Ten West Micropile Foundations

The population growth of the Southwest and changing electrical generation sources has led to changing electrical transmission needs throughout the regional grid.  DCR Transmission was formed to provide a connection between existing electrical substations in Tonopah, Arizona and Blythe, California to facilitate development of new renewable energy and storage resources and improve reliability and other public benefits.  The new 500kV transmission line spans 125 miles through remote and rugged terrain, avoiding the Kofa National Wildlife Refuge and all major population centers.  The selected route required traversing mountainous areas where access was limited, in some areas only permissible by helicopter.  The transmission line is supported on steel four-leg lattice towers, which are founded on conventional foundations in areas easily accessible, but requiring a different solution in the mountainous areas with limited access.

The prime contractor solicited Condon Johnson to provide a subcontracted design-build solution for the limited-access tower locations.  Condon Johnson engaged a local geotechnical engineer to design micropile foundation elements and Brierley to provide a connection interface between the micropile groups and transmission tower legs.  Due to the access limitations, poured concrete pile caps were not practical, so Brierley designed fabricated steel plate transfer assemblies that could be bolted to the micropile center bars and accept match-drilled bolted base shoes from the transmission tower legs.  Brierley divided the various tower types and leg reactions into two subgroups for simple and economical design and fabrication.  In areas where truck access was possible, the transfer assemblies were designed as a single component, but in locations where access was only possible by helicopter, the transfer assemblies were designed in multiple parts to accommodate the 4,000lb lift capacity of the available helicopter.  All components were specified to be hot-dip galvanized for maintenance-free durability.

Brierley coordinated with the contractor, foundation designer, and tower designer to ensure compatibility and efficiency of each aspect and ensure the solution was economical and constructible.  Brierley provided several initial concepts for the contractor to obtain fabrication estimates, and utilized the preferred version for final design.