Numerical Modeling of Columnar Reinforced Ground 1999 Kocaeli Earthquake Case History

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 damage beneath footings and mats. Nonlinear dynamic three-dimensional finite element analyses were conducted to model the reinforced ground at Carrefour. The results show that the primary benefit of the columns was different than first suspected. That is, we initially thought the higher composite stiffness of the reinforced ground led to reduced seismic shear stresses and shear strains in the soil mass. However, the numerical results show that the reinforced ground did not behave as a composite mass during shaking due to strain incompatibility between the soil and stiff columns. The results indicate that the columns did not significantly reduce seismic shear stresses and strains (and thus pore pressures) in the soil mass. The effectiveness of the jet-grouting at Carrefour was more related to the vertical support the columns provided that prevented seismically-induced settlements. The implication is that commonly-used design methods and assumptions may lead to overestimates of the effectiveness of ground reinforcement for mitigating seismic damage.