| Historical evidence from events as far back as the 1964 Niigata earthquake and most recently the devastating 1995 Hyogoken Nanbu (Kobe), Japan and 1999 Kocaeli, Turkey earthquakes indicates that sites with ground improvement as a liquefaction countermeasure suffer less ground deformation and subsidence than nearby unimproved areas. A thoroughly documented database of field case history performance has been compiled with over 100 cases from 15 different earthquakes in four countries. Generally, ground improvement is very effective at limiting ground strains, except in a few cases in which the remediation zone was not deep or wide enough. A series of dynamic centrifuge tests was performed to study the effect of depth and lateral extent of ground improvement on the settlement and acceleration of a structure on an embedded, shallow foundation. The results indicate that, for low levels of shaking, a reduction in settlement is achieved with increased improved zone depth. Some settlement occurs for improvement through the full potentially liquefiable thickness, even at low levels of shaking. The centrifuge tests show that in the free field, liquefaction can occur rapidly or slowly, from the top down or bottom up, and incrementally while excess pore water pressure is generated and dissipated at the same time. Thorough examination of the occurrence and timing of liquefaction in the free field and the sequence and degree of soil deformation under and near the structures and improved zones has revealed five mechanisms of settlement that fully explain the vertical and volumetric strains in the soil and the settlements of the structures. The dominant mechanism is associated with lateral spreading and high vertical strain of the unimproved, but not liquefied soil under the improved zone or structure when the adjacent free field soil liquefies. The contribution of long-term settlement due to post-liquefaction volumetric compression below the structure or improved zone is minor. Soil deformation and strain mechanisms are heavily influenced by the timing and relative location of initial liquefaction, the initial soil relative density and its distribution, the presence, absence, and stress of the structure and improved zones, and the shaking intensity and duration. |
