Drilled Shaft Difficulties and a Micropile Solution-Paper

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A replacement bridge over the Clark Fork River in Heron, Montana, was designed with eight permanently cased drilled shafts founded in bedrock. Although construction proceeded successfully at most locations, the casing for one Pier 2 shaft encountered a massive boulder in the sand-and-gravel overburden and could not be advanced safely. Further attempts risked destabilizing the temporary, pile-supported work trestle.<br /><br />The selected remediation retained the partially installed casing and placed seven 16-inch-diameter micropiles inside it. Each micropile extended through the overburden to bedrock, where it was anchored in a 20-foot rock socket. The micropiles were designed to meet the foundation’s structural and geotechnical demands, including lateral loading, using an iterative, three-dimensional analysis. Their tops were later embedded in the upper portion of the drilled shaft, which was completed with its reinforcing cage and concrete.<br /><br />Installation used a dual-rotary drill rig and a robust template to control micropile verticality. Temporary casing through the boulder allowed adjustments, while cement-bentonite grout filled the annular space around the permanent micropile pipes. A protruding weld seam inside the pipes initially interfered with drilling tools and had to be mechanically removed.<br /><br />The repair was successfully constructed, and the bridge opened to traffic. The project demonstrates that micropiles installed within an obstructed drilled-shaft casing can provide a practical alternative when advancing the original casing would create unacceptable risks. Close coordination among the designers, owner, general contractor, and specialty contractor was essential to refining the design and ensuring constructability.

Keywords

replacement bridge

Clark Fork River

Heron Montana

drilled shafts

massive boulder

micropiles

bedrock socket

dual-rotary drill rig

temporary casing

constructability