- 6 - K 0 過圧密粘土の有効応力経路と塑性ひずみ 今井 誉人 *1 吉富 隆弘 *2 赤石 勝 *3 外崎 明 *4 杉山 太宏 *5 Effective Stress Path and a Plastic Strain of K 0 Over-Consolidated Clay by Yoshito IMAI *1 , Takahiro YOSHIDOMI *2 , Masaru AKAISHI *3 , Akira TONOZAKI *4 and Motohiro SUGIYAMA *5 (Received on April 3, 2018 and accepted on May 10, 2018) Abstract Regarding plastic strain in predicting the subsidence of over-consolidated clay which is generally assumed to be an elastic body, Poisson’s ratio is required for elastic strain and an assumption about plastic potential is necessary for plastic strain. However, if the strain occurring in the over-consolidated clay is assumed to be the sum of the two components of elasticity and plasticity, the relationship between its plastic strain and the original yield function becomes complicated. This paper examines the relationship between plastic potential and effective stress path with respect to plastic strain of over-consolidated clay as a preliminary step in considering the problem of plastic strain which is assumed to be generated by changes in stress in the yield surface. The coefficient of earth pressure at rest calculated based on the assumption about plastic potential is completely different from the measured value, and it is clarified that the calculated strain amount is also unreliable. Keywords: Over-consolidated clay, Yield function, Plastic potential, Coefficient of earth pressure at rest 1.緒 言 一次元圧密状態に代表される 0 K 圧縮の有効応力経路 は,応力ひずみ関係に影響を与える.この 0 K 圧縮過程を 数値解析で再現するとき,構成式によっては計算される 水平方向有効応力 h が理論値や実測値と異なることが ある 1,2) .このような構成式によって計算された応力から 求めるひずみ量(変位量)は信頼できるものとはならな い.また,一般的には弾性体と仮定される 0 K 過圧密粘土 の沈下予測に塑性ひずみをも考慮しようとする場合,弾 性ひずみにはポアソン比,塑性ひずみには塑性ポテンシ ャルに関する仮定が必要になる.過圧密粘土の沈下予測 に塑性ひずみの発生までを考慮する必要性と重要性は明 確ではないが 2) ,過圧密粘土に生じるひずみを弾性・塑 性の二つの成分の和と仮定する場合,その塑性ひずみと 本来の降伏関数との関係もより複雑になる. この論文は,降伏面内の応力変化で発生すると仮定さ れる塑性ひずみの問題点を考える前段階として, 0 K 過圧 密粘土の塑性ひずみに関する塑性ポテンシャルと有効応 力経路の関係について検討している.塑性ポテンシャル に関する仮定次第で計算される静止土圧係数が実測値と 全く異なり,ひずみ量の計算結果も信頼できないことを 明らかにする. 2.試料および実験方法 東京近郊の沖積地盤で採取した 3 種類の粘土を用いて, 固定リング式の一次元圧密試験と三軸試験機を用いた 0 K 圧密試験を実施した.一次元圧密試験は,土被り圧を 解放された粘土の再載荷過程で発生する塑性ひずみの大 きさを推定することを目的に,標準圧密試験機 1 台とこ れよりも一回り大きな一次元圧密試験機(内径 8.4 cm, 高さ 2 cm)を 2 台準備した.内径 8.4 cm の試験機 2 台に 試料 H をセットして, 1 台は 20 kPa から 1280 kPa まで荷 重増分比 1 で段階載荷し原位置の圧密挙動とした.もう 1 台は室内圧密試験を想定して,土被り圧 p 0 まで載荷後 荷重を取り除き,応力を完全に開放した.その後,直径 6 cm の標準圧密試験機に再度セットして 10 kPa からの 段階載荷を行った. 0 K 圧密試験は, 0 K 過圧密粘土の有効応力経路を調べ ることが目的である.使用した試料の物理的性質は Table 1 に示すとおりである. *1 小野田ケミコ株式会社 *2 工学研究科建築土木工学専攻 *3 新日本開発工業株式会社 *4 金沢工業大学工学部環境土木工学科 *5 工学部土木工学科教授 Table 1 Soil parameters. Sample Gs w L (%) w p (%) Sand (%) Silt (%) Clay (%) S 2.67 83 21 5 41 54 M 2.69 136 84 8 28 64 H 2.66 121 52 10 72 18 東海大学紀要工学部 Vol.58,No1,2018,pp.6-10
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K0 過圧密粘土の有効応力経路と塑性ひずみ 今井 誉 …...Void ratio e Diameter 6 8.4 ( cm ) ei= 2.355 e0=2.362 p0 =78.5 kPa Sample H e=2.170 e=1.917 Table 2 Consolidation
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(Received on April 3, 2018 and accepted on May 10, 2018)
Abstract Regarding plastic strain in predicting the subsidence of over-consolidated clay which is generally assumed to
be an elastic body, Poisson’s ratio is required for elastic strain and an assumption about plastic potential is necessary for plastic strain. However, if the strain occurring in the over-consolidated clay is assumed to be the sum of the two components of elasticity and plasticity, the relationship between its plastic strain and the original yield function becomes complicated. This paper examines the relationship between plastic potential and effective stress path with respect to plastic strain of over-consolidated clay as a preliminary step in considering the problem of plastic strain which is assumed to be generated by changes in stress in the yield surface. The coefficient of earth pressure at rest calculated based on the assumption about plastic potential is completely different from the measured value, and it is clarified that the calculated strain amount is also unreliable. Keywords: Over-consolidated clay, Yield function, Plastic potential, Coefficient of earth pressure at rest
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Fig. 9 Comparison of effective stress paths and volumetric strain by FE analysis.