ASCE Manual of Practice No. 118 For Belowground Pipeline Networks For Utility Cables

U-Tech: Underground Technology Cutting Edge Technical Information for Utility Construction and Rehabilitation
By Dr. Lawrence M. Slavin, Dr. Oleh Kinash, Dr. Mohammad Najafi | January 2010 Vol. 65 No. 1
Figure 2 Typical Underground Conduit Construction (Courtesy of Underground Devices, Inc.)

• Trenching;
• Plowing; and
• Boring (e.g., piercing tools, horizontal directional drilling).

Section 3 of MOP No. 118 describes the equipment and procedures employed for each of these categories. In general, information is provided based on safety requirements of the National Electrical Safety Code (NESC), as well as construction practices provided by the Rural Utilities Service and the communications industry (e.g., Telcordia Technologies). In particular, there are requirements and recommendations for:

• Cable properties;
• Handling;
• Depth of cover;
• Physical separation;
• Routing; and
• Trench conditions.

mopFig 4.jpg
Figure 4 illustrates a typical set of the trench requirements.

Figure 4 Basic Requirements for Direct-Buried Distribution Cable Trench
(Reprinted with permission of the ASCE)
Cable installation methods in duct (subhed)

Section 4 of MOP No. 118 describes three basic methods for placing utility cables within ducts that have previously been installed in the field:

  • Pulling;
  • Pushing; and
  • Blown-cable.

All three methods have been successfully applied in industry, and each has its advantages, limitations, and range of application. The methods continue to evolve, with innovations based upon the cable characteristics and newly introduced products. Some techniques and equipment combine elements of these categories.

The most direct method for installing a cable within a duct is to connect a rope or pull-line to the leading end of the cable, using an appropriate type of grip, and to then pull the cable into the path. Mechanized equipment is often required since the pulling force may be significant. The force imposes a tensile load on the cable, which must not exceed its allowable tensile capability. The major disadvantage relates to possible placement distances. The tension or pull force at the leading end of the cable is required to offset the frictional drag forces that accumulate along the length of the cable. For the simple case of a cable resting on the bottom of a straight duct path, as illustrated in Figure 5, the frictional drag is essentially due to the weight of the cable in combination with the coefficient of friction at the cable-duct interface, which in many cases appears to be relatively low.

mopFig 5.jpg
Figure 5 Basic Cable Pulling Mechanism (Straight Path)
(Courtesy of Outside Plant Consulting Services, Inc.)