Ground Improvement Effectiveness for Liquefaction Mitigation at an Existing Highway Bridge: Fid-Bau Portal

Ground Improvement Effectiveness for Liquefaction Mitigation at an Existing Highway Bridge

Cooke, Harry G. / Mitchell, James K.

Prior field case history, physical modeling, and numerical modeling studies have shown the effectiveness of using ground improvement for reducing liquefaction-induced ground movements due to earthquakes and their effects on structures. However, in some of these studies even the reduced permanent displacements from using improved ground were larger than those typically tolerable for most existing highway bridges, and in other cases the treatment areas were larger than feasible or desirable at an existing bridge. In addition, few of the cases involved bridges. To investigate whether improved ground zones of limited size can potentially be used to reduce liquefaction-induced bridge movements to tolerable levels, a numerical modeling study was conducted consisting of two-dimensional, dynamic, effective stress analyses using FLAC and a non-linear, elasto-plastic soil model. Limited calibration and verification analyses of six centrifuge tests and a field case history showed the modeling method predicted liquefaction-induced ground and structure displacements within a factor of approximately two. Subsequent parametric study analyses of a highway bridge test case indicated densified or cemented zones extending completely through liquefiable soils could reduce the pier and abutment movements to tolerable levels. For the pier the effective treated zone was centered under the pier itself. For the stub abutment the improved zone typically extended from a location under the approach embankment behind the abutment to some distance beyond the toe of the embankment slope. The optimum width of treatment, beyond which further increases in width did not result in substantial reductions in structure movements, was dependent on the strength and stiffness of the treated zone.