Performance Analysis and Remedial Measures for Distressed Tank Foundation: A Case Study-Paper

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The study examines a 92 m-diameter, 20 m-high floating-roof oil tank in Nigeria that experienced excessive settlement during hydrotesting. The tank rested on a sand pad supported by stone columns installed in soft ground. Although design estimates predicted peripheral settlement of 130 mm, measurements reached 238 mm—above the 150 mm guideline—and differential settlement exceeded the 1:500 criterion at some locations.<br /><br />Further investigation identified highly compressible organic soil beneath the tank’s southwest quarter, beginning about 7 m below ground. This localized layer had not been identified in the original investigation, and routine single-column load tests did not capture its effects. The case highlights the need for location-specific soil investigations in geologically variable sites and, where appropriate, group load testing to assess deeper or localized weak layers.<br /><br />A performance assessment in accordance with API 653 found that planar tilt and out-of-plane settlement remained within acceptable limits, despite the high total settlement. Analysis also indicated that most primary consolidation occurred during the 90-day, staged hydrotest, with approximately 33 mm of further settlement anticipated at the affected edge. The tank was therefore approved to operate at full capacity.<br /><br />Remedial measures included installing seam plates to level the sand pad at locations with differential settlement and recommending continuous gravel or concrete ring walls, or piles, to limit lateral movement of organic soil. The seam plates were installed; the additional perimeter measures were not implemented because subsequent operation showed no unusual settlement. The authors emphasize careful settlement monitoring, staged hydrotesting, and thorough ground investigation to protect tank integrity and reliability.

Keywords

floating-roof oil tank

settlement

hydrotesting

sand pad

stone columns

soft ground

organic soil

API 653

differential settlement

ground investigation