With continued development in and around Branson, a new 7500 acre (3035 hectares) community called Grand Ridge is being built to provide houses, condominiums, retail stores and shops, hotels and at least two championship golf courses. In keeping with the development's clean lines and lush design, the electric distribution system was specified for underground installation. A conventional underground system, consisting of large conduit in the rocky Branson area, carried cost about US$50 per foot and carried no assurance that the installation could accommodate future load growth. One alternative was to copy distribution systems that are common in large metro areas, where services are provided by a network of tunnels under the city. This alternative, however, was prohibitively expensive because of the large scale of excavation that would have been necessary in rocky soil.

Looking at the Sidewalk The physical features of the Grand Ridge development included a four-lane road running through the middle of the project with a sidewalk on one side. The location of the sidewalk was a perfect right-of -way for the distribution circuit, provided that a suitable construction design could be developed. By installing the sidewalk as a precast raceway, the primary cables could be laid out on the ground within the raceway, eliminating the need for underground conduit. Pursuing this idea, Empire and Rose-Con Pipe Co., Springfield, Missouri, designed a sectionalized walk way that would have ample space for meeting the needs of the entire Grand Ridge infrastructure, both now and in the future. Design requirements included provisions for proper drainage and adherence to the State Highway H-20 loading specs to cover potential liability in the event that a car or truck drove across the sidewalk. In addition, the design included radius turns at curves in the sidewalk. Each top section of the sidewalk is 4 ft by 8 ft (1.2 m by 2.4 m) and each bottom section is 3 ft by 10 ft (.9 m by 3 m) along the inside dimension and 18 inches (46 cm) high. Each concrete section is 6 inches (15 cm) thick. The center portion of the box-like bottom section is open to the earth to accommodate entry of conduit runs from streetlights and other services. In addition, the central portion is filled with 6 inches (15 cm) of river rock below grade to control drainage.

Installation and Maintenance With the total height of the raceway measuring 24 inches (61 cm), 18 inches (45.7 cm) for the side wall and 6 inches (15 cm) for the thickness of the top slab, the bottom section was leveled at 10 inches (25.4 cm) below grade, leaving 14 inches (35.6 cm) above to match the elevation of the road. A grass strip was planted between the road and the sidewalk. Two 12-kV circuits, supplied from the overhead line connected to Empire's Riverside substation, were installed in the raceway. One circuit consisted of three single 4/0 AWG aluminum conductors and the other had three single 750 kcmil aluminum conductors. The cables have 175 mils of polyethylene insulation and 80 mils of jacketing with no shielding. Placement of the cables consisted of spooling the individual lengths of cable into the sidewalk raceway with no tension required for pulling. While the ampacities of the cables were based on cable-in-duct criteria, cooling from air circulating through the structure because of airways that existed as part of the drainage provisions could provide some additional benefit. In addition to the 12-kV circuits, GTE purchased space for its phone lines. Maintenance, in case a line is faulted, is expected to pose no problems because a fault locator with a radar unit can determine the exact location of the problem. Unlike the conventional system that requires the faulted section to be pulled out at a pull box and all three cables replaced, for the sidewalk installation, only the top section of the sidewalk needs to be removed to repair the single faulted cable.

Conclusion The unique design used for this special circuit proved to be particularly well suited for the project. Expectations are that the design will find other applications where it can be used, both on Empire's system and other electric utility systems as well. Fig. 1. Overall view of construction under way at the Grand Ridge project. Fig. 2. Center of sidewalk filled with river rock for drainage. Fig. 3. Cable being spooled off into the sidewalk raceway. Fig. 4. Sidewalk with some of the tops installed.

James E. Barker is a senior construction designer at Empire District Electric Co. where he has worked for 29 years. He worked at the company for 18 years as a lineman, while earning a diploma from ICS in electronics and a certificate for junior engineer. He is responsible for the design of overhead and underground distribution lines in the Branson area. He has previously published an article in T&D World on the design of pull boxes (July 1993).