Effects of drainage control on densification as a liquefaction mitigation technique

Abstract

Ground densification is among the most popular techniques for liquefaction mitigation employed in practice. Yet, the effects of densification, especially in combination with strategies that enhance or inhibit drainage to or from the densified area, on the performance of the soil-foundation-structure system are not well understood. This paper describes dynamic centrifuge experiments to evaluate these effects, considering 3- and 9-story, moment-resisting frame structures with different embedment depths, founded on layered liquefiable soil deposits. The experiments compared structures mitigated with densification alone, and mitigated with a combination of densification and either prefabricated vertical drains (enhancing drainage) or a flexible impermeable latex barrier (inhibiting drainage) around the densified area. Ground densification tended to reduce the foundation settlement, although not to acceptable levels (based on limiting values typically considered in design and performance assessment), but amplified the drift and acceleration demands on the superstructure. The addition of a flexible impermeable barrier around the densified area did not have a notable influence on foundation settlement. However, it increased the excess pore pressures under the edges of the 3-story structure by inhibiting outward flow, amplifying its foundation rotation compared to the case with densification alone. In the case of the heavier and deeper, 9-story structure, adding a barrier around the densified zone restricted the locally inward flow from the adjacent loose soil. This helped reduce net pore pressures under the edges of the structure after strong shaking, foundation rotation, and seismic demand on the superstructure. Enhancing drainage around the densified zone, on the other hand, notably reduced permanent foundation settlement and rotation during all motions nearly to acceptable limits, but amplified accelerations, imposing additional seismic demands on the structure, which could lead to damage if not considered in design. These results demonstrate the importance of considering the structure's dynamic properties and force-deformation behavior, foundation and ground motion properties, and soil-structure interaction when planning the geometry of ground densification and drainage.