CASE HISTORY: LARGE DIAMETER MICROPILES FOR THE HIGHWAY 53 RELOCATION PROJECT

摘要

The Highway 53 relocation project in Virginia, Minnesota, involves construction of a very tall bridgernacross a currently inactive iron ore mine pit. The inactive pit is partially flooded and serves as therncommunity’s drinking water source. The 1,132-ft long new bridge is a three-span, plate girder structure.rnTwo intermediate, approximately 200-ft tall piers are located within the pit.rnThe foundations for the two piers consist of groups of 30-in diameter piles which, despite therncontradiction in terms, are essentially very large micropiles. Extremely challenging and highly variablernsubsurface conditions exist at the site, ranging from uncontrolled iron ore mine waste fill to some of thernhardest rock on earth that can also be highly fractured and abrasive. Despite the conditions, largerndiameter (30-in) micropile foundations up to 175 ft deep were successfully installed using down-the-holernhammer methods and full-length permanent casing under an extremely aggressive construction schedule.rnFull-scale foundation load testing using the Statnamic rapid loading method was also conducted inrnsupport of the design and construction but under a separate pre-design phase contract. This paperrndescribes the unique aspects of this foundation project, including design, construction, and load testing ofrnthe large micropiles.