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Plate 1: Typical tractor and trailer set up for cable drum transportation (Photo courtesy of Prysmian Group)
4 4.1
Methodology for installation of cables into pre-installed ducts – Option 1
Jointing pit locations for subsequent phases may not correspond to the locations specified for East Anglia ONE as different cable technology or an alternative supplier could be used. When the jointing pit locations had been determined each would be excavated, the pre-installed ducts exposed and a lightly reinforced concrete slab would be cast for the base of the joint pit.
4.2
The cable drum would be delivered to one of the jointing pits and a cable pulling system would be installed into the trench. As with the direct lay this may comprise a steel bond and winching system with free spinning cable rollers placed along the bottom of the trench.
4.3
Once on site, the cable drum would be raised off the ground on hydraulic jacks to enable it to spin freely when pulled. The cable would then be pulled from the drum into the trench using the pre-installed rollers, with sufficient cable pulled through to the far jointing pit to allow for jointing onto the next section. The process would be repeated for the second cable to be installed in the duct.
4.4
The main variation for installing cables in pre-installed ducts as opposed to open trenches is that cable pulling in ducts relies on the use of biodegradable water based lubricant during the pulling process.
4.5
Plate 2 illustrates a typical cable pull. In this example the sides of the excavation have been softened from vertical sides back to a safe angle to reduce the risk of collapse, rather than protection through sheet piling. The cable is shown being guided into the previously installed duct with the aid of rollers weighted down with sand bags. Also shown is the concrete slab which is required for the base of the joint pit to provide a level working area prior to jointing.