A chain event of the 2016 Kumamoto earthquakes Seismic Performance caused considerable geotechnical damage related to liquefaction in many places around Kumamoto plain. Many low-rise Seismic Performance houses and traditional Japanese style houses, which were constructed on shallow foundation, suffered differential settlement and tilting due to liquefaction. To mitigate the building damages due to the liquefaction, a new countermeasure method of jet grout grid form with a horizontal slab is introduced in this study. The effectiveness of the proposed technique was evaluated through physical modelling and numerical modelling. As a part of the physical modelling, a set of 1 g shaking table tests for unimproved case and improved case were performed, in which the mitigation effects of the grid form with a horizontal reinforcing slab were examined based on the acceleration, excess pore water pressure ratio as well as ground settlement. Numerical simulation was also performed for assessing the effect of improved method on soil-structure interaction and building settlement during the earthquake. The physical and numerical results confirmed that the grid form with horizontal slab reinforced method is effective in settlement control and offers favorable contribution in liquefaction mitigation.

The 2016 Kumamoto Earthquakes are a series of strong earthquakes including a 6.5 magnitude foreshock and a 7.0 magnitude main shock, both struck at Kumamoto City, Kumamoto Prefecture on Kyushu Island, Japan. Subsequently, a chain of earthquakes triggered significant damage to infrastructures including the renowned cultural heritage of Kumamoto castle, buildings, roads and river embankments. In addition, there were geotechnical damages mostly related to liquefaction and associated lateral spreading in a few districts of Kumamoto City and in the port areas [1]. Especially, the Mashikitown and Minami ward of Kumamoto City experienced liquefaction extensively and intensively. Several traditional Japanese style houses and low-rise buildings suffered differential settlement and tilting as the consequences of liquefaction in those areas. Figure 1(a) indicates the building damage due to differential settlement observed in a residence in Makishi town, Kumamoto City. Furthermore, a new private hospital in Hirata district of Minami ward was one of the most affected buildings due to liquefaction. The ground subsidence about 40 cm was observed around the building as shown in Figure 1(b). According to the Kumamoto City report, approximately 2900 buildings suffered severe damage caused by liquefaction during the 2016 Kumamoto earthquake [2].

In view of the aforesaid statements, liquefaction can cause substantial damage to buildings in the form of ground subsidence and bearing capacity failure of the soil. In order to mitigate the damage caused by such future liquefaction, it is necessary to take appropriate remediation measures in geotechnical engineering practice. Several countermeasure methods have been developed to mitigate liquefaction and strengthen ground, such as gravel drain method, sand compaction pile method, cement deep mixing method, etc. Among many ground improvement techniques, a grid shaped cement mixing method is frequently used in Japan and its high applicability effect has been evaluated in many earthquakes [3] [4]. Many centrifuge models tests, large scale shaking table tests and numerical analyses were performed to assess the effectiveness of grid form method in liquefaction mitigation, proposing the design procedures and guidelines [5] [6] [7]. However, the number of researches for grid form method in liquefaction mitigation is very limited and some issues still remain in facilitating conventional grid form countermeasure method in residential areas. Firstly, the

construction cost is high, and it may not be economical to install under normal detached houses. Secondly, the equipment used in conventional method requires space and is difficult to utilize in residential areas. Thirdly, the construction of grid form directly under existing houses is not feasible, thereby grid spacing ratio is wide to avoid the house and it leads to increase the liquefaction potential. To overcome these problems, the present study is aimed at proposing a new grid from liquefaction countermeasure method with more economical and effective construction approach. The new countermeasure method of jet grout columns with horizontal slab introduced in this study offers low cost construction approach and fulfils the space restraint requirement. In addition, it is applicable to apply near boundaries of existing structures. Due to enforcement with the horizontal slab, it also enables to strengthen the soil and provide support to the structure similarly as the large diameter jet grout wall used in conventional method, despite having thinner walls (0.15 m). Moreover, the use of small grid spacing ratio (0.2) in this method helps to suppress the generation of excess pore water pressure.

This present study focuses on evaluating the effectiveness of proposed grid form countermeasure method with the horizontal slab in reducing liquefaction risk. In this paper, the previous researches and development of the new countermeasure method are introduced first. Subsequently in the later sections, the experimental modelling and numerical simulation performed were described in alignment with their corresponding results. For experimental modelling, 1 g shaking table tests for unimproved case and improved case were conducted under different dynamic loading conditions. The salient results of the acceleration response, excess pore water pressure ratio and settlement observed from both cases were discussed to assess the effectiveness of the soil improvement method. In numerical case, an effective stress-based model UBC3D-PLM was employed in liquefiable sand and the parameters used in this model were calibrated prior to the analyses to confirm the reliability of the numerical results. Herein, for both unimproved and improved ground, the focus is given to the significance of the settlement and distribution of shear strain under the building evaluated by using the numerical PLAXIS software.