â€œPrediction of Side Friction of Drilled Shafts in Very Dense Fine SANDâ€ UAE

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The presentation reports a geotechnical case study for nine large prestressed-concrete water-storage tanks in Ras Al Khaimah, UAE. Each tank is 85 m in diameter and 20 m high. The site’s 20 m elevation variation required cut and fill; filled areas were improved using deep dynamic compaction.<br /><br />Investigations—including boreholes, SPTs, CPTs, MASW and suspension logging—found slightly silty fine sand extending to at least 50 m, with no groundwater encountered in the boreholes. Below the shallow, variable cut-and-fill soils, the sand was very dense, commonly with SPT values of 50 or more and shear-wave velocities of about 400–500 m/s.<br /><br />The tender design specified large numbers of 750 mm and 900 mm drilled piles, approximately 30 m long. Although shallow foundations and ground improvement were considered as value-engineering alternatives, the contractor could not change the specified foundation system. The study therefore assessed whether pile lengths could be optimized.<br /><br />Pile resistance was evaluated using the conventional β-method for drilled shafts in sand and the IGM-3 Model 2 method, which treats very dense granular geomaterials as Category 3 intermediate geomaterials. The IGM-3 approach predicted higher side resistance and supported shorter piles than conventional sand-based estimates.<br /><br />Static load tests on two preliminary piles and 21 working piles, supplemented by dynamic tests, were interpreted using methods including Chin and Decourt. Limited settlement prevented reliable application of some conventional interpretation criteria. Overall safety factors were reported at 2–4, averaging about 3, with the specified factor of 2.5 confirmed for the installed piles.<br /><br />The study concludes that very dense sand (SPT N ≥ 50) should be distinguished from ordinary sand and may be assessed using the IGM-3 model. Applying this method enabled an approximately 25% reduction in nominal pile length and significant cost savings. Further local load-test documentation is recommended to validate the method.

Keywords

geotechnical case study

prestressed-concrete water tanks

deep dynamic compaction

very dense sand

SPT N50

drilled piles

IGM-3 Model 2

pile load tests

pile length optimization

Ras Al Khaimah UAE