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Case study on the deformation control method for embankment during liquefaction by using geosynthetics sandwiched with gravels

A raised embankment structure of 2m tsunami flooding height due to the Nankai Megaquake was planned in the development of parking lots for emergency vehicles by the West Nippon Expressway Company Limited. The reclaimed sandy soil layers with N value of 10 or less are distributed up to a depth of 12 m in this construction site. A liquefaction countermeasure becomes necessary since the ground liquefied and affected the embankment structure when Nankai Megaquake occurs. Although the ground improvement is conventionally used as a main liquefaction countermeasure, it is very costly to install. Considering the use of the parking lot even if sinking due to liquefaction occurs, the selection of construction method is carried out as it is important to suppress unequal settlement and make emergency vehicles to be going out after tsunami flood removed. In this project, the SECURE-G method was firstly adopted which is a structure using geosynthetics sandwiched with gravels was installed under the embankment. The excess pore water pressure around the gravel layer is quickly dissipated due to high water permeability of gravels. Furthermore, the composite structure of gravel with geosynthesis can perform as a rigid beam. As a result, the deformation of embankment is suppressed and its functions are secured. In the design, the deformation of embankment during liquefaction was evaluated using static finite element method (ALID) and the thickness of gravel layer and strength of geosynthetics satisfied the design requirements are decided. The paper describes the design and installation of the liquefaction countermeasure.