Research on Integrated Liquefaction Assessment for Stone Column Applied in Coral Sand Area within High Earthquake Magnitude Zone

Abstract

For deep and thick coral reef sand with high fine contents in high magnitude zone where the traditional vibro-compaction is not sufficient to compact, the stone column shall be applied as liquefaction mitigation measurement. Based on the analysis of the three major anti liquefaction criterion of stone column, an integrated evaluation method combined densification, shear stress distribution and drainage criterion for liquefaction mitigation was proposed, further a field test with 10.1% replacement ratio was launched. According to the analysis of the test result, the loose reclamation coral sand could be improved to an average 4.3 times SPT N value comparing. While for the medium dense coral sand, the densification among columns is limited, SPT N value is only increased by 1.3 times, and the densification decreases with fines content increasing. Further, the safety factor of liquefaction is only 0.70 and 1.01 when only considering densification criterion, or combined with shear stress redistribution criterion. However, if assessed by integrated assessment method which is mainly based on drainage criterion together with densification criterion and shear stress redistribution criterion, the calculated pore pressure ratio is still far less than the design value of 0.6 and that indicated the risk of liquefaction is still low. Moreover, when the bottom feed vibro-replacement process is adopted, the permeability coefficient of stone gravel is expected to reduce by half after stone column construction completion.