The Evolution of Sliding-Flow Structure under the Undrained Shearing in the Ring Shear Tests

Abstract

珪砂の非排水単調載荷応力制御リングせん断試験を実施し，粒状体のせん断変形過程を研究した。次の4つの変形状態を選んで研究を行った。即ち，"phase transformation"点と破壊点の間，破壊後の加速過程，定常状態到達直後の状態，およびせん断変位10mまでの定常状態でのせん断状態である。新しく作成したせん断ゾーンサンプラーを使って「非かく乱」サンプルを採取し調べたところ，上記各状態の進行と共にせん断ゾーンの厚さが増加する傾向にあり，せん断ゾーンは3つの部分に分割できることがわかった。この3つの部分とは即ち，コア，コアの上部，およびコアの下部である。特に最後の状態では(定常せん断中の分離作用によって)せん断ゾーン中の粗い粒子と細かい粒子が分離される傾向にあり，最終的には，「流れ構造」と一致する粒子の並行な配列構造が発達した。これはCasagrandeの流れ構造の研究と一致する。この構造が粒状体の残留挙動の原因である。即ち，非排水応力制御リングせん断試験では，粒状体の変形過程において，粒子の配向，配列が大きく影響する。粒子破砕が起きた場合，残留状態において細かい粒子はせん断ゾーンの下部に蓄積する。高速長距離地すべりのメカニズムや粒状体の進行性破壊の理解においても本研究成果は非常に重要なものである。

Undrained monotonic stress-controlled ring shear tests were conducted on silica sand to study the shear deformation process of granular materials. Four stages of deformation have been selected for this study namely the state between phase transformation and failure point, during the acceleration after failure, the moment the steady state appeared, and at the steady state shearing after shear displacement reached 10 m, which represent the beginning to the residual states of deformation processes respectively. Investigating the 'undisturbed' sample which collected by using a new shear zone sampler, revealed that the thickness of developed shear zone tends to increase with the progress of those stages and the shear zone could be divided into three parts I.e. the core, the adjacent zone above the core and the adjacent zone below the core respectively. At a particular final state, the shear zone has a tendency to become separated into parts of coarse and fine particles (by segregation process during steady state shearing) and finally the parallel orientation structure of the grains which correspond to the 'flow structure' was developed. This evidence is consistent with Casagrande earlier observations of flow structure. This structure is responsible to the residual behaviour of granular material. Therefore, under undrained stress-controlled test by using ring shear apparatus, the deformation process of granular material is predominantly by grain reorientation and rearrangement. If the grain crushing exist, the fine particles will be accumulated at the lower part of shear zone at residual state. Those results are very important for understanding the mechanism of rapid long runout landslides, and progressive failure on granular materials as well.