Stability Analysis of Geotextile-reinforced Slope Based on Japan Earthquake in 2011: Yuriage, Natori City Case

Abstract

The use of geotextile for slope strengthening was proven based on the investigation and report by Kuwano, Miyata, and Koseki (2014). Therefore, this research examines the impact of geotextile application on slope stability in newly-elevated area in Yuriage, Natori City, Japan. This project discussed the analysis of geotextile-reinforced slope by comparing the total displacement at the crest of slope between the compaction method as current design and the using of geotextile with further investigation by optimising the use of geotextile application.
A cross-section of slope with 0.435m height and 29 degrees slope angle were chosen for the geometry of analysis. Two loads acted on the slope such as the uniform load on top of the raised area and dynamic load from the earthquake were defined. Six layers that mostly consist of sandy soils were determined based on the SPT data in 16 locations. The average values of N-SPT were calculated for further parameter correlations based on these values. Furthermore, analyses based on the Limit Equilibrium method and Finite Element Method were selected to analyse the slope stability.
Limit Equilibrium Method using D-Geo stability software based on the Bishop method was taken to define safety factor of slope under uniform load (without earthquake load). The value of safety factor at 1.93 was obtained then compared to the result for the same problem based on the Finite Element Method using PLAXIS 2D with HS Small model that resulted at 2.07 safety factor. Further dynamic analysis was also completed using PLAXIS 2D with UBCSAND model to calculate the total displacement at slope crest without and with geotextile application. The strengthening method using geotextile (500 kN/m tensile strength, 5004 kN/m axial stiffness, 30m length in 5 layers) decreased the total displacement from 0.543m to 0.434m. However, the impact of geotextile is very limited in this case that only decrease 0.109m or about 20% from the total displacement without geotextile. Weak foundation, silty clayey sand layer (AC1) and liquefied layers (AC1 and fine sand AS22) below the slope were observed as the main reasons for this small impact of geotextile. Therefore, geotextile method is not recommended to use in this project given its very limited impact on the total displacement. Furthermore, the soil improvement methods were proposed to tackle these two reasons, such as replacing the soil material (AC1) with stronger soil for the foundation under the slope and/or construct sand pile to minimize the liquefied layer below the slope until AS21 depth, about 6 meters.
Further analysis based on combination of five values of geotextile strengths, four difference lengths, and three number of layers were made to get the optimum used of geotextile by comparing the total displacement at the crest. The first factor that has to be considered is the strength of geotextile because this factor gives more influence in the final total displacement. Secondly, with the high strength of geotextile, the length should be considered first rather than the number of layers based on the impact of both factor on the influence of strength to the total of displacement.