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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 H 1 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.

Fire retardant-treated (FRT) wood with a series of phosphoric acid and amino resin (leach-resistant type) or a mixture of phosphoric acid and boric acid (indoor use type) was weathered for up to 10 years at three places in Japan. Test results showed that FRT wood with a series of phosphoric acid and amino resin had high chemical retention and fire retardancy after 10 years compared with FRT wood with a mixture of phosphoric acid and boric acid. From 5 to 10 years, Decrease in fire retardancy of uncoated FRT wood treated with each fire-retardant was slower than up to 5 years. Fire retardancy of FRT woods with each fire retardant after 10 years, increased by coating with combination of fluorine resin-based and polybutadiene-based paint. FRT wood with a series of phosphoric acid and amino resin might keep initial fire retardancy for 10 years, if coated with the above paint. For uncoated and coated FRT wood with a mixture of phosphoric acid and boric acid, it showed that fire retardancy after 10 years was influenced by weathering sites. The condition for setting up the weathering rack was considered the reason for the influence of the weathering site on fire retardancy of weathered FRT wood.