Installing cables in trenches

The traditional method for installing cables in open country is to excavate a trench, line the bottom of the trench with a special filler, lay the cable on the bed and back fill with specialist filler. The backfill is usually a mixture of carefully graded aggregates which pack together closely and conduct heat away from the cables.

Trench with prefabricated protective concrete table duct

Here, workers can be seen standing on a concrete foundation, with a crane in the background lifting U-shaped concrete covers onto the foundation to create the duct.

The prefabricated concrete conduit being laid over the cable

Once the conduit is in place, the cable can be installed in behind it by dragging it through in a controlled process.

Cable reels lined up in a staging depot close to the installation area

Cables can be installed at high speed once a project gets going, so manufacture of the cable may start before the trench is started so that a stock of material is built up, ready to go into the trench or tunnel as soon as it is needed. By phasing the production and installation in this way, the project can be completed most time efficiently.

Detail of a cable unreeling machine

Laying cable by an existing highway reduces the cost of transporting equipment and cables compared with building temporary roads along the project

Cable being unreeled from a highway route.

By laying a cable next to a train track, the high pre-existing carrying capacity of the rail line can be used to save the costs of road transport and making access roads along the line of the project

Cable being unreeled from a train

Empty cable unreeling machinery is shown in the figure above.

Wide cable trench for dissipating heat

Backfilling a cable trench with specialist filling on top of 3 circuits of 3 phases each. Keeping the cables in a flat configuration maximizes the dissipation of heat into the surrounding environment and means that the cables can carry higher current loads.

Cables closely arranged in a trefoil configuration

These cables have been placed close together in a trefoil configuration to minimize the width of the trench needed to carry them.  This does mean that heat cannot escape so quickly, so that the carrying capacity of such a system will not be so high as for when cables are laid out flat.

Backfill going onto cable in a concrete duct

Filling in a cable trench with gravel on top of an engineered infill

Conventional trench being filled using an excavator

Back fill going onto single cable circuit

Here a backfill is going onto a single cable circuit in a concrete duct.  The backfill conducts heat away form the cables, into the concrete duct and to the soil beyond.

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