Vibro Stone Columns with Preloading – An Effective Foundation Solution for Large Storage Structures-Paper

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The paper presents a foundation solution for a large LPG mounded storage vessel built on soft, compressible coastal soils in North Chennai. The site contained about 7 m of soft to medium-stiff clay over loose-to-medium-dense silty sand. Under the structure’s 155 kPa load, untreated ground was expected to settle by more than 400 mm, exceeding the allowable long-term settlement of 50 mm. The loose saturated sand also posed a risk of earthquake-induced liquefaction.<br /><br />Vibro stone columns were installed beneath the mound footprint and extended beyond it to help improve bearing capacity, reduce settlement, and mitigate liquefaction. The columns were spaced at 2 m in a triangular pattern and extended to a minimum depth of 12 m. Design analysis estimated settlement of about 210 mm after ground improvement, so preloading was added to limit residual settlement.<br /><br />A surcharge equal to 60% of the structure’s loading—about 5.3 m of soil fill—was applied gradually and maintained for 45 days. Settlement was monitored at nine locations, and the degree of consolidation was assessed using Asaoka’s observational method. Monitoring indicated average settlement of about 230 mm under the surcharge, higher than predicted. Back-analysis suggested the soft clay was less stiff than initially assumed. The surcharge was removed after approximately 90% consolidation was achieved.<br /><br />The paper concludes that combining vibro stone columns with preloading can provide an effective foundation solution for large storage structures on compressible ground. Settlement monitoring and field-based verification are essential for refining design assumptions and confirming performance. Further staged mound construction and hydrotesting were planned, with monitoring to continue to ensure final settlements remain within the specified limit.

Keywords

vibro stone columns

preloading

soft clay

settlement control

liquefaction mitigation

mounded storage vessel

coastal soil improvement

Asaoka method

ground improvement

LPG storage foundation