STATNAMIC LOAD TESTING ON A 406 MM (16 IN) DIAMETER MICROPILE-Paper

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The Highway 53 relocation in Virginia, Minnesota, required a bridge across a flooded iron-ore mine pit. Its 60 m-high intermediate piers were designed on deep foundations amid highly variable mine-waste fill and fractured, abrasive bedrock. A pre-design test program evaluated construction feasibility and foundation capacity using Statnamic rapid load tests.<br /><br />This paper focuses on a 406 mm (16 in) diameter, 53.8 m-long, fully cased test pile. The upper 37–40 m consisted of dense, heterogeneous mine waste; beneath it lay weathered, fractured Biwabik Iron Formation, followed by strong, more competent bedrock. Initial attempts to install the casing with an Elemex bit failed when the ring bit detached. A Symmetrix bit succeeded, advancing the casing into competent bedrock. The pile was reinforced and grouted from the bottom up.<br /><br />A Statnamic test applied a derived static compressive load of 13.4 MN, producing 138 mm of pile-head displacement. The pile rebounded approximately 13 mm above its original position after unloading. This unusual response resulted from very high base resistance at the competent rock, low side resistance due to the slightly oversized installation hole, and substantial elastic shortening of the long, relatively small-diameter pile. The pile’s grout cracked under the load, so strain-gauge data could not reliably quantify side resistance; however, the cased pile did not undergo full plastic yielding.<br /><br />The test confirmed that large-micropile installation methods could construct the foundations despite difficult ground conditions, and that the Symmetrix system was effective. It also showed that foundation design was governed by structural strength rather than geotechnical capacity: base resistance exceeded the pile’s structural strength, and its maximum capacity was not fully mobilized during testing.

Keywords

Highway 53 relocation

Virginia Minnesota

bridge foundation

Statnamic load test

micropile

deep foundation

mine waste fill

Biwabik Iron Formation

Symmetrix drilling system

pile capacity