pilot borehole tol-
erances, water sources, and drilling fluid disposal requirements should
be included in the specifications or an accompanying job specific section
or a list of submittals. In addition to the technical specifications, the con-
tract documents should contain a plan and profile drawing. The drawings

FIGURE 5.11 Coating inspection.