Avista commissioned the rebuild of the Lolo-Oxbow transmission line, which runs from the Oxbow Dam on the Oregon-Idaho border to Lewiston, Idaho. The line crosses the Snake, Salmon and Imnaha Rivers.
Crux was selected to provide micropile design and construction services for four three-legged monopole structures requiring an alternative foundation option. All four were river crossing structures and too heavily loaded for direct embedded poles, which are limited in capacity by potential embedment depths, excavation diameters, and backfill materials. The sites were also in remote locations with unimproved access roads, making it unfeasible for concrete trucks to access and thereby rendering drilled shafts unfeasible. It was determined that micropiles could provide the most economical solution at these locations. Although single micropiles generally lack strength and stiffness to resist transmission pole loading, when used in a pile group the cumulative stiffness and strength of the foundation is greatly increased. Multiple small diameter micropiles were installed in a battered array and connected with a steel pile cap, providing a rigid foundation solution with strength and performance comparable to a traditional drilled shaft.
Crux crews accessed the sites by pickup, and equipment and materials were transported by a combination of truck, low pressure Morooka track carriers, forklift and crane. Three micropiles were installed at each pole leg to average depths of 35 feet. Mobile batch plants were utilized to complete grouting activities, and each foundation was capped with a double plate steel micropile cap.
Specific project challenges included working in remote locations, often three hours from crew lodging, and commuting these long distances repeatedly in harsh winter conditions. Crux crews successfully overcame these challenges with the support of an innovative in-house machine shop and outstanding project leadership.