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   Currently, most power engineering projects in China still adopt the direct burial laying method. This approach is deeply ingrained due to its perceived advantages of low cost and relatively simple procedures. However, it often overlooks critical factors such as understanding cable performance and consulting with cable suppliers. Key considerations include the type of cable sheath (e.g., anti-termite or rodent-resistant features, as underground environments may host termites, rats, and other pests that damage cables), the bending radius specified by the supplier, and whether the cable is armored, among other attributes.

If the cable is deemed suitable for direct burial, on-site surveys and verification should be conducted according to the construction drawings. After backfilling, it is advisable to install warning markers at cable joints, bends, or intersections with other lines to prevent external damage. However, direct burial is only applicable in areas with no or few termites, away from heat sources, on flat terrain, with low traffic density, no chemical corrosion, and no underground operations. Therefore, thorough geographical surveys are essential before opting for direct burial. Typically, 10kV and below cables with an armored layer can be considered for direct burial based on practical conditions.

New Laying Methods

1. Duct Laying

   Duct laying involves installing cables in pre-arranged underground pipes when multiple cables need to be laid. Key requirements include:

• The inner diameter of the ducts must be at least 1.5 times the outer diameter of the cable.

• Installation depth should be below the frost line.

• Ducts must slope towards manholes with a gradient greater than 0.2%.

• Gaps between ducts should exceed 20mm to ensure proper heat dissipation.

  During installation, ensure duct entrances are smooth, and the pipes are free of sharp edges or debris to prevent cable damage.

Advantages:

• Easy maintenance.

• Space reserved for additional cables.

• Small footprint.

• Flexible manhole design.

• Fire-resistant and mechanically robust.

• Suitable for areas with complex intersections, busy roads, or railways.

• Armored cables are not recommended for this method.

Disadvantages:

• Higher installation costs compared to direct burial.

• Poor heat dissipation, which may reduce the service life of XLPE-insulated cables by 35%–40% (typical design life is 30 years).

• Multiple joints increase risks of water leakage and pest damage.

• High maintenance costs.

2. Tunnel Laying

   Tunnel laying refers to installing cables in fully enclosed tunnels that accommodate 21 or more cables, equipped with inspection and maintenance passages. This method allows for easy addition or removal of cables without further excavation, offers strong mechanical resistance, and facilitates convenient inspections and repairs.

Key considerations for tunnel laying include:

• Voltage level, outer diameter, and insulation materials of the cables to determine spacing.

• Equipped with lighting, drainage, and dual ventilation systems (mechanical and natural) to reduce humidity.

• Smoke alarms, automatic fire extinguishers, and hydrants for safety.

• Some tunnels also feature temperature control, online monitoring, and anti-theft systems.

Disadvantages:

• High construction costs and long project cycles, leading many contractors to avoid this method to save costs and shorten deadlines.

Conclusion

   While duct and tunnel laying methods may have higher initial costs compared to direct burial, their benefits are significant. For instance, duct laying offers greater flexibility and excellent fire resistance, while tunnel-laid cables can extend service life by 10 years or more. Given these advantages, are duct and tunnel laying methods still considered expensive?