What are the differences between different types of underground communication pipelines?

At present, optical cables have already entered thousands of households, and cables attached to poles can often be seen in the wild. However, in many areas of the city, optical cables are not everywhere because they are mainly laid through underground communication pipelines (hereinafter referred to as “pipelines”). Pipelines are generally composed of a group of pipes buried underground and manholes at both ends of the pipe group, as shown in Figure 1.

A pipe group is usually composed of one or more plastic pipes. Manholes are divided into manholes and handholes according to their size; The volume of the manhole is relatively large, and the internal dimensions are usually not less than 2.2 meters x 1.4 meters x 1.8 meters (length x width x height). Construction personnel can work inside the manhole; The volume of the hand hole is relatively small, and the maximum size usually does not exceed 1.7 meters * 1.2 meters * 1.4 meters(length x width x height).

According to different construction scenarios, pipelines are usually divided into long-distance pipelines, municipal road pipelines, and grounding network pipelines. They have certain differences in terms of manhole type, pipe group combination, burial depth, and segment length.

Long Distance Pipelines

Long distance pipelines are generally constructed in the wild, mainly used as underground laying channels for long-distance (primary and secondary) optical cables or local network city to county optical cables. The pipe material of the pipeline is mostly 40/33 silicon core plastic pipe, as shown in Figure 2. The length of the pipeline should be 960 meters to 980 meters, and the optical cable should be laid inside the pipeline by air blowing.

Early long-distance pipeline roads were often built in farmland. To avoid affecting the cultivation of crops above the pipeline, the burial depth of long-distance pipelines (from the top of the pipeline group to the ground) is usually not less than 1.0 meter (slightly different in different sections and soil conditions), and the handhole is usually buried. The handhole cover plate is not less than 0.6 meters away from the ground surface, as shown in Figure 3. Obviously, it is very inconvenient to repeatedly open the buried handhole.

The internal dimensions of the long-distance pipeline handhole are shown in Table 1. Handhole cover plates are usually prefabricated with reinforced concrete. When the handhole is constructed in a location that does not affect crop cultivation, the handhole can be of a common type, with the handhole cover directly exposed to the surface, as shown in Figure 4. 

In recent years, the construction of long-distance pipelines has mainly along with the construction of transportation arteries. When constructed on national or provincial road shoulders, the burial depth of pipelines is usually not less than 0.7 meters. When constructed within the isolation zone of highways and high-speed railways, the burial depth is usually not less than 0.5 meters. The hand hole generally adopts the ordinary type hand hole shown in Figure 4, or the 700×900 type hand hole shown in Table 3. When using 700×900 type handholes for long-distance pipelines constructed in the wild, anti-theft manhole covers are generally used.

In some places, long-distance pipeline sections built on the shoulders of national and provincial roads are constructed according to the requirements of municipal road pipelines, which can result in the inability to lay optical cables using air blowing methods and reduce the efficiency of fiber optic cable deployment.

After entering the city, the long-distance pipeline is connected to the municipal road pipeline.

Municipal road pipelines

Municipal road pipelines refer to the pipelines that were laid along with the construction of urban trunk roads and branch roads. The main types of pipes used are 110/100 solid wall pipes, 7-hole plum blossom pipes (inner diameter 32mm), and/or grid pipes (4-hole/9-hole), as shown in Figure 5, the material of the pipes is mainly PVC-U.

The length of the pipeline section is determined based on the distribution of users along the pipeline and the location of branch pipelines, generally around 100 meters, with a maximum of no more than 200 meters. The minimum burial depth requirements for pipelines (from the top of the pipe group to the ground) are shown in Table 2.

The structure of pipeline manhole(handhole) mainly includes: foundation (including water tank), wall, covering, mouth (also known as “well neck”), mouth ring, and manhole cover, as shown in Figure 6. The manhole cover is usually circular.       

Figure 7 are on-site photos of manhole (handhole)covers, mouth,coverings, and walls.

The specifications of manholes (handholes) are related to the capacity of pipe groups. In GB 50373-2019, recommendations are made for the specifications of commonly used manholes (handholes)and the applicable capacity of pipe groups, as shown in Table 3. However, in practical engineering, the situation where the size of the manhole (handhole) is lower than the recommended specifications is very prominent, which also leads to the manhole always being filled with fiber optic cables, as shown in Figure 8.

The wall of the manhole (handhole) is mainly constructed using brick masonry, but the construction time is relatively long, and the quality  is difficult to guarantee. So, in recent years, the use of prefabricated manholes (handhole) has become increasingly widespread, as shown in Figure 9.    

Currently, comprehensive pipe galleries (see Figure 10) have been constructed on newly built roads in some cities to replace various underground pipelines for different purposes. However, the current construction and maintenance costs of comprehensive pipe galleries are much higher than traditional pipelines, and pipelines will remain the main channel for laying optical cables on municipal roads for a considerable period of time.

Resident network pipeline

The grounding network pipeline is a pipeline outside the municipal planning red line, mainly including communication pipelines within buildings, residential areas, and other areas within the building planning red line.

The grounding network pipeline often intersects more frequently with gas, water supply/drainage, and other pipelines, so plastic solid wall pipes, plum blossom pipes, and silicon core pipes with better bending performance are often selected as pipe materials. The burial depth requirements for the grounding network pipeline are shown in Table 2, but it is very common for the actual burial depth of the pipeline to be lower than the standard, as shown in Figure 11.

The number of pipe holes for the grounding network pipeline is usually small, and only hand holes are usually constructed. There are often many branches of optical cables in the residential network, and correspondingly, the density of hand holes is also high, as shown in Figure 12.

According to YD/T 5178-2017, the handholes for grounding network pipelines should be the same as those for municipal road pipelines, and the handholes in Table 3 should be used. However, in actual engineering, SK series wiring handholes are still widely used. The SK series wiring handhole has a shallow burial depth and low strength, and in actual use, there is a high probability of damage, as shown in Figure 13. It is no longer recommended to use it in YD/T 5178.

With the increase of urbanization rate, the proportion of fiber optic cables laid through pipelines is gradually increasing. However, the poor quality of pipeline construction has had a significant impact on the laying and maintenance of optical cables. I hope that in the near future, we can at least see that the quality of newly built pipelines is qualified.