Location: Northern British Columbia
The Northwest Transmission Line (NTL) is a new, 287 kV transmission line located in northern British Columbia. The approximately 214 mile (344 km) alignment connects the Skeena Substation, near Terrace, BC, to a new substation near Bob Quinn Lake. Once completed, the line will provide a clean, reliable source of power to this remote area of the province and open it up to planned industrial developments.
BC Hydro, Valard Construction LP, Crux Subsurface, Quanta Subsurface
Seven structures along the alignment were located in particularly rugged terrain and did not allow for drive-up access. Crux was selected to design and construct lattice tower micropile foundations for these helicopter-only sites. Other foundation types along the alignment included drilled shafts, screw piles, grillage and dug-in anchors, most of which required road access for the necessary installation equipment. Micropiles are lightweight elements and can be installed utilizing lighter weight equipment, eliminating the need for traditional access methods.
The choice to employ micropiles provided additional benefit when unexpected subsurface conditions were encountered during installation. Visual site reconnaissance had identified hard rock, but detailed geotechnical data was not available prior to construction. Once drilling began, weak, highly fractured material was encountered at several tower legs. Crux’s micropile installation methods allow for fast and efficient length modification onsite, enabling the desired capacity to be reached without having to redesign the foundation.
Steel pile caps were designed and utilized in place of concrete caps on all seven micropile foundations to eliminate footing excavations on steep slopes and provide a reduction in overall construction time. The steel caps could be manufactured offsite and were installed above-ground, whereas concrete would have required excavation and pouring to completed at each foundation site.
All micropile foundation work was completed on-schedule and the NTL is expected to be in service by spring 2014.