DESIGN OF GRID-WALL SOIL IMPROVEMENT TO MITIGATE SOIL LIQUEFACTION DAMAGE IN RESIDENTIAL AREAS IN URAYASU

Abstract

 In the land reclamation areas of Urayasu City, 8,700 small buildings including residential houses suffered severe damage due to liquefaction during the 2011 Tohoku earthquake. Following this event, Urayasu City adopted grid wall soil improvements as countermeasures to mitigate liquefaction. For the simplified grid wall soil improvement design, the spacing between the grid walls is restricted within L/H = 0.8, where L is the spacing between the grid walls and H is the thickness of the liquefaction layer. However, it is difficult to adopt L/H as a design guideline because the grid wall soil improvement has to be applied underneath existing houses. The construction of grid walls directly underneath houses is impossible. Consequently, the spacing between the grid walls increases. It is suitable to use the settlement of a house as a design guideline for the conditions in Urayasu. Therefore, the following guideline was adopted. For a shock of the same magnitude as that of the main shock in the 2011 Tohoku earthquake in Urayasu no obvious damage must occur to a residential house. To meet this guideline, FL must exceeds 1.0 in all liquefied layers, or the index of settlement D_cy must be within 5 cm and the non-liquefied layer H₁ must exceed 5.0 m. The finite element method with a quasi-three-dimensional analysis model can be used to estimate the settlement of houses. However, there are no examples that have adopted the settlement as a design guideline in grid wall soil improvement design. Thus, dynamic centrifuge model tests were conducted to investigate the relationship between the settlement of houses and the grid area. Existing design guidelines for grid wall soil improvement use FL or an excess pore pressure ratio. This study proposes a performance- based design method for grid wall soil improvement and verifies the validity of this approach.