The article Foresight Is More Than 20/20, in the May 2008 issue of OSP® magazine, describes a Buried Duct Network (BDN) intended to facilitate the addition of new utility cables (telephone, CATV, electric power) in buried plant applications. During recent months, the BDN has been successfully installed in several field trial sites, using both conventional open-cut trenching as well as horizontal directional drilling (HDD). The BDN system encourages and supports the installation of belowground utilities, minimizing subsequent reconstruction difficulties and hazards. It also provides an opportunity for conveniently and safely accomplishing cable upgrades, at a relatively low incremental cost. In addition, this R&D effort is supported by the U.S. Department of Transportation, and the FHWA Turner-Fairbank Highway Research Center.

A Different Kind of Hybrid
The proposed BDN solution is comprised of a duct network that helps ensure space-efficient, cost-effective joint-use buried facilities and practices are employed. Such facilities would be shared by communications and power utilities, or a subset of such participants. Figure 1 illustrates the generic BDN concept, representing an integrated system of ducts and handholes. A single main trench along the right-of-way (ROW) would be used for the participating utilities, as well as a service trench to each home (or small business) for residential type applications, as appropriate. The size and type of the indicated initially installed (direct-buried) distribution cables would be selected based upon the conventional engineering rules used by the utilities, in their initial attempt to meet present and future anticipated needs. Thus, the BDN upgrade capability would only be used to meet unanticipated needs that nonetheless sometimes arise, due to the desire to introduce new technologies, increase capacities, and/or replace degraded cables and wires.

The BDN is a hybrid between buried plant and an underground conduit system. It attempts to retain the relatively low cost of conventional direct-buried construction with the upgradability of underground conduit plant. The BDN concept may be adapted to a variety of configurations, including participation by communications companies only, avoiding some of the issues associated with sharing facilities with electric power. It is also possible that all the cable utilities, including communications and power, may agree to share the trench, but with the upgrade features of the BDN, including access to the handhole and ducts, restricted to the communications carriers. As experienced by the field trials to-date, the variety of alternatives for deployment of the BDN will continue to evolve.

Logistically, the initially installed distribution cables would be placed in the trench and directly buried, using present rules and practices, and terminated at the required equipment or (above- or below-ground) terminals. For residential or small business type applications, the initially required communications and electric power (120/240 volts) service wires/drops would be placed within individual small diameter (e.g., 1- to 3-in.) ducts, routed from the pedestal terminals mounted at the ROW to the residence or business.

In addition, two initially vacant, relatively large diameter (e.g., 4-inch) conduits or ducts are placed along the ROW, in the same main trench as the direct-buried distribution cables. There are no initial connections to the terminals, equipment, or hardware.

One duct could, for example, be used for future electric power upgrades or maintenance, and the other duct for communications (telephone and/or CATV), although the actual final implementation may vary, based upon the actual needs. Thus, if the demand for new communications cables requires the use of both ducts for this purpose, that is an option. Conversely, a power upgrade or maintenance activity may require the use of both ducts. The provision of only two main ducts is based upon the probability that not all utilities would require an upgrade in the foreseeable future, and that such duct capacity would therefore be sufficient for practical cable upgrade or
replacement needs.

The ducts are accessed at handholes placed at strategic locations along the ROW. Assuming several utilities shared responsibility for the initial installation of the BDN, a reasonable joint-usage agreement among these parties would allow for equitable cost sharing based upon the actual future occupancy of the ducts. Alternatively, if a government body (municipal, state, county) assumed responsibility/ownership for the ducts, that authority would administer and manage the ducts, making them available to various utilities, subject to appropriate fees and usage requirements, as appropriate.

The main ROW ducts may be of high density polyethylene (HDPE) or commonly used rigid polyvinyl chloride (PVC). The ducts would be sufficiently large to practically accommodate most utility cables and allow the option of further division into several paths for communications cables. This would be done via the subsequent installation of smaller innerducts, or textile fabric cells. At the time of future upgrade or replacement of any of the facilities, new distribution cable may be readily installed into one of the large ducts and temporarily accessed at the handhole for routing to the adjacent existing or future/replaced terminal or equipment. The connection to the equipment or hardware could be accomplished by local digging (hand tools, air, or water vacuum tools), requiring only minimal excavation. For a residential / small business type application, new service drops could be placed using the initially installed duct paths to the homes or small businesses, as described above. Thus, the ducts serve as a means of placing new cables or wires without requiring major construction activity.

Getting a Handhole On It
The BDN employs a joint-usage handhole to serve the needs of all participating utilities. For residential or small business applications, a handhole may be co-located with a group of above-ground service pedestal terminals. For non-residential applications (thoroughfares), the handholes would be located at required equipment or terminals or other convenient locations (e.g., to facilitate cable installation and/or accomplish a splice).

Figure 2 illustrates the handhole design as presently envisioned for the BDN. The bottomless feature allows water drainage, as well as the possible option of routing cables to the adjacent pedestals through the floor. Most important, the bottomless design allows the handhole to be readily placed directly over the two previously installed main distribution ducts routed along the ROW in the trench, with minimal effort. Two colinear “mouseholes” located at opposite ends capture each main duct passing through the handhole.

Knockout ports or slots are provided at the side of the handhole to facilitate future routing of cables to the various adjacent equipment or hardware terminals (existing or replaced), as required. The slots allow the option of bending a telephone cable (comprising a multitude of individual copper pair or fibers), after local removal of the relatively stiff protective outer jacket, and inserting the cable loop through the slot for placement within the adjacent pedestal terminal or equipment housing, without requiring the cable to be cut across its entire cross-section, or without the need to pass the free end out and back into the handhole and continuing along the route.

It is an important feature that, in general, the handhole not represent an access point for routine maintenance operations. The handhole is used only in the rare event of an upgrade, and will not contain any terminals or splices - only cables being routed to their nearby equipment or terminal point.

Verizon/Cable Company Field Trial
Recently, three field trials have been successfully completed. One of these projects was a cooperative venture along a state road in North Carolina including two service providers: Verizon and the local CATV (“Cable”) company.

Recently, State Road SR1223, in the City of Monroe, North Carolina, was being widened in conjunction with the installation of a related bypass in the area. Since the project already required moving existing utilities in the affected area, the Utility Relocation Manager for the project (Gus Kretschmer, The LPA Group of North Carolina) realized the need to future-proof the project. As a result, an approximately 1Kft long segment of BDN was installed, coordinated with the road construction.

The overall construction was cooperatively performed by three parties: Verizon’s contractor; the Cable company’s contractor; and the company responsible for most of the associated road-widening construction, Blythe Construction. Blythe Construction used its heavy equipment to open the basic trench, using a 3-foot wide bucket, which dealt easily with the rocky soil conditions in the area.

Verizon and the Cable company agreed to the joint-construction of the open-cut trench which would include their initial cables and the BDN conduits and handholes. This application would house both copper and fiber distribution cables. Interestingly, while the two communications companies agreed to jointly share the trench, they also agreed that Verizon would have access to the BDN facilities.

To begin, the Cable company directly buried two 2-inch innerducts, one of which represented a spare for future upgrades; the other would contain a fiber-optic cable.* Both innerducts contained a pull line.** Since the Cable company provided its own private upgrade capability, Verizon decided to place its two initial cables (fiber-optic and a 600-pair copper) within one of the two BDN conduits, allowing the remaining vacant conduit to serve its future needs.

The architecture for this trial included geometry and handhole locations, is shown in Figure 3. The actual final segment length was 720 ft. The handhole at one end was placed adjacent to existing pedestals of Verizon and the Cable company. The handhole at the opposite end was placed prior to a creek. The third handhole was conveniently placed near the mid-point, to facilitate subsequent cable placement for the relatively heavy 600-pair copper cable to be placed within one of the BDN conduits.

Figure 4 illustrates the trench cross-section, and the intended placement of the initial communications cables - to be installed following other road construction activities in the area.**

Since the application allowed low-cost open-cut trenching, conventional PVC pipe was conveniently employed. Two parallel conduit paths were constructed of 20-ft. lengths of 4-inch schedule 40 PVC pipe, assembled in the trench using adhesive joints (Figure 5).

For this application within the NC DOT right-of-way, three polymer concrete units (Figure 6) with a TIER 15 (ANSI/SCTE 77) rating were installed. These BDN handholes are 2½ feet in width and 4 feet in length, with a depth of 36 inches.

The handholes were placed over the previously installed conduits, which were captured by the mouseholes at the bottom of the units. In this case, Verizon decided to add longitudinal sweeps to the main BDN conduits, along the distribution path, to facilitate the pulling operation for installing their cables. Similarly, two lateral 4-inch PVC sweeps were added to facilitate the routing of these cables to the adjacent aboveground pedestal/terminal that was also installed during the project. These sweeps were larger than the standard knockouts provided, requiring the contractor to cut a larger window in the side of the handhole. This was readily accomplished. All conduits were plugged to prevent entry of soil and debris during the balance of the construction, and to avoid problems during the later cable installation process.

Since the BDN handholes are not intended to contain any splices or equipment, any such items must be housed independently by each utility. For this project, Verizon placed pedestals that could be accessed by the lateral sweeps. Separate housings serving the cable company cables were also installed to serve the cables within the innerducts. Such independent housings may be above-ground cabinets or pedestals, or below-ground or flush-mounted enclosures, such as other handholes (or mini-vaults), albeit of different size and configuration than those serving the needs of the BDN. In the case of the Cable company, a somewhat smaller mini-vault - approximately 2 feet wide, 3 feet long, with a depth of 36 inches - was installed in the vicinity of the BDN at the middle the route segment, as indicated in Figure 3, and shown in Figure 7.

Backfilling, Compacting, and Completion
Following placement of the conduits, handholes, innerducts, and mini-vault, the trench was backfilled. After filling to approximately half the depth of the trench, a compactor unit was lowered into the trench to assist in the compression of the loose soil. Warning tape was placed 12 inches above the ducts, as an effective means to alert possible future excavators that utility lines were immediately below.

Following backfilling and compaction at half the trench depth, the remainder of the trench was filled to grade level, and again compacted. Figure 8 shows the filled trench in the vicinity of the BDN handhole and Cable company mini-vault, at the middle of the route segment.

The BDN segment (Figure 3), including two vacant 4-inch PVC conduits and three handholes, plus local sweeps to adjacent terminals, as well as two vacant 2-inch HDPE innerducts, was installed within two full working days. The construction proceeded without mishap.

This effort demonstrated the ability of multiple communications carriers to efficiently and cost-effectively cooperate in placing their facilities, including the BDN, in a joint trench configuration.

About the Author
Dr. Lawrence M. Slavin is Principal of Outside Plant Consulting Services, Inc. He can be reached at tel: 973.983.0813, email: lslavin@ieee.org, and www.outsideplantconsulting.com. Dr. Mohammad Najafi, P.E. is Director of the Center for Underground Infrastructure Research & Education (CUIRE), The University of Texas at Arlington, Arlington, Texas. He can be reached at najafi@uta.edu and www.cuire.org.

End Notes:
* Direct-buried innerduct is typically categorized as “direct-buried cable”.
** The Cable company’s innerducts contain a metallic tracer wire embedded within the plastic wall to facilitate subsequent locating activities.

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