Cause Investigation of Abnormal Settlement Phenomenon in Pleistocene Clay Layers by Experimental Examination of Clay Structural and Mineralogical Composition Changes

本研究利用应变路径控制三轴仪对更新世粘土的K0固结特性进行了试验研究。阐述了该试验与常规试验相比的优点。利用BINDER单元系统测量剪切模量值，间接观察颗粒结构的变化。此外，还用扫描电子显微镜直接观察了颗粒结构。可以从这项工作中得出以下结论1。结果表明：1.应变路径控制试验具有以下优点：试件内应变和超孔压可自由控制，即使在大应变范围内也能保持均匀；可以精确控制粘土的排水模式和K0固结模式，缩短测试次数。这一应变路径控制的K_0固结试验结果表明，更新世粘土具有非常典型的固结特性，孔隙率降低了…在前期固结应力刚过后，更多的资金迅速释放。这种行为在重组高岭土粘土样品中并不明显。K0值先降至固结前应力，然后迅速增大，并趋于稳定。这种结果似乎是由于先期固结应力作用后粘性土发生结构性变化所致。在K_0固结试验过程中，剪切模数G逐渐增加到100(Mpa)，之后基本保持不变。G中这一变化点的孔隙率与先期固结应力后迅速变化的孔隙率完全一致。这种变化G的主要原因可以认为是在预固结应力作用后，更新世粘土的结构发生了显着的变化。利用扫描电子显微镜对更新世粘土的微观结构进行了观察。虽然在测试前的样本中观察到完整的硅藻，但在测试后的样本中观察到大量被破坏的硅藻。可以认为，先期固结应力后孔隙率迅速降低的原因之一是硅藻骨架被破坏。在本研究中，未观察到粘土颗粒连接结构的变化，认为这是先期固结应力附近孔隙比迅速减小的另一个原因。较少

In this research, the K_0-consolidation characteristics of Pleistocene clay were examined using the strain-path controlled triaxial apparatus. The advantages of this test are described when compared with the conventional tests. Shear modulus was measured using bender element system to observe the change of particle structures indirectly. In addition, the particle structures were observed directly using the scanning electron microscope. The following concluding remarks can be obtained from this work.1. The advantages of the strain-path controlled test were shown in the following : Strain and excess pore pressure in a specimen can control freely and can keep homogenous even in the large strain domain ; the drained mode and the K_0-consolidation mode of clays can be controlled exactly and their testing times can be shortened.2. The result of this strain-path controlled K_0-consolidation test showed the very characteristic consolidation behavior of Pleistocene clay ; the void ratio decreas … More ed rapidly just after the pre-consolidation stress. This behavior is not apparent in a reconstituted kaolin clay sample. The K_0 value decreased to the pre-consolidation stress, then increased rapidly after that, and became a steady value. It seems that such results were due to structural change occurring in the clay just after the pre-consolidation stress.3. The shear modulus G during the K_0-consolidation test gradually increased to 100 (MPa), then it almost kept constant after that. The void ratio of this changing point in G precisely agrees with the one changed rapidly just after the pre-consolidation stress. The main cause of this change G can be recognized to be structural changes of Pleistocene clay significantly just after the pre-consolidation stress.4. The microscopic structure of Pleistocene clay was observed using the scanning electron microscope (SEM). A large number of diatoms destroyed were observed in the sample taken after a test although intact diatoms were observed in the one before a test. It can be considered that one of the causes for the rapid reduction of the void ratio just after the pre-consolidation stress is the frame of diatoms destroyed. In this research, the change of link structures of clay particles was not observed, which was considered as another cause of the rapid reduction of void ratio near the pre-consolidation stress. Less