Common utility duct

A common utility duct, sometimes called a common utility conduit, is any structure – above, on, or below ground – that carries more than two types of public utility lines. However, the phrase often refers specifically to underground utility tunnels.^[1]

Advantages of common utility ducts

The advantages of such facilities are the reduction of maintenance manholes, one-time relocation, and less excavation and repair, compared to separate cable ducts for each service. One of the greatest advantages is public safety. Underground power lines, be the ducts common or separate, prevent downed utilities from blocking roads, thus speeding emergency access after natural disasters such as earthquakes, hurricanes, and tsunamis. When they are well mapped they also allow rapid access to all utilities without having to dig access trenches or resort to confused and often inaccurate utility maps.

The following table compares the features of housing utility networks in single purpose covered trenches with the features of common ducts:

Trench	Duct
Long-term collaboration has not always been a high priority. So robust, precise location notice for older utility trenchs was often not provided or maintained, and older trench locations are often unknown.	Ducts are often used where developing authorities value the long-term benefits of utility co-location. That focus on long-term collaboration often includes greater emphasis on making duct locations easily known.
Single-purpose trenches encourage a utility to follow a single-minded route to shorten runs and save initial installation costs for that particular utility. But uncoordinated routing encourages spatial chaos, using more space than if trenches were highly parallel, and greatly increasing the overall encumbrance on surrounding development.	Ducts demand coordinated, highly collinear routing, reducing the overall encumbrance on surrounding development.
Even if parallel, trenches occupy more surface area. The surface area they use encumbers the area available for all forms of property development and construction with the burden of avoiding or moving the utilities.	Ducts are typically cylindrical, greatly increasing the volume of network resources per unit of surface area occupied.
Access to a trenched network typically requires locating the utility network, cutting open the road or pavement surface, breaking open the concrete platform and excavating a trench, followed by reinstatement of the trench, concrete platform and road surface afterwards. (This is where most of the financial cost of network renewals and maintenance is incurred.) Road surfaces can be seriously damaged by frequent trenching, requiring more frequent resurfacing. In the process, pavement slabs are often broken and badly aligned. UK roads are subject to 5 million roadworks per year (mainly for utility works).	Utility networks in ducts typically include designed-in access points (like those now used by British Telecom). Where ducts and access points are installed, excavations are rare and recurring maintenance costs are lower.
Maintenance of networks in trenches requires re-digging and restoring the trench and any roadbed above it. Road users suffer repeated delays from roadworks, particularly in dense cities. Roadworks for trench adjustments also require large quantities of sand, aggregate, cement, tarmac and marking paint.	Ducts allow maintenance through their access points. Since access points mostly obviate new roadway intrusions, traffic delays from duct-related roadworks are greatly reduced. Not disturbing roadways means network adjustments require materials only inside the ducts.
Rural properties are often denied access to gas or cable telecom because the cost of new trench deployment cannot be economically justified independently of other networks. Rural networks for electricity and telecoms are often above ground, with increased risk of disruption, even though there are usually local underground water and gas networks serving the same properties.	Sharing the higher initial installation cost of ducts across all services could make rural service more economically feasible. Where ducts are used, all networks are typically underground in multi-purpose ducts. Redundant above-ground electricity and telecom poles are usually dismantled, increasing safety and reducing natural disaster impacts.
Without common utility ducts, new types of networks require new trenches or independent ducts. Such expansions have already included cable telephone and television networks. Proposed local heat transfer systems and more localised, reconfigured power generation systems would also require new trenches.	Common utility ducts are designed to accommodate anticipated new and evolving networks.
The high thermal conductivity of soil would require extreme insulation for heat transmission through trenched networks.	The low thermal conductivity of air in ducts allows heat transmission with less insulation and cheaper standoffs.

Examples of common utility ducts

Many examples of common utility ducts are found in Japan, where government officials have sought ways to reduce the catastrophic effects of earthquakes in their tectonically active country. Their use, however, is not limited to that country and there are many examples of such common utility ducts. These include:

• Incorporated with Xinyi^[2] and Sonshan MRT rapid transit lines in Taipei, Taiwan
• Azabu-Hibiya Common Utility Duct in Tokyo, Japan^[3]
• Minatomirai District lines in Yokohama, Japan^[4]
• Poundbury Village in Duchy of Cornwall, Prince Charles' master planned community in England incorporates common utility ducts^[5]
• "Utilidors" in Disney theme parks
• German cities such as Bremen. This city has near perfect surfaces on its footways, cycleways and streets. Virtually no repairs or disturbances are visible. The number of streetworks in progress is trivial. This can only be achieved by (a) vehicles not driving on footways or cycleways (b) utility ducts making it unnecessary to disturb the surfaces in order to access the networks below for repairs, maintenance and alterations. Utility networks are housed in a cluster of pipes which are located under footways and cycleways.

See also

• District heating
• Electricity distribution
• Empire City Subway
• Public utility
• Tunnel
• Utilidor
• Utility tunnel
• Dartford Cable Tunnel

References

1. ^ Taipei Gov Website
2. ^ MAA Website
3. ^ BigEmpire.com Website
4. ^ Yokohama Landmark Building Website
5. ^ Mitchell, Sandy. "Prince Charles is not your typical radical." National Geographic. May 2006. [1] Accessed online 9/14/06