The Takada River National Highway Office of Ministry of Land, Infrastructure, Transport and Tourism Hokuriku Regional Development Bureau is developing Joetsu Sanwa Road. This project in-volves the construction of embankments of up to 10m in height over a distance of 7km on a vast lowland flood plain. Because this site has cohesive strata extending to a depth of 60m, low improvement ratio deep mixing method utilizing geosynthetics was selected as the foundation improvement technique to ensure the stability of the road embankment and to control the deformation of the surrounding ground. To investigate the efficient ground improvement specifications, several test embankments with field observations (surface settlement gauges, displacement piles, differential settlement gauges, earth pressure gauges, strain gauges, and etc.) were constructed. Since conventional design methods tend to design very conservatively (very much on the safe side), it was decided to use finite element method (FEM) analysis in the design. By using the results of field observations, the soil parameters of the ground were obtained by back analyses in an elasto-plastic consolidation model. From the soil parameters, the optimum improvement rate, the length and the strength of the improved bodies that satisfy the design verification reference value were determined. The main reinforcement geosynthetics were laid in grids over the head of the improved bodies and their specifications were selected through the usage of 3D FEM analysis. This paper describes the economic specifications of the ground improvement technique and the geosynthetics based on the results of test em-bankments with field observations.
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