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.)

In practice, however, route curvature at discrete bends as well as cumulative curvature due to seemingly minor deviations from a nominally “straight” path lead, result in rapidly escalating tension. This is known as the “capstan effect,” illustrated in Figure 6, and results in increased frictional drag forces due to increased bearing pressure caused by tension pulling the cable snugly against the inside of curved portions. Such effects greatly limit practical pulling distances.

mopFig 6.jpg
Figure 6 Illustration of Capstan Effect
(Courtesy of Outside Plant Consulting Services, Inc.)

Cable pushers represent an alternative to pulling equipment and have the advantage of avoiding the need for a pull line. In principle, placement distances are limited by similar frictional effects as described above, including an analogous capstan effect due to increased bearing pressure caused by an axial compressive force pushing the cable snugly against the outside of curved portions. Unlike the tensile load on a cable, which is generally characterized by industry specified or reasonably well-defined limits, allowable axial compressive loads on cables are less well-defined. A typical failure mode of cables under axial compression comprises buckling and kinking, and is a function of the magnitude of the required push force, the bending stiffness of the cable, and the clearance in the duct.

In comparison to the familiar pulling or pushing methods, innovative “blown-cable” techniques have recently been developed. These methods utilize high pressure air introduced at the cable feed end of the duct which provides a forward thrust on the cable. In the “low-air-speed” alternative, a piston is connected to the leading end of the cable to apply a pull force, supplemented by a pushing force at the cable feed end. In the “high air-speed” method (Figure 7), the piston is omitted and the air is allowed to rush out the far end of the duct, resulting in a viscous drag force applied to the outer surface of the cable. The distributed nature of the latter force avoids the tension buildup associated with the capstan effect, resulting in placement distances relatively insensitive to route bends and curvature. The system also includes an essential cable pusher, which is advantageously employed to provide even greater placement distances. The original application for the blown-cable techniques was light-weight fiber-optic cables, but the equipment has been successfully utilized for the installation of other type cables, albeit typically at shorter placement distances.