Brownlee to Oxbow Transmission Line

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Client: Idaho Power Company
Location: Idaho, USA 

Project Description

Due to the rugged terrain of much of the project’s route, foundation design would be an important component of the Brownlee to Oxbow 230 kV line’s design and success. This was a primary reason Idaho Power Company (IPC) turned to POWER Engineers.  POWER Engineers has extensive experience siting transmission lines in the often difficult and complex landscape that characterizes IPC’s service area in southern Idaho and eastern Oregon.

The design required a detailed investigation of the rock and soil characteristics along the proposed route.  POWER Engineers teamed with Kleinfelder, Inc. and Crux Subsurface, Inc. to support the geological reconnaissance and geotechnical investigations.

After considering structural loads, access, soil/rock conditions, and site topography, POWER Engineers recognized the uniqueness of each site and required a site-specific foundation design and construction plan for each pole.  POWER Engineers chose an investigative program that combined soil/rock borings at representative structure locations with a geophysical survey at other intermediate locations.

Foundation selection and design

Before detailed design, a feasibility study determined the options for foundation types and a preferred type. POWER Engineers’ team reviewed the physical information, considered the types of access, and consulted foundation contractors to determine the best options for the project.

The team considered five foundation types and evaluated each for constructability, flexibility at each site, the availability of materials, and the required construction expertise. The team rejected three of the options: direct embedded steel poles, grouted anchor bolts, and spread footings. It selected a fourth option – concrete pad with grouted anchor bolts – to give the construction contractor an option in the field once actual soil and rock conditions were known. This option was not used, however.

As the preferred option, the team selected poured-in-place concrete caissons and prepared detailed designs using the LPILE foundation design program. The team placed each foundation in one of 24 groupings based on foundation loads and pier diameters. This approach allowed the construction crews to have maximum flexibility to match each foundation with field conditions and to minimize excavations for each site.

Project details