Lateral Spreading Mechanism in Flat Ground Adjacent to Slope Due to Post-Liquefaction Water Film Formation

Abstract

This paper investigates the mechanism by which significant lateral spreading occurs in flat ground due to liquefaction, using centrifuge model tests. Under conditions where a slope adjoins flat ground and a low-permeability surface layer exists above the liquefied layer, the excess pore water pressure varies even if the bottom of the surface layer is horizontal, excess pore water pressure varies due to differences in initial effective overburden pressure, resulting in a hydraulic gradient within the liquefied layer beneath the slope. After the end of shaking, the distribution pattern gradually changed so that the excess pore water pressure would equalize. As a result, water pressure increased beneath the toe of the slope and beneath the flat area, causing the excess pore water pressure ratio to exceed 1 by a significant margin. In the experiment designed to induce lateral spreading after the end of shaking, while horizontal displacement on the slope was small, horizontal displacement continued on the flat ground, resulting in lateral spreading. Observation of colored sand columns pre-installed in the ground confirmed that the lateral spreading occurred only in the surface layer of the flat ground. Based on the experimental results, we have proposed a mechanism for lateral spreading caused by liquefaction. After an earthquake, a water film forms beneath the low-permeability layer. Driven by the hydraulic gradient, the water in this film flows from beneath the slope to beneath the flat ground, causing the surface soil above the water film on the flat ground to flow laterally.