Introduction / 序論

There have been many reports of landslides occurring on natural slopes constituted of marine clay. This is thought to be caused by the effect of the cation concentration in the soil on the shear strength of the soil, but it’s not been supported theoretically. The purpose of this study is to clarify the mechanism of this strength change especially for expansive soil, and to make a constitutive model considering it.


Fig.1. Landslide caused by chemical effects

Modify conventional constitutive model / 既存構成モデルの改良

Referring to a previous study which observed changes in the soil particle structure of expansive clay due to the addition of salt – we hypothesized the following mechanism. That is, the interlayer distance of the expansive clay minerals changes due to changes of osmotic force caused by chemical effects. This changes the physical arrangement among the soil particles and affects the shear strength of the entire soil.
Furthermore, in order to consider this hypothesis in the conventional constitutive model, we express the critical stress ratio M*, which represents the soil strength, as a decreasing function of the interlayer distance of the expansive clay minerals. (Fig.2)



Fig.3. d-M* relation in the proposed model

Verification of the proposed model / 提案モデルの検証

We analyzed the previous experiment (Di Maio, 2004) using the proposed model. In the previous experiment, multiple specimens of expansive soil containing distilled water and 1M saline were made, and each was isotropically consolidated to a different pressure stage, sheared until collapse. Fig.3 shows a comparison between the results of the experiment and analysis at the time of consolidation and Fig.3 shows the results at the time of shearing. Judging from the figure, the model can qualitatively reproduce the strength change characteristic in addition to the volume change characteristic due to the chemical influence. Future tasks include improving the accuracy of the proposed model through further experiments and considerations, and large-scale analysis using the proposed model.

改良モデルを使って先行実験(Di Maio,2004)の解析をした。対象とする先行実験では間隙水が蒸留水と1M食塩水の膨潤性粘土の供試体を複数作ってそれぞれ別の圧力段階まで等方圧密した後、破壊に至るまで非排水せん断して応力の記録を取っている。図3に圧密時、図4にせん断時の実験と解析の結果を比較して示す。図から、モデルにより化学影響による体積変化特性に加え強度変化特性も定性的に再現できていることが分かる。さらなる実験と考察による提案モデルの精度向上や提案モデルを適用した大規模解析などは今後の課題である。

Fig.4. e-p’ relationship (Dotted lines express critical state)

Fig.5. Stress path during undrained shear