Seismic monitoring short-duration events: liquefaction in 1gmodels

Abstract

The duration of liquefaction in small models is very short, therefore special monitoring systems are required. In an exploratory sequence of liquefaction tests, S-wave transillumination is implemented with a high repetition rate to provide detailed information on the evolution of shear stiffness during liquefaction. These data are complemented with measurements of acceleration, time-varying settlement, excess pore pressure, and resistivity profiles. Measurements show that excess pore pressure migration from liquefied deep layers may cause or sustain a zero effective stress condition in shallow layers, that multiple liquefaction events may take place in a given formation for a given excitation level, and that unsaturated layers may also reach a zero effective stress condition. The time scale for excess pore pressure dissipation in fully submerged specimens is related to particle resedimentation and pressure diffusion; downward drainage from unsaturated shallow layers may contribute an additional time scale. High resolution resistivity profiling reveals the gradual homogenization of the soil bed that takes place during subsequent liquefaction events. The S-wave transillumination technique can be extended to field applications and implemented with tomographic coverage to gain a comprehensive understanding of the spatial and temporal evolution of liquefaction.Key words: densification, electrical resistivity, multiple liquefaction, pore pressure, shear wave, spatial variability.