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

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The case study describes liquefaction mitigation for a proposed minor bridge at a Seismic Zone IV site in India. Subsurface investigation found silty sand and silty-sand mixtures, with groundwater initially 1 m below ground level. Liquefaction analysis, assuming groundwater could rise to ground level and an earthquake magnitude of Mw 6.5, identified shallow soils to about 6.4 m as susceptible, particularly the upper layers.<br /><br />Rather than adopt deep pile foundations, which can be vulnerable to bending and loss of support in liquefied soils, the project team selected ground improvement. Dynamic compaction increased penetration resistance and improved deeper layers, but soils from ground level to 3.4 m remained liquefiable.<br /><br />The team therefore added soil replacement beneath the proposed foundation. A 2 m-thick zone, from 1.4 to 3.4 m depth, was excavated and replaced with compacted granular materials: a granular sub-base in the lower 0.5 m and suitable non-cohesive sandy soil above it, placed in compacted layers under continuous dewatering. The combined treatment improved bearing capacity and reduced settlement.<br /><br />The original single-span bridge with separate footings was changed to a monolithic reinforced-concrete box culvert, founded 1.4 m below ground level. The authors report that the treated ground’s bearing capacity and calculated settlement met the applicable requirements, allowing conventional shallow foundations instead of piles and providing a structure intended to accommodate possible post-earthquake settlement.<br /><br />The presentation considers the approach a successful, site-specific solution, while noting that long-term settlement and stability merit further study.

Keywords

liquefaction mitigation

ground improvement

dynamic compaction

soil replacement

seismic zone IV

shallow foundation

bearing capacity

settlement

bridge foundation

reinforced concrete box culvert