Case Study: Liquefaction Mitigation of Siesmic Zone IV Site By Dynamic Compaction and Soil Replacement-Paper

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The case study describes liquefaction mitigation for a proposed bridge in Seismic Zone IV in Uttar Pradesh, India. The site has a groundwater table about 1 m below ground level and predominantly sandy soils. SPT-based analysis, assuming a magnitude 6.5 earthquake and peak horizontal acceleration of 0.24g, identified liquefiable layers at approximately 1.45–6.40 m depth, with the shallowest layers posing particular concern.<br /><br />Rather than use deep pile foundations—which can be vulnerable to loss of soil support during liquefaction—the project adopted a combined ground-improvement approach. Dynamic compaction densified the deeper loose sandy soils. Post-treatment analysis showed that several layers between about 3.40 and 7.90 m became non-liquefiable, but high groundwater levels meant the shallow layers between 1.40 and 3.40 m remained susceptible.<br /><br />Those shallow soils were excavated and replaced: a 0.50 m compacted granular sub-base was installed beneath compacted, non-cohesive sandy granular fill. The foundation concept was also changed from a single-span bridge with independent footings to a reinforced-concrete box structure intended to act monolithically and accommodate potential post-earthquake settlement.<br /><br />The combined treatment improved the foundation stratum’s bearing capacity, and the calculated total settlement was reported to be within allowable limits. The author presents the approach as a site-specific, economical alternative to deep foundations, while noting that long-term settlement and stability warrant further study.

Keywords

liquefaction mitigation

bridge foundation

Seismic Zone IV

groundwater table

sandy soils

SPT analysis

dynamic compaction

ground improvement

excavation and replacement

reinforced-concrete box structure