Background / 背景
The Great East Japan Earthquake Disaster in 2011 affected several areas in Japan, causing severe damage to houses (e.g. tilting). As a possible countermeasure against liquefaction, which can be applied to the narrowly constructed residential areas, ground desaturation can be implemented.
2011年東北地方太平洋沖地震では，多くの地域で液状化現象が観測され，家屋や埋設物に甚大な被害を与えた．狭小な範囲に施工可能な液状化対策として，地盤の不飽和化が検討されている．
Apparatus / 試験機
Hence, the liquefaction resistance of soils from clean sands to silty sands under partially-saturated conditions was investigated using a Triaxial Apparatus with Linkage Double Cell System. A linkage rod moving simultaneously with the loading shaft is introduced to the reference tube, while the height of the top cap is reduced, and a longer stainless steel loading shaft is installed between the top cap and the load cell. The membrane filter technique was used to measure pore water pressure.
そのため，不飽和土用三軸試験装置（二重セル方式）を用いて，砂からシルト混じり砂の地盤材料の不飽和液状化強度特性を検証した．本研究で用いる二重セル方式の体積変化計測システムでは，リファレンスとなる水位面に載荷ロッドと同径の軸を挿入し，載荷軸と同時に動かすことで，水面形状による計測誤差をキャンセルできるように工夫している．また，下部ペデスタルの間隙水経路にメンブレンフィルターを用いることで，セラミックディスクを用いた場合と比べて試験時間が短縮できる．
Figure 1. a Triaxial Apparatus with Linkage Double Cell System
Correlation with LRR / 液状化強度比の相関性
To correlate with the Liquefaction Resistance Ratio (LRR), the effect of compressibility of pore air in the unsaturated soils was proposed as parameter by Okamura and Soga (2006) by calculating the potential volumetric strain, which is the volumetric stain of the specimens caused by pore air compression when the excess pore air pressure equals the initial confining pressure and is obtained by applying Boyle’s law. In this study, a new index called volumetric strain ratio (Rv) was proposed to correlate with LRR for various sands. According to the experimental data results, Rv exhibits improved correlation with LRR especially for soils with fines.
Okamura and Soga(2006)は，不飽和土中の間隙空気の圧縮性による潜在的な体積圧縮量（過剰間隙空気圧が初期拘束圧と同値になったと仮定し，ボイルの法則から得られる体積変化量）をパラメータとして， 液状化強度比（飽和土と不飽和土の液状化強度の比）の相関性を示している．本研究では，体積ひずみ比という新たな指標を定義し，異なる砂の液状化強度比の相関性を提案した．実験結果によると，体積ひずみ比は，特に細粒分を含む砂に関して，液状化強度比の相関性を改善できることが示された．
Figure 2.
Additional References
1)Wang H, Sato T, Koseki J, Chiaro G, Tan Tian J. 2016b. A system to measure volume change of unsaturated soils in undrained cyclic triaxial tests. Geotechnical Testing Journal 39(4): 532-542
2)Wang H, Koseki J, Sato T, Chiaro G, Tan Tian J. 2016a. Effect of saturation on liquefaction resistance of iron ore fines and two sandy soils. Soils and Foundations 56(4): 732-744
3)Okamura, M. and Soga, Y., 2006, “Effects of Pore Fluid Compressibility on Liquefaction Resistance of Partially Saturated Sand,” Soils and Foundations, Vol. 46, No. 5, pp. 695-700
