Direct burial optical cable installation involves burying the cable directly underground through trenching and grooving. Currently, long-distance trunk optical cable projects and local transmission network optical cable lines in suburban areas mostly utilize direct burial.
The construction steps for direct burial optical cable installation primarily include site resurvey, trenching, trench bottom preparation, cable laying, backfilling, and the installation of route markers.
1. Site Resurvey
Before construction begins, a site resurvey is conducted based on the construction drawings. The route and location of the cable must be verified, the spacing between the splice pits complies with the cable tray length, and measures such as crossing obstacles are considered appropriate. Directly buried optical cables must be parallel to and cross other facilities (buildings) at a minimum spacing. Appropriate measures should also be implemented for special structures to ensure the safety of both the construction and the cable.
2. Trenching
Trenching should be conducted after thorough survey and measurement, with further careful selection, measurement, and marking according to the route selection. In urban areas with a large number of underground facilities, manual excavation is generally employed. In rural areas with limited underground infrastructure, mechanical excavation is preferred.
-1. Digging the Optical Cable Trench
1) Cable Burial Depth
Direct burial of optical cables may be affected by numerous factors, such as ground vibration, mechanical damage, rodent infestation, and the depth of the permafrost layer. The depth of the optical cable trench should be determined primarily based on the ground pressure and vibrations to which the cables are subjected. Direct burial of optical cables at sufficient depth can prevent various mechanical damage. Furthermore, at a certain depth, the ground temperature stabilizes, minimizing the impact of temperature fluctuations on optical fiber transmission characteristics.
2) S-bend Retention
When laying optical cables on slopes with a gradient greater than 20° and a slope length greater than 30m, S-bend retention should be made. S-bends should also be made when entering an unmanned relay station. S-bends should also be made when crossing railways or highways.
-2. Digging the Splice Pit
The splice pit should be excavated in a manner that facilitates drainage and splicing. The depth of the joint pit is based on the depth of the optical cable trench; all joint pits should be on the same side of the optical cable line and on the outside of the road when close to the roadside.
-3. Route construction through obstacles
1) Pipe jacking When direct buried optical cables pass through railways, important highways and other areas where it is inconvenient to directly break the ground for excavation, the pipe jacking method can be adopted. The steel pipe can be pushed through from one end. Generally, a hydraulic pipe jacking machine is used to complete the process.
2) Pre-buried pipe When optical cables pass through highways, mechanized farming roads and streets, the pre-buried pipe method is generally adopted. Use tools such as steel chisels to excavate the road surface and dig out an optical cable trench that meets the depth requirements.
3. Trench bottom treatment In general areas, the trench bottom is filled with fine soil or sand. In areas where the soil is soft and prone to collapse, wooden stakes and blocks can be used as temporary retaining walls for protection.
4. Laying optical cables
-1. Preparation work Before laying optical cables, the following preparations should be made:
(1) Prepare the tools and equipment for laying optical cables. (2) Collect the cable reel list and deliver the cable to the designated location according to the order in the reel list. Designate a person to check the reel number.
(3) Repair the road for lifting and placing the cable. Safety measures should be taken in dangerous areas such as sharp turns and steep slopes.
(4) Clean the cable trench. Pad the bottom of the stone trench with fine soil or sand. In the cable trench on steep slopes, lay fixed crossbeams according to regulations.
(5) Inspect the cable. Report and repair any damage in a timely manner.
(6) Organize the laying personnel and stipulate a unified action signal for laying the cable.
-2. Requirements for laying the cable
When laying the cable, it is strictly forbidden to drag the cable directly on the ground. The laying speed should be uniform to avoid over-tightening or sharp bending of the cable. In areas with slopes of more than 30° and at turns on unplanned roads, the cable should be laid in an "S" shape, leaving enough length for easy relocation when the terrain changes. A 2m overlap should generally be left at the fiber optic cable joints. When splicing, an appropriate amount of excess should be left at each end, depending on the size of the splice pit.
-3. Specific Methods for Laying Optical Cables
Direct burial optical cables are primarily laid using mechanical traction and manual lifting. Mechanical traction uses a motor vehicle to pull a cable reel, automatically placing the cable in the trench. This method saves manpower, is highly efficient, and offers high quality, but is susceptible to terrain constraints. During manual laying, a single reel is first mounted on a jack, and then manually lowered into the trench at regular intervals.
5. Backfill
After the optical cable has been laid and tested for good optical and electrical properties, it can be covered with soil. This is commonly referred to as backfill in engineering.
6. Burying Route Markers
To demarcate the route of the optical cable, the specific location of line facilities, and facilitate routine maintenance and troubleshooting, direct burial optical cables should be laid with cable markers along their route. The marker is a reinforced concrete or stone sign that indicates the direction and special location of the optical cable, and is numbered with the relay section and marker number.