Hydro-trenching, or would you like broadband with your waste water collection?
9 March 2011
There is currently much interest in exploiting existing infrastructure to reduce both the cost and deployment time of Fibre To The Home (FTTH) broadband networks. This is especially so in heavily urbanised areas where the cost of installing new fibre infrastructure is increased significantly by:
- identification and avoidance of existing utility services
- traffic control (road closures)
- restoration of surfaces.
In addition to the direct costs of the deployment incurred by the network operator, local government authorities are also concerned with the externalities associated with conventional deployments, such as:
- traffic disruptions
- reduction in service life of roads due to the cumulative effect of multiple operators opening and closing roads
- aesthetic issues particularly in historically significant areas.
The technique of hydro-trenching involves the installation of fibre optic cables within functional water utility pipe infrastructure.
The European cities of Dusseldorf (Germany), Salzburg (Austria) and Berne (Switzerland) have made extensive use of existing sewer pipe networks to provide FTTH services. In these cities the primary motivation has been to avoid the very high restoration costs associated with the historic nature of the city areas.
In Australia, Brisbane City Council recently trialled a 1.4km fibre route along an existing sewer, however it was unclear what technology was used to deploy the infrastructure.
Urban pipe infrastructure
Most urban areas have three separate water pipe networks:
- potable (drinking) water distribution
- waste water (sewerage) collection
- storm water collection.
The first two pipe networks have virtually ubiquitous distribution to and from urban residential, commercial, educational, healthcare and industrial sites. Indeed the universal delivery of these services is the prime driver of the network design. The storm water network design tends to be primarily driven by the requirement to emulate natural drainage processes within the urban environment and is therefore not ubiquitous.
Waste networks are generally preferred over potable networks for implementing hydro-trenching techniques, to avoid any potential freshwater contamination issues.
Installation techniques for hydro-trenching
Most (around 97%) typical Australasian waste water systems consist of non person entry pipes with outside diameters (OD) between 150mm and 700mm (waste water pipes with a diameter of greater than 750mm are considered capable of person entry). Therefore the basic hydro-trenching technique involves the deployment of robotic technology to install fibre optic cable within a flexible protective sheath inside the pipe.
A number of installation techniques have been developed but the most widely accepted by waste water system operators is the use of an internally expanding clamp to fix the cable or cables to the top of the pipe. This technique does not harm or compromise the structural integrity of the existing pipe or liner (in more established waste water systems older pipes have been successfully rehabilitated by the installation of a plastic liner). Typically clamps are installed with a spacing of between two to five metres depending on the diameter of the pipe. The installation process is very sophisticated and both the clamp location and attachment points of the conduit to the clamps are individually selected during the robot’s preliminary mapping run through the pipe. This process ensures that lateral connections (pipes smaller than 150mm in diameter that provide most connections from households to the waste water main) to the waste water pipe are not obstructed by clamp installation (or the protective sheath).
The process also ensures that clamps are not installed across sewer pipe joints where small discontinuities in the surfaces may prevent the clamp sitting flush with the pipe and creating a particle trap. This is very important in waste water systems as such traps tend to accumulate material over time and eventually impede the maximum flow rate of the pipe. The effect is very similar to that of human hair accumulating in the waste water drain of a domestic shower.
The minimum internal diameter (ID) for robotic installation is currently considered to be 200mm. Manual techniques have also been developed to deliver fibre along waste water access laterals and other pipes smaller than 200mm. These manual techniques however are more labour-intensive and hence more expensive than robotic installation.
To ensure only a negligible reduction in flow capacity of the pipe it is recommended that no more than two 15.5mm conduits in a 200mm pipe are installed. This equates to a reduction in available cross sectional area of 1.2%. The fibre core capacity count of each 15.5mm conduit is 216. Installations of up to nine 15.5mm conduits have been successfully completed in waste water pipes of 350mm ID.
Access to the fibre is provided at designated junction boxes co-located with other waste water infrastructure such as access points (manholes).
The fibre conduits are attached to the clamps using clips and run along the top of the pipe. This ensures that remote robotic access for regular waste water system maintenance tasks (such as cleaning and CCTV inspection) is preserved.
It should be noted that conventional techniques are generally used to provide the final drop to the customer premises.
Case study: the Wellington city waste water system
The New Zealand capital Wellington was selected as a case study due to the ready availability of detailed information about the existing waste water infrastructure. The city has a very sophisticated geospatial information system (GIS) and asset management system that records the locations and attributes of all utility assets, and an extensive property information database.
The waste water system however shares a common history with other Australasian cities such as Sydney, Melbourne and Auckland, all of which first deployed closed underground wastewater collection and treatment systems during the 1890s. The materials used in the collection networks at that time were generally glazed vitrified clay pipes with some use of cast iron where increased structural strength was required. The use of earthenware piping continued until the widespread availability of concrete and composite plastic materials in the 1950s and 1960s. Even today significant quantities of earthenware materials remain deployed particularly in the more established areas of the cities (Exhibit 1).
We recently performed a study of the waste water network in Wellington Central Business District (CBD) utilising GIS analysis to assess the suitability of existing waste water pipe infrastructure for hydro-trenching.
We estimated the cost per metre of a single conduit install (up to 216 fibre cores) to be between NZD16.00 and NZD23.00 per metre while the cost of a two conduit install ranges between NZD21.00 and NZD29.00 per metre (Exhibit 2).
Installing ducts in trenches within urban areas on or near busy roads requires traffic management, identification and avoidance of existing services and surface restoration, all of which inflate the total cost of deployment. This is in addition to the labour-intensive process of trenching, laying and back-filling conventional ducting. These costs can collectively sum to more than NZD200 for every metre of trench deployed.
Our analysis indicated that 59% of the existing waste water pipe network in the Wellington CBD was suitable for hydro-trenching (Exhibit 3), and just over 80% of address locations were within 50 metres of a suitable waste water pipe.
Hydro-trenching clearly offers a cost-effective and minimally invasive solution for deploying networks, particularly in urban areas where suitable pipes are available.