STATNAMIC Case History

STATNAMIC CASE HISTORY

S9410: New Bern, North Carolina. On October 3, 1994, a STATNAMIC load test was conducted to an ultimate load of 21.1 MN (2,380 tons), the largest STATNAMIC test at the time. In all, three tests were conducted for the North Carolina Department of Transportation with the 30 MN as part of the I-70 load test program initiated to establish geotechnical design parameters for proposed drilled shaft foundations for the future Neuse River Bridge. Two axial compression tests were conducted on a 1.22 m diameter shaft and another on a 0.61 m diameter shaft. The 0.61 m shaft was laterally tested as well.

The site soil consisted of typical mature river delta formations overlying calcified sandstone bedrock referred to locally as marl. The overburden soils consisted of layered deposits of soft saturated clayey silts overlying thick deposits of dense fine occasionally silty sand. The overall depth of overburden remained fairly constant across the site. The marl bedrock, extending to depths beyond 100 metres, is considered highly porous, fissured, and subject to localized formation of significant cavities. The highly variable rock quality is known to produce vast changes in both rock and formation integrity over short distances.

The load-displacement behaviour of the two 1.22 m diameter shaft tests is shown above. Test 2 is offset by the net displacement of test 1 as in a cycled loading test. The estimated static load-displacement behaviour is calculated from the Unloading-Point method to account for damping and inertial effects. The static prediction curve shows generally elastic behaviour up to a loading of 11 MN. Beyond 11 MN, the predicted static behaviour shows an increased rate of displacement with increased loading, reflecting the onset of plastic soil behaviour within the pile-soil system. At a displacement of -14 mm, a loading of 12 MN was achieved.

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