Severe and nonuniform liquefaction damage of reclaimed ground contributed by interference between body waves and stratigraphic irregularity-induced surface waves

Abstract

The 2011 off the Pacific Coast of Tohoku earthquake caused extensive liquefaction damage to reclaimed land along the Tokyo Bay coast, even though it was approximately 400 km from the epicenter. The characteristics of the liquefaction damage include the fact that liquefaction occurred in soils with a high percentage of fine particles and that the distribution of liquefied and nonliquefied areas was nonuniform. The factors contributing to such nonuniform liquefaction damage included the heterogeneity of the ground materials and their depositional conditions, and the effects of the long earthquake duration. Although these points are certainly valid as reasons for the occurrence of severe liquefaction damage, they do not fully explain the mechanisms of the liquefaction of the fine-grained soils, or the localized extent of the liquefaction. To elucidate the severe and nonuniform damage, seismic response analyses of a multi-layered ground were conducted focusing on the stratigraphic irregularities in the ground beneath Urayasu city. The results showed that the thicker and softer sedimentary layers amplify the slightly long-period component of the seismic motion and increase the shaking at the ground surface. Moreover, the wave propagation in the ground became very complicated owing to the focal effect caused by the refractions and reflections of body waves at the stratum boundary, surface wave excitation at the base of the slope, and amplified interference between body and surface waves. This complex wave propagation contributed to nonuniform surface ground shaking and severe liquefaction damage. In addition, surface waves, which consist primarily of slightly long-period components, can propagate far and wide; as such, they triggered extensive damage owing to delayed shaking phenomena that continue even after the earthquake. The analysis results suggested that multidimensional elasto-plastic seismic response analyses considering stratigraphic irregularities are important for detailed seismic evaluation.

Comprehension of Seismic-Induced Groundwater Level Rise in Unsaturated Sandy Layer Based on Soil–Water–Air Coupled Finite Deformation Analysis

“…In this study, a one-dimensional analysis, which can easily comprehend water and air flow balances, was performed. Referring to geological columnar sections published by Chiba Prefecture [18,19] and Nakai et al [20], the ground was set up with a layer composition consisting of sand, clay, and bedrock.…”

“…In this study, a one-dimensional analysis, which can easily comprehend water and air flow balances, was performed. Referring to geological columnar sections published by Chiba Prefecture [18,19] and Nakai et al [20], the ground was set up with a layer composition consisting of sand, clay, and bedrock. Nakai et al [20] performed a two-dimensional analysis on liquefaction damage in Urayasu City during the 2011 Tohoku earthquake using the soil-water coupled finite deformation analysis code [21] incorporating the SYS Cam-clay model [12] and constructed and validated a ground model for Urayasu City.…”

“…A mesh of 1 horizontal and 1000 vertical finite elements was used under a plane strain condition. The periodic boundary condition [21] was set at the lateral sides and the viscous boundary condition [22] was set for the horizontal direction at the bottom (shear wave velocity V s = 400 m/s [20]). The displacement-fixed condition was set for the vertical direction at the bottom.…”

“…Table 1 shows the material constants and initial values for the elastoplastic constitutive equation SYS Cam-clay model [12]. The values were determined from the numerical simulations of mechanical tests of soils sampled from Urayasu City ground after the earthquake [20], and the initial values for specific volume, degree of structure, stress ratio, and degree of anisotropy were given to each layer uniformly while the initial overconsolidation ratio was distributed in the vertical direction according to the overburden pressure. The skeleton stress equation [24] was used in the elastoplastic constitutive model.…”

“…The other material constants related to the soil-water characteristic of the sand layer were determined by referring to a past study on experimental results of clayey sand [28] because the buried soil layer and alluvial sand layer in Urayasu City ground contained a large amount of fine grains [5] and we were unable to obtain the result of a soil water retention test using Urayasu sand. The other values were taken from Nakai et al [20]. The initial pore water pressure p w was set equal to the pore air pressure p a at the groundwater level, assuming the hydrostatic pressure distribution in the vertical direction.…”