Terrestrial backbones are an essential component of the telecom chain
Terrestrial backbones are an essential component of the telecom chain. They connect the main nodes of a country's network between major cities, and enable very high speed and high capacity links. They mainly aggregate traffic from different regions of the country and carry it to the next node or city. Although it is not the perfect comparison, but an analogy can be made between terrestrial backbones and highways of a country.
A costly deployment and uncertainty about work organization
The recommended 12 installation method for terrestrial backbone networks is to bury the fiber optic cables at a depth of around 1.20 meters along the entire route. This installation method guarantees a longevity of more than 30 years and a high availability of these infrastructures.
The investment for the deployment of a terrestrial backbone network is around 20,000 USD / km. Civil works represent a significant part of the initial investment. Insufficient preparation during the study phase is behind the difficulties faced during execution and sometimes leads to a significant extension of the projects or to their discontinuation.
The analysis of buried network construction delays or difficulties shows that they are mainly the result of:
- An approximate knowledge of the ground;
- a late awareness of societal and environmental impacts;
- a lack of knowledge of the nearby subsoil with the inability to mobilize the appropriate excavation means.
The environmental and geotechnical study phase must be anticipated
Building on its previous experiences, Sofrecom has developed an innovative methodology for the environmental and geotechnical assessment of routes by associating itself right from the study phase with several partners in order to keep these different aspects in check.
A first partner will provide local technical skills for a detailed assessment of the routes envisaged for the passage of the fiber: the road is photographed at regular intervals to analyze the most favorable side for fiber installation. Surface obstacles (bridges, river crossings, swamps, villages, road and railroad crossings, sacred places, protected sites, forests, etc.) are documented thanks to geo-localized photos, which allows to recommend a technical solution to their crossing.
Societal and environmental impacts will be analyzed by a specialized local partner holding the necessary accreditations. Opportunities along the route (villages, fuel stations) that will facilitate the execution of the excavation work and fiber optic installation are noted. This partner interacts with the authorities to specify the public right-of-way and the most suitable location for the cable burial.
Finally, a third partner, intervening remotely, has the geoscientific skills to recommend the sites for soil compaction tests and then to analyze the compaction of the soil to a depth of up to 1m50. He then confirms the relevant excavation means to carry out the trench according to the compaction and the excavation resistance of the soil:
- soft soil: it can be dug with a shovel,
- hard soil: use of a pickaxe,
- rocky soil: the use of jackhammers or mechanized excavators is then necessary.
Sofrecom confirms the feasibility of delivering these means to each area.
Geotechnical analysis of subsoils saves time and resources
Existing soil maps provide little information on rapid subsoil transitions and surface soil compaction. Consequently, they are rarely used by construction companies because their accuracy is considered insufficient. The visual observation of the outcropping rocks alone does not allow to anticipate the obstacles of the nearby subsoil.
The skills deployed in geoscience allow to improve the uncertainty on the subsoil compaction while reducing the number of measurement points, from the pre-feasibility stage of the project.
The current practice is to carry out soil compaction measurements with a penetrometer at a constant pace of 50 or 100 meters. However, this method consumes a considerable amount of time (each measurement takes about 20mn) and mobilizes expensive technical and human means.
The use of geosciences brings a precision of around one decameter. This innovation consists of adapting the density of measurement points on the basis of the geologist's overall understanding.
With comparable quality, the results are obtained with less resources since the density of measurement points with the penetrometer is of the order of one measurement every 2km on average (except dense urban areas) to be compared with one measurement every 100m according to a traditional methodology.
All the actions implemented during the study phase allow to de-risk the execution phase of the works and bring several benefits:
- anticipating the negotiations with the public authorities,
- promoting the project to local authorities
- saving time during the study phase by reducing the number of measurement points
- anticipating the excavation means necessary during the construction phase.
This methodology has already attracted an international investor and has been deployed over several thousand kilometers in Central Africa. It is particularly well suited for projects of over 500km
1 ITU-T Technical Report on Optical fibres, cables and systems. Chapter_03-Optical_cables_installation.pdf https://www.itu.int/pub/T-TUT-HOME-2015-OFCS « § 2.2.2 Buried network : « The minimum depth of buried cables is typically 1.20 m.”
2 When no alternative infrastructure (aerial installation on MV or HV cables for example) is available.