Case History: Large Diameter Micropiles for the Highway 53 Relocation Project-Paper

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The Highway 53 relocation in Virginia, Minnesota, required a 1,132-ft bridge across the flooded Rouchleau iron mine pit, a community drinking-water source. Two piers, each about 200 ft tall, stand within the pit and are supported by groups of large-diameter, fully cased micropiles installed up to 175 ft deep.<br /><br />Subsurface conditions ranged from highly variable mine-waste fill to exceptionally hard, fractured Biwabik Iron Formation bedrock. A preconstruction test program evaluated constructability and capacity using 16- and 24-in piles installed with down-the-hole hammer equipment. Statnamic load tests showed very high bedrock base resistance, but limited side resistance; at extreme loads, grout cracking also indicated that the steel casing and grout did not behave as a fully composite section. One attempted uncased rock socket collapsed, supporting the decision to use fully cased piles.<br /><br />Production piles were increased to 30 in in diameter and designed as end-bearing elements, with structural capacity governing. Groups of 14 and 19 piles support the two piers. Installation using a pilot-and-ring bit system and full-length permanent casing proceeded without significant problems, although drilling slowed substantially at depth in bedrock. Reverse-circulation drilling produced clean pile bases without additional cleanout.<br /><br />Instrumentation on two production piles confirmed low side resistance and showed negative skin friction developing through the mine-waste fill. The measured drag load was about 140 kips, below the 245-kip design estimate from neutral-plane analysis. The project demonstrated that large-diameter micropile methods can install casings through bouldery fill and penetrate very hard rock, while highlighting the importance of adequate compressed air, tooling condition, and accounting for drag loads in design.

Keywords

Highway 53 relocation

Virginia Minnesota

bridge foundation

Rouchleau iron mine pit

micropiles

fully cased piles

Biwabik Iron Formation

statnamic load test

negative skin friction

drag load