For longer distances, fiber optic cables are typically installed by hanging cables between poles (aerial), putting them on the seabed (submarine), or trench them in the ground (underground). The specific environmental conditions of a project will indicate which method – or combination of methods – is the optimal way to go. In this article, we take a closer look at the challenges and advantages with the underground installation.
Fiber optic network installation - a complicated task
In general, techniques that may need some necessary basic skills will in return offer huge benefits; Lower total cost of ownership, better performance and scalability.
Today there exist several fundamentally different cabling techniques for fiber optic networks. Some examples are;
- Direct buried cable installation
- Installation by blowing or pulling in cables in ducts
- Air-blown installation of tiny micro cables or blown fibers in microducts
Traditional optical fiber cables can be the safe, old-school technology that most installers practice, but the newer Air-blown fiber optic cabling systems for microducts is gaining more interest. The system offers low total cost of ownership as well as the capability to grow with user needs.
Regardless of the chosen cabling technique, there are some fundamental installation methods. These can be practiced with direct buried cable as well as for microducts in air-blown cabling projects.
It starts with the installation environment
In the ground, in water, aerial on the poles? Yes, this is the first question.
You may find this question rather self-explanatory, but please think twice!
If you have access to poles, then there are considerable savings in using these instead of making trenches for underground installations. Sometimes a shortcut through a bay with submarine cables will save you miles of digging, and if you can install cables under a bridge, then the extra cost for submarine cables can be eliminated. However, most installations are done in the ground, and this is the focus of this article.
There are five typically used ground-installation techniques and methods.
- Traditional trenching
- Mini trenching
- Micro trenching
- Plowing
- Drilling
Generally, # 1 is the most time consuming and expensive method and # 5 being the fastest and lowest cost.
So, why would you not always go for the most economical cost option? There are tons of parameters that define which method is best for a specific application. A low-cost technique like drilling cannot be used in areas with lot of rocks. It also requires investments in expensive equipment and skilled labour. When installation by plowing is quick, easy and low cost, making it ideal for rural areas, it is of no use in urban areas with asphalt or paving stones.
Choosing the right installation technique to minimize cost and maintain good quality is a skill and will be covered in other articles. Here, we will look into the methods used for fiber optic cables. Let’s start!
Excavating trench
When performing a direct buried duct, microduct assembly or cable excavation, it is of the highest importance to make sure that the trench is cleared from stones and sharp objects. Then the ducts are to be deployed on top of a fine bedding. After the deployment of the ducts they are covered with sand or fine soil on top. If there are more than one duct in the trench, one must lay sand between the ducts as well. Finally, a warning tape shall be put on the top of the cover layer. A warning tape of Al or plastics and a text is placed 10-20 cm above the duct to minimize the breaks from digging. If a plastic warning tape is used, it is possible to track the channel if the tape has a metallic wire. Ensure that the metallic wire is connected together if sections of warning tape is spliced.
Note: If a metallic wire is not present in the warning tape, it is a good idea to include duct with an integrated tracer wire. A simplified view of an excavated trench is shown in the figure below. Measures in mm.
Always check with the network owner or local regulators to verify the correct depth. For reference, some examples of typical D values are shown below:
- Ducts in green areas and pavements shall be on at least 0.35 m depth
- For ducts in driving areas the depth shall be at least 0.55 m
- In regions with ground frost, the trench should be 0.7 m deep or more
- Ducts installed in cultivated areas must be placed so deep that further cultivation does not jeopardize the optical installation, for example, at least 0.8 m
Plowing
Since the deployment using a plow has to be done in one go without stopping the machine, no obstacles must be in the route. Therefore, pre-plowing has to take place. The pre-plowing is to be done a minimum 100 mm deeper than the actual installation depth, as shown in the figure below.
After pre-plowing, the plowing of ducts can take place. This is done in one go using a static or a vibrating plow, see figure below. Vibrating ploughs cut the ground easier and refill more efficient than a static plough. Therefore, it is recommended to use vibrating ploughs.
Several ducts can be deployed at the same time, and warning tape, and preferably a trace wire is placed on the top.
When reinstating the ground after plowing, make sure to restore the ground to have similar looks and properties as before the plowing. This can, for example, be done by driving with the machine where the plowing took place. First on one side of the track, then on the other side, and lastly on the top, as shown below. Also, make sure to remove all visible rocks.
Mini Trenching and Micro Trenching
The mini- and micro trenching technique comes in different ways and uses various machines. The advantages of this technique over conventional cable laying technologies lie primarily in its speed of execution, lower cost, significantly lower environmental impact, and limited disruption to road traffic, and, as a consequence of the previous items, easiness in obtaining permits for the taking over of the public area. Typical mini- and micro trenching machines are shown below.
The difference with mini- and micro trenching is generally the trench profile. The smaller profile of the micro trenching offers lower cost, but also less space for ducts and more moderate protection. Typical trench profiles form mini- and micro trenching are shown below. Measures in mm.
When reinstating the asphalt after installing, make sure to restore the ground to have similar looks and properties as before the trenching. Reinstatement after mini trenching is done with concrete or foam concrete as backfill, with Bitumen on top. Micro trenching is done with a rubber strip on the top of a duct and then Bitumen on the top.
Horizontal directional drilling
Horizontal directional drilling is a beneficial method for deploying ducts. It is commonly used when deploying ducts under roads or water. The method is normally done in the following steps:
- Perform a survey with ground radar. This is to discover possible obstacles in the ground, for example, existing cables, water pipes, and sewers.
- Perform the drilling in a team of two persons; one using the drilling machine, and the other one using the locating device to track the drill head. The drilling begins with a small diameter pilot hole at the entry side of a project site. Bentonite is commonly used to stabilize the hole.
- Replace the drill head. Once the drill comes through, replace the drill head with a mandatory swivel and an optional reamer. The swivel makes sure that the duct does not rotate on the way back, and the reamer is used to widen the diameter of the drill hole.
- Attach a pulling grip and a swivel and tie the pulling rope to the swivel on the pulling grip with a strong knot.
- Pull through the duct (or cable) through the hole.
In order to make the installation fast and efficient, it is common to drill a hole in one direction, pull the duct through, then turn the machine around and drill a hole in the opposite direction and pull another duct through, see figure below. This way, the equipment is moved as little as possible, saving time, and, if applicable, reducing the cost of renting the drilling machine.
Then what?
After successful installation, it is time to do the rest. In access networks such as Fiber To The Home, it is essential to make duct branches to reach out with ducts to each end-user. Cabinets and closures for splitters need to be installed and finally, the cables or blown fibers are installed into the ducts. All these steps are equally important and will be handled in other articles.
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