Construction & Design of Overhead Transmission Lines
Client: Belize Electric Co. Ltd.
Burns & McDonnell performed conceptual and detailed design engineering for overhead transmission lines being installed by Belize Electric Co. Ltd., in order to transmit power top populated areas more than 90 miles from a hydro generating plant. Responsibilities included structure design, foundation design, conductor sag-tension design, insulation design, guy and anchor design, and structure spotting. We provided bills-of-materials, technical specifications and drawings for procurement of materials.
- Overhead transmission line design, conceptual and detailed
- Material procurement
- Construction services
- Structure design
- Foundation design
- Conductor sag-tension design
- Insulation design
- Guy and anchor design
- Structure spotting
Belize, through its utility Belize Electric Limited (BEL), contracted with the Belize Electric Company Limited (BECOL), to develop a hydro generating plant and transmit the energy to various areas, including Belize City, 90 miles away.
Electrical power in Belize had previously consisted of diesel generating stations located at the various population centers, with distribution lines serving the customers. Because Belize lacks an indigenous source of generator fuel, energy costs have been quite high. However, the country has abundant hydro resources in the mountainous rain forests near its border with Guatemala. The country, through its utility, Belize Electric Limited (BEL), contracted with the Belize Electric Company Limited (BECOL, a subsidiary of Dominion Energy Inc.) to develop a hydro generating facility on the Macal River, and to transmit the energy to various population centers, including Belize City which lies on the Caribbean coast 90 miles away.
BECOL contracted with Irby Construction Company to perform design and construction of the transmission lines and substations. Burns & McDonnell provided design of the transmission lines for Irby, as well as assisting with procurement of materials and providing engineering support during construction.
The project presented several engineering challenges:
- High winds: The project specifications required the lines to withstand hurricane wind speeds of 150 mph. Upon analysis by Burns & McDonnell, and discussions with BECOL and BEL, the design wind speeds were reduced to 110 mph except for the coastal zone, which remains at 150 mph.
- Varied terrain and land use: The line crosses coastal lowlands (where the ground elevation increases to only one foot above sea level in the first ten miles), urbanized areas, mangrove swamps, rolling farmlands, rain forests, and rugged mountains.
- Poor soil conditions: The coastal plain, over one-third of the line length, exhibited the most problematic soil conditions. Subsurface materials were marshes, very soft, silty, or clayey peat. Sufficient soils data were not available for design, nor was a qualified subsurface investigation firm available locally. Burns & McDonnell designed an on-site pole and anchor test program, which was performed by Irby, that allowed us to back figure the physical characteristics of the soils and develop the design parameters.
- Centerline changes: Many portions of the line were not initially released for construction by the government because of right-of-way problems. This required piecemeal design and construction, and required that Burns & McDonnell respond immediately with design revisions and bills-of-intersection per mile. Upon Burns & McDonnell's recommendation, it was agreed to select a new centerline, which was done as a joint effort by Burns & McDonnell, Irby, and the developer.
The predominant structure types for the project were wood-pole H-frames. Single-pole structures were used in populated areas, and other locations where right-of-way restrictions required their use. In order to support the H-frame structures in the soft soils, each pole of the frame was supported by power-installed screw anchors. The anchors were used to provide support for both uplift and compression. As many as three anchors per pole were used in the very soft soil conditions. Where single-pole structures were used in the weakest soils (i.e. in Belize City), wood poles were side-guyed to counter the overturning moments caused by the high wind design requirements.
Because of the coastal environment, special considerations were made in the design for insulator contamination.
Structure spotting was done with the TLCADD tower spotting program. Use of TLCADD not only provided improved design productivity and optimized line design, but also facilitated quick response to right-of-way changes.