Axial Capacity of Large-Scale Piles in Micaceous Soils-Paper

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The study assesses how mica-rich soils affect the axial capacity and stiffness of large driven piles, and whether established pile-design methods remain suitable in these soils. Mica’s platy particles can bridge between sand grains, increasing porosity and reducing shear-wave velocity, stiffness, and peak friction angle.<br /><br />The authors back-calculated results from compression and tension load tests on two 760 mm open-ended steel piles, each embedded 44 m in silty micaceous fine sand at Bangladesh’s Jamuna Bridge site. Mica content in the upper 50 m was reported as 10–25%. Measured capacities were compared with predictions from three CPT-based methods (UWA-05, ICP-05, and NGI-05) and the API-14 method.<br /><br />UWA-05 gave shaft-resistance estimates closest to measured values in both loading directions. ICP-05 and NGI-05 overpredicted shaft resistance by roughly 29–42%, while API-14 overpredicted it substantially more—about 78–132%. All methods underestimated base resistance, by approximately 50–72%. Consequently, predicted total compression capacities were generally conservative for the CPT methods and only moderately unconservative for API-14. Calculated axial-load distributions often differed from measurements, particularly for API-14. The API t-z load-transfer formulation also substantially overpredicted tension capacity, potentially because the piles were first loaded in compression.<br /><br />The authors suggest applying correction factors to shaft resistance when using methods that overpredict capacity in micaceous sands, while noting that UWA-05 performed well at this site. However, prediction differences were broadly within the scatter observed for piles in siliceous sands; the study therefore did not establish a general need to reduce calculated capacities solely because mica is present.
Keywords
mica-rich soils
driven piles
axial capacity
pile stiffness
micaceous fine sand
CPT-based methods
UWA-05
ICP-05
NGI-05
API-14