Observation of bifurcation of geomaterial in 3-D conditions through triaxial test using rectangular specimens

澄清和预测岩土材料的应变局部化是提高岩土工程问题计算精度必不可少的研究课题。以往关于应变局部化的数值研究大多是在平面应变条件下进行的，而试验研究大多是采用圆柱试样的三轴试验或平面应变试验进行的。对于地下应变局部化，二维分析与三维分析存在较大差异。考虑剪切带的形成，有必要对三维条件下的地基进行分析。为了将岩土材料的变形破坏行为作为真正的三维现象来掌握，本研究采用矩形粘土试件进行了三轴压缩试验。通过对数字照片的图像分析，可以得到试样侧面的剪切应变分布。所得结果如下：在低应变水平范围内，即使采用矩形试件，其应变速率依赖行为仍与常规三轴试验相同。应变速率影响试样的变形模式；较低应变率试验中，试件表面的应变局部化更为显著，特别是通常固结的粘土试件。而超固结粘土试样的应变局部化面积较大。最终的变形模式与试件的形状和尺度密切相关，而与固结条件无关。在应力-应变关系中，试样的应变软化更为明显，变形呈屈曲柱状。恰恰相反。试件的应力-应变关系几乎不发生明显的应变软化，存在多个剪切带并相互交叉。

Clarification and predictions of strain localization of geomaterials are indispensable research topics to improve an accuracy of computation of the geotechnical engineering problems. Most of past numerical studies concerning the strain localization have been done under the plane strain conditions and most of experimental researches are done by the triaxial tests using cylindrical specimen or by the plane strain tests. With respect to the strain localization in the ground, large difference between two- and three-dimensional analyses exists. It is necessary to analyze ground under three-dimensional condition to consider the shear band formation. In the present study, in order to grasp the deformation and failure behaviors of geomaterials as true three-dimensional phenomena, the triaxial compression test using the rectangular clay specimen was conducted. The distribution of shear strain observed in side of specimen can be obtained by the image analysis of digital photographs.The results obtained are as follows.In the range of low strain level, the stain rate dependency behavior was observed as same as the conventional triaxial test of clay even though the rectangular specimen was used. The strain rate affected deformation pattern of specimens ; lower strain rate tests had more significant strain localization on the surface of specimens, in particular the normally consolidated clay specimens. However, overconsolidated clay specimen had wider area which strain localization was observed. The final deformation patterns were closely related to the shape and scale of specimens rather than the consolidation conditions.In the stress - strain relations, the strain softening were more clearly observed in the specimens, which deformed like buckling column. On the contrary. the significant strain softening were hardly occurred on stress - strain relations in the case of the specimen, which several shear bands existed and crossed each other.