The amount of each category of duct which will require new construction, against that which can use existing ducts owned by other utilities (or rights of way) indicates how disruptive a trenching and duct laying programme would be.
BAH estimates that some 1064 km of new duct route will need to be constructed along or across pavements (including "lead-ins") and that the estimate for new duct route which crosses the roadway will total 46 km.
The average road crossing will be 16 metres from mid-pavement on one side to mid-pavement on the other (see discussion below of types of roads to be crossed). BAH therefore expects that some 2,875 (46 km/16 m) trenches will need to be dug across roads over the five year build period. If each road crossing takes 2 days (assuming an 8-hour working day), the average number of roads "open" at any time will be 2.3 roads per day, on the basis of a 5 day week, 50 weeks per year. Assuming the mini-trenching technique is used, and that no extraordinary complications arise, each team can lay approximately 150 m of duct per day on average. At least 5 teams would be required to complete the network in accordance with the schedule. Therefore, 750 m of road and pavement would be "opened" on an average day.
The cost of duct laying is minimized by using the shallow trenching technique. During conversations with the Highway Department, HCV, British Telecom International and various United States based cable companies, BAH has found no technical reason to reject shallow trenching. Most importantly, no long term structural instability results from using shallow trenching in the footways. However, objections to shallow trenching have been raised by the Highway Department on the grounds that once the duct has been laid via shallow trenching there may be problems when another utility, whose ducts pass underneath the cable operator's ducts, needs to open the pavement to repair or supplement its ducts. The difficulty arises in assessing who would be responsible for any damage incurred during this excavation process. The second objection is that permitting shallow trenching for cable would set a precedent which may be used by other utilities.
BAH's view is that it is not a strong enough objection. Any road excavations have the potential to damage existing ducts whether they be in shallow trenches or in the more conventional deep trenches. The cable operator's exposure to damage to its ducts should be subject to the same procedures to determine responsibility as are the other utilities.
BAH's conclusions on the composition of a typical kilometre of trunk cables are given below:
Construction Type
Machine Mini-Trenching Manual Mini-Trenching Conventional Trenching Road Crossing Existing Duct
Percentage of Total Network
48%
18%
268
38
5%
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