Cables in sewers the way forward
01 August 2002
Existing underground sewage and pipe networks could be used to house new-generation cables designed to sustain the IT revolution that is sweeping the world. Dr Jey Jeyapalan, PE, examines the technology involved, the concerns over the issue and the criteria for selecting the right sewers.
Hundreds of millions of people throughout the world are likely to be 'telecommuting' to work in less than a decade from now and this will increase productivity and quality of life significantly, saving energy, reducing pollution, and redistributing wealth and real estate values.
In North America alone, more than 110 million people are expected to 'telecommute' by 2010.
To meet the current and anticipated demand, the telecommunications industry has kept pace, creating technology that allows up to 10 trillion bits per second to travel through a single strand of fibre measuring a mere 7 microns, and this is equivalent to carrying 150 million phone calls simultaneously.
Some economists predict that without true broadband, our technology industry and economy could remain stalled, and currently the installation of an optical-fibre network to your office building or home is itself stalled.
The primary obstacle is 'the last mile'. This connection requires extensive construction, usually involving the excavation of city streets. Consequently, most cities discourage the new open-cut excavations that would bridge the gap over the last mile or last 10 feet.
This problem is worldwide in most major populated cities. The large cities in the Middle East are no different when it comes to the 'last mile' problem.
The solution
In North America alone, many trillions of dollars have been invested in the past century in building over 2 million km of sewers in the ground, while over 1 million km of natural gas pipes go into homes and businesses.
So it makes sense for either telecommunications companies, utilities, or city-owned telecom carriers to lease space in existing sewers and natural gas pipes for the laying of optical-fibre cables.
Owners of these underground utilities can generate a new revenue stream and telecommunication companies could install their optical fibre cables at an attractive cost.
But all parties must be aware of the need to follow proper standards of care to build networks in publicly-owned sewer systems and gas pipes.
How this works
There are at least five robot companies involved in this installation technology: CableRunner, DTI-CableCat, Ka-te, Nippon Hume, and RCC.
CableRunner uses a drill and dowel system in sewers of 250 to 700 mm in size. DTI-CableCat uses either a back-reamed anchor or an adhesive bed system in sewers of sizes 200 to 1,200 mm, while Nippon-Hume and RCC use drill and dowel systems for the same-sized sewers. Nippon-Hume uses a plunger pin stem for the anchor along with a two-part resin system while RCC uses a frictional stem anchor. Ka-te uses a stainless steel clamp and conduit system for sewers of sizes 200 to 700 mm.
In the drill and dowel system, a drill hole is made into the wall of the sewer pipe within the upper part and the cable is attached using either an anchor or a cable tray.
In the adhesive bed system, the sewer pipe surface is coated with a glue and either the cable is directly or attached or fitted into either trays or clips attached to the glue bed. In the clamp and conduit system, stainless steel rings are fitted inside the pipe where, the stainless steel conduits to house the optical fibre cables are clipped.
Each method has its advantages and disadvantages and is likely to find work in its own market niche where the users will take some responsibility for using suitable engineering criteria to pick the best to fit their budget and the needs of the project.
In addition, there are liner systems vying to do some of this as part of routine sewer maintenance programmes. There is a good chance that these liner companies will succeed if they are able to offer value-added relining systems for an attractive incremental fee to the city sewer agencies over the standard lining systems without cutting too much into the current functions of the sewers.
The concerns
As one example of the numerous questions and concerns raised by the possibility of placing optical-fibre cable in existing underground utility conduits, we'll look at the issues raised by sewer agency engineers, based on sound engineering principles, about allowing anything other than sewage in their sewers.
The operation and maintenance of sewage conveyance systems need active preventive maintenance and sound pipeline engineering input. If proper standard of care is not practised, it is only a matter of time until major problems will manifest and the sewer lease fee paid by a fibre installer to city hall will amount to nothing compared to the cost the public will have to bear to return the sewers back to normal.
Historical lessons learned more than 100 years ago in the Paris sewer tunnels when engineers attempted to place more than one utility in the same space must be studied thoroughly so as not to repeat the same mistakes.
Working in the sewer will affect the health, safety, and welfare of the people we serve and any short-sighted approach to selecting the sewers for installing and operating optical fibre cable, would expose all those in this new industry to an enormous liability.
Developing sound engineering standards falls well within this obligation. No two technologies operate the same way; no two technologies cost the same; and no two technologies provide the same end results.
If something were to go wrong with any one of the many technologies, in this author's view, even the ones that have the potential to serve intended functions, would be eliminated from the market place en masse'. And this will make the public wonder "why did our trained civil engineers licensed to protect the health and welfare of us all fail to follow appropriate standard of care''.
Sewer selection criteria
As an example, let us address one of the most pressing issues in every sewer owners' heart at the present time. If we were to consider allowing optical fibre cables in existing sewers, what criteria should a fibre cable installer use to select the sewers that are suitable for optical fibre deployment?
Operating an optical-fibre network in the sewers poses its own challenges. Proper civil engineering input is essential for the selection of the suitable sewer system for deployment.
The factors to consider in selecting the right sewer path are:
The primary access to the sewer for fibre cable installation using either a robot or other means is through the manholes at both ends of the reach. It is desirable that the length of the reach is shorter than what is reachable by the umbilical cable needed for the supply of air, electricity, and communications circuits. If man-accessible pipes were chosen, then this limitation would not apply.
Given the ubiquity of infiltration and inflow (I/I) problems, sewer designers have been able to count on only 85 per cent of the actual flow area to convey the flow. The impact of optical-fibre cables, their fasteners, and other deployment parts on flow characteristics need to be evaluated by running physical hydraulic studies with and without such components. Hydraulic studies of the flow conditions under the worst possible scenario based on past flow records in that sewer also needs to be done before the sewer is considered for optical-fibre cable installation.
An evaluation of the structural capacity of the sewer to carry the soil load, ground-water load, and live load needs to be conducted. This is to ensure that the current condition of the sewer is adequate to house the optical fibre network.
Sewers need to be cleaned periodically as part of their maintenance. Once optical-fibre cables are installed in the sewer, special precautions must be taken in choosing and applying suitable cleaning methods that would not cause damage either to the sewer wall or the optical fibre cables.
Periodic maintenance of the sewer will also involve inspection of the internal condition of the sewer system once the optical-fibre cables are installed. Special precautions need to be taken in choosing and applying suitable technology for sewer system inspection in order not to cause damage to either the sewer walls or the optical fibre cables.
Sewers require periodic maintenance involving anything from point repairs, grouting and relining, to total replacement. The current condition of the sewer system and its need for repair or rehabilitation during the design life of the optical-fibre network must be carefully evaluated.
It is not possible to work with certain sewer wall materials depending on the fibre installation technology used, and so the materials will need to be evaluated.
Sections of pipe that have grease accumulations of more than a suitable thickness within one year of cleaning should not be considered candidates for optical-fibre system installation until proper remedial action is taken.
Sewage carries many chemical reagents and the compatibility of fibre deployment materials and components with all such chemicals needs to be tested.
Optical-fibre systems should not be deployed in sewers with excessive calcium deposition.
Joint separations/offsets can lead to both infiltration and exfiltration. Structural damage to the sewer may result from pipe bedding material being transported into the pipe. Enough care should be exercised when such sewers are encountered before these are chosen for optical-fibre deployment.
Sewers should be free of excessive root intrusion to be eligible for installation of a optical-fibre system.
13. Condition of the manholes
Manholes should be in an acceptable physical condition for that sewer system to be used for optical-fibre cable installation.
Standards
Consensus standards developed within ASTM International and engineering guidelines developed in the American Society of Civil Engineers are needed to address technical issues such as these for every technology in the market. The standards we write need to be based solely on engineering merits.
* Dr Jeyapalan graduated from the University of California at Berkeley and was a civil engineering professor for 10 years. Dr Jeyapalan has been an international consultant for 30 years on design, engineering, construction, and standardisation for a wide range of technology on fibre optic networks, pipelines, trenchless works, pipeline rehabilitation, and failure investigation. He has testified as an expert on numerous disputes, claims, mediations, arbitrations, and lawsuits on failures of pipelines for water, sewage, oil, gas, chemicals, desalination, and hydropower.
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