ATC’s X-43 138 kV transmission line in central Wisconsin is close to 23 miles long, and featured aging poles originally installed in the 1940’s through the 1960’s.  System limitations had resulted in multiple occurrences of voltage collapse, and ATC commissioned a line rebuild with a maximum rating of 1030 amps for all seasons to resolve issues associated with system reliability, market congestion and pending asset renewals.

Crux provided foundation design and construction services at one particularly challenging H-Frame structure location within the Petenwell Substation, featuring access, subsurface, and schedule challenges.  The inability to mobilize large equipment within the substation and need to accommodate a nearby retaining wall were driving factors in selecting micropiles as a viable foundation alternative.  Micropiles are slender and minimally invasive elements which can be installed using compact drilling equipment, making them an ideal solution under the project circumstances.

Subsurface materials proved highly challenging with the presence of a thick, near-surface CLSM backfill material layer capping the underlying submerged and flowing sands.  The substation is located next to the Wisconsin River, resulting in a shallow water table approximately 12 feet below ground and contributing to the unconsolidated conditions.  As an added challenge, the project was under a tight schedule and would require 24-hour drilling to complete on time.

While Crux commonly employs battered piles and bolted pile head connections, an increase in foundation design loading late in the design phase required the implementation of welded pile head-to-steel pile cap connections.  This allowed the project team to meet strict foundation performance criteria while maintaining vertical pile placement within the relatively small structure footprint.

Ultimately, Crux installed 24 vertically oriented micropiles to 95-foot depths, successfully overcoming the challenging conditions and meeting the required schedule.