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Project: T174700 #50/145

Presently, electrical ducts are made primarily of plastic materials. Polymers such as polyethylene (PE) and polyvinyl chloride (PVC) are excellent candidates to replace historical cast concrete, cement or clay ducts used in distribution networks. These polymers present with high strength, low friction coefficient, corrosion resistance, light weight, easy handling, and superior weathering characteristics. Other polymers, such as fiberglass reinforced thermosetting resins, are being used in ducts as well. Ducts are used in two major methods for cable burial: direct buried ducts and concrete encased ducts.

With the direct buried duct method, cables are installed underground in polymeric ducts or sleeves. This method is usually used in rural or suburban areas where load conditions do not require additional reinforcements and the cables cannot be direct buried. If there is a road crossing, this method is not recommended. Direct buried ducts can be installed by a variety of techniques, including open trench (open cut), plowing, and directional drilling (horizontal directional drilling or directional bore). Among these techniques, open trench is the most common. Moreover, plowing and directional drilling installation techniques do not offer the short- and long-term support of open trench installations, where the bed and backfill can be engineered to provide the required support for the duct.

With concrete encased ducts, the cables are installed in a concrete encased duct bank which can be reinforced with rigid steel for additional strength. This method is usually used in urban areas with high load density, road crossings, and areas accessible to heavy equipment. Concrete encased ducts can be installed via two techniques: single pour (open cut) or tier-by-tier installation. Tier-by-tier installation is similar to the single pour installation, only it is done sequentially. The tier-by-tier method is based on pouring each tier independently and has the advantage of producing void-free, solid, homogenous concrete envelopes, compared to the single pour method.

A comprehensive questionnaire was distributed to various utilities/suppliers in order to collect information related to direct buried versus concrete encased ducts. The results of the survey showed that the direct buried duct method is dominant for primary cables, but the concrete encased duct method is used similarly for primary and secondary cables. Moreover, the dominant material in both techniques is PVC. Steel reinforced concrete ducts are used for the majority of primary cables and half of secondary cables. The main standard for duct specifications in the direct buried duct method is CSA C22.2 No.211.1-06; there is no standard for concrete encased ducts. Both methods are used in private properties, such industrial facilities or residential. The majority of the survey respondents mentioned they considered the soil load bearing capacity for direct buried ducts for secondary cables, and concrete encased ducts for both primary and secondary cables. In addition, the majority of respondents mentioned they do not receive any municipal guidance regarding both installation methods. The majority also mentioned they had experienced failure using the direct buried duct method. On the other hand, the majority of survey respondents did not mention failure experiences with concrete encased ducts. All in all, most respondents mentioned overall satisfaction with both methods. A direct comparison regarding the cost analysis of the two methods is difficult, because it depends on several factors, such as the contractor, site location, installation time, etc. But in general, the average installation cost for concrete encased ducts is higher than direct buried ducts.

Direct buried duct, concrete encased duct, primary cable, secondary cable