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Liquefaction Mitigation Using Jet-Grout Columns — 1999 Kocaeli Earthquake Case History

Martin, II, J. R. / Olgun, C. G.

The Kocaeli Earthquake (M = 7.4) struck Turkey on August 17, 1999 and caused significant damage along Izmit Bay. Following the earthquake, the authors investigated the field performance at improved soil sites. Of particular interest was the Carrefour Shopping Center that was under construction during the earthquake. The reclaimed site is underlain by strata of saturated soft clays, silts, and liquefiable loose sands. Small-diameter jet-grout columns had been installed at close spacings to reduce settlements and prevent liquefaction-related damages beneath footings and mats. Grouting had been finished for the main building which was 60% complete when the earthquake struck. Most of the site was still untreated. A post-earthquake inspection found that the treated area suffered no damage; whereas the unimproved sections, along with neighboring buildings on untreated ground, commonly suffered liquefaction-related settlements of 10–12 cm. Surprisingly, preliminary numerical analyses suggest that the main benefit of the columns was different than first suspected. That is, the reinforced ground probably did not behave as a composite mass during shaking due to strain incompatibility between the soil and stiff columns. This implies that the columns did not significantly reduce dynamic shear stresses and strains in the soil. Rather, the main benefit appears to have been the high vertical stiffness that prevented seismically-induced settlements. The implication is that commonly-used design methods may overestimate the benefit of ground reinforcement for mitigating seismic damages.